We have evaluated the use of nickel wool, nickelized carbon and a combination of both as catalysts in the on-line reduction of H,O, in a tubular micro-furnace, for 8% and 8"O measurement by continuous-fiow k~tope ratio mas spectrometry (CF-IRMS). The used was of a novel design whicb gives adequate resolution to overcome interference from 'He' on ' H ' H ' , allowing reduction to be performed in the presence of a He now. No measurable memory ef€ects were observed for 8% Using a nickel catalyst. A small, but significant degree of memory was observed for both 8'H and 6"O 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%0 for 6% and *0.4%0 for 8%. 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.
Continuous-flow isotope ratio mass spectrometry (CF-IRMS) allows more rapid analysis and reduces the need for pre-treatment of samples when compared with dual-inlet systems. When the sample conversion steps are included, there is little overall loss of precision when CF-IRMS is used. Continuous-flow systems require an inert carrier gas to transport bands of analyte gas through one or more online sample preparation stages and into the IRMS. Helium is the carrier of choice as a consequence of its inert nature, high ionization energy and low density, and is used for the analysis of I3C, "N, "0 and "S, following conversion to N,, CO, or SO, as appropriate.' Conventional designs of IRMS provide insufficient dispersion to satisfactorily resolve the ZH' H' ion beam from the tail of the much more intense 4He' ion beam, hence restricting the use of CF-IRMS in the determination of S' H. Argon may be used as the carrier gas to circumvent the lack of dispersion? but its high density, high mass and low ionization energy, relative to He, are detrimental to ion production. Methods for the separation of H, from He, such as a palladium filter, are a better solution, but preclude the analysis of permanent gases other than Until recently there was no suitably reliable and rapid method available for on-line preparation of H, from H20 that was appropriate for use in CF-IRMS. SZH in water samples is normally measured by dual-inlet IRMS following reduction of H,O to H, over a heated metal, either on-line with uranium3 or using zinc in a batch process!vs Problems inherent in the reduction of H20 with zinc include surface inhomogeneity and exchange of H, with the zinc, although much progress has been made towards their eradication.6.' Alternatively, equilibration of H20 with H2 may be undertaken, in the presence of a catalyst, and the resultant ' H : 'H ratio measured. Hokko beads, which consist of platinum supported on a porous hydrophilic polymer, operate at the vapour-gas interface to provide equilibrium of H,O with H2 in under 1 h.8*9 This rapid equilibration is highly temperature dependent, requiring temperature control to within kO.01 "C. Platinum-on-alumina physically isolated from the water, is an inexpensive alternative. Using platinum-on-alumina, equilibration is slow, taking 3 days to H22.