Dry purge for the removal of water from the solid sorbents used to sample volatile organic compounds from the atmospheric air

The dry purging technique used to remove water from the air and water sampl ing adsorbents in volatile organic compounds (VOC) analysis was investigate d. As a sampling simulation step, a fixed volume of humid air was passed th rough the tube filled with the sorbent bed. Desorption by the dry gas purgi ng followed. The concentration of water vapour in the gas at the outlet of the trap was directly measured in the course of all experiments. No more th an 300 ml of dry gas is enough for complete removal of water from Tenax, Ch romosorb 106, and Carbotraps B and C, even if large volumes of air at relat ive humidity as high as 95% are sampled. Adsorbed water can also be purged effectively from the carbon molecular sieves: Carbosieve S-III and Carboxen 569, 1000 and 1001. Carboxen 1000 is the easiest and Carbosieve S-III the most difficult case that requires the purging gas volume larger by about 60 -100%. Carboxen 569 and 1001 occupy an intermediary position. For carbon mo lecular sieves the dependence of water vapour concentration at the outlet o f the sampling tube on the dry gas volume is very characteristic: a long se gment that corresponds to the constant concentration is followed by a sharp decrease until the water is removed completely. The volume of dry gas nece ssary to achieve this task depends on the sample magnitude and relative hum idity and on the desorption temperature. The adsorbent mass exerts a very s mall effect. The latter phenomenon is unexpected but very important for ana lytical practice. Increase in the adsorbent mass prevents the losses of wea kly adsorbed analytes without the need to resort to increasing the purging gas volume. The water desorption process can easily be monitored and automa ted by placing a humidity sensor in the outlet channel of the purging gas.