On-line Reduction of H2O for δ2H and δ18O Measurement by Continuous-flow Isotope Ratio Mass Spectrometry

We have evaluated the use of nickel wool, nickelized carbon and a combination of both as catalysts in the on-line reduction of H2O, in a tubular micro-furnace, for δ2H and δ18O measurement by continuous-flow isotope ratio mass spectrometry (CF-IRMS). The IRMS used was of a novel design which gives adequate resolution to overcome interference from 4He+ on 2H1H+, allowing reduction to be performed in the presence of a He flow. No measurable memory effects were observed for δ2H using a nickel catalyst. A small, but significant degree of memory was observed for both δ2H and δ18O upon combining nickelized carbon and nickel, that was removed by two washes between analyses. On-line reduction is an accurate method of analysis of nL volumes of water, with a precision of ±4‰ for δ2H and ±0.4‰ for δ18O. The technique is fully adequate for determination of total energy expenditure by the doubly-labelled water method, being accurate over a wide range of enrichment.