Positive and Negative Chemical Ionization Mass Spectrometry

Abstract

Chemical ionization (CI) mass spectrometry developed from studies of ion-molecule reactions in simple hydrocarbon gases.⁽¹⁾ CI mass spectra depend upon the fact that the cross sections for ion-molecule reactions involving electron or ion transfer are considerably larger than the corresponding cross sections for interaction of molecules with free electrons.⁽²⁾ If a substrate is present in an ion source at concentrations ranging from 10^-3 to 10^-5 torr, and the same source contains a reagent gas at a pressure of approximately 1 torr, the initial ionization caused by interaction of high-energy electrons with the gases in the source will occur primarily in the reagent gas. Because the gas pressure in the ion source is maintained at approximately 1 torr, ions formed by interaction with the primary electron beam will experience up to thousands of collisions with neutral molecules prior to exiting the source. Since the cross sections for ion-molecule reactions are considerably larger than the cross sections for interactions of electrons and molecules, many of these collisions will be reactive, with the result that the spectrum of ions emerging from the source will be substantially altered from that obtained at low source pressures. The ionization process in CI mass spectrometry is best reviewed by examining the reactions that occur in the high-pressure mass spectrum of methane, one of the most common CI reagent gases.