Separation of isomeric amines with ion mobility spectrometry

Eight selected isomeric amines were ionized using atmospheric pressure chemical ionization and atmospheric pressure photoionization producing a protonated molecule [M+H](+) for each amine. The mobility of these ions was measured by ion mobility spectrometry. The amine compound class was shown to have an important role in mobility separation of the amines. 2,4,6-collidine, N,N-dimethylaniline and N-methyl-o-toluidine with highest observed mobilities have a N-heterocyclic aromatic ring, or are tertiary or secondary amines, respectively, whereas the rest of the compounds with lower mobilities were primary amines. It is suggested that the protonated -NH2 group (-NH3(+)) interacts more with the drift gas, and therefore the primary amines have lower mobilities. The effect of the drift gas was tested by mixing argon or helium with the nitrogen drift gas. The presence of argon shifted the mobilities towards lower values, while with helium the mobility shifted towards higher values. However, in neither case did this result in better separation of the unresolved compounds.