MASS-SPECTROMETRIC EVIDENCE FOR THE FORMATION OF SILAETHENE ACCOMPANYING THE FRAGMENTATION OF A NONESTER SILICON-CONTAINING PYRETHROID INSECTICIDE

The non-ester silicon-containing pyrethroid SSI-116 and its carbon analogue (ethofenprox, MTI-500) were investigated using a tandem quadrupole mass spectrometer incorporating a hexapole collision cell under positive-ion electron impact ionization conditions. Conventional mass spectrometry using the first quadrupole analyser only and tandem mass spectrometry on selected precursor ions and product ions, and also constant neutral loss scan experiments, were used. Accurate mass measurements on key ions were made using a high-resolution double-focusing mass spectrometer. Based an the experimental observations, mechanisms have been proposed to explain the major fragmentation pathways of SSI-116 and ethofenprox. The novel expulsion of silaethene, CH2=SiH2, as a neutral species from the silicon-containing insecticide and the formation of an ion at m/z 135, isomeric with the fragment ion, [C9H11O](+), observed in the spectrum of its carbon analogue, have also been rationalized. Minor fragmentation pathways, found only in the silicon-containing analogue, are also discussed.