Hydrogen-isotope analysis of nanomole (picoliter) quantities of H_2O

Abstract

We describe a technique for on-line reduction of water vapor carried in a He stream followed by continuous-flow mass spectrometry of evolved H_2 gas. This technique is appropriate for analysis of D/H ratios of ∼10^(−9) to ∼5 × 10^(−8) moles of water and has an external precision (1σ) typically varying between ±1.0 and 2.0‰. For specific ranges in sample size, temperature of the reduction reaction, and method of introducing evolved vapor into the mass spectrometer, this technique involves negligible analytical corrections (∼0 to 3‰) beyond those inherent to gas-source mass spectrometry of molecular hydrogen. We present an analysis of the D/H ratio of water released by stepped heating of a 10-mg fragment of the Shergottite-Nahklite-Chassignite meteorite ALH84001 and ∼1 to 2 μg of a terrestrial hydrous mineral (thermonatrite) as examples of possible applications making use of this technique.

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