Accuracy of δ 18 O isotope ratio measurements on the same sample by continuous‐flow isotope‐ratio mass spectrometry

Rationale The doubly labeled water method is considered the reference method to measure energy expenditure. Conventional mass spectrometry requires a separate aliquot of the same sample to be prepared and analyzed separately. With continuous‐flow isotope‐ratio mass spectrometry, the same sample could be analyzed sequentially for both 2 H and 18 O content and thus minimize sample requirement, reduce analytical cost, and avoid memory effect. Methods The 2 H contents of 197 urine samples collected from 22 doubly labeled water studies were determined using a Thermo Delta V Advantage continuous‐flow isotope‐ratio mass spectrometer. The 18 O content of these samples was measured either using a separate aliquot of the same sample using a VG Isogas gas‐isotope‐ratio mass spectrometer or using the same sample following the 2 H measurements on a Thermo Delta V continuous‐flow isotope‐ratio instrument. Results The δ 18 O values using the same aliquot of samples were accurate to 0.18 ± 2.61 ‰ (mean difference ± standard deviation (SD); 95% CI, –0.18 to 0.55 ‰; P = 0.33) compared with the values based on the standard conventional method. Bland and Altman pair‐wise comparison also yielded a bias of 0.18 ‰ with a 95% limit of agreement between –4.94 and 5.30 ‰. Conclusions The study demonstrated that continuous‐flow isotope‐ratio mass spectrometry is capable of producing accurate 18 O measurements on the same sample after 2 H measurements. The method greatly reduces the analytical cost and sample size requirement and could easily be adopted by any laboratories equipped with a continuous‐flow isotope‐ratio mass spectrometer. Copyright © 2015 John Wiley & Sons, Ltd.