For molecules possessing a permanent electric dipole moment the Stark effect may be used to perturb the molecular energy levels and to "tune" some absorption lines into coincidence with fixed frequency lasers. The same perturbation may also be used to modulate the absorption and allow the use of sensitive ac detection techniques. In this paper we report quantitative results for four organic gases of industrial importance: vinyl chloride, vinylidene chloride, acrylonitrile, and methanol. All have overlapping absorption bands in the CO<sub>2</sub> laser region. Because of the modulation techniques used, the observed signal as a function of electric field is the first derivative of the absorption line profile, and the peak-to-peak intensity of the derivative is the measured analytical signal. The analytical curve is a straight line over several orders of magnitude in concentration. In all cases, parts per million or sub-parts per million concentrations of the gases in air and/or nitrogen have been measured using a CO<sub>2</sub> laser. The paper will include a discussion of interferences and selectivity of the technique.
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