Isotope ratio monitoring gas chromatography mass spectrometry of D H by high temperature conversion isotope ratio mass spectrometry

Of all the elements, hydrogen has the largest naturally occurring variation s in the ratio of its stable isotopes (D/H). It is for this reason that the re has been a strong desire to add hydrogen to the list of elements amenabl e to isotope ratio monitoring gas chromatography/mass spectrometry (irm-GC/ MS), In irm-GC/MS the sample is entrained in helium as the carrier gas, whi ch is also ionized and separated in the isotope ratio mass spectrometer (IR MS), Because of the low abundance of deuterium in nature, precise and accur ate on-line monitoring of D/H ratios with an IRMS requires that low energy helium ions be kept out of the m/z 3 collector, which requires the use of a n energy filter. A clean mass 3 (HD+.) signal which is independent of a lar ge helium load in the electron impact ion source is essential in order to r each the sensitivity required for D/H analysis of capillary GC peaks. A new IRMS system, the DELTA(plus)XL(TM), has been designed for high precision, high accuracy measurements of transient signals of hydrogen gas. It incorpo rates a retardation lens integrated into the m/z 3 Faraday cup collector. F ollowing GC separation, the hydrogen bound in organic compounds must be qua ntitatively converted into H-2 gas prior to analysis in the IRMS, Quantitat ive conversion is achieved by high temperature conversion (TC) at temperatu res >1400 degrees C, Measurements of D/H ratios of individual organic compo unds in complicated natural mixtures can now be made to a precision of 2 pa rts per thousand (delta notation) or, better, with typical sample amounts o f similar to 200 ng per compound. Initial applications have focused on comp ounds of interest to petroleum research (biomarkers and natural gas compone nts), food and flavor control (vanillin and ethanol), and metabolic studies (fatty acids and steroids). Copyright (C) 1999 John Whey & Sons, Ltd.