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

Abstract

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 sp 3 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 (H 2 ) from the protonated ion [M + H] + . The results obtained from density functional theory (DFT) calculations supported this proposed formation mechanism of [M − H] + ion.

Proposal for a chemically consistent way to annotate ions arising from the analysis of reference compounds under ESI conditions: A prerequisite to proper mass spectral database constitution in metabolomics

“…The main bimolecular “in‐source” reactions encountered are addition‐elimination type reactions such as carboxylic acid esterification or ester hydrolysis, alkyl group exchanges allowing for instance amine alkylation, [2 + 2] and [2 + 4] cycloadditions, S N , S N Ar, or S E Ar reactions (as discussed above in the case of covalent adduct formation) as well as oxidation and reduction. For the latter cases, even though, H 2 and O 2 are not formal neutral gain or loss, they may be considered as such to simplify the resulting ion annotation because the mechanistic processes underlying the redox reactions are scarcely documented …”