Sensitive atmospheric pressure detection of nitroaromatic compounds and NOx(x= 1,2) molecules in an ionization chamber using resonance-enhanced multi-photon ionization

Abstract

Nitrobenzene and o-nitrotoluene were detected in trace concentrations in gas mixtures at atmospheric pressure in a simple unity-gain ionization chamber. The detection procedure relies on tunable ultraviolet laser 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 sampling volume. Additionally, nominally 1 ppm NO and NO₂ gas samples were analysed and the saturation fluence (via a resonant two-photon process at 226.3 nm) for NO⁺ production from NO gas was determined. Sensitivity 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 parameters are optimized.