High-precision D/H measurement from organic mixtures by gas chromatography continuous-flow isotope ratio mass...

Continuous-flow high-precision determination of D/H ratios from an organic mixture is described using gas chromatography coupled to a Pd filter system as an interface for isotope ratio mass spectrometry. A gas chromatograph and combustion and reduction furnaces are connected to a Pd filter via a postcolumn head pressure makeup gas to increase chromatographic sensitivity. This interface is evaluated using benzene as an internal standard in a mixture of ethylbenzene and cyclohexanone in hexane with analyte quantities of < 3 ng (< 300 pg of H). A calibration curve is constructed using four benzene samples over a range of -48 to 372/1000 (delta DSMOW), resulting in an average benzene D/H precision of SD < 5/1000 (delta DSMOW) and deviations of < 4/1000 from the calibration curve. Ethylbenzene and cyclohexanone of a single D enrichment are analyzed as unknowns in three sample mixtures with varying D-enriched benzene and result in precisions of SD < 5/1000. No apparent memory is observed between peaks of differently enriched analytes within the same chromatogram. All results are corrected for ion source nonlinearities characteristic of hydrogen analysis, using the internal peakwise correction algorithm, described previously. A small dependence of isotope ratio on palladium membrane temperature is demonstrated over a range of 4 degrees C; therefore, with tighter control of palladium temperature, precision can probably be improved. The data indicate that this system is useful for rapid continuous-flow IRMS analysis of D/H ratios from organics in complex mixtures characteristic of geological and biological samples.