Measuring the relative concentration of H2 and D2 in HD gas with gas chromatography

Frozen-spin polarized hydrogen deuteride (HD) targets are being prepared for photonuclear experiments at Jefferson Lab. The targets are frozen HD gas. To polarize and achieve the required spin-lattice relaxation times, the targets are made from high purity HD gas in which the residual H(2) and D(2) concentrations are known. To determine these residual concentrations, a gas chromatograph is used. The separation of the gas components is done in a column cooled to ≈120 K by immersing it in a mixture of liquid nitrogen and isopentane and using neon as the carrier gas. The different hydrogen isotopes have different transit times through the column and their arrival is registered by a thermal conductivity detector. The peaks in the chromatograms are fit using an exponentially modified Gaussian line shape to extract the areas of these peaks. The ratios of areas are corrected for differences in thermal conductivity to give the relative concentrations. Here, H(2) and D(2) concentrations on the order of a few percent have been measured with uncertainties of less than ± 5%. The linear response of the system is confirmed to within about ± 6.3%.