NEW METHODS FOR FULLY AUTOMATED ISOTOPE RATIO DETERMINATION FROM HYDROGEN AT THE NATURAL-ABUNDANCE LEVEL

A variety of methods for measurement of H-2/H-1 from H-2 are evaluated for their ability to be fully automated and for applicability to auto mated isotopic analysis of water and organic compounds. Equilibration of water with H-2 gas with the aid of a platinum catalyst has been com mercialized into a fully automated sample preparation device. A second and newer technique, involving injecting water, methane, or other vol atile organic compounds onto hot chromium in a reactor attached to the dual inlet system of a gas isotope ratio mass spectrometer, can be in tegrated with a conventional GC-autosampler to allow automated analysi s of a variety of substrates. Both techniques result in precisions aro und 1 parts per thousand(delta notation) on the VSMOW scale, and are f ast and accurate, and with appropriate mass spectrometers require only negligible scaling for the SLAP/VSMOW difference. Several experimenta l methods which show considerable promise employ ''isotope ratio monit oring'' (irm) inlet systems, in which a carrier gas is used for transp ort of H-2 to the mass spectrometer. Any such method has to address th e problem of He ions corrupting the measurement of the H-2 ions. One s uch approach uses a heated palladium membrane for selective introducti on of H-2 into the mass spectrometer, and a second involves modificati ons to the ion optics to control the stray helium ions. Both approache s have significant limitations that must be overcome before irm techni ques can be used in routine applications, in particular for measuring hydrogen isotopes From GC effluents (irm-GCMS).