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., delta H-2 less than or eq ual to 15000 parts per thousand, delta O-18 less than or equal to 1200 part s per thousand), 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 muL liquid water) samples show a clearly better accuracy for the H-2/H-1 ratio. In the case of O-18/O -16, the same level of accuracy is obtained as with conventional isotope ra tio mass spectrometer (IRMS) analysis. With LS, the precision is better for both O-18/O-16 and H-2/H-1. New is the ability to measure O-17/O-16 with t he same accuracy as O-18/O-16. A major advantage of the present technique i s the absence of chemical sample preparation. The method is proven to be re liable and accurate. and is ready to be used in many biomedical application s.