Recognizing alpha-, beta- or gamma-substitution in pyridines by mass spectrometry

A general mass spectrometric method able to recognize the site of substitution of monosubstituted pyridines is described. The method requires that the molecule under investigation forms, upon ionization and dissociation, the respective alpha-, beta- or gamma- pyridinium ion of m/z 78. Pyridinium ions are stable and common fragments of ionized and protonated pyridines and are found to function as appropriate structurally diagnostic fragment ions. They can be identified by their characteristic and nearly identical collision-induced dissociation behavior and distinguished by the combined use of two structurally diagnostic ion/molecule reactions with acetonitrile and 2-methyl-1,3-dioxolane. alpha-, beta- or gamma-substitution in pyridines can, therefore, be securely recognized via an MS-only method based on structurally diagnostic ions and by the inspection of a single molecule (no need for intracomparisons within the whole set of isomers).