Dual-laser cavity ring-down spectroscopy for real-time, long-term measurement of trace moisture in gas

Cavity ring-down spectroscopy (CRDS) is a highly sensitive technique that enables the measurement of trace species in gases at or lower than the nmol mol−1 (ppb) level in amount-of-substance fraction (mole fraction). In particular, CRDS is a very powerful tool for trace-moisture measurement. In real-time, long-term measurement of trace moisture using CRDS, the accuracy is limited by undesired changes in ring-down time over a period of time (background drift). In this study, we introduced a dual-diode laser system in CRDS for trace-moisture measurements to compensate for the background drift. The frequency of the first laser was tuned to the on-peak position of a water absorption line to measure trace moisture, while that of the second laser was tuned to an off-peak position to track the background drift. The two laser diodes were alternately used to measure the ring-down times at on- and off-peak positions by rapidly switching the two laser beams. The effectiveness of dual-laser CRDS for real-time, long-term measurements of trace moisture was demonstrated by experiments based on a primary trace-moisture standard in the range of 8 nmol mol−1–630 nmol mol−1. The use of the primary trace-moisture standard in the experiments guaranteed the metrological traceability of the measurement results to the international system of units (SI), making the measurement results reported in this study highly reliable. The expanded uncertainty obtained using dual-laser CRDS at 8 nmol mol−1 was more than four times better than that of the primary trace-moisture standard, implying the potential of a trace-moisture standard based on dual-laser CRDS whose uncertainty in a low range is better than that of other generator-based trace-moisture standards.