Volatile organic compounds (VOCs) are measured with sorbent tubes followed by thermal desorption (TD) analysis. Water vapor in the atmosphere affects sampling breakthrough and results in low collection efficiency. This paper reports the effect of relative humidity (RH) and temperature on the collection of 57 ozone precursors with a multi-bed tube composed of three different strengths of adsorbents (Tenax TA, Carbograph 1 TD, and Carboxen 1003). Unacceptable breakthrough values for volatile C 2 -C 5 aliphatic compounds were observed under high (>60%) RHs. The breakthrough volumes (BV) for C 2 aliphatic compounds were reduced 13 -22 fold under 90% RH.Dry-purge with inert helium gas removes excessive water content before the TD analysis but also causes a maximum of 40% loss of target analytes. Condensation is another pathway for water retention. The tube temperature should be regulated at 5-10 • C above the air temperature to minimize condensation under RHs >30%.

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