Positive and negative ion chemical ionization mass spectrometry of trimethylsilyl derivatives of pyrrolizidine alkaloids using NH+ and OH− as the reactant ions

Abstract

It is shown that the combination of positive ion and negative ion chemical ionization (PINICI) in gas chromatography/mass spectrometry of trimethylsilyl (TMS) derivatives of pyrrolizidine alkaloids (PAs) offers a rapid tentative structure elucidation of such compounds in plant material. PICI with NH3 as the reactant gas gives abundant [MH]+ ions; elimination of the necic acid produces a positive ion, corresponding to the necine base, which may carry a substituent at C7. NICI with OH− as the reactant ion cleaves the ester bonds, which may be present at C7 and C9 of the necine base or at the β-C atom of the necic acid. This is illustrated by means of the fragmentation pattern of the PICI- and NICI-mass spectra of echinatine-(TMS)3 7-acetyl-lycopsamine-(TMS)2, β-acetyl-echinatine-(TMS)2, supinine-(TMS)2 and curassavine-(TMS)3 (or stereoisomers). In this way, a number of 7-angelyl/tiglyl (e.g. symphytine), β-angelyl/tiglyl and β-(iso)valeryl (not reported before) derivatives of echinatine (or isomers like lycopsamine and intermedine) and of PAs with a saturated necine base, like trachelanthamine (or stereoisomers), could be shown in Symphytum taxa, Anchusa officianlis L. and Eupatorium cannabinum L.