Development of primary standard gas mixtures containing 10 µmol/mol oxygen in nitrogen with verification by a two-oven GC-TCD system for effective separation of oxygen from argon in presence of plenty of nitrogen

Abstract

Primary standard gas mixtures (PSMs) containing 10 µmol/mol oxygen in nitrogen were prepared in aluminum alloy cylinders from pure nitrogen and oxygen gases according to ISO 6142-1. We developed a system consisting of a gas chromatograph with a thermal conductivity detector (GC-TCD) and two ovens to verify precisely the amount-of-substance fraction of oxygen in the PSMs at a level of 10 µmol/mol. As pure nitrogen gas usually contains argon as an impurity, it is necessary to separate the oxygen peak from the argon peak in order to verify the amount-of-substance fraction of oxygen in the PSMs. Although the use of packed columns is essential in order to achieve high precision using a GC-TCD system, it is notoriously difficult to separate oxygen and argon by this method. To overcome this problem, we used two gas chromatograph ovens: The first oven separates oxygen and argon from the nitrogen, whereas the second oven—held at −15 °C or below—separates oxygen and argon. The two-oven GC-TCD system was able to achieve good resolution between the oxygen and argon peaks. The relative uncertainty in the determination of the oxygen fraction obtained using this system was 0.35 %. In order to avoid adsorption of oxygen onto the inner surface of the aluminum cylinder, we pretreated each cylinder by filling it with a standard gas mixture of oxygen in nitrogen at an oxygen partial pressure similar to that of the PSMs. The cylinders were then evacuated and used for the preparation of the target PSMs. From long-term monitoring using the GC-TCD system, we found the PSMs containing 10 µmol/mol oxygen in nitrogen to be stable for at least 30 months. The expanded combined uncertainties for the PSMs (k = 2) were found to be 0.026 µmol/mol to 0.036 µmol/mol, according to ISO 6142-1.