This paper demonstrates our optical measurement system based on near-infrared tunable diode laser absorption spectrometry and reports the results of trace moisture determination in nitrogen and ammonia gases. A near-infrared InGaAsP distributed feedback diode laser operating at room temperature was employed as the optical source. We used a dual-cell detection strategy to cancel common mode noise from the diode laser and remove the effect of the residual moisture absorption in the beam path outside the sample cell. We also used this method to successfully eliminate the interfering absorption of matrix gas molecules such as NH(3). The detection limit of H(2)O absorption of 4 ppb in nitrogen and 12 ppb in ammonia was obtained using a single-pass absorption cell of only 92 cm in length and the average results of 10 scan measurements. This system has characteristics of both the high sensitivity and capability of in situ and real-time measurement.

The performance of a purifier in reducing moisture in hydrogen chloride (HCl) was evaluated using near-infrared laser absorption spectrometry. The moisture concentration in HCl passed through the purifier was lower than the detection limit (3σ) which was estimated to be 95 ppb. Despite the fact that the moisture concentration in the inlet stream increased from 1 ppm to 11 ppm, the moisture concentration in the outlet stream was maintained below the detection limit. Therefore, it is quite important to use the purifier in order to obtain dry HCl in a stable manner.