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

Abstract

The dry purging technique used to remove water from the air and water sampling adsorbents in volatile organic compounds (VOC) analysis was investigated. As a sampling simulation step, a fixed volume of humid air was passed through the tube filled with the sorbent bed. Desorption by the dry gas purging 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 than 300 ml of dry gas is enough for complete removal of water from Tenax, Chromosorb 106, and Carbotraps B and C, even if large volumes of air at relative 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 molecular sieves the dependence of water vapour concentration at the outlet of the sampling tube on the dry gas volume is very characteristic: a long segment that corresponds to the constant concentration is followed by a sharp decrease until the water is removed completely. The volume of dry gas necessary to achieve this task depends on the sample magnitude and relative humidity and on the desorption temperature. The adsorbent mass exerts a very small effect. The latter phenomenon is unexpected but very important for analytical practice. Increase in the adsorbent mass prevents the losses of weakly adsorbed analytes without the need to resort to increasing the purging gas volume. The water desorption process can easily be monitored and automated by placing a humidity sensor in the outlet channel of the purging gas.