Electrospray ionization mass spectrometry of 3-formylchromones and 3-formylcoumarins shows that collision-induced dissociation of the protonated ions results in elimination of H¬¬2 as a major fragmentation route. This loss of H2 yields an unstable carbonyl cation, which is susceptible to rapid reaction with water to form a protonated carboxylic acid. This process can take place in the electrospray source and in the collision cell. Consequently, the fragment ions observed in the MS/MS spectrum include those derived from the aldehyde and from the corresponding carboxylic acid. Elimination of CO from the original protonated ions is observed as a minor route in these aldehydes, while other aldehydes exhibit loss of CO and little loss of H2. Studies reveal that aldehydes which protonate predominantly on their carbonyl group or contain a remote basic site for stable protonation, such as a pyridine ring, undergo loss of CO. But aldehydes which contain a vicinal group that forms a hydrogen bridge with a protonated formyl group undergo significant loss of H2, which leads effectively to oxidation and to more complex MS/MS spectra. In the chromone and coumarin derivatives, this complication arises when the formyl group is located on a ring position adjacent to the oxo group. DFT calculations show that elimination of H2 from the protonated aldehyde leaves behind a relatively stable acylium ion, better described as a ketene cation. Hydration of this cation can lead to more complex MS/MS spectra, which may distort MRM results and lead to erroneous identification in library search results.
Citation: Rapid Communications in Mass Spectrometry
Pub Type: Journals
Aldehydes, collision-induced dissociation, fragmentation, tandem mass spectrometry, formylchromones, formylcoumarins, H2 elimination