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

The amount of trace moisture is precisely determined in N-2 and HCl gases b y means of near-infrared diode-laser absorption spectroscopy. As an optical source, we use a near-infrared InGaAsP distributed-feedback diode laser wh ich operates at room temperature. A multi-pass sample cell designed for cor rosive gases is employed to enhance molecular absorption. For the purpose o f reducing the etalon noise that is usually encountered in multi-pass absor ption spectroscopy, we adjust the base path length in accordance with the b andwidth of a molecular spectral line so that the etalon frequency does not critically overlap with that of the molecular spectrum.. The remaining noi se is further reduced by applying multi-stage smoothing. Trace moisture con tent as small as 2.3 ppb is detected with a 20-m absorption length.