Atmospheric Pressure Ionization (ISI and APCI)

Abstract

Chemical ionization (CI) is based on the transfer of charged species from a reagent ion to an analyte molecule. This process produces an analyte ion that can be analysed by a mass spectrometric analyser. The ions formed in positive mode are usually adducts of the analyte molecule with small cations such as H⁺, NH ⁺₄ , Na⁺ and, in negative mode, result from loss of protons, electron capture, adduct formation with small anions such as Cl^-, HCOO^- CH₃COO^- and so on¹. CI spectra are usually simple with little fragmentation and the technique can be very sensitive, as the ion current is concentrated in a few species. CI requires a higher pressure with respect to that found in the common Electron Ionization (EI) sources and in the mass analyser. It is very efficient in producing ions, as the rate constants for the ion-molecule reactions are generally the collision rate of the reagents. On the basis of the last observation, we can postulate that CI performed at atmospheric pressure would have the highest efficiency: the creation of ions at atmospheric pressure is quite straightforward and the major difficulty is transferring the ions produced into the high vacuum (10^-5-10^-6 Torr) region of the mass analyser.