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

The adsorption of water vapour in the sorbents used to sample volatile orga nic compounds (VOCs) from the atmosphere was investigated by frontal gas ch romatography. Air of 95% relative humidity (RH) was passed through the sorb ent bed and the uptake of water was monitored at the outlet of the trap. Gr aphitized carbons and non-polar polymeric sorbents, such as Tenax and Chrom osorb 106, show poor water trapping of generally less than 5 mg of water pe r 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, adso rb substantial amounts of water, corresponding to the volume of micropores. An important feature is a lack of adsorption at low RHs. Measures to preve nt water adsorption in sampling even very humid gases are advanced. The RH of sampled gas should be decreased below a threshold value (RHthr). The RHt hr is 50% for Carboxen 1000 and less than that for the other sorbents studi ed. Practical implementation of the suggested method is discussed.