Metrological traceability in gas analysis at I.N.Ri.M.: gravimetric primary gas mixtures

The role played by chemical measurements is fundamental in all scientific fields from environment to food, to pharmaceutical, and indeed important decisions and the orientation of future strategies depend on traceable measurement results. Coherent sets of gas mixtures, with known concentration and measurement uncertainty, are used as reference standards in important applications as the calibration of analytical instrumentation for atmospheric pollutants monitoring and the check of purity of gases used in surgery. In this paper the realization of primary reference gas mixtures (PRMs) in cylinders at I.N.Ri.M is described. The preparation method adopted is the gravimetric one that can be used for different analytes within a wide concentration range achieving a small uncertainty and the metrological traceability to SI units and to pure substances. The method may be applied to stable gases, and chemical conditions must previously be fulfilled to avoid reactions of gas components with the container walls and with critical impurities. It consists in introducing determined amount of gases in a previously evacuated cylinder in two or more steps, weighing the cylinder before and after each step. The preparation procedure followed at I.N.Ri.M. was validated taking part in international comparisons arranged in the MRA framework. The facility used was entirely realized at I.N.Ri.M. and it enables to carry out all the preparation steps from the evacuation of the cylinders to their accurate weighing. Since the mass determination of small amount of gases with high accuracy is the critical step, particular care is given to the cylinder weighing in good thermal stability conditions. To decrease the buoyancy effect, the mass of the cylinder to be weighed is compared with a reference cylinder, then both are alternatively and automatically exchanged and weighed by means of a device that minimizes the operator influence. In addition, to optimize the mass comparator performance, the mass difference between the two cylinders is kept within 1 g by adding calibrated mass standards. The obtained PRMs concentration is checked by chemical analysis and uncertainty on composition is calculated applying the uncertainty propagation law, on the basis of the gravimetric preparation data.