High-Precision D/H Measurement from Hydrogen Gas and Water by Continuous-Flow Isotope Ratio Mass Spectrometry

Abstract

Two instrumental approaches are described for continuous-flow high-precision determinations of D/H ratios from hydrogen gas or via on-line reduction of water. In the first system, Ar is used as a carrier gas, with a Ni reduction furnace and a water trap to remove minor levels of unreduced water that are a potential source of memory effects. Precisions of SD < 10/1000 (delta DSMOW) over a 600/1000 range from -55 to +532/1000 are obtained for liquid water (0.4 microL). Linearity is excellent over 4 orders of magnitude of D concentration in tap water (r2 > 0.9999), although precision degrades at enrichments delta DSMOW > 5000/1000. In the second system, a heated Pd metal foil functions as a filter to admit purified hydrogen into the mass spectrometer. Hydrogen gas injections are made into flowing Ar and are directed to the Pd filter (approximately 330 degrees C) which passes hydrogen isotopes only while diverting the carrier flow to waste. Precisions of these measurements are SD < 6/1000 over the D enrichment range -213 to 340/1000, with excellent linearity (r2 > 0.9999) and accuracy (< 2/1000). Similar precision is obtained using the on-line reduction apparatus and a water trap prior to the Pd filter with injections of 0.4 microL of liquid water, with acceptable linearity (r2 > 0.999) over 3 orders of magnitude of D concentration. Neither system shows any sign of memory effects when water is analyzed. The data indicate that either one of these systems is a useful means for continuous-flow IRMS of D/H isotope ratio determinations.