A comparison of direct thermal desorption with solvent extraction for gas chromatography-mass spectrometry analysis of semivolatile organic compounds in diesel particulate matter

Direct thermal desorption-gas chromatography-mass spectrometry (DTD-GC-MS) is a technique that is finding application in the characterisation of the semivolatile organic carbon fraction of ambient and combustion source particulate matter (PM) collected on filters. In this study, three DTD-GC-MS methods were assessed and compared to a conventional solvent extraction method for analysis of a mixture of target analytes in solution and of diesel PM collected on quartz filters. The target analytes included n-alkanes, hopanes, steranes and polycyclic aromatic hydrocarbons. This study showed that while the three DTD-GC-MS methods were generally comparable to the solvent extraction method, (1) the choice of calibration strategy and calibration materials has a significant impact on the measured accuracy of a method; (2) very large variations were seen in all methods for the more volatile compounds such as C₁₀ to C₁₃ n-alkanes and naphthalene; (3) accuracy, defined as difference from the known concentration of a liquid sample, ranged from 5% to 32%; (4) precision, defined as the relative standard deviation, ranged from 4% to 16%. The average difference of DTD-GC-MS results from the solvent extraction results for the diesel PM filters ranged from 20% to 40%. This difference was driven by the large number of target analytes present at relatively low concentrations (<25 pg/mm²) and their corresponding higher variability. Differences in performance among the compound classes were noted. Minimum detection limits for the DTD-GC-MS methods were on the order of 0.1 to 1 pg/mm² and were as good as or better than those obtained for the solvent extraction method.