On-line quantitative determination of2H/1H isotope ratios in organic and water samples using an elemental analyser coupled to an isotope ratio mass spectrometer

Abstract

A rapid continuous-flow technique for the quantitative determination of hydrogen isotope ratios in organic materials and water, at natural abundance levels, is described. Samples are pyrolysed in a helium stream at 1080°C over an inert form of carbon. Hydrogen is separated from the other pyrolysis gases by gas chromatography and enters the ion source of the isotope ratio mass spectrometer (IRMS) via a crimp. The 2H/1H ratio of the gas was determined by the simultaneous integration of the m/z 2 (1H1H) and m/z 3 (2H‰1H) ion beams over time. δ2H‰ values of organic and water samples were determined by comparison with organic and water standards, respectively. The precision of the on-line δ2H‰ measurement is <3.3‰. The accuracy of the method was demonstrated by the analysis of a laboratory standard water (NTW), a European Community Bureau Reference material, N,N-tetramethylurea (TMU) and an International Atomic Energy Association polyethylene reference material (IAEA-CH-7). The measured values were NTW -39.4‰, TMU -144.1‰ and IAEA-CH-7 -100.7‰ (reference values are NTW -38.5‰, TMU -147.4‰ and IAEA-CH-7 -100.3‰). No apparent memory effect was observed when measuring samples at the natural abundance level. Calibration of organic measurements with water reference materials did not give satisfactory results, probably owing to differences in their respective pyrolysis pathways. The potential of the technique for food analysis was demonstrated by measuring the δ2H‰ values of vanillin samples derived from natural and synthetic sources and by the analyses of hexamethylenetetramine synthesized from beet, cane and apple juice sugars. © Crown copyright 1998. Reproduced with the permission of the Controller of Her Majesty’s Stationery Office.