While conducting composition measurements on helium-hydrogen mixtures using a Varian gas chromatograph at WSU’s Analytical Chemistry Service Center, I discovered that the ratio of orthohydrogen–parahydrogen has a significant effect on the measurements. An in depth discussion of the allotropic forms of hydrogen can be found in the previous post “Why equilibrium hydrogen doesn’t exist”. In my system, gaseous hydrogen is condensed in a copper test cell at 20 K. An ortho-para catalyst is placed in the bottom of the test cell to ensure all of the hydrogen is converted to parahydrogen. Helium gas is then introduced into the test cell to the desired pressure. The amount of helium that dissolves into the liquid is measured by extracting a liquid sample through a tube at the bottom of the test cell where it is vaporized and collected in a gas sampling bag. The composition of the sample is then analyzed using gas chromatography. The total composition of the first helium-hydrogen measurements were only totaling between 80% – 90% instead of the expected 100%. The GC column was packed with a hydrogen compatible material so it was unlikely the equipment was causing the discrepancy. We double checked that the primary standard gas mixtures were still obtaining correct measurements, they were. The only difference was that the gas standards contained normal hydrogen (since they were maintained at room temperature) and the samples being collected were parahydrogen. By adding an ortho-para catalyst just before the mixture was collected in the sampling bag, I was able to convert the hydrogen back to the normal composition. After this was implemented, every gas sample measurement was within the uncertainty of the equipment. Once again, the subtle differences between orthohydrogen and parahydrogen cannot be overlooked even in a process as standardized as gas chromatography.