Comb-assisted cavity ring-down spectroscopy for ultra-sensitive traceable measurements of water vapour in ultra-high purity gases

Abstract We report on the development of a comb-assisted cavity ring-down spectrometer for trace water mole fraction determinations in high purity gases. By tuning laser light coincidence with H 2 O absorption line at 1.3946 µ m, we were able to determine sub-ppm relative concentrations vapour N sub-percent statistical uncertainty. The sensitivity was carefully assessed, yielding detection limit about nmol/mol.