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.