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 the laser light in coincidence with a H2O absorption line at 1.3946 µm, we were able to determine sub-ppm relative concentrations of water vapour in N2 with a sub-percent statistical uncertainty. The sensitivity of the spectrometer was carefully assessed, yielding a detection limit of about 2 nmol/mol.

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