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

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 (RH thr ). The RH thr is 50% for Carboxen 1000 and less than that for the other sorbents studied. Practical implementation of the suggested method is discussed.

Analytical artifacts caused by the presence of water vapor in the headspace of food products

“…1) and the powdered Parmesan cheese in the analyses performed with a dry purge step. These modifications appeared despite the low water retention on the non-polar porous polymer of the Tenax adsorbent [7,20]. Thus the areas of approximately 40 % of the peaks were strongly modified and 21 out of 79 peaks disappeared from the chromatographic profiles (Tab.…”

Estimations of isoprenoid emission capacity from enclosure studies: measurements, data processing, quality and standardized measurement protocols

“…In particular, different adsorbents vary greatly in water vapor adsorption capacity (Dettmer and Engewald, 2002;Helmig and Vierling, 1995). Typically, graphitized carbon blacks and Tenax-type of polymeric adsorbents, which retain molecules by pure physical adsorption and do not tend to form hydrogen bonds with water (hydrophobic adsorbents), have low water adsorption capacity, while carbon molecular sieves have high water adsorption capacity (Ciccioli et al, 2002;Engewald, 2002, 2003;Gawlowski et al, 1999;Helmig and Vierling, 1995), likely reflecting the presence of surface oxides in carbon molecular sieves leading to hydrogen bond formation (Dettmer and Engewald, 2002) or due to generation of strong adsorption fields inside the micropores of 5-7Å as the result of overlapping dispersion forces of neighboring pore walls (Ciccioli et al, 2002;Gawlowski et al, 1999). For adsorbents with high water affinity, water vapor can reduce BVOC adsorption efficiency by blocking adsorption sites and thus, reducing the surface area available for BVOC adsorption (Ciccioli et al, 1992;Helmig and Vierling, 1995).…”

Speciation of volatile organic compounds from poultry production

“…In fact, volatile fatty acids (VFAs) have not been detected with canister sampling and analysis from animal feeding operations (Koziel et al, 2005;Blunden et al, 2005;Filipy et al, 2006) and they are known to be a significant VOC associated with animal production (Zahn et al, 1997;Zahn et al, 2001;Ngwabie et al, 2008). Even alternative air sampling techniques such as sorbent tubes (USEPA Method TO-17) have their own short comings especially when sampling in humid environments using molecular sieve sorbents (Helmig and Vierling, 1995;Gawlowski et al, 1999;Trabue et al, 2008a). Rabaud et al (2002) noted problems with excess water when sampling air from a dairy facility in California.…”