Early experiments in gas phase ion/molecule reactions were done in modified analytical mass spectrometers and occurred to small extents because the pressures were low (< 10-3 mm Hg) and the reaction times were short (~ s). Only very fast reactions, occurring at nearly every collision, could be observed. Subsequently, as instruments were built to operate at higher pressures and longer times, reactions could be studied to large extents of conversion, consecutive reactions were observed, and equilibrium conditions achieved.
As source pressures were increased, the effects of small concentrations of impurities were (re-)discovered and chemical ionization mass spectrometry, CIMS, was developed as an analytical application of ion/molecule reactions. CIMS and GC/CIMS are now routine analytical techniques. CI mass spectra are often less complex than EI mass spectra and CIMS is frequently used for quantitation in complex mixtures. GC/CIMS with selective reactant ions can be used to identify different classes of compounds. GC/CIMS is a convenient way to study the ion chemistry of complex, but volatile, molecules.
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