D and 18 O enrichment measurements in biological fluids in a continuous‐flow elemental analyser with an isotope‐ratio mass spectrometer using two configurations

In doubly labelled water studies, biological sample enrichments are mainly measured using off‐line techniques (equilibration followed by dual‐inlet introduction) or high‐temperature elemental analysis (HT‐EA), coupled with an isotope‐ratio mass spectrometer (IRMS). Here another continuous‐flow method, (CF‐EA/IRMS), initially dedicated to water, is tested for plasma and urine analyses. The elemental analyser configuration is adapted for each stable isotope: chromium tube for deuterium reduction and glassy carbon reactor for 18 O pyrolysis. Before on‐line conversion of water into gas, each matrix is submitted to a short and easy treatment, which is the same for the analysis of the two isotopes. Plasma is passed through centrifugal filters. Urine is cleaned with black carbon and filtered (0.45 µm diameter). Tested between 150 and 300 ppm in these fluids, the D/H ratio response is linear with good repeatability (SD < 0.2 ppm) and reproducibility (SD < 0.5 ppm). For 18 O/ 16 O ratios (from 2000 to 2200 ppm), the same repeatability is obtained with a between‐day precision lower than 1.4 ppm. The accuracy on biological samples is validated by comparison to classical dual‐inlet methods: 18 O analyses give more accurate results. The data show that enriched physiological fluids can be successfully analysed in CF‐EA/IRMS. Copyright © 2006 John Wiley & Sons, Ltd.