Fragmentation of methyl abscisate and pentafluorobenzyl abscisate in methane electron capture negative ionization tandem mass spectrometry

The methyl and pentafluorobenzyl esters of the plant hormone abscisic acid were subjected to methane chemical ionization tandem mass spectrometry (MS/ MS). The spectra obtained allow the unambiguous identification of abscisic acid in a few milligrams of plant material, In full-scan mode pentafluorobe nzyl (PFB) abscisate (ABA) gave three significant ions at m/z 263 ([M - PFB ](-)), m/z 219 ([M - PFB - CO2](-)) and m/z 153 ([M - PFB - side chain](-)) . Each of these was subjected to MS/MS and structures were assigned to the product ions using the labelled analogues, PFB[1'-O-18]ABA, PFB[4'-O-18]ABA , PFB[side-chain-H-2(4)]ABA and PFB[ring-H-2(6)]ABA. Similarly, in full-sca n mode, methyl abscisate gave three significant ions at m/z 278 (M-), m/z 2 60 ([M - H2O](-)) and m/z 245 ([M - H - CH3OH](-)) and in MS/MS the use of the methyl esters of the above labelled analogues, and [H-2]methyl abscisat e, allowed structures to be assigned to the product ions. Using these resul ts it will be possible to dissect abscisic acid so that most labelled atoms from C-13-labelled substrates will be able to be uniquely identified from a few nanograms of C-13-labelled abscisic acid. If sufficient incorporation of C-13-labelled substrates can be obtained it should be possible to inves tigate the pathway(s) of abscisic acid biosynthesis using less than 1 g of plant material. Copyright (C) 1999 John Wiley & Sons, Ltd.