Analysis of organophosphate pesticides by a trochoidal electron monochromator mass spectrometer system

Abstract

An electron monochromator mass spectrometer was used to study the resonant electron energies versus negative ion masses of the organophosphate insecticides dicapthion, EPN, ethion, fenitrothion, leptophos, leptophosoxon, paraoxon, and parathion. Each compound yielded a unique two-dimensional electron energy/mass spectrum. The most abundant ions are produced with electrons of energies between 0.03 and 1 eV, but ions result also from capture of electrons with energies ranging to 8.5 eV. Both resonance electron capture ions and dissociative electron capture ions are produced with electrons of energies ranging from 0.03 to 8.5 eV, and ions may have as many as three observable resonance states from which they are formed. Substituted thiophenoxide ions are postulated to arise by rearrangement of the parent thiophosphate ions through a spiro intermediate. Most fragment ions can be rationalized as arising through simple homolytic cleavage of the parent radical anions.

Electron monochromator-mass spectrometer instrument for negative ion analysis of electronegative compounds

“…Negative ion resonances (NIR) for all of the resonance and dissociative electron capture processes can be plotted as a function of electron energy (53). Such plots constitute a total electron capture cross-section with respect to electron energy, and are thus a probability of electron capture [P(e)] vs. electron energy [el distribution €or that compound.…”

“…These compounds include dicapthion, EPN, ethion, fenitrothion, leptophos, leptophosoxon, paraoxon, parathion, and a range of parathion analogs (Chart 1). Most of these compounds show stable molecular radical anions and all show a wide range of fragment anions (53,58) when analyzed with the EM-MS instrument.…”

“…The thiophenoxide anion can arise only via a rearrangement reaction, whereas simple bond cleavage between the phosphorous and oxygen atoms could account for the production of the phenoxide anion. A spiro intermediate followed by fragmentation via pathway b (Scheme 3) has been proposed for the production of the thiophenoxide anions (53 spiro intermediate via pathway a, but the electron energies required for the formation of these two anions were generally different; thus, these data suggest different mechanisms for their formation.…”

Analytical applications of electron monochromator-mass spectrometry

“…During the 1990s, Deinzer et al made significant improvements to Illenberger and co‐workers' design and increased the ionization current to a maximum of 2 µA while maintaining an electron resolution of ±0.1 to ±0.4 eV (Illenberger, Scheunemann, & Baumgärtel, ; Oster, Kühn, & Illenberger, ). Over 3 years, Laramée et al described the design, tuning, and application of electron monochromator‐mass spectrometry (EM‐MS) that would later serve as the basis for the JEOL USA, Inc., commercial development (Laramée, Kocher, & Deinzer, ; Laramée & Deinzer, ; Laramée et al, ).…”

“…In additional GC/EM‐MS analyses, REs of organophosphate pesticides, two of which are shown in Figure a (parathion) and Figure b (paraoxon) (Laramée et al, ), were studied. Each marker on the graph represents a RE at which the molecule underwent dissociative electron capture and the m / z of the resulting fragment.…”