Observation of the intermediates of in-source aldolization reactions in electrospray ionization mass spectrometry analysis of heteroaromatic aldehydes

Electrospray ionization mass spectrometry (ESI-MS) analyses of 2-(1,2,4-triazole-1-yl)-6-methyl-3-quinolinecarboxaldehyde were carried out by using an ion trap mass spectrometer in a positive-ion mode. Interestingly, several unusual [M + 15](+), [M + 33](+) and [M+ 47](+) ions were observed with a high abundance in the ESI-MS spectrum when methanol was used as the ESI solvent. However, only the protonated molecule was obtained with acetonitrile as the ESI solvent. These unusual ions have been proposed as the intermediates of an aldolization reaction occurring in the ESI source, which have been validated by a tandem mass spectrometry experiment, high-performance liquid chromatography/mass spectrometry analysis and theoretical calculations. A full understanding of this reaction can contribute to the avoidance of analysis errors in the ESI-MS analysis of unknown heteroaromatic aldehydes.