ISOTOPE RATIO MEASUREMENTS USING GLOW-DISCHARGE MASS-SPECTROMETRY

Glow discharge mass spectrometry (GDMS) has been evaluated for its ability to measure isotope ratios in solids. Isotope ratios were measured for B, Cu, Sr, Ag, Sb, Re and Pb in concentrations ranging from 15 ppm to pure metal. External precision of better than 0.03% has been achieved for isotope ratios measured using pure solid elemental samples; typical precision is better than 0.1% for elements present in concentrations greater than 0.5 wt.%. For elements present in concentrations of 10-20 ppm, precision was about 1%. Isotopic bias was < 1%, and day-to-day variations in isotopic bias for a single element are as large as variations in isotopic bias between different elements, suggesting that most isotopic bias is dominated by instrumental parameters. Values of isotope ratios show a dependence on the pressure of the Ar discharge gas, suggesting that improvements in sample geometry and discharge stability have the potential to allow GDMS to achieve even better precision in the measurement of isotope ratios.