Adsorption of water vapour in the solid sorbents used for the sampling of volatile organic compounds

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Abstract
The adsorption of water vapour in the sorbents used to sample volatile organic compounds (VOCs) from the atmosphere was investigated by frontal gas chromatography. Air of 95% relative humidity (RH) was passed through the sorbent bed and the uptake of water was monitored at the outlet of the trap. Graphitized carbons and non-polar polymeric sorbents, such as Tenax and Chromosorb 106, show poor water trapping of generally less than 5 mg of water per gram of sorbent. Polar polymeric sorbents, e.g. Chromosorb 108, Porapak T and Porapak N, sorb more water which is, however, weakly bound and easily removed by purging the sorbent bed with a dry gas. Carbon molecular sieves, e.g. Carbosieve S-III, Carboxen 569, Carboxen 1000 and Carboxen 1001, adsorb substantial amounts of water, corresponding to the volume of micropores. An important feature is a lack of adsorption at low RHs. Measures to prevent water adsorption in sampling even very humid gases are advanced. The RH of sampled gas should be decreased below a threshold value (RHthr). The RHthr is 50% for Carboxen 1000 and less than that for the other sorbents studied. Practical implementation of the suggested method is discussed.

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