An online method combining a thermal conversion elemental analyzer with isotope ratio mass spectrometry for the determination of hydrogen isotope composition and water concentration in geological samples.

An online continuous-flow method, combining a thermal conversion elemental analyzer (TC/EA) with isotope ratio mass spectrometry (MS), is evaluated for the determination of both the hydrogen isotope composition and the water concentration of hydrous and nominally anhydrous minerals. The technique involves reduction of hydrous minerals or nominally anhydrous minerals by reaction with glassy carbon at 1450 degrees C in a helium stream. The product gases, H2 and CO, are separated on a gas chromatographic column prior to analysis in the mass spectrometer. Calibration curves for the H concentration analysis were generated from a standard of benzoic acid (C7H6O2) that has an H concentration of 5.0 wt%; the analytical uncertainties were better than +/-0.05% in our runs. Two standards of material with given D values, polyethylene IAEA-CH-7 and biotite NBS-30, were tested for the purpose of calibrating a natural garnet 04BXL02 representing nominally anhydrous minerals. Preheating at 90 degrees C for 12 h was found to be suitable for removing adsorption water on the sample surface. This results in constant D values and total H2O contents for the garnet, with weighted means of -94 +/- 1 and 522 +/- 11 ppm (wt), respectively. The TC/EA-MS technique allows routine analysis of sample sizes as small as 0.01 microL H2O. For natural minerals, absolute reproducibilities for D values are +/-0.5 to +/-2 (1) and relative uncertainties for total H2O concentrations are at levels of +/-1% to +/-3% (1). Therefore, this online method can be used for the quantitative determination of H isotope composition and H2O concentration of either hydrous or anhydrous minerals.