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

Electron monochromators designed for the production of low-energy electrons (0-15 eV) with nearly monoenergetic distributions have been available for many decades. The concept of adapting the electron monochromator as an ion source onto mass spectrometers for the purpose of electron capture negative ion-mass spectrometric (ECNI-MS) analyses is only now being realized. Two different analyzers, a quadrupole and a double focusing sector instrument, have recently been retrofitted with electron monochromators to test their utility as analytical instruments for the detection of environmental compounds and chemical agents. Electron energy scans of compounds in these classes reveal unique negative ion resonances, which can be used as an additional analytical dimension of information for compound identification and confirmation. Electron currents of 430 µA at 0.03 eV electron energy are now available from the electron monochromator, which will provide sufficient electron flux to meet modern standards for trace level analyses. The narrowest electron energy spread achieved has been ±0.07 eV (fwhm). The electron monochromator-mass spectrometer (EM-MS) instrument has been interfaced to a gas chromatograph (GC), and this system (GC/EM-MS) was used to record ion chromatograms of mixtures of polychlorinated compounds. Regioselective ion loss, resulting from dissociative electron capture by the parent molecule, is electron-energy dependent and can be monitored with the EM-MS instrument. Finally, positive ion spectra produced with monoenergetic electrons have also been recorded. © 1997 John Wiley & Sons, Inc.

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

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.

Capillary gas chromatographic introduction of environmental compounds into a trochoidal electron monochromator/mass spectrometer.

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.

Application of a trochoidal electron monochromator/mass spectrometer system to the study of environmental chemicals.

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.

Regioselective chloride ion loss from chlorine-37 enriched polychlorodibenzofurans by negative-ion mass spectrometry.

Regiospecific chlorine-37 enriched dichloro-, trichloro-, tetrachloro-, and pentachlorodibenzofuran isomers were investigated under electron capture negative-ion mass spectrometry (ECNIMS) conditions. The relative chloride ion loss from the enriched positions was determined from the chloride-37 and chloride-35 ion abundances after correcting for the isotopic enrichment. Regioselective chloride anion losses were observed with the carbon 3-chlorine bond (beta-chlorine) generally found to be the most labile. The carbon-chlorine bond cleavages can be loosely associated with calculated carbon-chlorine bond residual charges.

Evidence for radical anion formation during liquid secondary ion mass spectrometry analysis of oligonucleotides and synthetic oligomeric analogues: a deconvolution algorithm for molecular ion region clusters.

It is shown that one-electron reduction is a common process that occurs in negative ion liquid secondary ion mass spectrometry (LSIMS) of oligonucleotides and synthetic oligonucleosides and that this process is in competition with proton loss. Deconvolution of the molecular anion cluster reveals contributions from (M-2H).-, (M-H)-, M.-, and (M + H)-. A model based on these ionic species gives excellent agreement with the experimental data. A correlation between the concentration of species arising via one-electron reduction [M.- and (M + H)-] and the electron affinity of the matrix has been demonstrated. The relative intensity of M.- is mass-dependent; this is rationalized on the basis of base-stacking. Base sequence ion formation is theorized to arise from M.- radical anion among other possible pathways.

Negative ion fast atom bombardment mass spectrometry of oligodeoxynucleotide carbamate analogs.

Oligonucleotide analogs, in which carbamate rather than phosphodiester linkages form the backbone, have been analyzed by negative ion fast atom bombardment mass spectrometry. These oligodeoxynucleotides, dimers (982.4 daltons) to 11-mers (4356.5 daltons), show intense [M - H]- ions and some degree of cleavage of the sugar-carbamate backbone structure in repeating fashion on both sides of the carbonyl groups. Sequencing of the shorter chain oligonucleotide analogs, up to six bases long, is demonstrated by complete 3' and 5' terminal sequence fragment ions. Longer chain oligomers show partial sequencing information.

Relationship of molecular radical anion abundance and calculated lowest unoccupied molecular orbital energies for polychlorinated dibenzofurans and dibenzodioxins in electron capture negative ion mass spectrometry: evidence for negative metastable ions.

Relationships were found between experimentally measured molecular radical anion abundances and calculated lowest unoccupied molecular orbital energies (epsilon LUMO) for polychlorodibenzofurans and polychlorodibenzo-p-dioxins. Anion abundances were measured using standard mass spectrometric techniques, while epsilon LUMO were calculated by the 'Complete Neglect of Differential Overlap' method. Polychlorodibenzofurans with calculated epsilon LUMO greater than or equal to 1.6 eV show 0% molecular radical anion and those with epsilon LUMO less than or equal to 1.4 eV show greater than or equal to 80% molecular radical anion abundance. Similarly, the molecular radical anion is absent for polychlorodibenzo-p-dioxins with calculated epsilon LUMO greater than or equal to 2.0 eV. A trend towards greater molecular radical ion relative abundance appears for 2.0 eV greater than or equal to epsilon LUMO greater than or equal to 1.0 eV and a maximum is reached around 1 eV, whereupon the molecular ion abundance diminishes with lower epsilon LUMO. B/E linked scan analysis indicates that chlorodioxins with epsilon LUMO less than 1 eV give increasing amounts of metastable anions.