Hydralazine derivative of aldehyde: A new type of [M - H]+ ion formed in electrospray ionization mass spectrometry.

Hydralazine has been widely employed in the development of drugs, derivatization reagents, and ligands. In the present work, we reported a new type of dehydrogenated ion [M - H]+ that was produced from the hydralazine derivative of hexanal in electrospray ionization mass spectrometry (ESI-MS). The formation of [M - H]+ ions in the ESI-MS was found to be independent on the mobile phase composition of the liquid chromatography and ESI source parameters. A series of hydralazine derivatives of aldehyde were investigated to confirm this phenomenon. The results showed that hydralazine derivatives of aldehydes that contained an sp3 hybridization carbon with a hydrogen at the α-position of aldehydes could form the unexpected [M - H]+ ions, whereas hydralazine derivative of acetone could only generate [M + H]+ ion in the ESI-MS. We proposed the possible formation mechanism of [M - H]+ ion for the hydralazine derivatives of aldehydes: the [M - H]+ ion was possibly formed by the loss a hydrogen molecule (H2 ) from the protonated ion [M + H]+ . The results obtained from density functional theory (DFT) calculations supported this proposed formation mechanism of [M - H]+ ion.