Analytical methods for the measurement of hydrogen isotope composition and water content in clay minerals by TC/EA

Abstract

Measurements of water content and hydrogen isotopic composition in water-bearing silicate minerals are often made using a TC/EA coupled to a gas chromatographic column and a mass spectrometer. Alternative vacuum-based analyses via a reaction with Zn reagent require larger sample amounts and longer durations of analysis. For clay minerals the TC/EA method can also be applied, but the presence of adsorbed surface or interlayer water, if not removed successfully, may typically result in lower measured δD values. Even if these water phases are removed prior to analysis, certain clays re-adsorb atmospheric moisture within a few minutes. This effect is especially pronounced in the smectite group or other highly hygroscopic materials and needs to be corrected. A method adapted for the TC/EA, with evacuation of the sample prior to analysis on a vacuum line is described, which allows the accurate analysis of the structurally bound water in hygroscopic minerals. The results from comparative measurements using TC/EA and vacuum methods of water extraction indicate that evacuation at a temperature of 250°C for 3h leads to a relatively fast and effective removal of adsorbed water. Subsequent storage in vacuum and quick transfer to the autosampler or inlet of the TC/EA prevents rehydration by water vapor in air.