Investigations into trace detection of nitrocompounds by one- and two-color laser photofragmentation/fragment detection spectrometry

Trace concentrations of nitrogen dioxide (NO2), nitromethane (CH3NO2), and 2,4,6-trinitrotoluene (TNT) are detected by both one- and two-color laser p hotofragmentation/fragment detection (PF/FD) spectrometry using one or two lasers. The PF/FD methods studied are (1) one-laser, one-color photofragmen tation of the analyte molecule at 227 or 454 nm with subsequent detection o f the characteristic nitric oxide (NO) photofragment by one- or two-photon laser-induced fluorescence using its A(2)Sigma(+)-(XII)-I-2 (0,0) transitio ns near 227 nm; (2) one-laser, two-color PF/FD, where a 355 nm laser beam i s used for additional analyte photofragmentation and NO is detected by both one- and two-photon LIF as in the previous case; (3) two-laser, two-color PF/FD, where the pump and probe beams are time delayed; and (4) one-laser, one-color PF/FD at 355 nm, where the 355 nm beam photofragments the target molecule and the prompt emission from electronically excited NO (A(2)Sigma( +)) is monitored in the range of 200-300 nm. PF/FD excitation and emission spectra are recorded and also simulated with the use of a computer program based on a Boltzmann distribution analysis with transition probabilities, r otational energies, and rovibrational temperatures as input parameters. The effects of laser wavelength, laser pump energy, time delay between pump an ti probe beams, and analyte concentration on PF/FD signal are investigated and reported. Limits of detection [signal-to-noise (S/N) = 3] for the nitro compounds range from low ppb, to ppm, for 10 s integration time and laser e nergies of similar to 5 mJ and 100 mu J for the Dump and probe beams, respe ctively. These results are presented and compared to other PF/FD methods fo r nitrocompound monitoring.