Abstract: | 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 [CH5]+, [C2H5]+, and [C3H5]+, then react “chemically” with the substrate M in fast acid/base type reactions to form ions of the type [MH]+, [M(C2H5)]+, 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. |