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 concentrati ons are needed with high accuracy. For this purpose we developed a diffusio n device combined with a dynamic dilution system. Pure liquid compounds are placed in glass vials. They diffuse through a capillary on top of each via l into a diffusion chamber flushed with synthetic air. In an additional dyn amic dilution step the final concentration is adjusted with a flow of purif ied synthetic air to typical mixing ratios between several ppt (v/v) and pp b (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 lit tle 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, f rom 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 d evice was checked with different standard mixtures with mixing ratios rangi ng from 0.32 ppt (v/v) (tribromomethane) to 1353 ppt (v/v) (n-pentane). The regression analysis of peak area versus concentration showed excellent agr eement among the standards for each compound with correlation coefficients (r(2)) between 0.9826 and 0.9998. The temporal stability of the diffusion d evice was determined from more than 270 measurements of one standard mixtur e. The reproducibility of the peak areas ranged between 2.2% and 12.7% depe nding on the compound. (C) 1999 Elsevier Science B.V. All rights reserved.