Cryogenic trapping for determination of odor concentration

The sensory testing method applied under Japanese law to measure odor concentration has a lower detection limit of 10 in the specified Odor Index. To measure odor below the limit, a condensing procedure using solid sorbents (Tenax-TA, Unicarbon B and Carbosieve SIII) has been developed and used in Japan. This procedure however cannot condense all odorous substances, and is specifically unsuited to hydrogen sulfide, methyl mercaptan, dimethyl sulfide, ammonia, and other typical odorous substances. In the present study, cryogenic trapping was tested to improve recovery rate. As water in sample air causes choking of the trap tube, vacant pre-columns to condense the water were connected to the Tenax-TA-packed column. The columns were chilled with liquid oxygen before passage of 100 L of sample air. The columns were then heated to 200°C under passage of 50 mL/min of nitrogen carrier gas to desorb odors. The desorbed gases were captured in sampling bags made of polyethylene terephthalate film. The total volume of desorbed gases was approximately 1 L. The method showed good recovery rates for hydrogen sulfide, methyl mercaptan, dimethyl sulfide and ammonia, and was useful for determining low-level odor concentrations during measurement of odor in ambient air at various sites in Osaka City.