Deuterium and oxygen-18 determination of microliter quantities of a water sample using an automated equilibrator

We describe a modified version of the equilibration method and a correction algorithm for isotope ratio measurements of small quantities of water samples. The delta D and the delta O-18 of the same water sample can both be analyzed using an automated equilibrator with sample sizes as small as 50 FLL. Conventional equilibration techniques generally require water samples of several mu L. That limitation is attributable mainly to changes in the isotope ratio (O-18/O-16 or D/H) of water samples during isotopic exchange between the equilibration gas (CO2 or H-2) and water, and therefore the technique for mu L quantities of water requires mass-balance correction using the water/gas (CO2 or H-2) mole ratio to correct this isotopic effect. We quantitatively evaluate factors controlling the variability of the isotopic effect due to sample size. Theoretical consideration shows that a simple linear equation corrects for the effects without determining parameters such as isotope fractionation factors and water/gas mole ratios. Precisions (1-sigma) of 50-mu L meteoric water samples whose isotopic compositions of -1.4 to -396.2%o for delta D are +/- 0.5 to +/- 0.6%o, and of -0.37 to -51.37%o for delta O-18 are +/- 0.01 to +/- 0.11 parts per thousand. Copyright (C) 2007 John Wiley & Sons, Ltd.