Measurement of δ18O, δ17O, and 17O-excess by Off-Axis Integrated Cavity Output Spectroscopy and Isotope Ratio Mass Spectrometry

Water stable isotopes have for many years been used to study the hydrological cycle, catchment hydrology, and polar climate. Recently, there has been mounting interest in measurement and use of the less-abundant 17O isotope in addition to 2H and 18O. Off-axis integrated cavity output spectroscopy (OA-ICOS) measures δ18O, δ17O, and 17O-excess in liquid water without sample preparation or highly-trained operators. OA-ICOS allows measurements to be made on a relatively compact and affordable instrument by researchers collecting the samples. Numerous (514) high-throughput measurements of the international water standard GISP are used to demonstrate the precision and accuracy of OA-ICOS: δ18O =-24.74 × 0.06 ‰ (HWHM) and δ17O = -13.12 × 0.04 ‰ (HWHM). For comparison, the IAEA value for δ18O of GISP is 24.76 × 0.09 ‰ (1σ) and an average of previously reported values for δ17O of GISP is -13.12 × 0.06 ‰ (1σ). Repeated (26) high-precision measurements of GISP provide a 17O-excess of 23 × 9 per meg (HWHM); an average of previously reported values for 17O-excess is 22 × 11 per meg (1σ). The precision of OA-ICOS measurements of δ18O, δ17O, and 17O-excess can be further enhanced by additional averaging. Comparison with two independent isotope ratio mass spectrometry (IRMS) laboratories shows that OA-ICOS has equivalent accuracy and precision as the current fluorination-IRMS techniques in δ18O, δ17O, and 17O-excess. The capability to accurately measure δ18O, δ17O, and 17O-excess in liquid water inexpensively and without sample preparation is expected to enhance both the number and breadth of applications of δ17O and 17O-excess for understanding of the water cycle, atmospheric convection, and climate modeling among others. 17O-excess measurement accuracy demonstrated by measurements of GISP and four commercially-available USGS standards by OA-ICOS and two independent IRMS labs. Error bars represent one standard error of the mean. The line behind the GISP columns shows the collected average of 3 previously reported IRMS measurements.