US Patent 9110008 Method for isotopic analysis of water in bodily fluids

Biological fluid samples containing proteins (e.g., blood plasma or saliva) are prepared for isotopic analysis by precipitating the proteins while leaving the isotopic ratio unaffected. This precipitation can involve adding metal ions, salts, organic solvents, or organic polymers. The sample is then centrifuged to allow transfer of the supernatant for isotopic analysis, e.g. by tunable diode laser absorption spectrometry to obtain a quantitative measure of the 2H and 18O isotope levels in the water relative to reference standards.