MONITORING OF VAPOR-PHASE NITRO-COMPOUNDS USING 226-NM RADIATION - FRAGMENTATION WITH SUBSEQUENT NO RESONANCE-ENHANCED MULTIPHOTON IONIZATION DETECTION

The development of a novel technique for sensing trace vapors of NO2-containing compounds is reported. This technique is based on the use of one laser operating at 226 nm to both photofragment the target molecule and detect the characteristic NO fragment, formed from a rapid predissociation of NO2, by resonance-enhanced multiphoton ionization (REMPI) and/or laser-induced fluorescence (LIF) via its A2SIGMA+-X2PI (0,0) band origin. The analytical utility of this technique is demonstrated on a number of compounds, including dimethylnitramine, nitromethane, nitrobenzene, trinitrotoluene (TNT), and 1,3,5-trinitrohexahydro-1,3,5-triazine (RDX), employing molecular beam sampling with time-of-flight mass spectrometric analysis of the jet-cooled analyte seeded in an atmosphere of buffer gas. With the present system, limits of detection of 8 and 24 parts per billion (ppb) are obtained for ROX and TNT, respectively, using only approximately 100 muJ/pulse of laser energy. The limits of detection of the other compounds studied are also presented and discussed.