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Nitro-PAH and Nitroaromatic Analysis by Direct Formation of Negative Ions

Some of the more toxic compounds found in particulate matter produced from a diesel engine are the nitrated polycyclic aromatic hydrocarbons (NPAH). These compounds generally show mutagenic and carcinogenic activities. Other nitro compounds such as drugs and explosives are analytically important. This report describes the use of a trochoidal electron monochromator for the analysis of unsubstituted PAHs and the corresponding nitro- PAHs as well as a variety of other nitro-substituted compounds including three common explosives. Isomer distinction is accomplished by using a tunable-energy electron monochromator. A surprising observation is that CID fragmentation is found to depend on the electron-capture resonance for precursor-ion formation.

A new high-current trochoidal electron monochromator1 has been designed and interfaced to a JEOL AX505 magnetic-sector mass spectrometer. The electron monochromator provides a means for controlling the ionizing electron energy over the 0 eV to 30 eV range with an electron energy resolution of +/- 0.3 eV and output currents in excess of 200 μA. This permits the direct formation of negative ions without using an energy-moderating buffer gas, and it provides a means for measuring electron capture resonances and forming ions in specific energy states. The electron monochromator has demonstrated a detection limit of 10 fg of hexachlorobenzene (splitless injection) with a signal-to-noise ration of 31:1. A linear response has been measured for octafluoronapthalene with concentrations ranging over seven orders of magnitude. Linked-scan MS/MS and selected reaction monitoring have been used to examine energy-dependent fragmentation.

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