The gas laboratory team at METAS, the Swiss Federal Institute of Metrology, is currently developing new facilities to generate reference gas mixtures for a variety of reactive compounds, at concentrations measured in the atmosphere and in a SI-traceable way (i.e. the amount of substance fraction in mole per mole is traceable to SI-unit definitions). The production process is dynamic, i.e. the gas standard is produced continuously over time.
Here we present the realisation of such standards for water vapour in the range 1-10 µmol/mol (ppm), for volatile organic compounds (VOCs) such as limonene, alpha-pinene, MVK, MEK, in the nmol/mol (ppb) range and for greenhouse gases such as SF6 and other fluorinated gases (F-gases) in the pmol/mol (ppt) range. The matrix gas can be nitrogen, synthetic air or clean dry air. It is purified beforehand from the compounds of interest to the required level, using commercially available purification cartridges.
The principle is to dilute the pure substance (water, SF6, etc) with purified matrix gas while controlling both flows. The source of the substance of interest is a permeator, i.e. a reservoir containing the pure substance and losing it over time by permeation through a membrane. The permeator is placed in a permeation chamber temperature- and pressure-controlled, with a constant carrier gas flow. The mass loss is continuously monitored over time by a magnetic suspension balance. This primary mixture is then dynamically diluted to reach the required amount of substance fraction. All flows are piloted by mass flow controllers which makes the production process flexible and easily adaptable to generate the required concentration. All parts in contact with the gas mixture are passivated using coated surfaces, to reduce adsorption/desorption processes as much as possible. Moreover the presented technique can be adapted and applied to a large variety of molecules (e.g., NO2, other VOC and refrigerants, NH3...) and is particularly suitable for gas species and/or concentration ranges that are not stable in cylinders.
Two types of setups are currently developed: one already built and fixed in our laboratory in Bern as well as two portable generators that are still under tests and could be used anywhere in the field. The permeation chamber of the portable generators has multiple individual cells allowing the generation of mixtures up to 5 different components if needed.