SENSITIVE ATMOSPHERIC-PRESSURE DETECTION OF NITROAROMATIC COMPOUNDS AND NOX (X=1,2) MOLECULES IN AN IONIZATION-CHAMBER USING RESONANCE-ENHANCED MULTIPHOTON IONIZATION

Nitrobenzene and o-nitrotoluene were detected in trace concentrations in gas mixtures at atmospheric pressure in a simple unity-gain ionizat ion chamber. The detection procedure relies on tunable ultraviolet las er radiation that identifies the characteristic wavelength dependence of the NO+ ion fragment. Sensitivity levels are 0.25 +/- 0.05 ppm for nitrobenzene and 0.75 +/- 0.1 ppm for o-nitrotoluene, with the current limitations being background ionization, laser power and small sampli ng volume. Additionally, nominally 1 ppm NO and NO2 gas samples were a nalysed and the saturation fluence (via a resonant two-photon process at 226.3 nm) for NO+ production from NO gas was determined. Sensitivit y levels of 120 and 50 ppb were estimated from the data on NO2 and NO, respectively. It is shown that an increase in sensitivity of an order of magnitude is possible in all instances if the experimental paramet ers are optimized.