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
