Diffusion technique for the production of gas standards for atmospheric measurements

For the calibration of gas chromatographic measurements of volatile organic compounds in ambient air samples, standard gas mixtures at low concentrations are needed with high accuracy. For this purpose we developed a diffusion device combined with a dynamic dilution system. Pure liquid compounds are placed in glass vials. They diffuse through a capillary on top of each vial into a diffusion chamber flushed with synthetic air. In an additional dynamic dilution step the final concentration is adjusted with a flow of purified synthetic air to typical mixing ratios between several ppt (v/v) and ppb (v/v). The diffusion rates are determined from the mass loss of the vials. Extensive tests over 21 months showed that the diffusion rates varied little with time, between 1.4% and 3.1%, depending on the compound. The system proved to be suitable for compounds with a wide range of boiling points, from 305 K (1,1-dichlorethene) to 418 K (1,2-dimethylbenzene) The diffusion device was applied to a gas chromatographic system with a flame ionization detector and an electron-capture detector. The linearity of the diffusion device was checked with different standard mixtures with mixing ratios ranging from 0.32 ppt (v/v) (tribromomethane) to 1353 ppt (v/v) (n-pentane). The regression analysis of peak area versus concentration showed excellent agreement among the standards for each compound with correlation coefficients (r(2)) between 0.9826 and 0.9998. The temporal stability of the diffusion device was determined from more than 270 measurements of one standard mixture. The reproducibility of the peak areas ranged between 2.2% and 12.7% depending on the compound. (C) 1999 Elsevier Science B.V. All rights reserved.