QUANTITATIVE-ANALYSIS OF TRACE MOISTURE IN N-2 AND NH3 GASES WITH DUAL-CELL NEAR-INFRARED DIODE-LASER ABSORPTION-SPECTROSCOPY

This paper demonstrates our optical measurement system based on near-i nfrared tunable diode laser absorption spectrometry and reports the re sults of trace moisture determination in nitrogen and ammonia gases. A near-infrared InGaAsP distributed feedback diode laser operating at r oom temperature was employed as the optical source. We used a dual-cel l 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 NH3. The detection limit of H2O absorption of 4 ppb in nitrogen and 12 ppb in ammonia was obtained using a single-pass absorption cell of on ly 92 cm in length and the average results of 10 scan measurements. Th is system has characteristics of both the high sensitivity and capabil ity of in situ and real-time measurement.