Chemical Ionization—A Mass‐Spectrometric Analytical Procedure of Rapidly Increasing Importance

The mode of ionization of a molecule has a strong influence on its behavior in the mass spectrometer and thus on the information that can be obtained from its mass spectrum. In chemical ionization a reagent gas, e.g. methane, is first ionized by electron impact. The ions formed in ion‐molecule reactions, in particular [CH 5 ] + , [C 2 H 5 ] + , and [C 3 H 5 ] + , then react “chemically” with the substrate M in fast acid/base type reactions to form ions of the type [MH] + , [M(C 2 H 5 )] + , etc. , which subsequently fragment to various extents. Alternatively, chemical ionization can be effected by charge exchange, in that ions of a reagent gas, e.g. [He] +• , react with the substrate M to form molecular ions [M] +· . Chemical ionization can thus be conducted in a more or less mild fashion and the extent of the fragmentation can be controlled over a very wide range.