Measurement of δO, δO, and O‐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 lessabundant O isotope in addition to H and O. Off-axis integrated cavity output spectroscopy (OA-ICOS) is demonstrated for accurate and precise measurements δO, δO, and 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: δOVSMOW−SLAP = −24.74 ± 0.07‰ (1σ) and δOVSMOW−SLAP = −13.12 ± 0.05‰ (1σ). For comparison, the International Atomic Energy Agency (IAEA) value for δOVSMOW−SLAP is −24.76 ± 0.09‰ (1σ) and an average of previously reported values for δOVSMOW−SLAP is −13.12 ± 0.06‰ (1σ). Multiple (26) high-precision measurements of GISP provide a O-excessVSMOW−SLAP of 23 ± 10 per meg (1σ); an average of previously reported values for 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 δO, δO, and O-excess. The ability to measure accurately δO, δO, and O-excess in liquid water inexpensively and without sample conversion is expected to increase vastly the application of δO and O-excess measurements for scientific understanding of the water cycle, atmospheric convection, and climate modeling among others. S isotopic compositions of water, particularly δH and δO, have long been used to improve understanding of the hydrological cycle, catchment hydrology, and polar climate, e.g., ref 3. The combination of the measurements, d-excess = δH − 8δO, roughly describes the degree to which δH values vary from what would be expected at low and temperate latitudes if equilibrium processes were solely responsible for the relationship between δH and δO values. Kinetic fractionation processes, such as diffusive processes associated with evaporation, are partially responsible for the variation in d-excess, which varies as a function of both evaporative temperature and humidity. Given its sensitivity to environmental conditions of evaporation, d-excess has been used extensively to characterize climate and hydrological conditions today and in the past, e.g., refs 3−6. Recently, there has been increasing interest in measurement and use of the less-abundant O isotope in addition to H and O. Similarly to d-excess, the deviation from an expected relationship between O/O and O/O ratios has been defined by O-excess = ln(δO + 1) − 0.528 ln(δO + 1), and variation in O-excess in waters is dominated by kinetic fractionation effects. However, O-excess is relatively insensitive to temperature and thus can be used as a direct proxy for humidity at the region of evaporation. Combined measurements of d-excess and O-excess allow researchers to understand more fully the contributions of both temperature and relative humidity to water isotope fractionation, thus improving atmospheric models and understanding of climate processes, e.g., ref 8. Recent advances using fluorination coupled to isotope ratio mass spectrometry (IRMS) have made it possible to make precise measurements of O-excess in waters; these measurements have been applied to ice, meteoric waters, and African precipitation. The variation in O-excess in meteoric waters is generally a very small quantity (reported in per meg, parts per million), and meaningful measurements require extremely high precision. Received: July 29, 2013 Accepted: September 16, 2013 Article