TRACE DETECTION OF NITROCOMPOUNDS BY ARF LASER PHOTOFRAGMENTATION IONIZATION SPECTROMETRY

A new method for detecting trace vapors of NO2-containing compounds ne ar atmospheric conditions has been demonstrated with the use of one-co lor-laser photofragmentation/ionization spectrometry. An ArF laser is employed to both photolytically fragment the target molecules in a col lision-free environment and ionize the characteristic NO fragments. Th e production of NO is hypothesized to result from a combination of two NO2 unimolecular fragmentation pathways, one yielding NO in its X2PI electronic ground state and the other in its A2SIGMA+ excited state. I onization of ground-state NO molecules is accomplished by resonance-en hanced multiphoton ionization processes via its A2SIGMA+ <-- X2PI (3, 0), B2PI <-- X2PI (7, 0) and/or D2SIGMA+ <-- X2PI (0, 1) bands at 193 nm. The analytical utility of this method is demonstrated in a molecul ar beam time-of-flight apparatus. Limits of detection range from the p arts-per-million (ppm) to parts-per-billion (ppb) level for NO, NO2, C H3NO2, dimethylnitramine (DMNA), ortho- and meta-nitrotoluene, nitrobe nzene, and trinitrotoluene (TNT). Under effusive beam experimental con ditions, discrimination between structural isomers, ortho-nitrotoluene and meta-nitrotoluene, has been demonstrated with the use of their ch aracteristic photofragmentation/ionization mass spectra.