Improving the analytical flexibility of thermal desorption in determining unknown VOC samples by using re-collection

The thermal desorption (TD) technique has long suffered from the 'one-shot' problem, whereby the entire sample is consumed in a single analysis, and thus no sample remains for repeated analysis. Recent developments in TD equipment allow for the quantitative re-collection of split samples during thermal desorption, which can be utilised for archiving or immediate analysis. However, the performance of TD systems for re-collecting different volatile organic compounds (VOCs) has rarely been demonstrated. This study provides a systematic investigation into the re-collection efficiency for over 90 VOCs on a TD unit under different conditions. An analytical method was developed based on multi-sorbent tubes and TD-GC/MS, which could quantitatively measure 92 VOCs with good sensitivity (method detection limit between 0.01 and 2 ng) and precision (< 10%). Satisfactory re-collection performance (recoveries within 100% ± 20%) was found for over 70 compounds under different split modes for multiple times, and the single (outlet) split mode was preferred in this regard, in order to avoid significant uncertainties in the results. Thermal labile, polar or reactive compounds such as alcohols and ketones were generally not compatible with re-collection, as they were either lost due to thermal decomposition or formed as system artefacts. In addition, bromochloromethane should not be used as an internal standard when performing sample re-collection, since it will experience significant loss during repeated analysis and lead to overestimation for corresponding compounds. Finally, the re-collection was tested with low-concentration field samples to resolve the unexpected water problem in analysis. Although higher uncertainties were expected in the re-collected samples, the results provided good information on overall concentration variations at the sampling site, thereby instilling confidence in the results obtained from the primary analysis.