Prevention of water vapour adsorption by carbon molecular sieves in sampling humid gases

The water uptake by the solid sorbents Carbosieve S-III, Carboxen 569, 1000 and 1001, all of which are used for sampling of volatile organic compounds from the atmosphere. was examined using a direct experimental approach. The content of retained water is affected both by the trap temperature and the initial water vapour concentration in the sampled gas. Two different adsorption mechanism are operative. At low relative humidities (RH) only active polar centres are involved. This adsorption is so weak that negative water interferences can easily be managed. Another mechanism, the micropore volume filling, involves substantial amounts of water, becomes operative once the threshold value for relative humidity (RHth) is surpassed. RHth is 45+/-3% for Carboxen 1000 decreasing to 35+/-3% for the three other sorbents studied. A novel but simple strategy was tested for water management: moderate heating of the trap during the sampling (a warm trap method). The temperature elevation required depends on the RHth characteristic for the specific sorbent, and RH and the temperature of the sampled gas. Usually the 5-15 degrees C elevation is sufficient; only under extreme RH conditions is an elevation of 20 degrees C necessary. The diagrams are given to determine this elevation. Since the sample RH is significantly decreased at an elevated temperature the negative effect of water uptake on the safe sampling volume is alleviated. Consequently the sampled gas volume can be as large as desired which decreases detection limits.