Determination of trace moisture in gases by diode-laser multi-pass absorption spectroscopy

Abstract

The amount of trace moisture is precisely determined in N2 and HC1 gases by means of nearinfrared diode-laser absorption spectroscopy. As an optical source, we use a near-infrared InGaAsP distributed-feedback diode laser which operates at room temperature, A multi-pass sample cell designed for corrosive gases is employed to enhance molecular absorption. For the purpose of reducing the etalon noise that is usually encountered in multi-pass absorption spectroscopy, we adjust the base path length in accordance with the bandwidth of a molecular spectral line so that the etalon frequency does not critically overlap with that of the molecular spectrum. The remaining noise is further reduced by applying multi-stage smoothing. Trace moisture content as small as 2.3 ppb is detected with a 20-m absorption length.