We present a novel approach to study the organic composition of aerosol filter samples using thermal-desorption proton-transfer-reaction mass spectrometry (TD-PTR-MS) in the laboratory. The method is tested and validated based on the comparison with in situ TD-PTR-MS measurements. In general, we observe correspondence within the levels of uncertainty between in situ and offline TD-PTR-MS measurements for compounds desorbing at temperatures above 100 degrees C and for quartz fiber filters that were sampled for more than one day. Positive sampling artifacts (50-80%, with respect to the in situ measurements) from adsorption of semivolatile organic gas phase compounds are apparent on filters sampled for one day. Detailed chemical analysis shows that these positive sampling artifacts are likely caused by primary emissions that have not been strongly oxidized. Negative sampling artifacts (7-35%, with respect to the in situ measurements) are observed for most filters sampled for two and three days, and potentially caused by incomplete desorption of aerosols (in particular, nitrogen-containing organics) from the filters during the offline measurements and chemical reactions on the filters. (C) 2014 Elsevier Ltd. All rights reserved.