Modulation cancellation method for isotope 18O/16O ratio measurements in water.

The application of an innovative spectroscopic balancing technique to measure the isotope 18O/16O ratio in water vapor is reported. Quartz enhanced photoacoustic spectroscopy has been employed as the absorption sensing technique. Two isotope absorption lines with the same quantum numbers, with very close lower energy levels, have been selected to limit the sensitivity to temperature variations and guarantee identical broadening as well as relaxation properties. The sensitivity in measuring the deviation from a standard sample δ18O is 1.4‰, in 200 sec of integration time.