New Methods for Fully Automated Isotope Ratio Determination from Hydrogen at the Natural Abundance Level

Abstract

A variety of methods for measurement of ²H/¹H from H₂ are evaluated for their ability to be fully automated and for applicability to automated isotopic analysis of water and organic compounds. Equilibration of water with H₂ gas with the aid of a platinum catalyst has been commercialized into a fully automated sample preparation device. A second and newer technique, involving injecting water, methane, or other volatile organic compounds onto hot chromium in a reactor attached to the dual inlet system of a gas isotope ratio mass spectrometer, can be integrated with a conventional GC-autosampler to allow automated analysis of a variety of substrates. Both techniques result in precisions around 1‰ (δ notation) on the VSMOW scale, and are fast and accurate, and with appropriate mass spectrometers require only negligible scaling for the SLAP/VSMOW difference. Several experimental methods which show considerable promise employ “isotope ratio monitoring” (irm) inlet systems, in which a carrier gas is used for transport of H₂ to the mass spectrometer. Any such method has to address the problem of He ions corrupting the measurement of the H₂ ions. One such approach uses a heated palladium membrane for selective introduction of H₂ into the mass spectrometer, and a second involves modifications to the ion optics to control the stray helium ions. Both approaches have significant limitations that must be overcome before irm techniques can be used in routine applications, in particular for measuring hydrogen isotopes from GC effluents (irm-GCMS).