APPLICATION OF TUNABLE DIODE-LASER ABSORPTION-SPECTROSCOPY TO TRACE MOISTURE MEASUREMENTS IN GASES

We have applied tunable diode laser absorption spectroscopy (TDLAS) to trace moisture determination in hydrogen chloride gas and in nitrogen. To the best of our knowledge, these are the first TDLAS measurements to combine operation in a reactive gas matrix with low ppb moisture measurements. Gas handling methods, which were previously applied to trace moisture measurement using other equipment, are combined with a TDLAS system that uses second derivative detection and signal averaging techniques. The system is calibrated by direct introduction of a moisture standard into the absorption cell, and its response time has been evaluated using a gas handling system capable of rapid switching between humidified and dry gas streams. The reproducibility (2 X standard deviation of the signal) of the system is currently 0.4 ppb moisture for analyses in nitrogen, when 10 runs of 5 s each are averaged. The sensitivity (slope of the calibration curve) is considerably reduced in HCl, and the reproducibility in HCl is 4 ppb. For the absorption line used, the pressure broadening coefficient of moisture in HCl has been shown to be approximately three times its value in nitrogen, which partially accounts for the reduction in sensitivity in HCl, The current limitation on measurable moisture levels is the background of the system. The lowest background levels measured to date are 10 ppb in nitrogen and 63 ppb in HCl.