Stable isotope ratio measurements on highly enriched water samples by means of laser spectrometry.

We demonstrate the feasibility of using laser spectrometry (LS) to analyze isotopically highly enriched water samples (i.e., delta2H < or = 15,000/1,000, 18O < or = 1,200/1,000), as often used in the biomedical doubly labeled water (DLW) method to quantify energy metabolism. The method is an important extension of a recently developed infrared laser direct absorption spectrometer. The measurements on highly enriched, small-size (10 microL liquid water) samples show a clearly better accuracy for the 2H/1H ratio. In the case of 18O/16O, the same level of accuracy is obtained as with conventional isotope ratio mass spectrometer (IRMS) analysis. With LS, the precision is better for both 18O/ 16O and 2H/1H. New is the ability to measure 17O/16O with the same accuracy as 18O/16O. A major advantage of the present technique is the absence of chemical sample preparation. The method is proven to be reliable and accurate and is ready to be used in many biomedical applications.