Determination of flavone, flavonol, and flavanone aglycones by negative ion liquid chromatography electrospray ion trap mass spectrometry

Eleven naturally occurring flavonoid aglycones, belonging to the representa tive flavone, flavonol, and flavanone types were separated by high performa nce liquid chromatography and analyzed on-line with negative ion electrospr ay ionization tandem mass spectrometry (ESI-MS/MS). In order to resolve the MS/MS spectra obtained, each compound was reinvestigated by direct loop in jections using an ion trap mass spectrometer. The MS " spectra obtained all owed us to propose plausible schemes for their fragmentation supported by t he analysis of five complementary synthetic flavonoid aglycones. The negati ve ion ESI-MS/MS behavior of the different aglycones investigated in this s tudy revealed interesting differences when compared with the previously des cribed patterns obtained using various ionization techniques in positive io n. Thus, concerning the retro Diels-Alder (RDA) fragmentation pathways, sev eral structurally informative anions appeared highly specific of the negati ve ion mode. In addition, a new lactone-type structure, instead of a ketene , was proposed for a classic RDA diagnostic ion. We also observed unusual C O, CO2, and C3O2 losses which appear to be characteristic of the negative i on mode. All these results and these unusual neutral losses show that the n egative ion mode was a powerful complementary tool of the positive ion mode for the structural characterization of flavonoid aglycones by ESI-MS/MS. ( C) 2001 American Society for Mass Spectrometry.