A Correction for the Cylinder Wall Adsorption in Forensic Ethanol Gas Standards

Forensic ethanol gas standards are used for, among other, the calibration and metrological verification of evidential breath analysers as described in OIML-R126. A correction for the amount fraction ethanol in forensic gas standards due to cylinder wall adsorption is described. The correction was developed for both the national primary measurement standards as well as for derived primary reference materials. A novel method based on the well-known decanting principle was developed and assessed using two suites of gas mixtures with ethanol amount fractions between 50 μmol mol−1 to 1000 μmol mol−1 in nitrogen. From the results, it is inferred that the initial adsorption loss is a function of the amount fraction and an interpolation formula was developed accordingly. To account for differences in adsorption between cylinders, a mixed effects model was used to describe the adsorption loss data with an excess standard deviation to account for between-cylinder effects.