ROLE OF MOLECULAR-OXYGEN IN FRAGMENTATION PROCESSES OF ABSCISIC-ACID METHYL-ESTER IN ELECTRON-CAPTURE NEGATIVE IONIZATION

Ion‐molecule reactions with oxygen promote fragmentation in the electron capture negative ionization mass Spectrometry of methylated abscisic acid. It is shown that low‐energy collisions of the molecular anion (m/z 278) with O2 in the center quadrupole of a triple quadrupole mass spectrometer result in the formation of an adduct ion which subsequently fragments to yield two prominent ions, one at m/z 141 which contains the side‐chain and a second at m/z 152 which includes the ring. Two minor products are represented by peaks at m/z 179 and 136. These ion‐molecule reactions can also occur in the ion source when oxygen is present, which may cause fluctuations in relative intensities of the fragment ion peaks. The principal fragments of mass 141 and 152, and also those of mass 179 and 136, do not form in the absence of O2. Product ion mass spectra following collisions of M−· with 18O2 in the center quadrupole show that the side‐chain fragment ion shifts from m/z 141 to m/z 143, indicating retention of an oxygen atom from the gas‐phase reaction with molecular oxygen. It is proposed that on reaction with oxygen, a peroxy radical anion intermediate at m/z 310 is formed which then fragments to yield an enolate anion (m/z 141) or a quinone‐like radical anion (m/z152). The induced fragmentations of the 2‐cis and 2‐trans isomers of methylated abscisic acid appear to be identical. Analysis of isotopically labeled compounds of both abscisic acid and structurally similar metabolites reveal that oxygen‐activated fragmentation in these selected cases provides diagnostic ions which allow one to determine the position(s) of isotopic enrichment in the molecule. Copyright © 1990 John Wiley & Sons, Ltd.