Ion/molecule reactions in a miniature RIT mass spectrometer

Abstract

Ion/molecule reactions were explored in a newly developed miniature mass spectrometer fitted with a rectilinear ion trap (RIT) mass analyzer. The tandem mass spectrometry performance of this instrument is demonstrated using collision induced dissociation (CID) and ion/molecule reactions. The latter includes Eberlin transacetalization reactions and electrophilic additions. Selective detection of the chemical warfare simulant dimethyl methyl phosphonate (DMMP) was achieved through selective Eberlin reactions of its characteristic phosphonium fragment ion CH₃OP⁺(O)CH₃ (m/z 93), with 1,4-dioxane or 1,3-dioxolane. Efficient adduct formation as a result of electrophilic attack by the phosphonium ion on various nucleophilic reagents, including 1,1,3,3-tetramethyl urea, methanesulfonic acid methyl ester, dimethyl sulfoxide and methyl salicylate, was also observed using the RIT device. The product ions of these reactions were analyzed using CID and the characteristic fragmentation patterns of the ionic addition products were recorded using multiple-stage experiments in the miniature RIT instrument. This study clearly demonstrates that a small, home-built, miniature RIT mass spectrometer can be used to perform analytically useful ion/molecule reactions and also that instruments like this have the potential to provide a portable platform for in situ detection of organophosphorus esters and related compounds with high specificity using tandem mass spectrometry. Copyright © 2005 John Wiley & Sons, Ltd.

INTRODUCTION

Mass spectrometry is an effective method for chemical analysis and for the study of ion chemistry in the gas phase. Chemical species are identified on the basis of their molecular weights and their fragmentation patterns, using single stage (MS) or multistage (MSⁿ) mass spectrometry experiments. Besides the wide range of laboratory applications that are rapidly being extended from chemistry to biology, pharmacology and medicine, in situ chemical monitoring using mass spectrometry has recently received considerable attention. This is due to the increasing need to identify chemical compounds in a variety of technological process and circumstances, including food quality regulation,1 environment protection,2, 3 space exploration,4–6 forensic analysis and public safety arenas.7–11 These applications typically require rapid detection and identification with high confidence of particular compounds, including specific chemical warfare agents, toxic industrial compounds and explosives at trace concentrations. The specific goals of this type of application are to identify targeted chemicals at low concentrations (ppb or lower12, 13) in short times (ca 10 s or less14, 15) and with a low rate of false positives or negatives. The goal of timely analytical information demands that the analytical instrument be brought to the sample rather than bringing the sample to the instrument, as is traditionally done for in-lab analysis. Portable mass spectrometers are being developed to allow this capability for in situ analysis. Examples are provided by commercial instruments based on conventional quadruple ion traps and (CIT) cylindrical ion traps.16 In addition to instrumentation of appropriate size and portability, a key issue for in situ analysis at low levels is to provide a low false-positive rate to improve the specificity of the analysis of samples, which are often of high complexity. This requires mass spectrometric methods that allow distinction between targeted analyte and other compounds that may be very similar in structure and molecular weight,13 and may include isomers.17, 18 Tandem mass spectrometry has traditionally been used in such cases to increase signal-to-noise ratios by minimizing chemical noise. Hence the emphasis in this study on the use of MS/MS experiments in a miniature portable mass spectrometer.

Recent efforts at miniaturization of mass spectrometers include the extreme cases in which micro, on-chip instruments are being developed,19–21 although only mini mass spectrometers with analyzer sizes in the 1 cm range, power in the 100 W range and total instrument weight on the order of 10 kg are available as functioning mass spectrometers.19, 20, 22–24 Among the different types of mass analyzers that could be chosen for use in a miniature mass spectrometer, the ion trap has uniquely attractive features. These include the facts that (1) mass analysis is possible at relatively high pressure (1 mTorr); (2) the already small size of the mass analyzer facilitates reduction in size of the other components of the instrument and (3) it is intrinsically a tandem mass spectrometer. The CIT, a geometrically simplified form of Paul Trap,25 has been selected as the mass analyzer for a miniature instrument26 that has recently been commercialized. Even more recently, the rectilinear ion trap (RIT), an analyzer with much improved performance when compared with a CIT of the same size, has been described.27 This analyzer was used to construct an RIT mass spectrometer (RIT-MS) with a small pumping system and miniaturized components.28 Analyzers made up of thousands of micron-scale CITs21 have also been fabricated and are currently being tested. Similarly, meso-scale (sub mm, but not micron size) RIT analyzers, present as arrays, have been fabricated using rapid polymer prototyping.29 Miniature mass spectrometers with RIT mass analyzers are expected to be valuable for in situ chemical analysis because of their intrinsic advantages in ion trapping capacity and ion transfer efficiency,27 and their simple mode of operation.28

As a complementary method to collision induced dissociation (CID), which uses the fragmentation pattern of the molecular ion to identify chemical structure, ion/molecule reactions performed on mass-selected ions represent an alternative tandem mass spectrometry experiments that provides the chemical specificity that is essential in the types of high-value applications being discussed. Like CID, ion/molecule reactions have also proven effective in providing data that are characteristic of particular chemical structures,30, 31 even though they have been far less widely used for this purpose. The combination of ion/molecule reactions and CID has also been used.30, 32–34 The fact that the study of ion/molecule reactions using ion traps is already well established is evident, for example, in the work of Tabet and others on stereochemical and isomer differentiation,35, 36 and in the work of Vachet37 on the chelation of metal-containing ions. In addition, fundamental kinetics and thermochemical studies are being done using ion/molecule reactions in ion traps,38 with the ability to change the charge state of trapped ions adding an important new dimension to the information available from these devices.39 A variety of ion/molecule reactions have been carried out in a miniature CIT mass spectrometer.40 Among those reactions, the reactions with phosphonium ions are of particular interest owing to the fact that organophosphorus chemical warfare agents and pesticides yield characteristic phosphonium ions in their mass spectra.41 Previous studies31, 42 have shown that the Eberlin transacetalization reaction of phosphonium ion can be used to identify the chemical warfare stimulant, dimethyl methyl phosphonate (DMMP), in mixtures with high selectivity. In this study, we demonstrate the capabilities of a miniature RIT-MS in performing ion/molecule reactions and in recording tandem mass spectra. The chemical warfare simulant, DMMP, is used as a representative analyte and is identified using the Eberlin reaction as well by electrophilic addition reactions.

EXPERIMENTAL

The miniature RIT mass spectrometer used here is composed of a sampling system, a thermionic emission filament for internal electron ionization/chemical ionization (EI/CI), an RIT mass analyzer and a DeTech Quad2000 (Detector Technology Inc., Palmer, MA) high-pressure electron multiplier (EM) for ion detection. The vacuum system consists of a small Pfeiffer TC100 turbo pump (Pfeiffer Vacuum Inc., Nashua, NH) and a compact KNF UN813.44-stage diaphragm pump (KNF Neuberger Inc., Trenton, NJ). As used in this study, each of the two rectangular y-electrode pairs comprising the RIT is 50 mm in length and 10 mm in width, while the x-electrodes are 7.6 mm in width (the definition of the directions is the same as used previously27). A 1-mm diameter hole is made in the center of each endcap electrode. An entrance/exit slit (40 × 1 mm) is centrally located in the x-electrodes, while ions are ejected in the radial direction and collected by a detector set in front of the slit in the left plate of the x-electrodes. The filament (FF700, Scientific Instrumentation Services, Ringoes, NJ) is biased by an adjustable DC power supply to provide electrons of 13–20 eV ionization energy, generally orthogonally above the entrance slit so as to be able to enter the RIT to produce internal ionization. A four-way reducing cross (NW40, MDC, Hayward, CA) is used as the vacuum chamber with a background pressure of 1.0 × 10⁻⁶ Torr. The RF frequency is 881 kHz and the maximum RF voltage is 1500 V. The control system and data processor were home-made and are described in detail else where.28

Experiments were carried at room temperature. Mass spectra were recorded using a scan function with three segments, comprising a 500 ms period for ionization, a 15 ms ion cooling period and a 15 ms RF ramp for mass analysis. In MS/MS and MSⁿ experiments, ions of interest were first isolated using a 4 ms, notched waveform inverse Fourier transform (SWIFT, calculated by ITSIM 5.043, 44) and then allowed to react with the neutral reactant molecules at a constant RF voltage. Reaction times were optimized to allow overall reaction yields of about 70% or more, as judged by comparison of the final and original ion abundances. The product ions were isolated again using a notched SWIFT waveform and fragmented by CID using resonance excitation of the isolated ions to generate the product ion MS/MS spectra. The typical bandwidth for each notch is 10 kHz. The frequency used for excitation AC was 300 kHz and the amplitude was optimized for each experiment.

All chemicals were obtained from Sigma-Aldrich (Milwaukee, WI) and were introduced without further treatment as headspace vapors at ambient temperature from the neat liquids stocked in finger tubes. Vapors of gaseous samples, such as DMMP, methyl salicylate and other analytes, were introduced into the vacuum system through two separate sampling tubes controlled by two leak valves (Granville–Phillips, series 203, Helix Technology, Mansfield, MA), so that the pressure of DMMP and the other reactant can be maintained easily at 8.0 × 10⁻⁶ Torr and 2.0 × 10⁻⁵ Torr, respectively (uncorrected, as read using a model 354 Bayard–Alpert ion gauge, Granville-Phillips, Helix Technology, Mansfield, MA). A third leak valve was used to introduce the buffer gas, normally helium, into the manifold, allowing a total manifold pressure about 8.0 × 10⁻⁵ Torr for CID process.

RESULTS AND DISCUSSION

The chemical agent simulant, DMMP, was ionized via internal EI in the RIT and the spectrum shown in Fig. 1 was recorded. In addition to the signal due to protonated DMMP [DMMP + H]⁺ (m/z 125), the phosphonium ion CH₃OP⁺ (O)CH₃ (m/z 93) was generated by fragmentation of [DMMP + H]⁺ (m/z 125) by loss of CH₃OH. This fragment ion was also observed in the MS/MS spectrum of [DMMP + H]⁺ (m/z 125) (Fig. 1(b)). The peak of m/z 111 corresponds to the adduct of the phosphonium ion (m/z 93) with trace amounts of water present in the ion trap.45 Loss of methanol from the ion of m/z 111 gives rise to the ion of m/z 79. The phosphonium ion CH₃OP⁺ (O)CH₃ (m/z 93) was isolated and used as the reactant ion in the Eberlin reaction as well as in the electrophilic additions studied.

Figure 1. (a) Mass spectrum of DMMP recorded using a miniature RIT-MS; (b) CID mass spectrum of protonated DMMP (m/z 125).

Eberlin reaction of phosphonium ion in the RIT

The prototype Eberlin reaction (Scheme 1) is the transacetalization of acylium ions with 1,3-dioxolanes to form cyclic ionic acetals in the gas phase. This reaction parallels the transacetalization between aldehydes/ketones with the formation of cyclic acetals in solution and is also related to the classical acetalization of aldehydes/ketones with diols. Discovered a decade ago,34 the Eberlin reaction has been extended to many other amphoteric ions having both a Lewis acidic and basic site; they include thioacylium (R–C⁺S),34 carbosulfonium (H₂CS⁺—R),46 sulfinyl (R–S⁺O),47 borinium (RO)₂B⁺,48 arylnitrenium (ArNH⁺),42 silylium (RO)₃Si⁺48 and phosphonium (R₂P⁺O)31, 48, 49 ions.

In the present study, Eberlin reactions of the DMMP-derived phosphonium ion (m/z 93) with six different neutral reactants (Scheme 2) were performed in the miniature RIT mass spectrometer to examine the reactivity of the phosphonium ion and the quality of the data obtainable from a miniature instrument. The product ion spectrum showing the reaction of this phosphonium ion with a typical neutral reactant, 1,4-dioxane, is illustrated in Fig. 2. The overall yield (estimated from peak areas of the product and reactant ions) for the reaction with 1,4-dioxane was ca 72% after a reaction time of 200 ms. An increase in reaction time to 500 ms resulted in an increase in the overall reaction yield of 84%. The yield of the Eberlin reaction under typical conditions is ca 50% largely because of competitive proton transfer reactions. The phosphonium ion CH₃OP⁺ (O)CH₃ (m/z 93) was isolated by application of a SWIFT waveform (0.15 V amplitude, 255–265 kHz notch, 4 ms), as shown in Fig. 2(a). All ions observed in the single-stage mass spectrum were then ejected from the ion trap, except for the ion of interest at m/z 93. After isolation and a reaction time period of 200 ms in the presence of 1,4-dioxane at a pressure of 2.0 × 10⁻⁵ Torr, the transacetalization product at m/z 137 was observed (Scheme 2). The peak at m/z 181 in the reaction product spectrum corresponds to the ion–neutral cluster of the phosphonium ion with 1,4-dioxane. The structure of the product ion (m/z 137) was examined using a triple stage (MS³) experiment. To perform this experiment, a second SWIFT waveform (0.15 V amplitude, 150–160 kHz notch, 4 ms, Fig. 2(c)) was used to isolate the product ion of m/z 137, generated from reaction of the isolated phosphonium ion of m/z 93 with 1,4-dioxane, and an excitation AC (50 ms, 0.25 V, 155 kHz) was applied to cause its dissociation. The dissociation of the product ion of m/z 137 yielded fragment ion of m/z 93 (this spectrum is the sequential product ion type50, 51of MS³ spectrum, Fig. 2(d)). The data is in agreement with pervious observations made using conventional laboratory-scale instruments31, 40 and confirms the structure of the product ion as well as the quality of the data from the Eberlin reaction in the miniature RIT-MS. Other peaks observed in the sequential product ion CID spectrum (Fig. 2(d)) include the ions of m/z 79, 111 and 125. The ion of m/z 111 is generated from the secondary reaction of the fragment ion m/z 93 with water present in the ion trap, and the loss of methanol from the ion of m/z 111 gives rise to m/z 79. The appearance of protonated DMMP ion (m/z 125) in the MS³ spectrum is probably due to proton transfer between a fragment ion such as m/z 79 with neutral DMMP still present in the ion trap.48, 52 The characteristic fragmentation that can be induced is expected to be useful in improving the specificity of in situ chemical analysis using the RIT.

Figure 2. (a) Isolation of the phosphonium ion (m/z 93) generated from DMMP; (b) MS/MS spectrum showing the ion/molecule reaction of the phosphonium ion with 1,4-dioxane in RIT; (c) mass spectrum showing the isolation of the Eberlin reaction product ion of m/z 137; (d) MS³ mass spectrum showing CID of the ion of m/z 137 to generate in turn m/z 93.

Figures 3(a–e) show product ion spectra for other Eberlin reactions of phosphonium ion (m/z 93) with different neutral acetal reactants, including 1,3-dioxane, 1,3-dioxolane, 4-methyl-1,3-dioxolane, 2-methyl-1,3-dioxolane and 2-phenyl-1,3-dioxolane (Eqns 2–6, Scheme 2). In each case, the characteristic peaks due to the products of the Eberlin reaction with the phosphonium ion (m/z 93) are clearly observed. The Eberlin reaction products, which are either m/z 137 or 151, were isolated with SWIFT waveforms and then subjected to CID to yield characteristic fragment ions. The results are summarized in Table 1. The formation of the protonated reagent molecular ions was also observed in Fig. 3, again most likely owing to secondary proton transfer processes between the reagent neutrals and proton-carriers which remain unknown. Similar phenomena have been previously reported.31, 52 Note that the combination of ion/molecule reaction chemistry and the CID analysis of the reaction products provides an effective way to use mass spectrometry to distinguish such neutral acetal isomers as 1,4-dioxane, 1,3-dioxane, 4-methyl-1,3-dioxolane and 2-methyl-1,3-dioxolane.

Figure 3. MS² mass spectra showing the ion/molecule reactions of the phosphonium ion (m/z 93) with (a) 1,3-dioxane; (b) 1,3-dioxolane; (c) 4-methyl-1,3-dioxolane; (d) 2-methyl-1,3-dioxolane; (e) 2-phenyl-1,3-dioxolane.

Table 1. Eberlin reactions of the phosphonium ion CH₃OP⁺ (O)CH₃ (m/z 93)

Chemical reagents	Structure	Molecular weight	m/z of ionic products	m/z of CID fragment ions
1,4-Dioxane		88	137	125, 111, 93, 79
1,3-Dioxane		88	151	141, 125, 115, 93, 89
1,3-Dioxolane		74	137	125, 109, 93, 79
4-Methyl-1,3-dioxolane		88	151	125, 109, 93
2-Methyl-1,3-dioxolane		88	137	125, 89
2-Phenyl-1,3-dioxolane		150	137	125, 109, 93

Electrophilic addition reactions of phosphonium ions in the RIT

The ion/molecule reaction capabilities of this miniature RIT mass spectrometer were also demonstrated with a series of electrophilic addition reactions of the phosphonium ions. Phosphonium ions are reactive electrophiles53 because of the strong polarity of the PO bond; for instance, phosphonium ions can attack electron-rich substrates via electrophilic addition to form a covalently bonded complex.33, 45, 54 The specific fragment ions resulting from such a covalently bonded ionic product might provide valuable information for the identification of the phosphonium ion. The electrophilic addition ion/molecule reactions of phosphonium ion (m/z 93) with four different neutral nucleophiles, 1,1,3,3-tetramethyl urea (TMU), methanesulfonic acid methyl ester, dimethyl sulfoxide and methyl salicylate, were explored in the mini RIT-MS. The MS³ data on the product ions from these reactions are summarized in Table 2.

Table 2. Electrophilic addition reactions of the phosphonium ion CH₃OP⁺ (O)CH₃ (m/z 93)

Chemical reagents			m/z of ionic products	m/z of CID fragment ions
Name	Structure	Molecular weight
1,1,3,3-Tetramethyl urea		116	209	194, 117, 85
			241	209, 194, 117, 85
Methanesulfonic acid methyl ester		110	203	171, 111, 79
Dimethyl sulfoxide		78	171	139, 119, 79
			213	125, 79
2-Hydroxybenzoic acid methyl ester		152	245	125, 152, 93, 120

A typical case is the mass spectrum that was collected for the reaction of the phosphonium ion m/z 93 from DMMP with 1,1,3,3-tetramethyl urea (Fig. 4(a)). The electrophilic addition product of the phosphonium ion with 1,1,3,3-tetramethyl urea, [93 + TMU]⁺, was observed at m/z 209. The overall yield for the product of this reaction was estimated to be 88–94% when using a reaction time between 200 to 500 ms. Other peaks at m/z 72, 117, 125 and 241 correspond to (CH₃)₂NCO⁺, [TMU + H]⁺, [DMMP + H]⁺ and [DMMP + H + TMU]⁺. The association product (m/z 209) of the phosphonium ion with 1,1,3,3-tetramethyl urea represents the dominant peak in the spectrum. Upon CID (Fig. 4(c)), instead of fragmenting back to the phosphonium ion (m/z 93), the product ion m/z 209 yields fragments of m/z 85, 117 and 194, indicating that it is covalently bound rather than being a weakly bonded adduct. This result is in agreement with the previous reported result of electrophilic addition reactions of phosphonium ions in the gas phase.33, 45 It is also a very useful result for chemical analysis, since there is no simple relationship between the masses of the product and reactant ions.

Figure 4. (a) MS² mass spectra showing the ion/molecule reactions of the phosphonium ion (m/z 93) with 1,1,3,3-tetramethyl urea; (b) isolation of the electrophilic addition product ion of m/z 209; (c) MS³ data showing CID of the ion of m/z 209.

Two possible structures for this particular addition product, a and b, are shown in Scheme 3. The adduct is likely to be generated by the attack of the phosphonium ion on either the nitrogen or the oxygen atom of TMU. There is evidence that both processes occur; thus, fragment ions of m/z 117 and 194 can be generated with the loss of CH₃^• and (CH₃O)P(O)(CH₂) from ion a, while ion b fragments to give rise to an ion of m/z 85 via β-elimination of DMMP. The latter fragmentation process involves an interesting formal methoxyl group abstraction from TMU by the phosphonium ion, a process that might be useful in selectively identifying phosphonium ions. An analogous characteristic methoxyl group abstraction by the phosphonium ion was observed to be responsible for the formation of the fragment ion of m/z 79 during the dissociation of the electrophilic adduct of the phosphonium ion and methane sulfonic acid methyl ester (m/z 203) (Table 2). It is likely that both these reactions are driven by the formation of the strong PO bond (high bond energy, ca 96 kcal/mol55). It is also noted that ions of m/z 241 were formed during CID (Fig. 4(c)) with a mass-to-charge ratio even higher than the precursor ion m/z 209. Presumably, this ion was generated by a secondary reaction of the fragment ion at m/z 117 with neutral DMMP present in the trap.

CONCLUSION

This study clearly demonstrates that a small, home-built, miniature RIT mass spectrometer can be used to perform ion/molecule reactions of mass-selected ions as well as other tandem mass spectrometry experiments. It provides data of reasonable quality in short periods of time suggesting that instruments like this have the potential to be used for in situ detection of organophosphorus esters and related compounds with high specificity.

The Eberlin reaction is a particularly useful way to detect phosphorus esters and is easily performed in the RIT-MS, which also allows product structures to be confirmed by subsequent CID spectra. It can be speculated that the demonstrated Eberlin reactions and other diagnostic MS/MS experiments could be carried out in parallel using multiplexed instruments,56, 57 with one sample but a different reaction occurring in each reaction/mass analysis channel, to further enhance the selectivity in the detection of organophosphorus chemical-warfare agents and other toxic compounds in complex mixtures. The characteristic formal methoxyl abstraction by phosphonium ion during the fragmentation of electrophilic adducts of phosphonium ions with TMU or methanesulfonic acid methyl ester is also potentially valuable in the selective detection of phosphorus esters in complex matrices.

Acknowledgements

This work was supported by the Purdue Research Foundation Trask Fund, DETRA (N00164-00-C-0047) and the National Science Foundation (CHE 04-12782).