Measurement of δ18O, δ17O, and 17O-excess in water by off-axis integrated cavity output spectroscopy and isotope ratio mass spectrometry.

Stable isotopes of water have long been used to improve understanding of the hydrological cycle, catchment hydrology, and polar climate. Recently, there has been increasing interest in measurement and use of the less-abundant (17)O isotope in addition to (2)H and (18)O. Off-axis integrated cavity output spectroscopy (OA-ICOS) is demonstrated for accurate and precise measurements δ(18)O, δ(17)O, and (17)O-excess in liquid water. OA-ICOS involves no sample conversion and has a small footprint, allowing measurements to be made by researchers collecting the samples. Repeated (514) high-throughput measurements of the international isotopic reference water standard Greenland Ice Sheet Precipitation (GISP) demonstrate the precision and accuracy of OA-ICOS: δ(18)OVSMOW-SLAP = -24.74 ± 0.07‰ (1σ) and δ(17)OVSMOW-SLAP = -13.12 ± 0.05‰ (1σ). For comparison, the International Atomic Energy Agency (IAEA) value for δ(18)OVSMOW-SLAP is -24.76 ± 0.09‰ (1σ) and an average of previously reported values for δ(17)OVSMOW-SLAP is -13.12 ± 0.06‰ (1σ). Multiple (26) high-precision measurements of GISP provide a (17)O-excessVSMOW-SLAP of 23 ± 10 per meg (1σ); an average of previously reported values for (17)O-excessVSMOW-SLAP is 22 ± 11 per meg (1σ). For all these OA-ICOS measurements, precision can be further enhanced by additional averaging. OA-ICOS measurements were compared with two independent isotope ratio mass spectrometry (IRMS) laboratories and shown to have comparable accuracy and precision as the current fluorination-IRMS techniques in δ(18)O, δ(17)O, and (17)O-excess. The ability to measure accurately δ(18)O, δ(17)O, and (17)O-excess in liquid water inexpensively and without sample conversion is expected to increase vastly the application of δ(17)O and (17)O-excess measurements for scientific understanding of the water cycle, atmospheric convection, and climate modeling among others.