Gas mixtures of ammonia in nitrogen are often used as the first step in the measurement of ammonia in ambient air. These measurements comprise a wide range of concentrations and therefore gas mixtures in cylinders are usually diluted to reach the required concentration. To be in compliance with EU regulation the uncertainty of a 30 micromol/mol concentration should be no larger than 0.3 micromol or 1% relative. Until recent years VSL could supply Primary Reference gas Mixtures for ammonia in the range 10 – 300 micromol/mol with an uncertainty of 3%rel. at the 30 micromole concentration.
Within the “Metrology for ammonia in ambient air” project (MetNH3; European Metrology Research Programme) more accurate gaseous reference materials for ammonia in nitrogen in cylinders are developed. One of the targets in the project is to achieve an uncertainty of 1% rel. at the 30 micromol/mol concentration for ammonia gas mixtures in cylinders.
Both the cylinders used and the measurement procedure are the most important factors contributing to the uncertainty. Different types of cylinder surface passivation were tested both on short and long term stability at 10 and 100 micromol/mol concentrations. For improvement of the analytical measurement procedure new types of tubing as well as the effect of using one reducing valve in combination with a multi position valve were tested. The optimal combination results in a reduced repeatability of the analysis of 0.2% which means that differences in stability can be assigned to individual cylinders.
Together these factors result in more accurate traceable gaseous reference materials for ammonia mixtures in cylinders and the required 1% relative at the 30 micromol/mol concentration seems possible on short notice. An international comparison between the national metrology institutes will be organized to underpin this.
I have been working at VSL for over 30 years mainly in gas analysis and purity analysis with an emphasis on reactive and sticky components like hydrogen sulfide, ammonia and water. Lately I am also responsible for the quality control for reference materials supplied to customers.
I have been working on MetNH3 and will be the VSL contact person for the new EMPIR project : metrology for hydrogen vehicles (MettroHyVe)
Since last year I am convenor for the working group on gas analysis of Euramet.