Note: Reliable, robust measurement system for trace moisture in gas at parts-per-trillion levels using cavity ring-down spectroscopy.

ABSTRACT

We report a simple, robust cavity ring-down spectroscopy system to reliably measure trace moisture in gases at parts-per-trillion (ppt) levels. The performance of the system was evaluated on the basis of experiments performed in a manner traceable to the International System of Units. The obtained result was in good agreement with the primary trace-moisture standard at 12 nmol/mol (12 ppb) in N2 in amount-of-substance fraction. Measurement capability of residual moisture in high-purity dry N2 at ∼130 pmol/mol (130 ppt) was demonstrated, and background noise of 5.3 × 10(-12) cm(-1) was attained, corresponding to a minimum detectable H2O of 5 pmol/mol (5 ppt).