Differentiation of isomeric heptylamines by in-source collision-induced dissociation of [M + H]+ ions

Rationale: 1,3-Dimethylamylamine (1,3-DMAA), 1,4-dimethylamylamine (1,4-DMAA) and 2-aminoheptane are isomeric heptylamines commonly used as stimulants. The fragmentations of their [M + H]⁺ ions (m/z 116) are almost identical and consist of the formation of a fragment ion C₄ H₉ ⁺ at m/z 57. It is interesting to check if analysis of electrospray ionization (ESI) mass spectra of these three amines enables their differentiation. In order to better understand the fragmentation of isomeric heptylamines, 1-, 3- and 4-aminoheptane were also included in the study.

Methods: ESI in-source collision-induced dissociation (CID) mass spectra were obtained with a Waters/Micromass ZQ2000 mass spectrometer (single quadrupole-type instrument) at a cone voltage of 12-34 V. The influence of three commonly used solvents on the fragmentation of [M + H]⁺ ions was tested, namely methanol, methanol/water (1/1) and acetonitrile/water (1/1).

Results: Only one quantitative difference was observed in the mass spectra of 1,3-DMAA, 1,4-DMAA and 2-aminoheptane, namely formation of a product ion at m/z 57 from the [M + H]⁺ ion of 1,3-DMAA is more efficient than the formation of this ion from [M + H]⁺ ions of 2-aminoheptane and 1,4-DMAA. In-source CID of 3- and 4-aminoheptane consists of (besides the formation of ion C₄ H₉ ⁺ ) the formation of a low-abundance, but clearly seen, fragment ion at m/z 114 (loss of H₂ ).

Conclusions: Differentiation of 1,3-DMAA from 1,4-DMAA and 2-aminoheptane seems to be possible by in-source CID of their [M + H]⁺ ions. However, comparison of the mass spectrum obtained for the analysed sample with that of a standard seems to be necessary. Only 3- and 4-aminoheptane may be easily differentiated from the other heptylamines by the presence of fragment ions at m/z 114.