https://doi.org/10.15255/KUI.2002.032
Published: Kem. Ind. 53 (6) (2004) 265–272
Paper reference number: KUI-32/2002
Paper type: Review
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Chemical Ionization Mass Spectrometry

S. Kazazić and S. Martinović

Abstract

Chemical Ionization (CI) is applied to samples similar to those analyzed by electron ionization and is primarily used to enhance the abundance of the molecular ion. Chemical ionization uses ion-molecule reactions in the gas phase to produce ions from the sample molecule. The chemical ionization process is initiated with a reagent gas such as methane, isobutane, or ammonia, which is initially ionized by electron impact ionization. A high gas pressure in the ionization source results in the propagation of ion-molecule reactions between the reagent gas ions and reagent gas neutrals. Some of these ions can react with the analyte molecules to produce ions. This ion-molecule reaction is a much lower energy process than EI and results in either a protonated molecule, a deprotonated molecule (a negative ion – mass spectrometry can be used for the detection of either positive or negative ions), an adduct ion (where the reagent ion adds to the analyte molecule), or a molecular ion (brought about through charge transfer from the reagent ion to the analyte molecule). If we ionize methane gas at a relatively high pressure (0,1 Torr) by electron impact, then we initially create the molecular ion. However, the methane is at a high pressure, so there is a distinct possibility of the molecular ion colliding with another methane molecule. Therefore, if a small amount of the sample to be analyzed is introduced into the ion source in the vapor phase, it is the species such as CH5 + that act as the means of ionization of the sample. Chemical ionization is a relatively soft ionization method, and in fact, it represents the first soft ionization introduced to mass spectrometry. While the CI process itself generates ions of relatively low internal energy, thus exhibiting a low level of fragmentation, evaporation of the analyte prior to ionization remains the critical step. Therefore, CI will allow molecular ion recognition only if the analyte can be evaporated without decomposition.

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Keywords

mass spectrometry, ionization methods, chemical ionization, ion-molecule reactions, reagent ions