Chemical Ionization Mass Spectrometry: Theory and Applications
Published Online: 15 SEP 2006
Copyright © 2000 John Wiley & Sons, Ltd. All rights reserved.
Encyclopedia of Analytical Chemistry
How to Cite
Munson, B. 2006. Chemical Ionization Mass Spectrometry: Theory and Applications. Encyclopedia of Analytical Chemistry.
- Published Online: 15 SEP 2006
Chemical ionization mass spectrometry (CIMS) is a technique for forming ions of the compound of interest (analyte, A) by ion/molecule reactions from reactant ions of a reagent gas that is generally present in a much greater abundance than the analyte. The reactant ions are generally produced by electron ionization (EI) of the reagent gas. The ions produced by EI often react with the large excess of the reagent gas to form the actual reagent ions that react with the analyte. CIMS is performed with both positively and negatively charged reactant ions. The most common ion/molecule reactions in CIMS are proton transfer (which forms AH+ or [A − H]− ions), hydride transfer (which forms [A − H]+ ions), charge or electron transfer (which forms A+• or A−•ions), and adduct formation or attachment (which forms [A + R]+ or [A + R]− ions). Fragment ions from decompositions of these AH+, A±•, [A − H]±, and [A + R]± ions are frequently observed. The extent of fragmentation can be controlled by the choice of reagent gas used and can be predicted to some extent from ionic thermochemical data. Collisionally stabilized electron capture at high pressures (to form A−• ions) is often classified as chemical ionization (CI). The most common use of CIMS in analytical mass spectrometry is to obtain simplified mass spectra of compounds, often one species spectra, which can be used for quantitative analysis of mixtures. CIMS can be performed with any type of mass spectrometer (quadrupole, magnetic, time-of-flight (TOF), Fourier transform ion cyclotron resonance (FTICR), ion trap) and CIMS capabilities are routinely available on many commercial instruments. With ion trap or FTICR mass spectrometers, it is possible to select specific reactant ions. It is generally considered that CI and EI sensitivities are approximately the same in the positive ion mode and that electron capture CI at high pressures for compounds with high electronegativities gives a much greater sensitivity than other EI or CI techniques. CIMS has the same general limitations as electron ionization mass spectrometry (EIMS) on the volatility and thermal stability of the compound being analyzed. However, direct insertion of the sample into the source of the mass spectrometer allows the analysis of relatively involatile and thermally unstable compounds.