A trochoidal electron monochromator has been interfaced to a mass spectrometer to perform electron capture negative ion mass spectrometric (ECNIMS) analyses of environmentally relevant chemicals. The kinetic energy of the electron beam can be varied from 0.025 to 30 eV under computer control. No reagent gas is used to moderate the electron energies. An electron energy spread of +/- 0.1 to +/- 0.4 eV full width at half-maximum (fwhm) can readily be obtained at a transmitted current of 2 x 10(-6) A, improving to +/- 0.07 eV at 5 x 10(-7) A. Comparisons of ECNI results from the electron monochromator/mass spectrometer system with those from a standard instrument that uses a moderating gas show similar spectra for heptachlor but not for the s-triazine herbicides, as for example, atrazine. This compound shows numerous adduct ions by standard ECNIMS that are eliminated by using the electron monochromator to generate the mass spectra. Isomeric tetrachlorodibenzo-p-dioxins show distinct differences in the electron energies needed to produce the maximum amount of parent and fragment anions. Multiple resonance states resulting in stable radical anions (M.-) are easily observed for nitrobenzene and for polycyclic aromatic hydrocarbons. Ionic products of dissociative electron capture invariably occur from several resonance states.
A gas chromatograph was interfaced to an electron monochromator/quadrupole mass spectrometer. The new system was tested for the analysis of environmental compounds. Detection sensitivity for hexachlorobenzene (HCB) through the gas chromatograph was 5 pg or better, and a mass-resolved molecular ion cluster for this compound on the fly was achieved with 10.8 ng of sample. An ion chromatogram was obtained using 45 ng of Aroclor 1254, and the extract from a trout muscle sample recently collected in the Arctic yielded a chromatographic profile similar to that observed using negative ion chemical ionization mass spectrometry. A mixture of HCB and 2,4,6-trinitrotoluene (TNT) were shown to be distinguishable at 2.4-eV electron energy. The energetics of regioselective fragmentation of [4-15NO2]TNT by dissociative electron capture can be determined on the fly. Complete negative ion gas chromatography/mass spectrometry (GC/MS) spectra were obtained for hexafluorobenzene and TNT by simultaneously ramping the electron energy from -2 to 15 eV and scanning the mass over a 200-Da range.
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