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Detection and Screening of Chemicals Related to the Chemical Weapons Convention

Chemical Weapons Chemicals Analysis

  1. George M. Murray

Published Online: 16 SEP 2013

DOI: 10.1002/9780470027318.a0403.pub2

Encyclopedia of Analytical Chemistry

Encyclopedia of Analytical Chemistry

How to Cite

Murray, G. M. 2013. Detection and Screening of Chemicals Related to the Chemical Weapons Convention. Encyclopedia of Analytical Chemistry. .

Author Information

  1. University of Tennessee Space Institute, Tullahoma, TN, USA

  1. Update based on the original article by Maarten S. Nieuwenhuizen, Encyclopedia of Analytical Chemistry, © 2000, John Wiley & Sons, Ltd

Publication History

  1. Published Online: 16 SEP 2013

1 Introduction

  1. Top of page
  2. Introduction
  3. Detection Roles and Requirements
  4. Detection of Chemical Warfare Agent Vapors
  5. Detection of Chemical Warfare Agent Liquids
  6. Detection of Solids Contaminated with Chemical Warfare Agents
  7. Detection of Industrial Gases or Vapors
  8. Reagent-based Tests
  9. Point Detection Instruments
  10. Standoff Detection Instruments
  11. Research and Development
  12. Principle Chemical Sources of Sensor False Alarms
  13. Abbreviations and Acronyms
  14. Related Articles
  15. References

Verification procedures dealing with chemical warfare agents (CWAs) and other chemicals listed in the schedules of the Chemical Weapons Convention (CWC)1 may consist of the following:

  1. Location of the area of interest for inspection scenarios dealing with CWA storage, production, destruction and industrial facilities, and old and abandoned chemical weapons sites, as well as investigations to obtain information relevant to alleged use.
  2. Sampling and transportation, which involves procedures to send materials from the location area to a receiving area, which can be on the site itself or a chemical analysis laboratory elsewhere.

The Organisation for the Prohibition of Chemical Weapons (OPCW), which was established to administer implementation of the CWC, conducts verification activities and as a result numerous inspections take place. Detection plays a major role in many of the possible inspection scenarios and is a most important component of the health and safety policy of the OPCW. The hazard from CWAs, and other very toxic compounds, may originate from vapors or aerosols, liquids (neat liquids as well as solutions), or solids (neat as well as contaminated solid materials), which may be converted into each other, for example, liquids may evaporate and vapors may adsorb to solids. In a number of the scenarios mentioned, these compounds may be encountered by a member of an OPCW inspection team. As a result, an inspector could become contaminated with toxic materials through a number of mechanisms. Those mechanisms may involve breathing, touching, eating, or drinking, which are all related to the main entry routes of toxic materials into his/her body: the respiratory tract, the digestive tract, or the skin.

All operations involving OPCW personnel are carried out in such a way as to minimize exposure as far as possible, afford reasonable safety, and minimize operational risks. Therefore, for his/her own safety, an inspector needs to protect himself/herself against the hazards he/she may encounter by taking protective measures, such as wearing protective clothing and a gas mask.

The chemicals of interest to the OPCW are listed in three so-called schedules, which are laid down in the Annex on Chemicals of the CWC. These schedules not only include CWAs but also many CWA-related compounds.

Within the framework of verification procedures, detection is somewhat similar to detection in the framework of chemical defense, that is, the detection of CWAs. Many examples exist nowadays of commercially available military equipment that is capable of performing different detection tasks.2 The various types of equipment include manually operated so-called wet-chemical detection kits as well as advanced automated equipment. Military detection equipment is usually designed to be quick and easy to operate, of limited size and weight, and robust.

At this point, the use of military detection equipment for OPCW purposes should be considered. Considering the scheduled chemicals of the CWC, as well as specific requirements for verification purposes as laid down in OPCW's list of equipment,3 the following conclusions can be drawn:

  • Military detection equipment can be used to fulfill a number of requirements, especially in the case of production, storage, and destruction of CWAs, during inspections for alleged use, as well as at old and abandoned chemical weapon sites.
  • Military detection equipment is only suited for the detection of a limited list of CWAs and certainly not all chemicals of interest to the CWC, such as CWA precursors or degradation products. Military detection equipment is designed to operate against accepted wartime risks. During peacetime operations, such as during OPCW inspections, the acceptability of casualties resulting from those operations is much lower. As a result, protection from the use of detectors should be much better than in the case of military use and, therefore, basic requirements, such as response time and sensitivity, are more stringent.
  • Detection equipment that will detect all CWAs listed in Schedule 1 of the CWC is required.
  • Because there are many hazards not related to CWAs, especially during industrial inspection scenarios, the introduction of additional detection methods is necessary. This includes equipment to safeguard an inspector from all kinds of industrial chemicals as well as from explosive or flammable vapor mixtures or low oxygen content of the air.

During the preparative period of the OPCW, many discussions took place with respect to the choice of inspection equipment, including detectors. Eventually, a list of inspection equipment was approved3 detailing operational requirements (general and specific), common evaluation criteria, and technical specifications. These items of equipment were purchased and are used during inspections.

Detection equipment as agreed by the OPCW includes detectors for CWAs in the form of vapors (Section 3), based on both wet-chemical detection principles and physical−electronic techniques; in the form of liquids (Section 4), both freestanding liquids and liquids present in closed vessels; or in the form of contaminated solids (Section 5). According to the OPCW list of equipment for the detection of industrial chemicals (Section 6), only detection tubes will be discussed, because the CWC limits industrial chemical detectors to equipment that does not collect samples, that is, disposable devices. Section 6 includes detectors for explosive or flammable vapor mixtures or lack of oxygen.

2 Detection Roles and Requirements

  1. Top of page
  2. Introduction
  3. Detection Roles and Requirements
  4. Detection of Chemical Warfare Agent Vapors
  5. Detection of Chemical Warfare Agent Liquids
  6. Detection of Solids Contaminated with Chemical Warfare Agents
  7. Detection of Industrial Gases or Vapors
  8. Reagent-based Tests
  9. Point Detection Instruments
  10. Standoff Detection Instruments
  11. Research and Development
  12. Principle Chemical Sources of Sensor False Alarms
  13. Abbreviations and Acronyms
  14. Related Articles
  15. References

2.1 Roles of Detection

The first and foremost important role of detection refers to safety. In many cases, an inspector's sensing organs will not be able to tell him/her when to take protective measures. This is clearly expressed in Table 1, which lists the so-called maximum allowable concentrations (MACs) for short-term exposure of CWAs. The data were derived from the occupational health literature for this type of agent or estimated from the toxic properties of the agents involved. Furthermore, Table 1 lists the limit of smell (LS) of the agents, which have been derived from either systematic studies or accidents that have taken place in the past. In most cases, the LS is higher than the MAC.

Table 1. Comparison of MACs of CWAs and LS
AgentCAS numberMAC (mg m–3)LS (mg m–3)
  1. LS, limit of smell; L, lewisite-1; HD, sulfur mustard; GA, tabun; GB, sarin.

Phosgene(75-44-5)0.43
Hydrogen cyanide(74-90-8)111
Cyanogen chloride(506-77-4)0.63
L(541-25-3)0.515
HD(505-60-2)0.051
GA(77-81-6)0.0210
GB(107-44-8)0.015

Because an inspector cannot smell the danger, he/she would have to wear protective equipment all of the time, which is a physical burden and is not a feasible option. The only solution is to obtain proper information about the hazardous status of the environment by employing technical means. This is called detection.

Detection may play an additional role in location of the area or object, which is of most interest from an inspector's point of view, in the event that sampling and analysis are required during an inspection. Indiscriminate sampling for the different inspection scenarios obviously leads to an excess of samples. Large numbers of samples take a considerable amount of time to analyze, slowing down the inspection process considerably, and conflicting with the often limited duration of the inspection. The inspection team must await the results of the analyses before it can act on findings or discard samples. Therefore, the use of detection equipment may accelerate the process by acting as a preselector to find “hot spots.” The number of samples will be limited or the quality of the samples may increase.

An additional role of detection refers to the screening of samples. One can imagine that during an inspection, not all samples are taken as a result of a previous detection, because (rarely) the detector may not be present or the detector may not be easily operable near the sampling site (e.g., because of the location), or when samples are taken at random (e.g., when the amount of time is limited). In that case, the samples, when collected at a central place on the inspection site, may be screened using detection equipment in order to be classified in accordance with the detection result. This may be important if a selection of samples has to be made or prioritized.

2.2 Specific and General Requirements

In view of the extreme toxicity of CWAs, it should be obvious that the sensitivity of detectors should be high and their response rates rapid. This applies especially for detectors that detect CWAs in open contact with the environment. In fact, a dose-related short response time is an important requirement: the detector is allowed to respond less rapidly at low concentration because the dose an inspector may obtain in the period during which he/she is unprotected (response time of the detector in addition to the time needed for putting on protective gear) may be acceptably low. For example, for vapors of nerve and blister agents in air, the lower limit of detection (LOD) is in the range of 5–20 µg m−3 and 200 µg m−3, respectively, with response times of <2 min and a clear-down time, which is the time needed for the signal to arrive at its “zero” value, of <10 min, preferably <2 min.

Assuming a detector that is sufficiently sensitive and the response is fast, it should also be very selective. On the one hand, when detectors are used for verification purposes, they should be capable of detecting a broad range of chemicals relevant to the purpose of the inspection. On the other hand, they should show a minimum of false indications. False indications can mean two things. One is called a false-positive signal: a signal occurs when there should not be one. This will cause irrelevant protective measures to be taken. Although, this actually leads to overprotection, in the long run, the results may still be dangerous. Through some psychological mechanism, people tend to ignore signals when the false signal rate is too high and as a result no protective measures may be taken when a real alarm occurs. The second false indication is called a false-negative signal: no signal appears when there should be one. This may be caused by the presence of one or more other chemicals in combination with CWAs or other compounds requiring detection. It is quite obvious that the false-negative alarm is even more dangerous than the false-positive alarm.

In addition to the requirements for sensitivity, speed of response, and selectivity, there are many general requirements for transportable devices to be employed in the field in toxic environments. These include criteria such as set-up time, portability, operability by personnel in full protective gear, power supply (preferably battery powered), reliability, low pressure (transport by aircraft), temperature (−25°C to 45°C), and humidity (up to 95% relative humidity (RH)). Chemical hardening, including the possibility of proper decontamination, is also important in this respect. Furthermore, data storage by this type of equipment is not allowed, nor may it collect samples in any way, nor bring the sampled material outside of the inspected area.

3 Detection of Chemical Warfare Agent Vapors

  1. Top of page
  2. Introduction
  3. Detection Roles and Requirements
  4. Detection of Chemical Warfare Agent Vapors
  5. Detection of Chemical Warfare Agent Liquids
  6. Detection of Solids Contaminated with Chemical Warfare Agents
  7. Detection of Industrial Gases or Vapors
  8. Reagent-based Tests
  9. Point Detection Instruments
  10. Standoff Detection Instruments
  11. Research and Development
  12. Principle Chemical Sources of Sensor False Alarms
  13. Abbreviations and Acronyms
  14. Related Articles
  15. References

Before World War II, the presence of vapors of CWAs or other toxic chemicals was detected with biological indicators such as the human nose or the use of small animals (e.g., birds were taken to the battlefield). The days have passed that these methods can be employed. With respect to detection in the vapor phase, two approaches have been identified. One approach is called the chemical approach, that is, the application of detection kits. The other, more modern, approach employs physical−electronic devices. Detection kits differ from physical−electronic devices in their operation: the former are usually rather laborious to operate. Furthermore, in the case of detection kits, the extensive use of (bio)chemicals may cause problems from the point of view of logistics as well as shelf life. On the other hand, detection kits are often more selective, although considerable progress has been made with the various physical−electronic systems in that respect. The latter systems are usually faster and more flexible as they may be adapted to detect other compounds or classes of compounds.

3.1 Wet-Chemical Detection Systems for Chemical Warfare Agent Vapors

According to the OPCW, detection kits employing wet-chemical detection schemes must be capable of detecting blister agents, nerve agents, and the so-called blood agents (agents causing blocking action of hemoglobin in the blood) as a minimum. The sensitivity, expressed as a lower LOD, should be 0.01 mg m−3 for nerve agents, 0.07 mg m−3 for mustards, 0.7 mg m−3 for lewisite-1 (L), 1 mg m−3 for cyanogen chloride (CK), and 6 mg m−3 for hydrogen cyanide (AC), with a response time of 5–15 min per class of agent.

When using a wet-chemical vapor detection kit, a rather complicated process yields a positive or negative detection result. First of all, a sample is taken by drawing air through an adsorbing material using a manual pump. The adsorbing material may be present in a tube of which both ends need to be broken before use, or on a small pad, both of which may be placed in front of the pump. Then, one or more reagents are added to the sample according to a fixed procedure. In the case of detection tubes, one or more of the reagents may also be present in the tube contained in small ampoules, which can be broken successively after sampling has taken place. After some time, a color develops, which tells the operator whether the detection result is positive or negative.

Later, in order to illustrate some of the chemistry involved, a number of detection schemes are treated in more detail. Two reaction schemes are shown dealing with the detection of organophosphorus (OP) compounds (nerve agents), including the Schoenemann reaction and the enzymatic detection principle. A third detection scheme deals with the detection of alkylating compounds such as mustards.

3.1.1 The Schoenemann Reaction

The Schoenemann reaction, originating from 1944, is based on the fact that peroxophosphonates oxidize amines much easier than other peroxy ions (Scheme 1).4 First, the OP compound reacts with hydrogen peroxide or sodium perborate in an alkaline solution (pH 9–10). Then, the resulting peroxophosphonate reacts with a leuco dye, such as benzidine or o-dianisidine, to form an orange-brown-colored reaction product. In certain modified reactions, the amine is replaced by a precursor of a chemiluminescent compound (e.g., luminol)5 or a fluorescent compound (e.g., indole).6 The colorimetric variant of the Schoenemann reaction has been applied to many kits using detection tubes or detection pads.

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Scheme 1. The Schoenemann reaction for the detection of organophosphorus compounds.

3.1.2 Enzymatic Detection

The use of enzymes in the diagnosis of diseases is an important benefit of biochemical research and clinical analysis since the 1940s. The real impact of enzymes in chemical analysis was not felt until the early 1960s. In that period, the use of enzymes for the detection of many types of compounds, including the so-called anticholinesterases, was initiated.7 During the action of nerve agents on the human body, the active center of the enzyme acetylcholinesterase is phosphonylated or phosphorylated. Once the enzyme is deactivated in this way, it can no longer fulfill its function of hydrolyzing acetylcholine, which plays an important role in the transfer of nerve stimuli. Consequently, the acetylcholine concentration is increased and the specific symptoms of poisoning appear. The enzymatic detection method employs the enzyme-inhibiting properties of certain OP compounds. This kind of detection method can be regarded as a first example of a biosensor. In this detection method, the naturally involved acetylcholine is replaced by a chromogenic substrate, for example, 2,6-dichloroindophenyl acetate (Scheme 2). Normally, the enzyme will catalyze the hydrolysis of the ester group of the substrate, which causes a distinct color change from orange red to blue. As soon as the enzyme is inhibited, this hydrolysis reaction will not proceed and the color will remain red. This difference in color between the active and inhibited states of the enzyme can be observed visually or spectrophotometrically. The latter technique made the automated detection of nerve agents possible.

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Scheme 2. The colorimetric version of the enzyme inhibition method for the detection of organophosphorus compounds.

3.1.3 Detection of Alkylating Compounds

Many CWAs other than OP compounds have in common their strongly alkylating properties, that is, a good so-called leaving group, such as the chlorine atoms of sulfur mustard (H or HD), CK, phosgene (CG), and lewisite. A detection method employing these alkylating properties, especially for the detection of mustards, was postulated in the 1950s, although the related chemistry had been known in the 1920s.8 As an example, the reaction of HD with (4-nitrobenzyl)-pyridine, often called DB3, is shown in Scheme 3. Upon reaction with HD, a change from colorless to blue is observed after treatment with sodium hydroxide, whereas reaction with CK yields a yellow-orange color.

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Scheme 3. The reaction of HD with (4-nitrobenzyl)-pyridine. R = ClCH2CH2SCH2CH2.

3.1.4 Detection Kits

The detection reactions described earlier have been applied in military detection kits. Within the military context, the content of a specific detection kit is strongly dependent on the operational use that is foreseen, as well as correlation with results of other detection techniques, varying from individual devices to automated detectors, reconnaissance vehicles, remote sensing devices, and even field laboratories. As a result, the kits differ with respect to the kind of CWA they are designed for, the kind of detection chemistry that is performed, the operating procedures, the sensitivities, the number of detections available per CWA, and the layout of the kit as well as size, weight, shelf life, and price.

In Section 1, it has already been stated that in the context of the OPCW, military detection kits are not always suitable, especially because their LODs are directly related to acceptable risks. Some military detection kits almost match the sensitivity requirement. In Table 2, the LODs of a vapor reconnaissance kit used by the Netherlands Armed Forces are listed as an example. In addition, the table lists the MAC that is already listed in Table 1, indicating that as a result of compromises related to the military use, the LODs are not always sufficient from a nonmilitary point of view. In order to lower the LODs of this type of wet-chemistry-based detection kits, one solution is simply to increase the sampling time. As a result, more air is sampled and more contaminants are allowed to adsorb. Disadvantages are the increased amount of time needed for sampling and the increased occurrence of false positives.

Table 2.  LODs of the Netherlands Vapor Reconnaissance Kit for a Number of CWAs Including MAC Values for the Agents Listed in Table 1
AgentMAC (mg m–3)LOD (mg m–3)
Phosgene0.45
Hydrogen cyanide115
Cyanogen chloride0.61
L0.53
HD0.050.2
GA0.020.002
GB0.010.002

Apart from the rather complicated detection kits, individual handheld detectors have also been developed, employing the same kind of chemistry. In the military context, these are mostly used as end-of-alarm indicators. A variety of these so-called residual vapor detectors have been developed in several countries. As an example of an individual nerve agent detector, the so-called “button” detector was developed by the TNO Prins Maurits Laboratory (now TNO Earth, Environmental and Life Sciences) in the 1980s, although it is no longer in service. This cone-shaped detector consists of a plastic holder containing two separated air-permeable reagent papers. One paper (the enzyme paper) is impregnated with the enzyme butyrylcholinesterase, which acts in a similar way to acetylcholinesterase, and silica. The other paper (the substrate paper) contains the enzyme substrate 2,6-dichloroindophenyl acetate (see Section 3.1.2). The detector also contains a reservoir that releases a reagent solution when punctured. When air is drawn through the enzyme paper, the OP compounds are adsorbed onto the silica. After some time, the “button” is pressed. The reagent solution is released, the reagent papers are wetted, and at the same time pressed together initiating the enzymatic detection reaction. After 2 min, the blue color of the decomposed substrate can be clearly observed, or not in the case of a positive detection result.

3.2 Automated Chemical or Physical−Electronic Detection of Chemical Warfare Agent Vapors

Since the 1970s, several automated chemical or physical−electronic chemical vapor detectors have been developed for military, industrial, or environmental use. Although the systems are usually rather complex, they are designed to be easy to handle. They have been designed as chemical vapor warning devices in order to tell persons when to take protective measures. These systems usually operate in an automated, continuous, stand-alone way. Other types of equipment often apply the same detection technologies in another way, that is, as chemical vapor monitors, which are usually handheld devices. From the point of view of the various CWC verification scenarios, monitors are much more relevant than warning devices. Military warning devices are designed to be operational for a relatively long period of time in a stand-alone mode in order to protect troops against an attack with CWAs. Military monitoring equipment is used in close vicinity to, and handled by, the user.

The most important vapor detection techniques for the detection of CWA are based on electrochemical detection, flame photometry, and ion mobility spectrometry (IMS) and examples of these techniques are used by the OPCW. They are discussed in more detail in the following sections.

3.2.1 Electrochemical Detection

One electrochemical method of detection is based on the reaction of some OP compounds with certain oximes, such as isonitrosobenzoyl acetone (IBA), to liberate cyanide ions under the influence of a hydroxide ion and a good leaving group. In the case of V-type nerve agents, a reactive leaving group is introduced by reaction with silver fluoride on a conversion filter before the reaction with IBA. The reaction intermediates initially yield one cyanide ion per molecule of CWA. Subsequently, a cyanide ion also reacts with IBA under the influence of hydroxide ions to yield two cyanide ions in return. Thus, the initial level of cyanide ions is doubled, which can be seen as chemical amplification. The cyanide can be detected via a colorimetric reaction with p-nitrobenzaldehyde or electrochemically.

On the basis of this electrochemical method of detection, a miniature CWA detector called individual chemical agent detector (ICAD) was developed in the United States. The detector consists of a reusable electronic module (processor, audible alarm, and warning light) and a disposable sensor module containing a battery and the sensor cells.

Another approach to electrochemical detection is the use of ion-selective electrodes (ISEs). In the case of sarin (GB) and soman (GD), hydrolysis of the nerve agents yields a fluoride ion that can be detected with a fluoride-selective electrode, whereas tabun (GA) yields a cyanide ion that can be detected with a cyanide-selective electrode.

The enzyme inhibition approach can also be combined with electrochemical detection. When using acetyl or butyryl thiocholine as the substrate, a potentiometric detector can detect the activity of the enzyme. The concentration of converted substrate is a measure of the presence of the enzyme inhibitor.

3.2.2 Flame Photometry

The application of flame photometric detector (FPD) was first reported in 1966 as a selective gas-chromatographic detector.9 Normally, the concentration of OP compounds other than CWAs in the atmosphere is very low. Therefore, a more or less selective method could be developed based on the detection of phosphorus using flame emission. When phosphorus-containing compounds are burnt in a hydrogen-rich flame, excited HPO species are formed, whereas sulfur-containing compounds form excited S2 species. When these species return to their ground state, light is emitted in the wavelength range 400–550 nm. In particular, the intensive flame emission near 526 nm is attributable to the HPO species and, therefore, characteristic of phosphorus, whereas flame emission from the S2 molecule can be measured at 394 nm. Flame photometric devices only detect compounds containing phosphorus and/or sulfur and, therefore, do not cover the whole range of CWAs and related compounds. In the case of the nerve agent O-ethyl-S-2-diisopropylaminoethyl methylphosphonothiolate (VX), which contains both phosphorus and sulfur, a response is shown in both the “P-channel” and the “S-channel.” A more recent development, the pulsed FPD, can detect additional elements, including nitrogen and arsenic.

In Figure 1, the basic scheme of an FPD device is shown. Air is drawn into the detection chamber using an air pump. The air contaminants are burnt in a hydrogen-rich flame. In the chamber, a photometric cell in combination with optical filters detects light emission at the relevant wavelengths.

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Figure 1. Schematic representation of an FPD device (A = air pump, B = reaction chamber, C = flame, D = hydrogen supply, E = photometric cell, F = electronics, G = display).

CWA detectors for military purposes have been developed employing this method in a number of countries. The most advanced system, developed in France, is a four-channel (P, S, As, HNO) handheld system called AP4C, based on the previous AP2C. The main problem, however, for this type of detection device is the hydrogen supply, which requires an extra logistic pathway in addition to electrical power, which is used by any kind of physical−electronic system. LODs for modern military flame photometry-based detectors are approximately 0.002 mg m−3 for nerve agents and approximately 0.1 mg m−3 for HD with almost real-time responses. The AP4C is also capable of detecting some toxic industrial chemicals (TICs).

3.2.3 Ion Mobility Spectrometry

IMS refers to the principles, practice, and instrumentation for characterizing chemical substances through the measurement of gas-phase ion mobilities. Although mobility measurements of gas-phase ions have been studied by physicists since the beginning of the twentieth century, no attempt was made to employ these phenomena for chemical detection purposes. In the 1960s, numerous devices employing relatively simple ion separations (i.e., without the use of high vacuum and magnetic fields as in mass spectrometry (MS)) were described, mostly in a military context, until in 1970, the development of IMS began.10 In modern analytical IMS methods,11 ion mobilities are determined from ion velocities measured in a drift tube. All processes occur commonly at ambient pressure in air or nitrogen. Ion mobilities are characteristic of substances and provide a means for detecting and, at least partially, identifying vapors.

A general description of IMS is given in this section. In Figure 2, a schematic layout of an IMS device is shown indicating the reaction region and the drift region. This IMS device is the chemical agent monitor (CAM) used by many armed forces in the world as well as by the OPCW.

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Figure 2. Schematic representation of an IMS detection device. (Reproduced by permission of Smiths Detection.)

Ambient air is drawn into the inlet of the detector by a pump. This air, with its possible contaminants, first passes through a dust filter and subsequently comes into contact with a silicone membrane. The membrane allows some of the target material to permeate while excluding (to a higher degree) a number of other contaminants found in air, such as water vapor. Ions are generated by subjecting the permeated air to a 63Ni radioactive source emitting β-radiation. The β-particles ionize the air molecules to form a population of highly mobile positive and negative ions, the so-called reactant ions, among which are [(N2) x ·H(H2O) n ]+ and [(N2) x  · (H2O) n  · O2] (x = 1–3; n = 1–3). Charge transfers occur by multiple collisions of these reactant ions with molecules of contaminants if present in the air. These complicated reactions result in the formation of relatively stable ionic clusters. These so-called product ions are usually relatively large compared to the (air) reactant ions. The ions are swept through the reaction region by an electrical field.

Figure 3 represents the formation of a positively charged ion cluster in air containing water (Equation a), and the formation of negatively charged ion clusters in air containing water as a result of ion transfer (Equation b), charge transfer (Equation c), or dissociative charge transfer (Equation d).

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Figure 3. Reaction equation for the formation of positively charged (a) and negatively charged (b, c, and d) ion clusters in air with water (M = analyte molecule, X and Y = analyte molecule fragments; x = 1–3; n = 1–3).

Entrance of the ion clusters into the drift tube is restricted by a metal grid, which acts as a shutter. The shutter grid opens periodically (typically for 0.3 ms in every 30 ms) allowing the ion clusters to enter the drift region. The shutter grid provides control over the length of the periods and the intervals between them. An electric field (commonly 200 V cm−1) imparts a constant force to the ion clusters leading to separation in accordance with their respective mobilities (commonly 1–2 cm2 V−1 s−1). Ion velocity is inversely dependent on the effective collisional cross section of an ion. This makes IMS an ion-size analyzer. Ions with the highest mobility travel faster and advance in front of those with lower mobilities. At the end of the drift tube, the ions collide with a collector giving rise to current pulses. Plotting this current, which is a measure of the intensity of the ions, versus the drift time results in an ion mobility spectrum, which was previously known as plasma chromatogram. Although the latter term better represents the separation of the cluster ions, the technique is nevertheless called spectrometry, which may be confusing from a physical point of view.

The IMS plot will not always show one single peak. First of all, not all reactant ions are converted into clusters and, therefore, a residual reactant ion peak (RIP) is present, which can be used for internal reference purposes. Secondly, one single compound may generate a number of equally stable clusters with different mobilities as a result of their composition, that is, the so-called monomer clusters containing one analyte molecule and some reactant species, as well as larger dimer clusters resulting from interaction between the monomer cluster and one more analyte molecule, resulting in a lower mobility (larger drift time). The relative height of the various peaks is usually concentration dependent, whereas peak shapes are affected by the complexity of the cluster mixture and the kinetics of the reactions between the clusters as compared to the IMS timescale.

Two IMS modes may be employed: a positive mode detecting positively charged clusters originating from compounds with a high proton affinity, such as OP compounds, or a negative mode detecting negatively charged clusters originating from compounds with a high electron affinity such as organochlorine compounds (mustards, H).

Figure 4 shows a number of IMS plots recorded by the TNO Prins Maurits Laboratory with a CAM. The IMS plots were obtained using dedicated data acquisition software. Normally, only qualitative and semiquantitative information is provided on a liquid crystal display screen in the form of a bar graph. In Figure 4(a), a positive mode IMS plot of the nerve agent GB (0.2 mg m−3 at 45% RH and 22°C) is depicted showing both the monomer and the dimer peaks along with the RIP. In Figure 4(b), the negative mode IMS plot of the blister agent HD is depicted (0.6 mg m−3 at 45% RH and 21°C). In this plot, a cluster is also shown with a mobility larger than RIP. This is the result of HD fragmenting into smaller ions such as Cl. Figure 4(c) shows the IMS plot of the blister agent L (0.3 mg m−3 at 60% RH and 21°C) indicating both monomer and dimer clusters as well as a cluster related to the decomposition product.

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Figure 4. IMS spectra of (a) GB (positive mode; 0.2 mg m–3), (b) HD (negative mode; 0.6 mg m–3), and (c) L (negative mode; 0.3 mg  m–3). (Data recorded at the TNO Prins Maurits Laboratory.)

Owing to the short opening time of the shutter grid, only about 1% of the total ion current is collected. This affects the LOD, a disadvantage that is claimed to be more than compensated for by the high selectivity of IMS technology. This allows the manufacturer to set the alarm levels relatively low, bringing the LOD to the required level. The disadvantage of setting the LOD low is that more interferences may occur. In practice, both false positives (signal without analyte present) and false negatives (no signal when analyte is present) tend to occur, especially when the sensitivity is set relatively high. Operator training may be very helpful in dealing with these phenomena. In addition, the use of different physical−electronic techniques together with IMS may yield a large synergistic effect in the area of false signal rejection. During United Nations Special Commission (UNSCOM) missions in Iraq following the 1991 Gulf War, CAM and the FPD AP2C operated “hand in hand.”

With respect to IMS, it should be mentioned that worldwide developments continue, aimed at improved IMS, exploring nonradioactive ion sources, such as corona discharge, different reagent chemistries to improve selectivity, improved data-handling systems and miniaturization, as well as coupling of IMS to gas-phase separation techniques such as gas chromatography (GC). In particular, the search for nonradioactive ion sources is important from the point of view of the OPCW because the presence of a radioactive source can cause transport problems on international deployment. Furthermore, dedicated IMS systems for compounds other than CWAs are already available, whereas flexibility is increased by using writable memories, which enable reprogramming from one target analyte to another. The current state of the art incorporates high-resolution IMS with GC and an MS (GC/HRIMS/MS).12

4 Detection of Chemical Warfare Agent Liquids

  1. Top of page
  2. Introduction
  3. Detection Roles and Requirements
  4. Detection of Chemical Warfare Agent Vapors
  5. Detection of Chemical Warfare Agent Liquids
  6. Detection of Solids Contaminated with Chemical Warfare Agents
  7. Detection of Industrial Gases or Vapors
  8. Reagent-based Tests
  9. Point Detection Instruments
  10. Standoff Detection Instruments
  11. Research and Development
  12. Principle Chemical Sources of Sensor False Alarms
  13. Abbreviations and Acronyms
  14. Related Articles
  15. References

This section deals with the detection of neat CWA liquids (detection of CWAs dissolved in water is not considered here), including the detection of freestanding CWA liquids, as well as the so-called nondestructive evaluation (NDE) techniques for the detection of CWA liquids contained in various kinds of vessels. NDE is a screening technique, which aims at obtaining more information (including some information about the chemical nature) about the liquid inside the vessel in order to determine if it contains a CWA or not. Depending on the scenario, this information is relevant in many ways. For instance, the information may contribute to the decision whether sampling, that is, opening of the vessel and taking a sample, is necessary.

4.1 Detection of Free-standing Chemical Warfare Agent Liquids

According to the OPCW, neat liquid G-type nerve agents, V-type nerve agents, and mustards should be determined quickly and separately. To detect neat liquids, several types of detection paper, as well as powders or chalks, employing colorimetric reactions have been developed. The reactions should not be compared with the relatively complicated reaction schemes used in wet-chemical detection of vapors (see Section 3.1). In general, the matrix material (paper, chalk, or powder) is impregnated with one or more reactive dyes, which develop a color change upon reaction with a CWA. These methods may be especially useful within the context of the verification of alleged use and during inspections of military production, chemical weapon stockpiles, or CWA destruction facilities. Liquid detection paper is easily used as an initial screening method for droplets or larger amounts of liquid, which are suspected to consist of CWAs. Nerve agents and blister agents are detected quickly in a more-or-less selective way. Some liquid detection methods are able to detect the three different categories of CWA with one and the same device, because of the presence of three different dyes impregnated in the matrix. It should be noted, however, that some non-CWA liquids may also cause development of color.

In contrast to the advanced vapor detection equipment, there is a lack of direct physical−electronic equipment for the detection of freestanding CWA liquids, especially when small amounts of liquid are lying on the ground or on material, or even absorbed in the pores, cracks, and crevices of material. Some systems have been proposed to fill this gap, most of them employing active laser-based detection methods, sometimes even preceded by chemical treatment of the surface to generate chemiluminescent compounds, as in the case of the Schoenemann reaction mentioned in Section 3.1.

It should be clear that, from an operational point of view, the simple methods for the detection of freestanding liquids or contaminated surfaces do not always suffice. In order to overcome this problem, an indirect approach is often used. Assuming any liquid that is accompanied by a certain amount of vapor, one could try to reveal the presence of a liquid by tracking the vapor all the way to the source. This approach only works if the vapors are not evenly distributed in the atmosphere. Only in the case of vapor concentration gradients (plumes) may one track the vapor in the direction of higher concentrations toward the source.

At this point, it should be stressed that this indirect approach is limited in the case of CWAs dissolved in liquids, because the vapor pressure will be much lower, allowing only a very small amount of agent vapor to be released into the atmosphere. Also at low temperatures and with agents that possess a low volatility (e.g., the nerve agent VX), or that are very strongly adsorbed to the surface, the release of vapor into the atmosphere may be very limited, whereas the so-called contact hazard may still be significant. CWAs will still be transferred from the liquid or the contaminated surface through the skin of a relatively warm bare hand.

The OPCW requirements for vapor detectors to detect chemicals in a liquid form are 0.01 mg  cm−2 for nerve agents and 0.08 mg  cm−2 for HD. Military vapor detection equipment can be used for this indirect approach. After proper training, one can use a flame photometric or IMS-based device to screen surfaces in a very controlled manner in order to follow concentration gradients and find liquid spots, or leaking tanks or munitions.

It may also be possible to transfer liquid to the detector by picking it up with an absorbent material and releasing the vapor by heating the material in front of the inlet of a vapor detection device. Although the number of manipulations is increased and special tools are needed, the chances of locating the liquid or contaminated surfaces in a sensitive and selective way are raised significantly. One of the commercially available FPD devices, the previously mentioned AP4C, employs a small battery-powered accessory to absorb liquids and subsequently release them into the inlet of the detector.

4.2 Nondestructive Evaluation of Chemical Warfare Agent Liquids

As stated in the previous section, there are several ways to monitor the environment for the presence of CWA liquids. In certain types of inspection, for example, at declared or suspected chemical weapons stockpiles, or in the case of unexploded munitions or at destruction facilities, an inspection team may be faced with the issue of verifying the contents of a munition, reactor, tubing system, or container. Dealing with chemical munitions is especially difficult. The objects are hazardous both chemically and explosively and, therefore, safety concerns are very important. Some of the problems an inspector is confronted with are many chemical munitions appear identical to conventional munitions; agent containers or storage tanks are not unique to that purpose; munitions can be multipacked and containerized. Nevertheless, the verification of the contents, both qualitatively and quantitatively, forms a major task for an inspector. One could try to open the object by drilling or punching a hole to access the liquid or facilitate the release of vapor, but one can imagine that this may be impossible, very dangerous, at least very time consuming, and require special skilled people (explosive ordnance personnel). Therefore, several techniques have been developed to obtain information about the content of an object without opening or damaging the object. A convenient suite of NDE equipment is available for the characterization of munition fills.13 Acoustic, X-ray, and radiometric techniques are discussed in more detail.

Decisions about inspection alternatives are in general complex because of the numbers and types of objects, storage configurations, inspection objectives, and the level of confidence required. Table 3 summarizes the relationship between information that may be derived, the method employed, and the applicable munition configuration.

Table 3. Relationship between Information Derived (increasing information content from top−down), Method Employed, and Applicable Munition Configuration
Increasing information contentDetection methodMunition configuration
SolidHuman earFree standing
Solid versus liquidAcoustic or X-rayFree standing
Physical propertiesAcousticFree standing
Integral fill signatureAcousticFree standing
Inventory of elements in the fillNeutron activationAny configuration
Molecular structure determinationSampling and analysisAny configuration
4.2.1 Acoustic and Ultrasonic Detection Methods

As a minimum, the human ear can act as an NDE device to determine whether either freestanding liquid or solid material is present. When tapped with a metal object, such items will give a bell-like sound if they are empty or liquid filled. It should be noted that the human ear approach may be somewhat difficult when wearing protective clothing, including a gas mask. Acoustic and ultrasonic systems with their enhanced analysis ability can detect liquid-filled items, determine the fill levels, and obtain signatures that can lead to distinguish the CWA fill type. When other than freestanding munitions are present, and when a more direct indication of the fill type is desired, the neutron interrogation systems can be applied.

The major limitation of the acoustic and ultrasonic techniques is that access to the primary wall of the container is required. Shipping containers and packaging effectively preclude the use of acoustic probing techniques.

In the case of acoustic resonance spectroscopy (ARS), objects respond with resonant sound when mechanically excited.14 Two piezoelectric transducers are used: a transmitter and a receiver. By employing the piezoelectric effect, which is a long-known phenomenon, electrical energy is converted by the transmitter into mechanical energy in the form of acoustical waves.15 Properties such as frequency (in the 1–30 kHz range) and amplitude of a sweep of acoustic waves with different wavelengths are modulated by the physical properties and geometry of objects with which the waves interact on their way to the receiver, where the mechanical energy is converted back to electrical energy employing the same piezoelectric effect. In the output spectrum thus obtained, signals only become distinguishable (up to two orders of magnitude above the noise level) when a resonance condition determined by the fill and the fill level is reached. As a result, the resonance spectrum provides a unique signature for objects and their contents. Because the acoustic signature is based on whole-body vibrations of the object, the exact placement of the transducers on the object is not very critical.

According to the OPCW, ARS must be able to discriminate between solid- and liquid-filled munitions with an acceptable degree of confidence, and determine the fill level of a container.

Figure 5 shows the response of the nerve agents GB and VX in the same type of projectiles. It should be stressed that this does not mean that some degree of absolute identification can be performed with this kind of equipment. As the total system is responding, the orientation of the item is not critical and even standoff excitation is possible. ARS can be best used to measure the degree of similarity between two or more filled items. Thus, if the resonance spectrum of a certain type of shell filled with, for instance HD, is known, the spectra of other shells of the same type with an unknown fill can be compared with it. Furthermore, this method can be employed to determine the thickness of the walls of a container. In the case of relatively old munitions or storage tanks, this will be an especially useful approach.

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Figure 5. Acoustic resonance spectra of two 155-mm shells, one filled with the nerve agent VX and the other filled with the nerve agent GB. (Reproduced from Sinha and Apt14 by permission of the University of California, Lawrence Livermore National Laboratory (LLNL), and the US Department of Energy (DOE).)

Ultrasonic pulse echo (UPE) interrogation of physical structures is a very mature technology.16 The basic principle of this technique is similar to that of sonar. A high-voltage transducer, coupled to the object to be investigated, provides a square-wave pulse of ultrasonic energy that travels through the wall and into the contents. A search unit made from a piezoelectric material is used to convert the reflected sound waves back into electrical signals. Pulse reflections and times of echo returns are a function of the physical properties of the object. The frequencies used are in the megahertz range. Key characteristics are that pulses are reflected from any material boundary, echoes are returned sooner in the case of solids, and powder fills damp out multiple echoes. All interactions provide indications of the nature of the fill. The simple presence or absence of an echo is a clear indication of whether a fill is a liquid or a solid. The times of the echo returns can indicate structural or fill variations. The fill level can be determined by sweeping the device alongside the object.

According to the OPCW, UPE equipment must be able to sort like items by comparison of sound propagation time with an acceptable degree of confidence and be able to determine the liquid fill level of a container.

In Table 4, the differences in speed of sound, expressed as their transit times, of the nerve agents GB and VX as well as a surrogate munition (ethylene glycol and water) in 155-mm shells are clearly indicated.

Table 4. Transit Times for Different Fills of a 155-mm Shell
Fill typeMean transit time (ms)
  1. VX, O-ethyl S-2-diisopropylaminoethyl methylphosphonothiolate.

GB66.7
VX47.9
Ethylene glycol/water surrogate58.3
4.2.2 X-ray Photography

Another way of performing NDE is the use of low-power portable X-ray analysis. Commercial X-ray equipment is available for this purpose. The density of the material, which an X-ray meets on its way from the source to a detector, determines the amount of radiation that actually hits the detector. This affects the degree of whitening of a photosensitive material. This technique operates in the same way as X-ray photography for medical purposes.

Both X-ray photography and X-ray video may be employed. X-ray photography is cheaper and yields better resolution, whereas X-ray videos are more expensive but the operation is real time and the resolution is adequate. X-ray photography is routinely used to locate items in overpack containers in order to visualize the presence of steel, or at least high-density objects, in plastic or wooden containers or boxes. Furthermore, it can be used to image the internal structure of munitions (X-rays of 150–300 kV can penetrate steel casings up to 1.5 cm thick) and assess the presence or absence of burster charges.

X-ray interrogation can also be used to discriminate between solid and liquid contents by observing the solid–gas or solid–liquid interface. By moving an object such as a shell with respect to the Earth's gravitational field, one can easily distinguish a solid from a liquid because the solid–gas interface will hardly move, whereas the liquid–gas interface will move. The information obtained, that is, whether the shell contains a solid or a liquid, is an important part of the tiered inspection process as shown in Table 3.

4.2.3 Radiometric Detection Methods

Neutron activation analysis (NAA) is a more powerful NDE technique than acoustic spectroscopy and X-ray interrogation, although more time consuming, costly, and subject to stringent safety regulations.17-19 Instrumentation, including portable systems such as portable isotopic neutron spectrometry (PINS), is available for identifying the presence of individual elements indicative of CWAs inside thick-walled steel vessels, such as artillery shells, and even through secondary containers. Both CWAs and high explosives are rich in the elements carbon, hydrogen, and oxygen, but CWAs also contain rather unique combinations of the elements arsenic, chlorine, phosphorus, and sulfur. By determining the atomic ratios present, phosphorus smoke munition and high explosives can easily and automatically be distinguished from G-type or V-type nerve agent munitions. The detector operates in a near-real-time manner. The object to be investigated is irradiated, typically, with low-energy neutrons (binding energies inside the nucleus are 7–10 MeV, except for hydrogen, where it is 2.2 MeV) produced by an isotopic neutron source such as 252Cf. The source is placed near the munition being surveyed. Each second, approximately one million neutrons are emitted by the source, some of which penetrate the casing and interact with the contents: nuclei are excited and decay in less than a picosecond to the ground state, with the emission of characteristic γ-rays. With a high-purity germanium γ-ray detector, requiring constant low temperature (77 K), the energies and intensities of γ-rays released by neutron interactions are measured. The energy of the γ-radiation is characteristic of elements present in the object. In Table 5, a number of characteristic energies as well as the relative elemental composition of a number of CWAs are shown. These energies have been measured and cataloged since the mid-1950s.20

Table 5. Characteristic γ-ray Energies of Some Key Elements for NAA Detection, and Relative Elemental Composition of a Number of CWAs and the High Explosive TNT
Energy (keV)ElementReactionGBVXHDLTNTRemark
279.5As n,nγZ 36.1 L  
93.5Ti n,nγZ Smokes    
1266.1P n,nγZ22.111 Nerve  
    6 Agents  
2233.4  n,nγZ      
3900.3  n,nγZ      
195.9Cl n,γ  44.751.3 L, HD, smokes, bleach 
6110.9  n,γ       
2211.8Al n,γ  Casings    
197.1F n,nγZ13.6 GB   
582.1  n,γ       
3683.9C n,γ 34.349.40.211.437.0Many agents and materials
4945.3  n,γ       
2223.3H n,γ 7.19.75.01.02.2Calibration line
2230.2S n,nγZ 12.020.1  HD, VX, smokes
5240.5  n,γ       
7631.1Fe n,γ  Steel containers    
7645.5  n,γ       
10318.3N n,γ  5.0 18.5HN3, X, explosives 

Figure 6 illustrates the difference between the γ-ray spectrum of HD in a ton container and that of a high explosive in a 155-mm shell.

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Figure 6. γ-Ray spectrum of sulfur mustard in a ton container (a) and high explosives in a 155-mm shell (b). (Reproduced from Caffrey et al.20 by permission of the University of California Lawrence Livermore National Laboratory and the US Department of Energy.)

Some difficulty in easy access to randomly chosen items, such as shells at the back of a bunker or in the middle of a stack, can be envisaged. Furthermore, the presence of a radioactive source could potentially cause transport problems on international deployment. According to the OPCW, neutron interrogation NDE equipment must have levels of emitted radiation such that extensive shielding is not required to limit exposure to safe levels at a reasonable standoff, be capable of detecting chemical elements, and/or ratios of chemical elements in chemicals related to the CWC, including phosphorus, sulfur, arsenic, nitrogen, chlorine and, to the extent possible, carbon, hydrogen, fluorine, bromine, and iodine.

For fast and large-scale screening of objects, an important NAA method is based on hydrogen concentration measurement (HCM). In this method, fast neutrons are emitted from a neutron source (252Cf). Upon collision with protons, the neutrons slow down into so-called thermal neutrons, which are in thermal equilibrium with the environment. The neutrons can leave the object and be measured by a detector, usually consisting of a scintillator, converting neutron energy into light, and a photomultiplier tube for amplification of the light emission. Under controlled circumstances, the number of neutrons detected per unit of time relates to the number of hydrogen atoms per unit of volume. Therefore, calibration for each type of object and fill is required.

Without HCM, the ratio of elements, such as the P/S ratio, cannot be accurately determined with a single measurement as hydrogen significantly diminishes the flow of fast neutrons. A typical measuring time is <1 min, compared to the 30 min required for other NAA methods. With HCM, discrimination can be made between, for instance, different nerve agents or high explosives, provided that the contents are relatively pure. Because HCM is so fast, the fill level may also be measured, especially when UPE fails in the case of double-walled containers where the required contact between the object under investigation and the transducer is not possible.

One major drawback of the HCM technique is the fact that L cannot be detected readily because of its very low hydrogen content.

The OPCW requires HCM to be capable of determining hydrogen concentration within the fill of munitions or other containers at an accuracy of better than 10%. Furthermore, it must be capable of allowing discrimination between explosives and CWAs, such as nerve agents or mustard, as well as discriminating between representatives of the latter class of agents.

5 Detection of Solids Contaminated with Chemical Warfare Agents

  1. Top of page
  2. Introduction
  3. Detection Roles and Requirements
  4. Detection of Chemical Warfare Agent Vapors
  5. Detection of Chemical Warfare Agent Liquids
  6. Detection of Solids Contaminated with Chemical Warfare Agents
  7. Detection of Industrial Gases or Vapors
  8. Reagent-based Tests
  9. Point Detection Instruments
  10. Standoff Detection Instruments
  11. Research and Development
  12. Principle Chemical Sources of Sensor False Alarms
  13. Abbreviations and Acronyms
  14. Related Articles
  15. References

There are a few direct detection methods able to determine whether solid materials are contaminated. Relatively large amounts of suspect liquids can at least be observed visually. A hazard resulting from contaminated solid materials may not be obvious because the CWA may be absorbed or adsorbed, but the contact hazard is still considerable. Therefore, for solid materials, the level of protection should be raised and gloves must be worn at all times.

An indirect approach is monitoring of surrounding air with a vapor detection system. However, this presents the same limitations as mentioned in Section 4.1 for the indirect detection of liquid CWAs. The reader is referred to Section 4.1 for further information about the use of vapor detectors for the detection of contaminated solids or for the checking of decontaminated solids. The AP4C system includes an attachment for sampling and monitoring contaminated solids.

6 Detection of Industrial Gases or Vapors

  1. Top of page
  2. Introduction
  3. Detection Roles and Requirements
  4. Detection of Chemical Warfare Agent Vapors
  5. Detection of Chemical Warfare Agent Liquids
  6. Detection of Solids Contaminated with Chemical Warfare Agents
  7. Detection of Industrial Gases or Vapors
  8. Reagent-based Tests
  9. Point Detection Instruments
  10. Standoff Detection Instruments
  11. Research and Development
  12. Principle Chemical Sources of Sensor False Alarms
  13. Abbreviations and Acronyms
  14. Related Articles
  15. References

In the previous sections, emphasis was put on the detection of CWAs. However, a number of inspection scenarios are not CWA related, but may take place in a toxic environment generating safety problems. Although toxicity may not be as high as in the case of CWAs, the scope of chemical hazards, which may be encountered by an inspector, will be very wide. In the chemical industry, safety of personnel is a very important issue. Within the field of occupational safety and health, many toxic chemicals or classes of chemicals have been identified and much toxicological information has been generated in order to set exposure limits for people working in environments containing those chemicals. As in the case of protection against CWA, a hazard has to be detected in order to protect against it. Therefore, detection equipment for many different chemical hazards has been developed and used on a large scale. The same general problems, such as selectivity, sensitivity, false-alarm rates, and stand-alone compared with manually operated equipment, that were discussed for CWA vapor detection and equipment are applicable.

A basic subdivision of the field is made into direct-reading colorimetric indicators and direct-reading instruments. From the point of view of the OPCW, two classes of equipment are very important: direct-reading colorimeters, especially glass detection tubes, and equipment for the detection of flammability and of explosive vapor mixtures or lack of oxygen.

6.1 Glass Detection Tubes

Glass detection tubes are used to test the environment for the presence of toxic gases or vapors. Detection is indicated by the length or shade of color change of the detector tube after sampling and shows some kind of concentration indication in parts per million, milligrams per cubic meter, or percent volume. After use, the tubes are discarded.

Glass detection tubes containing solid and/or liquid chemicals are convenient and compact detectors. Several companies employ a number of basic reaction schemes for the detection of many organic and inorganic compounds. Some reactions, among others, are

  • reduction of chromate or dichromate to chromous ions for the detection of alcohols, aldehydes, ketones, esters, hydrocarbons, or sulfur dioxide;
  • reduction of iodine pentoxide with fuming sulfuric acid to iodine for the detection of aromatic hydrocarbons, acetylene, carbon monoxide, or vinyl chloride;
  • reduction of ammonium molybdate with palladium sulfate to molybdenum blue for the detection of ethylene, butadiene, hydrogen sulfide, and sulfur dioxide;
  • reaction with potassium palladosulfite for the detection of carbon monoxide or AC;
  • color change of pH indicators for the detection of acidic and alkaline compounds;
  • reaction with o-toluidine for the detection of chlorine or chlorinated organic compounds, bromine, or nitrogen dioxide;
  • reaction with tetraphenylbenzidine for the detection of halogens, halogenated organic compounds, nitric oxide, or acetonitrile.

Use of these detection tubes is extremely simple. Before use, both ends are broken off and the glass tube is placed in a dedicated holder fitted with a squeeze bulb or piston pump. The recommended air volume is drawn through the tube by the operator, closely following the manufacturer's instructions. The operator then reads the concentration in air by examining the exposed tube. The length of the colored stain is a semiquantitative measure of the concentration.

In some cases, direct sampling may be a problem, in which case, the tube may be placed near the sampling point and the pump operated at some distance. A rubber tube extension with the same inside diameter as the sampling tube may be inserted between the detector tube and the pump. When hot air samples need to be taken, cooling may be essential. In that case, a special probe of glass or metal may be attached to the inlet of the tube. When employing special accessories, it is very important that the air components are not adsorbed on the materials used. Therefore, the previously mentioned rubber extension is placed between the tube and the pump and not in front of the tube.

The selectivity of tubes is a major consideration with respect to applicability and interpretation. In some cases, lack of selectivity permits detection of a class of chemical compounds rather than a single compound.

Although detection tubes are usually advertised as being capable of use by unskilled operators, and the procedures are indeed very simple, many limitations and potential errors are inherent in this method. The results may be dangerously misleading and sometimes need to be evaluated by a knowledgeable person.

Figure 7 shows an example of the layout of a detection tube for AC, a TIC also in the CWC schedules. The detection range is 5−50 ppm for two pump strokes and 2–12 ppm for 10 pump strokes. The analyte reacts with mercury chloride to yield hydrogen chloride, which causes a pH-sensitive dye to change from blue to yellow. The type of reaction employed is not sensitive for hydrogen, hydrocarbons, and carbon monoxide up to 50 vol.%; carbon dioxide up to 15 vol.%; halogenated hydrocarbons, nitriles, carbon disulfide, and acetic acid up to 1 vol.%; ammonia and sulfur dioxide up to 1000 ppm; and hydrogen sulfide and hydrogen chloride up to 300 ppm.

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Figure 7. Layout of a detection tube for hydrogen cyanide (1 = sealed tube tips, 2 = marking area, 3 = white protective layer, 4 = blue indicating layer in parts per million, one scale for two pump strokes and the other scale for 10 pump strokes, and 5 = flow direction arrow).

6.2 Detection of Explosive Vapor Mixtures, Flammability, or Lack of Oxygen

The purpose of a flammability/explosive/air-quality monitor is to determine whether safe entry into confined spaces with potentially flammable/explosive atmospheres or insufficient air quality is possible.

A general chemical hazard unrelated to a specific chemical compound is the presence of combustible gases, that is, the hazard that a mixture of oxygen and one or more chemical compounds (hydrogen or volatile hydrocarbons) might explode upon some kind of ignition mechanism. Explosive mixtures sometimes behave very subtly with respect to ignition. A very simple human action taking place inside or near the gas mixture, such as a scratch or a spark resulting from static electricity, may cause the mixture to explode.

A somewhat related issue is lack of oxygen. As a result of a combination of lack of fresh air flowing through a building and release of gases or vapors driving out the oxygen, or smoldering fires or biological activities consuming the oxygen, the oxygen level may no longer be sufficient for inspectors to operate normally. Many companies sell explosive vapor monitors, but they employ much the same operating principle. Most combustible or explosive gas detectors are based on the principle that gases will react with oxygen in the presence of a heated catalyst mounted on a pellistor. Oxidation of a gas further raises the temperature of the pellistor causing, in turn, an increase in resistance, which is detected by a Wheatstone bridge and indicated on a display as a gas concentration. This detection principle has some limitations as some specific gas mixtures may poison the catalyst. Leaded gasoline and certain compounds containing silicone are suspect in this respect. For this reason and because of the fact that the lower explosion limit (LEL) may differ from one gas to another, explosive gas detectors require frequent calibration and tuning to the LEL of the expected worst case. Oxygen in the air (normal concentration 20.9% by volume) is usually detected electrochemically in the 0−25% oxygen range. A number of instruments combine explosive vapor detection and oxygen detection in a general-purpose detector.

The following sections provide more detailed descriptions of detection equipment and its use in scenarios, both military and civil, other than OPCW inspections. Further details of a wide variety of detection equipment can be found by a simple search of the Internet. Manufacturer's websites are a good source of information.

7 Reagent-based Tests

  1. Top of page
  2. Introduction
  3. Detection Roles and Requirements
  4. Detection of Chemical Warfare Agent Vapors
  5. Detection of Chemical Warfare Agent Liquids
  6. Detection of Solids Contaminated with Chemical Warfare Agents
  7. Detection of Industrial Gases or Vapors
  8. Reagent-based Tests
  9. Point Detection Instruments
  10. Standoff Detection Instruments
  11. Research and Development
  12. Principle Chemical Sources of Sensor False Alarms
  13. Abbreviations and Acronyms
  14. Related Articles
  15. References

Historically, most first responders to toxic chemical releases have relied on reagent test kits to identify the chemical. When a liquid or gaseous agent is placed in contact with certain reagents, a change in color, fluorescence, or chemiluminescence may occur. Some reagents react with a variety of agents to give qualitative information, while others simply give a “yes” or “no” response. These reagents are supplied in a variety of forms such as papers, tubes, or vials. Some systems are semiautomated whereby the ambient air is drawn into a chamber in a continual flow by an electric air pump. The air then passes through an electrolyte medium and, if the target agent is present, it reacts in solution, generating an electrical, colorimetric, or fluorescent signal to indicate its presence. Such automated systems are now becoming obsolescent in the defense arena as they become replaced by very effective technology that requires less maintenance, uses fewer reagents, generates fewer false alarms, and offers greater accuracy. For example, the UK-manufactured Nerve Agent Immobilized Enzyme Alarm & Detector (NAIAD), since replaced by IMS-based detectors, could detect any type of nerve agent (even currently unknown types) because it accurately mimicked the natural process of acetylcholinesterase inhibition. It was also capable of detecting blood agents, such as AC.

7.1 Detection Papers

See also Section 4.1.

The US Army issues M8 and M9 detection papers to troops (Figure 8). In the commercial sector, the civilian equivalent in the form of chemical agent liquid detectors (C8, M-9, and 3-way) is distributed by Tradeways Limited (Anapolis, Maryland, USA). Adhesive-backed papers from the M8 chemical-detection booklets can be stuck to surfaces and clothing to provide warning of a liquid attack. The M8 paper gives immediate, qualitative verification of the presence of liquid V- and G-type nerve agents and H-type blister agents. Yellow coloration on the paper signifies the presence of a nonpersistent (G-type) nerve agent, red signifies the presence of a blister agent, while olive green or black signifies a persistent (V-type) nerve agent. However, false positives can occur with liquid pesticides, antifreeze, or petroleum. The M9E1 tape comes in an adhesive-backed roll, which can be worn on clothing, and detects the presence of liquid V- and G-type nerve agents and H- and L-type blister agents. However, detection papers are not as reliable as other means of detection because they depend on liquid agent contacting the surface of the paper; no type of paper detects traces of chemical agent vapors. Some solvents and standard decontaminating solutions cause false-positive reactions with the M8 paper. Extremely high temperatures, scuffs, certain types of organic liquids, and decontamination solution number 2 (DS2) can cause false-positive reactions with the M9 paper. The M9 paper does not distinguish between the types of agent involved, only that an agent or agents may be present. Similar detector paper booklets, type X-1 and X-3, are in service with the Chinese armed forces.

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Figure 8. M8 detection paper.

7.2 Detection Tubes

Detection tubes are provided by several vendors, most prominently Dräger, Gastec, and RAE systems. The reagents are usually redox active and are class sensitive, not specific to individual agents. Many Dräger tubes indicate easily oxidizable molecules by the reduction of I2O5 to I2. In another example, the Dräger phosphoric acid ester tube contains the enzyme cholinesterase, so as to imitate nature; inhibition of the cholinesterase results in a color change from yellow to red, as indicated by phenol red. While this tube can reliably detect nerve agents, it will also respond to other cholinesterase inhibitors such as some pesticides. The chip measurement system (CMS) utilizes the same color-changing reaction technology as the tubes but in a “chip” format. The system combines the color-changing chip with an optical reader, a mass flow controller, and a pump in a lightweight handheld system (Figure 9).

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Figure 9. Detection tubes instruction card.

7.3 Detection Kits

The M256A1 Chemical Agent Detector Kit (Figure 10) is a portable, disposable chemical agent detector kit that can detect and identify nerve, blister, or blood agent vapors. It is typically used to determine when it is safe to unmask after a chemical agent attack. Each kit consists of 12 disposable plastic sheet sampler detectors, one booklet of M8 paper, and a set of instruction cards. Each sampler detector incorporates a square test spot for blister agents, a circular test spot for blood agents, a star-shaped test spot for nerve agents, and a lewisite-detecting tablet and rubbing tab. The test spots are made from standard laboratory filter paper. Eight glass ampoules are provided, six containing reagents for testing and two in an attached chemical heater. When the ampoules are crushed between the fingers, channels formed in the plastic sheets direct the flow of liquid reagent to wet the test spots. Each test spot or detecting tablet develops a distinctive color that indicates whether a chemical agent is present in the air or not.

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Figure 10. Chemical Agent Detector Kit C-2.

The M18A2 Chemical Agent Detector Kit (currently called ABC-M18A3) is aimed at both the collection and the identification of CW agents, as well as TICs. The kit contains sealable sampling tubes for the safe transport of unidentified, but suspect, samples to a laboratory. It uses both detector tubes and paper tickets to detect and classify dangerous concentrations of lethal chemical agents in the air, as well as liquid chemical agent contamination on exposed surfaces. Agents detected are CK, HD, nitrogen mustards (HN-1 and HN-3), phosgene oxime (CX), AC, CG, lewisite, ethyl dichloroarsine, methyl dichloroarsine, the G-series nerve agents, and VX. The use of an eel enzyme in place of the horse enzyme for the nerve agent test provides an improvement to the M256A1 Kit by detecting lower levels of nerve agent. Any type of mustard is also detectable as long as vapor is present. Both of these kits are commercially available; the former is sold as the Chemical Agent Detector Kit C-2 (pictured later). At present, there are no detection kits for the arsenical vomiting agents, tear gases, and incapacitating agents.

All of these individual detection devices are simple, very sensitive, and give a very rapid response. The drawback to the simplicity is that these devices generally do not identify the specific agent that is present, only the class of agent; the drawback to the sensitivity is that these devices can give false-positive readings. Therefore, separate and orthogonal systems are needed to verify the presence of an agent and to identify the specific agent.

8 Point Detection Instruments

  1. Top of page
  2. Introduction
  3. Detection Roles and Requirements
  4. Detection of Chemical Warfare Agent Vapors
  5. Detection of Chemical Warfare Agent Liquids
  6. Detection of Solids Contaminated with Chemical Warfare Agents
  7. Detection of Industrial Gases or Vapors
  8. Reagent-based Tests
  9. Point Detection Instruments
  10. Standoff Detection Instruments
  11. Research and Development
  12. Principle Chemical Sources of Sensor False Alarms
  13. Abbreviations and Acronyms
  14. Related Articles
  15. References

Most of the instruments and devices available for detecting CWAs are continuous vapor samplers for point detection. There are three major types of point detection instruments: ion mobility spectrometers, gas chromatographs, or surface acoustic wave sensors (SAWSs). Normally, these devices are designed to draw air directly from the atmosphere, but some have the option of using sorbent tubes and other concentration methods. Major considerations are weight, power requirements, ease of operation, data logging, and occurrence of false-negative/positive responses.21, 22

8.1 Ion Mobility Spectrometry

The most common device used at present for chemical monitoring by the military is the ion mobility spectrometer. Many types of these devices are available for detection of both CWAs and TICs. The applications of IMS have been detailed in a review.23 See also Section 3.2.3 for a description of the principles of IMS.

8.1.1 IMS Examples

The GID-3 (Smiths Detection) is a British-made ion mobility spectrometer for battlefield detection of the most common chemical agents. The GID-2A is a similar portable and rugged device designed for fixed locations and unattended operation; in the United States, it is known as the XM22 ACADA (Automatic Chemical Agent Detection Alarm) (Figure 11). GID-3 is able to detect the most common nerve and blister agents and can be programmed to detect other agents such as blood and choking agents and chlorine gas. The GID-3 has successfully undergone extensive environmental testing against damage due to shock, vibration, and electromagnetic pulse (EMP), both for operating and for storage. The related CAM handheld units (see later) from the same manufacturer operate on similar principles and are mass produced.

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Figure 11. The GID-3 Automatic Chemical Agent Alarm (ACADA) system.

The ACADA variant of GID-3 replaced the M8A1 ACADA System, a portable chemical agent alarm also based on IMS, which was used during the 1991 Gulf War. The ACADA is a higher resolution IMS, which reduces the chance of false positives through the use of dual 4-in.-long analysis cells. The dual cells are used to examine positive and negative ions simultaneously. The unit can make some detections within a few seconds of exposure and can clean itself out for a fresh detection within 60 s. Its sensitivity to nerve agents is 0.1 mg  m−3 and to blister agents is 2.0 mg  m−3. At these levels, the unit is designed to register “no false positives” to shipboard contaminants, such as paints, oils, and flame retardants. It samples 4 L of air per minute without a preconcentrator. It is an upgrade to the Improved (Chemical Agent) Point Detection System (IPDS) currently in use by the US Navy.

The Improved Chemical Agent Monitor (ICAM, also known as CAM2) and Enhanced Chemical Agent Monitor (E-CAM) are handheld ion mobility spectrometers. Air is drawn into the IMS unit and ionized by a weakly radioactive source. A computer examines the pattern of the time(s) of flight of ion clusters generated from a sample, determines the level of chemical agent present, and indicates the level of hazard on a display or by an alarm. Additional programming can be included to extend the range to cover other agents, and certain harmless chemical simulants used as training aids, and for testing the correct functioning of the device. Levels of detection are shown in Table 6.

Table 6. Some Military Chemical Warfare Agent Detectorsa
EquipmentAgentSensitivityTimeOperational or maintenance/limitsNotes
  1. a

    Adapted from Ref. 22.

M-8 paperNerve-G100-μ drops<30 sDisposable/handheld, dry undamaged paperChemical agent detector, potential for false positives
 Nerve-VX100-μ drops   
 Mustard-H100-μ drops   
M-9 paperNerve-G100-μ drops<20 sDisposable/handheldAdhesive-backed dispenser roll or books
 Nerve-VX100-μ drops 3-year shelf life 
 Mustard-H100-μ drops Carcinogen 
 Liquids only    
M-18A2 Detector KitNerve-GB0.1 mg  m−3 2−3 minDisposable tubes25 tests per kit;
 Nerve-VX0.1 mg  m−3  HandheldDetector tubes, detector tickets, and M-8.
 Mustard-H, HN, HD, HT0.5 mg  m−3    
 Lewisite-L, ED, MD10.0 mg  m−3    
 Phosgene-CG12.0 mg  m−3    
 Blood-AC8.0 mg  m−3    
 Liquid, vapor, aerosol    
M-256A1 Detector KitNerve-G and VX0.005 mg  m−30.02 mg  m−3 15 minDisposableEach kit contains 12 disposable, plastic sampler detectors and M-8 paper.
 Mustard-HD2.0 mg  m−3 Series is longerHandheld 
 Lewisite-L9.0 mg  m−3  5-year shelf life 
 Phosgene oxime-CX3.0 mg  m−3 AC  
 Blood-AC, CKVapor or liquid8.0 mg  m−3 25 min  
M-272 Water Test KitNerve-G and VX0.02 mg  m−3 7 minPortable/lightweightUsed to test raw or treated water; type I and II detector tubes, eel enzyme detector ticket; kit conducts 25 tests for each agent
 Mustard-HD2.0 mg  m−3 7 min5-year shelf life 
 Lewisite2.0 mg  m−3 6 minUSN, USMC 
 Hydrogen cyanide20.0 mg  m−3 7 min  
CAM chemical agent monitoringNerve-GA, GB, VX0.03 mg  m−30.1 mg  m−3 30 sHandheld/portable battery operated 6–8 h continuous use. Maintenance requiredRadioactive source. False alarms with perfume, exhaust paint, additives to diesel fuel
 Blister-HD and HNVapor only ≤1 min  
ICAMNerve-G and V0.03 mg  m−3 10 s4.5 poundsAlarm only
Improved chemical agent monitorMustard-HD0.1 mg  m−3 10 sMinimal trainingFalse positives common
ICAM-APDNerve-G0.1 mg  m−3 30 s12 pounds including batteriesAudible and visual alarm
Improved chemicalNerve-V0.04 mg  m−3 30 sLow maintenance 
Agent monitorMustard-H2.0 mg  m−3 10 sMinimal training 
Advanced point detectorLewisite-L2.0 mg  m−3 10 s  
ICADNerve-G0.2−0.5 mg  m−3 2 min8-oz pocket-mountedAudible and visual alarm
Miniature chemicalMustard-HD10 mg  m−3 (30 s for high levels)4-month serviceMarines
Agent detectorLewisite-C10 mg  m−3  No maintenanceNo radioactivity
 Cyanide-AC, CK50 mg  m−3 2 minMinimal training 
 Phosgene-CG25 mg  m−3 15 s  
M-90 D1CNerve-G, V0.02 mg  m−3 10 s15 lb. with batteryIon mobility spectroscopy and metal conductivity technology can monitor up to 30 chemicals in parallel. Alarm only
Chemical agentMustard0.1 mg  m−3 10 sRadioactive source exempt from licensing. Minimal training 
DetectorLewisite0.8 mg  m−3 80 s  
 BloodN/A   
 Vapor only    
M-8A1 alarmNerve-GA, GB, GD0.2 mg  m−3 ≤2 minVehicle battery operatedRadioactive source (licensing required)
Automatic chemicalNerve-VX0.4 mg  m−3  Maintenance requiredAutomatic unattended operation; remote placement
Agent alarmMustard-HD10 mg  m−3 ≤2 min  
 Vapor only ≤2 min  
MM-120−30 CWA<10 mg  m−2 of surface area≤45 sHeater volatizes surface contaminantsGerman “Fuchs” (FOX Recon system/vehicle)
Mobile mass spectrometry gas chromatographVapor    
RSCAAL M-21Nerve-G90 mg  m−3  Line-of-sight dependent 10-year shelf lifePassive infrared energy detector
(a/k/a JLSCAD)Mustard-H2300 mg  m−3  Two-person portable tripod 
 Lewisite-L500 mg  m−3   3 miles; visual/audible warning from 400 m
 Vapor    
SAW Mini-CADNerve-GB1.0 mg  m−3 1 minMinimal trainingAlarm only; false alarm from gasoline vapor, glass cleaner
 Nerve-GD0.12 mg  m−3 1 minField use 
 Mustard-HD0.6 mg  m−3 1 min1 pound 
 Vapor  No calibration 
ACADA (XM22, GID-3)Nerve-G0.1 mg  m−3 30 sVehicle mounted, battery poweredRadioactive source (license required)
 Mustard-HD2 mg  m−3 30 s Minimal training
 Lewisite   
 Vapor   
Field mini-CAMsNerve-G, V<0.0001 mg  m−3 <5 minDesigned for field industry monitoring (10 lb.)8-h training
 Mustard-HD<0.003 mg  m−3 <5 min 24 h/7 days
 Many others<0.003 mg  m−3 <5 min  
Viking 573 GC/MS (Bruker Daltonics)Nerve-G, V<0.0001 mg  m−3 <10 minField use, but 85 poundsNeeds 120VAC, 40-h training
 Mustard-HD<0.003 mg  m−3 <10 min  
 Many others    
Agilent 6890 GC with flame photometric detector (FPD)Nerve-G, V<0.0001 mg  m−3 <10 minDesigned for laboratory useGas, air, 120VAC
 Mustard-HD<0.0006 mg  m−3 <10 min 40-h training
 Many others    

8.2 Gas Chromatographs

The primary military use of GC is for monitoring of storage sites. GC, when combined with MS (GC/MS), is specified by the OPCW as the analytical method of choice when testing samples at suspected sites of chemical agent contamination. The principles of GC and the specific detectors employed will be discussed more thoroughly in the section on commercial systems (see Section 8.4). Two particular detection technologies are used at CWA depots and demilitarization plants. The first of these systems is similar to the point detection systems in that it is automated, rapid, and sensitive. The second system is used to verify the results obtained with the first; it requires sampling and subsequent analysis at an on-site or off-site laboratory.

The Miniature Continuous Agent Monitoring System (MINICAMS ®) utilizes an automated near-real-time gas chromatograph (Figure 12). An air sample is drawn through a preconcentrator loop filled with an adsorbent. Periodically, the system is switched so that a carrier gas stream flows through the preconcentrator loop as it is heated, carrying the adsorbed sample into the GC. The GC separates the chemical compounds in the sample and, while the sample components are being separated, the preconcentrator loop begins to collect the next sample. MINICAMS® detects agents with an FPD (see Section 3.2.2). The entire cycle from sample collection to detection typically requires 3 to 5 min. MINICAMS® is a refinement of the larger Automatic Continuous Agent Monitoring System (ACAMS) detector, which is still in use in some locations. The commercial form of this instrument is manufactured by O.I. Analytical, the latest version being the MINICAMS® Series 3001.

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Figure 12. MINICAMS® Series 3001 Continuous Air Monitor (O.I. Analytical).

The Depot Area Air Monitoring System (DAAMS) is a system used to confirm the detection of an agent by the MINICAMS®. Larger air samples are drawn continuously through the DAAMS preconcentrator tube, which contains a correspondingly larger quantity of adsorbent. At either a predetermined time, or when a confirmation of a MINICAMS® result is required, the DAAMS tube is transported to a laboratory, where the sample is desorbed into a laboratory gas chromatograph.

8.3 Surface Acoustic Wave Sensors

SAWS devices work in the same manner as quartz crystal microbalances (QCMs) but at higher frequencies and with greater sensitivities. One example is the SAWRHINO, a SAWS-based detector that incorporates a separation step by first collecting vapors through condensation and then desorbing them over a 3-SAW detector to give a time versus temperature trace. In this way, the detector combines the separation aspects of GC with the detection and identification capabilities of a multi-SAWS detector. As with most SAWS devices, the detection of chemical agents relies on the application of pattern recognition algorithms. SAWSRHINO uses the responses generated from the three variously coated SAWS devices as the input data for a neural network algorithm to determine a recognition pattern. Military SAWS devices include the SAWCAD, MiniCAD, and, as discussed later, the Joint Chemical Agent Detector (JCAD), which combines the SAWS detector with an electrochemical cell.

8.4 Commercial Devices

Most military devices are now available in commercial versions. This section discusses existing commercially available devices that could be applied to chemical agent detection but are not necessarily marketed for that application.

8.4.1 Gas Chromatographs

All of the detection methods discussed later benefit greatly from a separation of components before detection. GC is a very efficient separation technique, which generally allows separation of analytes from background species. The detection methods outlined earlier can be applied to not only detect species but also discriminate to some degree against types of compounds that might interfere. The problems associated with current gas chromatographs, with respect to continuous monitoring, are size and speed. These problems could be addressed by appropriate redesign and miniaturization. GC may be combined with the following detectors (see Table 7).

Table 7. Properties of Common GC Detectors
DetectorPrinciple of operationSelectivitySensitivity (g s−1)
Thermal conductivityHeat dissipationUniversal10−8
Flame ionizationH2O2 flame 2000 °C plasmaResponse to organic compounds but not to permanent gases or water9 × 10−16
    
Electron capture 3H 63Ni

N2 + β [RIGHTWARDS ARROW] ee + sample [RIGHTWARDS ARROW] loss of ion

Response to electron-adsorbing compounds (especially halogens, nitrates, and conjugated carbonyls)2 × 10−14 for CCl45 × 10–14 for CCl4
    
ThermionicAlkali modified H2O2 Flame 1600°C plasmaEnhanced response to phosphorus compounds and nitrogen compounds4 × 10–14 P containing compounds
P-containing compounds N-containing compounds  7 × 10–12 N containing compounds
Photoionization

He + β [RIGHTWARDS ARROW] He*sample He* > 1o

Universal − responds to all compounds2 × 10–14 for methane
AEDFlame or plasmaUniversal − responds to all compounds1 × 10−9
8.4.2 Thermal Conductivity Detector

Traditionally, the thermal conductivity detector (TCD) has been recognized as a universal detector, but one of limited sensitivity. Several manufacturers have invested in miniaturization of the TCD, with improvements in sensitivity resulting in lower detection limits. For example, Agilent (previously Varian) sells a multichannel GC (490 Pro Micro GC) incorporating a MEMS-based TCD with sensitivity of about one parts per million.

8.4.3 Electron Capture Detector

The electron capture detector (ECD) is designed to detect molecules with highly electronegative elements such as halogens. The electrons are typically provided by a radioisotope such as 63Ni or tritium (3H) absorbed into a palladium host. The presence of compounds with highly electronegative elements can be detected at extremely low levels. Again, high selectivity is dependent on a prior separation.

8.4.4 Thermionic Detector

The thermionic or alkali flame ionization detector (FID) is a variant of the FID that exhibits enhanced sensitivity to nitrogen- (N) and phosphorus (P)-containing compounds. It is essentially a hydrogen flame around a pellet of an alkali metal salt, with an electric potential across the flame. Organic compounds in the air are drawn into the flame and ionized. The alkali metal enhances the response to N- and P-containing compounds and, as many chemical agents contain N and P, it is selective for their detection. This detector is rarely used without prior separation by GC.

8.4.5 Flame or Low-energy Plasma Photometer

Most chemical agents contain specific elements in common. G and V agents all contain phosphorus, and the mustards contain sulfur or nitrogen. The combustion of these materials yields excited atoms that emit light characteristic of these elements. The emissions are viewed through an interference filter by a photodetector. As an alternative to a flame, low-powered, inductively coupled (or microwave) plasmas have been used as emission sources. These sources have been combined with photodiode array detectors and are known as atomic emission detector s (AEDs). This combination allows the detector to acquire a full spectrum, providing information on most of the elements in a compound. The combination of GC and an AED can often provide an approximate empirical formula. This type of detector was previously marketed by Hewlett-Packard but is now available from joint analytical systems.

8.4.6 Photoionization Detector

The presence of organic compounds in air can be detected by using a light source with sufficient energy to ionize them, resulting in gas-phase electrical conductivity. Selectivity of detection is obtained by tuning the light source to the correct energy. Unfortunately, all compounds with ionization energies lower than that supplied by the lamp will be detected. This method then becomes reliant on a separation step (i.e., GC) to provide good identifications.

8.4.7 Mass Spectrometer

The mass spectrometer is often considered to be the best GC detector, as it can usually provide unambiguous identification of eluted components. A variety of mass spectrometers have been used as GC detectors, with the quadrupole mass selective detector (MSD) the most popular. Quadrupole mass spectrometers are selective ion filters that can scan a suitable range of mass/charge (m/z) ratios in milliseconds. The popularity of the quadrupole is due to its economy, speed, and ability to tolerate higher pressures than some other mass spectrometers, making it especially suitable as a detector for GC. Quadrupole mass spectrometers are tuned such that the spectra produced are similar to those obtained from a magnetic sector mass spectrometer, allowing both types to use the same libraries of spectra for compound identification. The primary use of these devices is to detect (organic) molecules that on application of electron ionization produce fragmentation of the molecule. This approach can be applied to chemical agent detection with the addition of detection algorithms designed to recognize the fragmentation pattern of the agents. Because of their rapid scanning capability, mass spectrometers are compatible with fast GC methods.

An alternative to electron ionization is chemical ionization (CI). This reduces fragmentation and increases sensitivity by preserving the molecular ion. CI simplifies identification but complicates the hardware system, as a cylinder of reagent gas is required (e.g., methane, isobutane, or ammonia). The combination of GC and MS can usually provide the selectivity needed for unambiguous identification.

Tandem mass spectrometry (MS/MS) is an approach that uses one stage of MS to select a specific ion from a compound to be further fragmented and mass analyzed by a second stage of MS. As more qualitative data are produced, this method may be used in future detection systems as algorithm developments progress. Its main advantage is potentially much higher selectivity.

Two other MS detectors used for GC are the quadrupole ion trap and the time-of-flight (TOF) mass spectrometer. These systems are available for laboratory use but no portable instruments are commercially available.

8.4.8 Miniature Mass Spectrometers

Several vendors offer portable mass spectrometers, or gas chromatographs equipped with mass spectrometers, as detectors for chemical agents.24 The Bruker Daltonics Viking 573 is essentially a miniaturized laboratory-based system made more robust for transport and field use, although still requiring ∼120 VAC power. It makes use of the Agilent 5973 MSD and utilizes Agilent ChemStation software. The Inficon Hapsite is a custom-made portable GC-MS instrument shown in Figure 13.

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Figure 13. Inficon Hapsite custom-made portable GC-MS.

The Smiths Detection GUARDION employs high-speed, high-resolution GC, and a miniaturized toroidal ion-trap mass spectrometer to identify volatile and semivolatile organic compounds in complex gases, vapors, liquids, and solids. GUARDION is hand portable and ruggedized for use in a contaminated “hot zone” or extreme environments. It redefines size, weight, and speed for hand-portable GC/MS technology and is ready to operate within 5 min from a cold start. The CUSTODION™, a solid-phase microextraction (SPME) fiber syringe, is used for simple sample collection and injection into the GC/MS system. Advanced low thermal mass GC technology enables rapid heating and cooling of the GC column, allowing 12–15 samples to be analyzed per hour.

The Bruker Daltronics MM2 is a quadrupole mass spectrometer with a membrane inlet. It can be equipped with flexible accessories, such as a surface probe and a gas chromatograph with thermal desorption capability. The detection limit for volatile organic compounds and volatile agents is in the low parts per billion to low parts per million range depending on the analytical procedure, for example, adsorbent enrichment (low parts per billion range) or online monitoring (very low parts per million range) from ambient air.

FLIR Systems produces a monitor called the ChemSense 600. It utilizes the same cylindrical ion trap (CIT) technology as the Griffin family of gas chromatograph−mass spectrometers providing the ability to perform multidimensional analysis or MS/MS. Application to chemical weapons simulant testing has been published in a detailed account by Cooks and coauthors.25

A thermal desorption system used for near-real-time monitoring when interfaced to a GC or GC/MS is produced by Markes International (Figure 14). The TT24-7 thermal desorption system is a robust, transportable unit, convenient for installation on-site or in mobile response vehicles, as well as in conventional laboratories. The TT24-7 incorporates two electrically cooled traps, which operate in tandem to ensure 100% sampling efficiency with no dead time. Air/gas is first sampled onto the channel A focusing trap at electronically controlled flow rates up to 1 L min−1. At the end of the sampling time, channel B is switched in-line, while channel A is prepurged, desorbed, and analyzed. When sampling switches back to channel A, channel B is desorbed and analyzed. Trap heating rates approaching 100°C s−1, combined with a reverse flow of carrier gas, ensure efficient backflush desorption of analytes over a wide boiling range, giving narrow peak widths and high sensitivity with mass spectrometric or selective GC detectors.

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Figure 14. Markes TT24-7™ GC-MS continuous monitor.

8.4.9 Surface Acoustic Wave Sensors

The SAW MiniCAD is a pocket-sized instrument, which can automatically monitor trace levels of toxic vapors of both HD and G-type nerve agents with a high degree of specificity. The instrument is equipped with a vapor-sampling pump and a thermal concentrator to provide enriched vapor samples to a pair of high-sensitivity-coated SAW microsensors. All subsystems are designed to consume minimal amounts of battery power. Optimal use of the SAW MiniCAD requires that a suitable compromise be made between the conflicting demands of response time, sensitivity, and power consumption. The SAW MiniCAD is able to achieve a higher sensitivity with an increased vapor sampling time; however, a faster response can be achieved at a lower sensitivity setting. Testing of the SAW MiniCAD has been performed with GD, GA, and HD. These tests were performed at a variety of concentrations and humidity levels. There were no significant effects noted due to changes in humidity levels for any of the chemical agents tested.

9 Standoff Detection Instruments

  1. Top of page
  2. Introduction
  3. Detection Roles and Requirements
  4. Detection of Chemical Warfare Agent Vapors
  5. Detection of Chemical Warfare Agent Liquids
  6. Detection of Solids Contaminated with Chemical Warfare Agents
  7. Detection of Industrial Gases or Vapors
  8. Reagent-based Tests
  9. Point Detection Instruments
  10. Standoff Detection Instruments
  11. Research and Development
  12. Principle Chemical Sources of Sensor False Alarms
  13. Abbreviations and Acronyms
  14. Related Articles
  15. References

The M21 Remote Sensing Chemical Agent Alarm (RSCAAL) is based on a passive infrared (IR) detector. The incoming IR signal is compared against known agent spectra; when a match is detected, a display is illuminated and an alarm sounds. The display also indicates in which of the seven fields of view (spread over a 60-degree arc) the agent was detected. This allows the operator to track a moving agent cloud. The M21 is capable of detecting nerve and blister agents in the vapor phase from a distance of up to 5000 m; however, it must have a direct line of sight to the agent cloud in order to function. It is being replaced by a newer version, the General Dynamics Joint Service Lightweight Standoff Chemical Agent Detector (JSLSCAD). This is a passive FTIR (Fourier transform infrared) spectrometer with 360° coverage in a 60° cone angle and a 5-km range, reduced in humid or obscuring conditions. The unit utilizes a highly sensitive HgCdTe detector, which has caused some difficulties owing to the need to cool the spectrometer. It detects nerve, blister, and blood agent vapor clouds in unattended operation, using automatic warning and reporting through the JWARN (Joint Warning and Reporting Network) system. A commercial, passive IR detector called RAPID (Remote Air Pollution Infrared Detector) is available from Bruker Daltonics. The RAPID was designed for mounting on a vehicle. It is relatively small, relatively light, and scans a complete 360° rotation in 3 s. Light detection and ranging (LIDAR), although not a new technology, and Differential Absorption LIDAR (DIAL) systems are still under development.

10 Research and Development

  1. Top of page
  2. Introduction
  3. Detection Roles and Requirements
  4. Detection of Chemical Warfare Agent Vapors
  5. Detection of Chemical Warfare Agent Liquids
  6. Detection of Solids Contaminated with Chemical Warfare Agents
  7. Detection of Industrial Gases or Vapors
  8. Reagent-based Tests
  9. Point Detection Instruments
  10. Standoff Detection Instruments
  11. Research and Development
  12. Principle Chemical Sources of Sensor False Alarms
  13. Abbreviations and Acronyms
  14. Related Articles
  15. References

10.1 Military Research and Development

The critical need for improved hazardous chemical sensors has spurred a large investment in research and development by many nations and supranational groups such as North Atlantic Treaty Organization (NATO). This investment is made apparent by the large number of different devices currently under development. While the military is either performing or sponsoring many developments in chemical agent detection, their investment has focused more on engineering and less on innovation. The common theme in the United States is a “joint effort,” aimed at consolidation of technologies from all branches of the military. However, many of these projects are undeveloped or based on incremental increases in technology and cannot be expected to yield much improvement in the near term; civilian-sponsored investigations are far more likely to make major breakthroughs in technology in the near term.

Examples of military systems are discussed in the following sections.

10.1.1 Automatic Chemical Agent Detection Alarm

The M22 is an advanced, point sampling, chemical agent alarm system employing IMS. It is designed to detect nerve and vesicant agents.

10.1.2 Improved Chemical Agent Point Detection System

The IPDS also employs IMS and is an improved point detection system. In addition to G and VX nerve agents, the IPDS is designed to detect vesicant agent vapors. Because it is a shipboard instrument, it is much larger and needs more power than portable IMS devices. The IPDS-LR uses the Bruker Daltonics RAID-S2 IMS-based detectors.

10.1.3 Joint Chemical Agent Detector

The JCAD will employ surface acoustic wave (SAW) technology to detect nerve and blister agents. It is designed to be lightweight and portable and to reduce false alarms. The JCAD will also allow detection of new types of nerve agent.

10.1.4 Joint Chemical and Biological Agent Water Monitor

The Joint Chemical and Biological Agent Water Monitor (JCBAWM) will be a portable device to detect, identify, and quantify both chemical and biological agents in water. It will allow the user to sample water and obtain a digital readout of the contents. The technology to be employed in this monitor is still under review.

10.1.5 Joint Service Lightweight Standoff Chemical Agent Detector

See also Section 8.4.1. The JSLSCAD is a passive, IR detection unit employing FTIR spectrometry. The device is designed to detect nerve and blister vapor clouds at a distance of up to 5 km.

10.1.6 Shipboard Automatic Liquid Agent Detector (SALAD)

The technologies to be used in the shipboard automatic liquid agent detector (SALAD) have recently been reviewed, but no decision has been made on the final selection. The instrument is designed to be an automated, externally mounted liquid agent detector capable of detecting G-type and VX nerve agents as well as vesicant chemical agents.

10.1.7 The Special Operations Forces Non-Intrusive Detector and the Swept Frequency Acoustic Interferometry Detector

Special Operations Forces' (SOF) Non-intrusive Detector and the Swept Frequency Acoustic Interferometry (SFAI) Detector are portable, handheld acoustic instruments developed specifically to enable rapid detection and identification of CW agents within munitions, railcars, ton containers, etc.

10.2 Civilian Research and Development

A wide variety of approaches to hazardous chemical detection are being investigated. Examples of some of the more promising approaches are outlined later. While many of the methods being investigated are only suitable for use in the laboratory at the present time, they might be rapidly converted to field monitors through miniaturization.

10.2.1 Capillary Electrophoresis

Capillary electrophoresis (CE) has been used by several laboratories to analyze mixtures of chemical agents. CE is an attractive approach because of its high separation efficiency and fast analysis time. The difficulty in applying CE to chemical weapons detection is twofold. First, there is a lack of selective detectors for use with CE. Most CE detection is by UV absorbance, either directly or indirectly by including a UV-absorbing additive to the running buffer. Direct UV detection at 200 nm was used by Cheicante et al. to determine sulfur-containing CW agents using a variant of CE called micellar electrokinetic capillary chromatography (MECC).26 Mercier et al.27 used indirect photometric detection to analyze nerve agent decomposition by indirect UV detection, while Nassar et al.28 used a similar approach for aqueous solutions and soil extracts with analysis times as short as 3 min. The problem with nonselective detection will probably be addressed by the application of selective GC detectors such as the pulsed flame photometric detector. Another issue is a lack of automated CE systems. This is an area of very active research.29 Groups at Sandia and Oak Ridge National Laboratories in the United States are working to make miniature CE devices in the “lab on a chip” format. Again, the incorporation of a selective detector will be needed, but the CE on a chip can easily be made in an array with multiple detectors for unambiguous identification.

10.2.2 He or Ar Afterglow Detection

Inductively coupled plasma emission spectrometry is a standard method for trace elemental analysis. While sensitive and selective, these instruments are large and require considerable support solutions and gases. Fortunately, there exists a lower power version of this method, which is available in the form of a GC detector called the AED (see Section 8.4). The multielement capability of this detector gives a rough empirical formula for each peak detected and thus reduces analytical ambiguities. Creasy et al. used GC-AED to analyze environmental samples for CW-related compounds.30

10.2.3 Ion Chromatography

Ion chromatography (IC) can be used to measure ionic concentrations in solution. It requires a fairly large amount of solvent and, usually, a suppressor column to neutralize acidity before detection by conductivity. These disadvantages have been reduced by new reagent-free systems utilizing eluant regeneration. For analytical purposes, many analyses previously performed by IC are now performed using CE. However, Katagi et al. used IC with indirect photometric detection to analyze nerve agents and hydrolysis products in human serum.31

10.2.4 Molecularly Imprinted Polymer Sensors

The process of molecular imprinting creates a selective binding site for a specific molecule.32 The site is created by using the molecule of interest as a template in a copolymerization process. The removal of the template leaves a cavity that is selective for rebinding the template molecule. The process of molecular imprinting can be used to improve the selectivity of many existing sensors that rely on selective coatings such as QCM and SAWS. However, the inclusion of a chromophore in the imprinted site can lead directly to an optical sensor for the molecule of interest. If the optical sensor is based on a form of luminescence, highly selective and sensitive sensors can be produced. Jenkins et al. produced a fiber optic-based luminescence sensor designed to measure the hydrolysis product of GD in water. The sensor exhibited high selectivity, no interference from OP herbicides or pesticides, and high sensitivity, with an LOD of 600 fg mL−1 in water.33

10.2.5 Cavity Ring Down Spectroscopy

This is a method of measuring small amounts of optical absorbency with a pulsed light source and a significantly higher sensitivity than obtainable in conventional absorption spectroscopy. The cavity ring down (CRD) technique is based on the measurement of the rate of absorption rather than the magnitude of absorption of a light pulse confined in a closed optical cavity with a high Q-factor (Q-factor indicating a high quality or highly reflective optical resonator). The advantage over normal absorption spectroscopy results from (i) the intrinsic insensitivity of the CRD technique to light source intensity fluctuations and (ii) the extremely long effective path lengths (many kilometers) that can be realized in stable optical cavities. The technique has been applied to solid optical resonators with selective coatings.34

10.2.6 Surface Plasmon Resonance

This detector is based on the collective oscillations of the free electron plasma at a metal surface. Typically, a prism is coated with a metal film and the film coated with a chemically selective layer. The surface is illuminated by a laser and the amount of material adsorbed by the coating affects the angle of the deflected beam. This platform is theoretically similar in sensitivity to a QCM. This is another platform whose selectivity is based on the coating. The typical coating is bound antibodies, thus this device becomes a platform for immunosensors.35 Surface plasmon resonance (SPR) has been applied to detection of biological agents.

10.2.7 Porous Silicon Interferometer

A device based on a porous silica substrate used as a Fabry−Perot interferometer (etalon) has been reported as a sensor for OP nerve agents. The porous silica is coated with a surfactant and a copper hydrolysis catalyst. The mode of operation is through the loss of finesse of the etalon due to the production of HF by the catalytic decomposition of the agent and subsequent reaction of HF with the silicon surface producing roughening. The device, though novel, only applies to fluorine-containing compounds.36

10.2.8 Solid-Phase Microextraction

Recently, the use of fibers coated with a variety of chromatographic stationary phases has become popular as a means of sample acquisition and preconcentration. These fibers can be used for gaseous headspace sampling or as aqueous solution extractants. They are most commonly applied as sampling devices for GC but are also used as samplers for high-performance liquid chromatography (HPLC) and capillary electrochromatography (CEC). The advantages of sampling with SPME for GC are preconcentration and solvent peak elimination. Stuff et al. used SPME air sampling for GC analysis of blister agents.37

10.2.9 Biosensors

In general, the term biosensor has come to mean any sensor that uses biomolecules, such as antibodies, for chemical recognition. This form of recognition has been long applied as immunoassays for a variety of compounds.38 Biosensors can be made using a variety of the measurement platforms described previously. There are limitations when applying biosensors to the detection of CWA and toxins owing to the inability of the organism used to make the antibodies to survive exposure to the toxic chemical. Additionally, the binding event is often irreversible making biomolecule recognition difficult to apply to real-time monitoring. A review by Paddle lists available biosensors and describes their modes of operation.39

10.2.10 Conductive Polymer Sensors

Conductive polymers such as polypyrrole or polyaniline have been applied to sense vapor and gases. These sensors change their conductivity when exposed to organic vapors owing to effects the vapors have on the availability of charge carriers. One way to obtain selectivity for these materials is to specifically engineer or dope the polymers with recognition sites. The other approach to selectivity is to make an array of different polymers and measure their relative responses to a variety of compounds. The later approach is one of the methods for making an “electronic nose.” While the electronic support for this approach to multianalyte sensing is less complex, it is unlikely to be as sensitive as the QCM, SAWS, or SPR devices. One way to enhance the sensitivity of these materials is to use a large surface area. This idea is addressed in the work of Collins and Buckley by using conductive polymer coating for fabrics.40

10.2.11 Microelectromechanical Systems

Under G. E. Spangler, Technispan has used microelectromechanical systems (MEMS) fabrication techniques to develop miniaturized GC systems for integration with IMS systems (such as the CAM or ACADA). Defense Advanced Research Projects Agency (DARPA) has funded research and many classical laboratory techniques are being pursued; however, no complete system has yet been produced.

10.2.12 Surface Acoustic Wave Sensors

SAWS technology has been applied to develop gas sensors for a wide variety of analytes.41 The key to specificity is the production of coatings that selectively bind analyte species.42 Unfortunately, an immunoprotein, the ultimate in chemical selectivity, is not completely specific to a nerve agent and typically binds irreversibly. Thus, some additional selectivity factor is needed to make a sensor that is truly specific. One method uses a lanthanide chelate coating to give a thermodynamic affinity for OP compound coordination.43 A variety of lanthanide chelates have been used in tests, with variable but consistent affinities and sensitivities. The most distinct characteristic shared by all devices based exclusively on thermodynamic affinities is a clear lack of specificity. While the sensors will detect nerve agents in a sensitive and quantitative manner, any other moiety with a similar chemical functionality will give a similar or, in the case of many of the current sensors, an overwhelmingly greater response. This is unfortunate since OP pesticides, such as dichlorvos, the active ingredient in many flea collars and fly strips (e.g., No Pest®), and malathion, used in fly sprays and in agriculture, are highly prevalent compounds. Thus, the high probability of a false-positive test renders the current line of sensors less attractive. An emerging technology that could greatly improve the selectivity of polymer coatings for SAWS devices is molecular imprinting.32

10.2.13 Fourier Transform Infrared

Spectroscopic techniques have higher information content than other methods of detection. The potential exists for the identification of many chemicals. The difficulty in the application of spectroscopic sensing to continuous monitoring is to obtain and process all of the information available in a short period of time. This requires a fast scanning instrument combined with a computer, which both operates the instrument and identifies the compounds detected. As spectra will be a composite of several species, some chemometric algorithm must be applied to deconvolute the spectra. The alternative is to find a unique spectral attribute that is indicative of the chemicals of interest. For example, nerve agents have a phosphorus−oxygen double bond (Figure 15) with a characteristic vibrational frequency. By monitoring this frequency, nerve agents could be detected. However, there are problems with this approach. The first problem stems from the narrow linewidths for vibrations in the gas phase. Although the functional group vibrational frequencies are close (within 100 cm−1), the linewidths may be as narrow as 1.0 cm−1 and the IR source linewidth must be matched to include the entire band of interest. A second problem stems from vibrational frequencies of other functional groups overlapping the band. Thus, for less ambiguous detection, a multiband approach is needed. This brings things back to the complete spectrum approach. As with the other detection strategies, interferences will be encountered from chemically similar materials. However, if the sophistication of the system is high enough, the correlations of the fingerprint region of the IR spectrum can discriminate against similar compounds. The difficulties encountered are the resulting size and the expense of the detection system. Some developing systems may mitigate these issues, but there are currently no commercially available monitors with this sophistication. A potential additional benefit of this IR spectral approach may be the detection of biological agents.

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Figure 15. Chemical structures of nerve agents.

10.2.14 Raman Spectroscopy

Raman scattering is the result of inelastic scattering of photons with molecules; the energy of scattered photons is changed by an amount equal to a molecular rotation, vibration, or the sum of a vibration and rotation, or in rare cases by an electronic excitation (usually, 10−3 less intense). The Stokes lines (red shifted) are more intense than the anti-Stokes lines because most of the molecules are in the ground vibrational level at room temperature. The anti-Stokes lines decrease in intensity with increase in energy due to the Boltzman population of the vibrational level. Raman is a weak effect and was not generally practical before the invention of lasers, but there are a variety of Raman techniques, which give increased sensitivity, such as resonance Raman. Resonance Raman improves sensitivity from about 0.1 M to about 10−6 M.44

10.2.15 Surface-Enhanced Raman Spectroscopy

It was discovered that samples adsorbed on certain rough metal surfaces give unusually intense Raman spectra. This is thought to be due to electric fields occurring at the surface of the metal. This explanation is further supported by the fact that colloidal suspensions of metal particles also produce the effect and that the effect is dependent on the size of the metal particle. The effect is also observed in the case of certain formally forbidden electronic transitions. The importance of this effect is that detection limits for surface-enhanced Raman spectroscopy (SERS) can be as low as 10−9 to 10−12 M.

10.2.16 Confocal Raman Microscopy

This Raman technique is similar to IR microscopy in that the information produced is indicative of molecular vibrations. The Raman spectrum is an intensity compliment to the IR spectrum due to differences in selection rules. Raman spectra obtained with a confocal microscope have an advantage in spatial resolution (about 1 μ) over IR microscopy (10 s of microns) as the signal is not a function of path length. Sample preparation is minimal; material is placed on a slide and moved under the objective. Because the laser is tightly focused, very small particles can give good Raman spectra, allowing the Raman microscope to achieve high sensitivity for minute particles.

10.2.17 Portable Handheld Raman Spectrometers

Several vendors offer portable Raman spectrometers. One of these spectrometers, the Delta Nu Observer V. 2, has been demonstrated to be able to detect agents (GB and GD) on surfaces. Additional handheld Raman instruments include the Thermo Scientific TruScan and TruScan RM.

11 Principle Chemical Sources of Sensor False Alarms

  1. Top of page
  2. Introduction
  3. Detection Roles and Requirements
  4. Detection of Chemical Warfare Agent Vapors
  5. Detection of Chemical Warfare Agent Liquids
  6. Detection of Solids Contaminated with Chemical Warfare Agents
  7. Detection of Industrial Gases or Vapors
  8. Reagent-based Tests
  9. Point Detection Instruments
  10. Standoff Detection Instruments
  11. Research and Development
  12. Principle Chemical Sources of Sensor False Alarms
  13. Abbreviations and Acronyms
  14. Related Articles
  15. References

The most difficult problems in chemical analysis are those that involve “real samples.” Real samples are by their nature heterogeneous and complex. The atmosphere is an especially difficult matrix due to the plethora of organic compounds that exist as trace constituents. When this atmosphere is brought inside, the number of trace chemicals is increased. Even the most sophisticated analyst, using state-of-the-art chemical instrumentation, will encounter difficulties in identifying and quantitating a trace chemical. Therefore, building an automated chemical sensor system to perform all of these functions is a daunting task and no perfect solutions are yet available. Obviously, the key to unambiguous trace detection is chemical selectivity.

Chemical selectivity can be obtained in a variety of ways. When the analyte of interest is a trace constituent, laboratory analysis methods inevitably begin with a separation process. The modern chemical separation technique for atmospheric samples is GC. GC is an efficient separation process, but high efficiency requires time. More rapid types of GC sacrifice efficiency for speed and try to compensate for loss in selectivity by using a selective detector. Consequently, continuous monitoring is impractical and, at best, a periodic sampling is all that can be obtained. Therefore, that period must be kept short if a human response is required. As seen earlier, a monitor consisting of a gas chromatograph with a flame photometric detector is useful for nerve agents. However, such a system could be fooled by chemically related pesticides of similar mass and polarity. Nevertheless, if the system is used in a facility for storage or destruction of chemical weapons, the high sensitivity of the system would be a useful guard against small leaks or short-term events. Thus, each detection system is a compromise determined by the application. Additionally, the combination of GC and FPD is much better than either of the devices used alone. Ideally, these “criteria of selectivity” should be compounded to the point that no false-positive detection can occur. Multiple criteria of selectivity may be obtained by a single method that has many orthogonal components or by combining two techniques that have a different physical basis for detection. For example, if a selective adsorbent is used to collect the sample for the GC-FPD system, the chance of false positives drops dramatically.

The reliable detection of chemical agents requires a high degree of selectivity. Owing to the extreme toxicity of most of these agents, this requirement must be coupled with an ability to detect very small quantities. In most chemical instrumentation, unambiguous identification and the detection of small quantities must meet a compromise. It is for this reason that so many chemical agent detection schemes rely on a combination of techniques or “hyphenated methods,” such as GC/MS. Unfortunately, many of the instruments for hyphenated techniques can be large and cumbersome, and require a skilled operator. Therefore, the “state of the art” in chemical agent detection typically follows one of the following two paths: (i) miniaturization and automation of a classical hyphenated technique or (ii) the development of new technologies, which have an intrinsic selectivity and sensitivity link.

11.1 Methods

Chemical interferences are dependent on the detection method. This results from the fact that many of the detection methods are not molecule specific. For example, many detection papers change color based on a physical property shared by many similar chemicals. Thus, the topic of interferences will be discussed in a method-specific manner. This discussion has been limited to the sensor systems that are candidates for use as continuous real-time monitors.

11.1.1 Surface Acoustic Wave Sensors

As described earlier, SAWS detectors measure the mass of materials that adhere to the surface of the device. These devices are typically used in pairs, with one surface kept clean to act as a reference. The selectivity of a SAWS device is entirely due to the selectivity of any surface coating. In the case of CW agent detection, typical coatings are organic polymers. These polymers are designed to have favorable interactions with certain chemical functional groups associated with the chemical agent being sought. The interactions range from weak van der Waals and dispersion forces to stronger hydrogen bonding. Unfortunately, many chemically similar compounds will also bind to the SAWS. For example, nerve agent sensor SAWS are coated with acidic groups to interact with the gas-phase basicity of the phosphonate esters. The problem is that not all phosphonate esters are CW agents; phosphonates are used in a variety of industrial processes, such as water treatment, in detergents, and in the oil industry. Other problems arise from humidity. As the polymer can hydrogen bond with water, humidity must be measured and its effects subtracted. In the SAWS community, these problems are addressed in the classical manner of using the chemical separation techniques as described earlier, or by using an array of SAWS devices with multiple coatings and applying pattern recognition algorithms to aid selectivity. The arrays are still subject to interferences from similar compounds, can participate in chemical reactions, and cause false positives. The most recent recommendations are to use SAWS in conjunction with a verification sensor that works by another physical principle; an orthogonal technique such as IMS. This would greatly reduce the false-positive rate but comes with a much higher cost. In general, SAWS suffer from interferences by chemically similar compounds, hydrocarbons, and moisture.

11.1.2 Ion Mobility Spectrometry

The selectivity of IMS is dependent on several key steps. The first step in the process is ionization. The process of ionization can be made selective for a class of compounds by either physical or chemical processes. Photoionization is an example of a selective physical means of ionization. Using lamps with differing energies can select for compounds with lower or higher ionization thresholds. However, for IMS, ionization is normally achieved by using a 63Ni radioactive source. The β-radiation electrons from the 63Ni impact the nitrogen in air and create ions. In air, these ions react by a complicated mechanism to eventually produce protonated water clusters from atmospheric moisture. The water clusters react with other gas-phase molecules to produce positive ions. A reagent gas (pure chemical) is usually added to the ion source to affect selectivity. For example, nerve agents are basic in the gas phase and hence accept protons easily. Acetone is added to the ion source to eliminate ionization of compounds that are weaker proton acceptors than acetone. The result is that fewer species are ionized and potential interferences are eliminated. The next step in the IMS process is the migration of the ions in the electric field. This process is designed to separate ions based on their mobility. Atmospheric pressure electromigration results in broad peaks due to collisions with the gas molecules in the drift region. Thus, IMS spectra (plasma chromatograms) may be poorly resolved. This problem is exacerbated by the clustering phenomena noted in the ionization process. Additionally, impurities in the gas (air), which occupies the drift region, can cause irreproducibility. This can be eliminated if purified air is used in the drift region, but is impractical for field-deployed devices. Separation occurs in the drift tube based on an ionic mobility, which has components of m/z ratio and geometry. The geometry component comes from the collisions that occur between sample molecules and the air, thus compounds with the same mass may have significantly different migration times. As a result of these processes, narrower peaks are obtained with laboratory IMS instruments than with field units. Thus, IMS detectors designed for field use are prone to interferences from a variety of sources and exhibit relatively high rates of false-positive and false-negative detections.

IMS can be combined with GC to give much more reliable detection. As the separation of chemicals takes time when traditional chromatography is applied, response time becomes a problem. Fast GC, as described earlier, may be a means to get around this problem; however, IMS does not appear to be a useful detector for fast GC because it is limited to a simple Faraday cup detector by the presence of gas at atmospheric pressure. This detector is much less sensitive than those employed by traditional gas chromatographs, such as mass spectrometers. Probably, the best way to employ IMS for the detection of chemical agents would be to use the IMS as a plasma chromatograph coupled to a small quadrupole mass spectrometer equipped with a sensitive electron multiplier or multichannel plate detector. This should give a rapid response instrument with much greater detection reliability, although with the possibility of decreased robustness due to complexity.

11.1.3 Gas Chromatography

Monitors that employ GC have a sampling interface that traps vapors and preconcentrates a sample before injection. This process is allowed by the periodic nature of the analysis and is matched to the required time for the chromatograph to perform the separation process. A balance must be made between the separation processes and the required speed of analysis. The selectivity of the gas chromatograph is tuned by the choice of stationary phase used in the capillary column. The materials used to coat the capillary are similar to the polymer coatings described for SAWS and other sensors. Temperature programming of the column is not often employed in a continuous monitor due to the time required to cool the column before the next injection, although fast GC methods with low thermal mass columns help in this respect.

The ultimate selectivity of GC is determined by the detector. The most selective detectors are spectroscopic, such as FTIR or MS. Automated systems can employ chemometric algorithms to discriminate unresolved chromatographic peaks. These combinations are expensive and require significant computer support. As such, they are more likely to be used in a laboratory for confirmation. Efforts to convert this approach to field units are still under development. The MINICAMS® described earlier, based on a flame photometric detector, is a reliable and selective monitor but requires 3−5 min to make a determination. Gas chromatographs also require a source of purified carrier gas, and flame detectors require additional hydrogen and air for operation. GC-based devices will have the fewest false positives and the most sensitive detection levels. The only significant interferences would be from compounds very similar to the agents, such as pesticides and herbicides.

11.1.4 Fourier Transform Infrared Spectroscopy

A major problem for chemical agent detection (Figure 15) is that very similar compounds are common in the environment, such as the OP-based insecticides (Figure 16). For example, dichlorvos, the pesticide used in flea collars, is extremely similar in structure and atom content to the nerve agents. Using a gas chromatograph with FTIR detection is currently a laboratory option only. Because FTIR detectors are intrinsically slow, scanning speed is restricted, and fast GC techniques are excluded because few spectra will be obtained in the timescale of a single chromatographic peak.

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Figure 16. Chemical structures of two common insecticides.

11.1.5 Raman spectroscopy

Owing to the similarity with FTIR, Raman spectroscopy is susceptible to the same sorts of interferences found in FTIR. There is also the problem of matrix fluorescence, which can result in baseline anomalies that confuse identification algorithms. Raman and FTIR are complementary methods and are often used together.44

11.1.6 Mass Spectrometry

See Section 8.4. MS can be applied to CW agent detection with the addition of detection algorithms designed to recognize fragmentation patterns of the agents. The problem, as for IR spectra, is that it is difficult to interpret spectra from a combination of compounds without prior separation. Similar solutions are often applied: selective ionization methods (e.g., CI), monitoring for several ion masses simultaneously, or using a separation method such as GC. In this way, highly selective detection with minimal interferences can be achieved. Unlike FTIR, mass spectrometers can scan rapidly, so that fast GC methods are applicable. As mentioned previously, selectivity can be enhanced using tandem methods, but their implementation in a detection system may be difficult. Ion sources can be modified to give greater specificity. Smith et al. have used ion traps to allow self-ionization by VX resulting in pseudomolecular ions that reduce ambiguity.45, 46

11.2 Specific Chemical Interferents

11.2.1 Insecticides

OP pesticides were developed in Germany during the 1930s and 1940s. They readily undergo biodegradation (unlike the organochlorides, they do not bioaccumulate) and are toxic to a wide range of insects. Unfortunately, some are also toxic to humans. One very toxic pesticide is parathion, as little as 2 mg  can kill a child and several hundred children have died from exposure. Less toxic is malathion (LD50 = 1375 mg  kg−1), which is hydrolyzed by carboxylase enzymes. Mammals have these enzymes, whereas insects do not. However, these enzymes can be inhibited by other organophosphates. In the human body, these pesticides are converted to oxons, which are acetylcholinesterase inhibitors. Some commonly used pesticides (for example, malathion and the carbamate carbaryl, or Sevin®) and some common therapeutic drugs (the carbamates pyridostigmine (Mestinon) and physostigmine (Antilirium)) also inhibit acetylcholinesterase (Figure 17). While the OP pesticides cause the same biological effects as nerve agents, there are some important differences in the duration of biological activity and response to therapy. This is due to the phosphorus−sulfur functionality instead of the phosphorus−oxygen or phosphorus−fluorine function of nerve agents.

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Figure 17. Some additional organophosphorus pesticides.

11.2.2 Herbicides

Glyphosates (Roundup®) are a member of the organophosphate class of herbicides. They are nonnitrogen-based herbicides that inhibit synthesis of essential amino acids and promote destruction of photosynthetic pigments in foliage. They are colorless to white, odorless, crystalline, or powdery solids, which are applied from the air or ground. They are nonselective, postemergence, and broad-spectrum herbicides used to control annual and perennial grasses, sedges, broadleaf, and emergent aquatic weeds. They also serve as insecticides for fruit tree insects. Their structural similarity to nerve agents can be seen in Figure 18.

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Figure 18. Glyphosate (Roundup®).

Carbamates are a class of herbicides, which inhibit seedling growth (Figure 19). The class includes pesticides such as carbaryl (Sevin®), aldicarb, and pirimicarb. They are more degradable than organophosphates and have lower dermal toxicities. Their toxicity is also due to the inhibition of acetylcholinesterase but they do not penetrate the central nervous system, so most effects are respiratory in nature. An acetylcholinesterase, which has been carbamylated, can undergo spontaneous hydrolysis in vivo, which reactivates the enzyme leading to less severe or shorter duration symptoms. Carbaryl has a low toxicity in mammals; however, pirimicarb is highly toxic to mammals, but not readily absorbed through the skin.

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Figure 19. Carbamate herbicides.

11.3 Summary

As discussed earlier, the most likely interferences come from compounds that chemically resemble CW agents. The compounds that are similar to CWAs are some insecticides and herbicides. A difficulty in selecting against these interferents stems from the fact that some pesticides could be used as terrorist weapons. Therefore, the issue of selectivity is of prime importance. All sensor detection methods have some susceptibility to interferences. The highest degree of false-positive rejection occurs when a method combines separation with selective detection. These methods are generally too slow to give a timely warning, or require large and expensive instruments. The best compromise may be to use two types of inexpensive point sensors that function by different physical mechanisms to give orthogonal data.

For the interested reader, some articles based on presentations given at a workshop on the impact of scientific developments on the Chemical Weapons Convention provide useful overviews.47-50

12 Abbreviations and Acronyms

  1. Top of page
  2. Introduction
  3. Detection Roles and Requirements
  4. Detection of Chemical Warfare Agent Vapors
  5. Detection of Chemical Warfare Agent Liquids
  6. Detection of Solids Contaminated with Chemical Warfare Agents
  7. Detection of Industrial Gases or Vapors
  8. Reagent-based Tests
  9. Point Detection Instruments
  10. Standoff Detection Instruments
  11. Research and Development
  12. Principle Chemical Sources of Sensor False Alarms
  13. Abbreviations and Acronyms
  14. Related Articles
  15. References
ACADA

Surface Acoustic Wave Sensor

ACAMS

Automatic Continuous Agent Monitoring System

AC

Hydrogen Cyanide

AED

Atomic Emission Detector

ARS

Acoustic Resonance Spectroscopy

CAM

Chemical Agent Monitor

CEC

Capillary Electrochromatography

CE

Capillary Electrophoresis

CG

Phosgene

CIT

Cylindrical Ion Trap

CI

Chemical Ionization

CK

Cyanogen Chloride

CMS

Chip Measurement System

CRD

Cavity Ring Down

CWAS

Chemical Warfare Agents

CWC

Chemical Weapons Convention

CX

Chip Measurement System

Daams

Depot Area Air Monitoring System

Darpa

Defense Advanced Research Projects Agency

Dial

Differential Absorption Lidar

E-Cam

Enhanced Chemical Agent Monitor

Ecd

Electron Capture Detector

Emp

Electromagnetic Pulse

FID

Flame Ionization Detector

FPD

Flame Photometric Detector

FTIR

Joint Service Lightweight Standoff Chemical Agent Detector

GC

Gas Chromatography

HCM

Hydrogen Concentration Measurement

HPLC

High-Performance Liquid Chromatography

IBA

Isonitrosobenzoyl Acetone

ICAD

Individual Chemical Agent Detector

ICAM

Improved Chemical Agent Monitor

IC

Ion Chromatography

IMS

Ion Mobility Spectrometry

IPDS

Improved (Chemical Agent) Point Detection System

IR

Infrared

ISE

Ion-Selective Electrode

JCAD

Joint Chemical Agent Detector

JCBAWM

Joint Chemical and Biological Agent Water Monitor

JSLSCAD

Joint Service Lightweight Standoff Chemical Agent Detector

JWARN

Fourier Transform Infrared

LEL

Lower Explosion Limit

LIDAR

Light Detection And Ranging

LOD

Limit Of Detection

LS

Limit Of Smell

MAC

Maximum Allowable Concentration

MECC

Micellar Electrokinetic Capillary Chromatography

MEMS

Microelectromechanical Systems

MINICAMS

Miniature Continuous Agent Monitoring System

MS/MS

Tandem Mass Spectrometry

MSD

Mass Selective Detector

MS

Mass Spectrometry

NAA

Neutron Activation Analysis

NAIAD

Nerve Agent Immobilized Enzyme Alarm & Detector

NATO

North Atlantic Treaty Organization

NDE

Nondestructive Evaluation

OPCW

Organisation for the Prohibition of Chemical Weapons

OP

Organophosphorus

CMS

Chip Measurement System

PINS

Portable Isotopic Neutron Spectrometry

QCM

Quartz Crystal Microbalance

RAPID

Joint Warning and Reporting Network

RH

Relative Humidity

RIP

Reactant Ion Peak

RSCAAL

Remote Sensing Chemical Agent Alarm

SALAD

Shipboard Automatic Liquid Agent Detector

SAWS

Surface Acoustic Wave Sensor

SERS

Surface-Enhanced Raman Spectroscopy

SFAI

Swept Frequency Acoustic Interferometry

SOF

Special Operations Forces

SPME

Solid-Phase Microextraction

SPR

Surface Plasmon Resonance

TCD

Thermal Conductivity Detector

TIC

Toxic Industrial Chemical

TOF

Time-of-Flight

UNSCOM

United Nations Special Commission

UPE

Ultrasonic Pulse Echo

References

  1. Top of page
  2. Introduction
  3. Detection Roles and Requirements
  4. Detection of Chemical Warfare Agent Vapors
  5. Detection of Chemical Warfare Agent Liquids
  6. Detection of Solids Contaminated with Chemical Warfare Agents
  7. Detection of Industrial Gases or Vapors
  8. Reagent-based Tests
  9. Point Detection Instruments
  10. Standoff Detection Instruments
  11. Research and Development
  12. Principle Chemical Sources of Sensor False Alarms
  13. Abbreviations and Acronyms
  14. Related Articles
  15. References
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  • 2
    T.J. Gander (ed), Jane's NBC Protection Equipment 1996–1997, Jane's Data Division, Coulsdon, UK, 139174, 1996.
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  • 4
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  • 5
    J. Goldenson , ‘Detection of Nerve Gases by Chemiluminescence’, Anal. Chem., 29, 877879 (1957).
  • 6
    B. Gehauf , J. Goldenson , ‘Detection and Estimation of Nerve Gases by Fluorescence Reaction’, Anal. Chem., 29, 276278 (1957).
  • 7
    C. Gelman , D.N. Kramer , ‘Enzymatic Methods for Detection of Anticholinesterases’, US Patent 3049411, 1962.
  • 8
    J. Epstein , R.W. Rosenthal , R.J. Ess , ‘Use of γ-(4-Nitrobenzyl)pyridine as Analytical Reagent for Ethyleneimines and Alkylating Agents’, Anal. Chem., 27, 14351439 (1955).and references cited herein
  • 9
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