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 H-2 and O-18 content and thus minimize sample requirement, reduce analytical cost, and avoid memory effect. METHODS: The H-2 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 O-18 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 H-2 measurements on a Thermo Delta V continuous-flow isotope-ratio instrument. RESULTS: The O-18 values using the same aliquot of samples were accurate to 0.18 2.61 parts per thousand (mean difference +/- standard deviation (SD); 95% CI, -0.18 to 0.55 parts per thousand; 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 parts per thousand with a 95% limit of agreement between -4.94 and 5.30 parts per thousand. CONCLUSIONS: The study demonstrated that continuous-flow isotope-ratio mass spectrometry is capable of producing accurate O-18 measurements on the same sample after H-2 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 (c) 2015 John Wiley & Sons, Ltd.