Detection of nitrogen dioxide by the laserexcited fluorometry

Abstract

A very sensitive apparatus was constructed for measurements of trace concentrations of NO₂ in the air. The apparatus consisted of a nitrogen-laserexcited dye laser, a low-background sample cell, a double monochromator, and a gated photon counter. Two monitoring techniques have been investigated: a direct method and a trap method. The direct method is to measure the fluorescence signal of the flowing sample directly. The minimum detectable concentration was 0.6 ppm. The advantage of this method is its reliability which comes from the calibration of the absolute value of NO₂ concentrations through the simultaneous determination of the intensity of Raman lines of N₂ and O₂. In the trap method, the gas sample was flowed through a liquid nitrogen trap. After several minutes of trapping, the sample was degassed and then expanded into the fluorescence cell. Not only the increase of concentration but also the increase of fluorescence quantum efficiency of NO₂ upon the reduction of pressure allowed the detection of lower concentration; the detection limit was 0.02ppm. The present method is more reliable than the reported methods, because the monochromator is useful in removing any interferences of the fluorescent contamination and in reliable calibration of concentration by comparing the intensity with the Raman line.