Concentrations of H<sub>2</sub>CO in the part-per-billion by volume (ppbv) range are determined in gaseous mixtures containing large excesses of NO<sub>2</sub>, CH<sub>3</sub>CHO, and N<sub>2</sub> by the use of photoacoustic spectroscopy (PAS). Mixtures containing H<sub>2</sub>CO as a minority component are prepared by gas permeation methods and examined by pulsed laser PAS in the ultraviolet and visible spectral regions. The relative concentrations of the major components of each mixture are an order of magnitude or more larger than the H<sub>2</sub>CO component under study. PA spectra with 0.5-cm<sup>−1</sup> resolution in the 302.5-303.8 nm region are presented in Fig. 3 and are used to identify spectral signatures needed for selective detection. The dependence of PA signals on concentration is determined separately for each of the three polyatomic components in samples with N<sub>2</sub>. An experimental protocol for quantitatively measuring H<sub>2</sub>CO concentrations near 50 ppbv in NO<sub>2</sub>, CH<sub>3</sub>CHO, and N<sub>2</sub> mixtures is described.

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