Evaluation of wood treatment effectiveness using thermal desorption-pyrolysis gas chromatography-mass spectrometry

A thermal desorption-pyrolysis gas chromatography with mass spectrometric detection (TD-Pyr-GC-MS) method was developed that enables a comprehensive analysis of preservatives as well as native wood constituents in treated wood. The developed method is based on thermal desorption, i.e., extraction of wood preservatives, followed by pyrolysis to investigate changes in the occurrence of characteristic wood building blocks (e.g., guaiacol derivatives). We implemented combined MS data acquisition (i.e., SITI) consisting of selected ion monitoring (SIM) and total ion current (TIC) and allowing for low limits of detection of preservatives in SIM mode and TIC detection of any evolving species. The matrix detection limits were 0.3-20 ng depending on the preservative, requiring only 100 µg sample size. The method quantified wood preservatives in aged wood samples in various states of deterioration in a range of 18-166 µg/g. A decline in the concentration of some of the target preservatives, like tebuconazole, in the environmentally weathered samples was observed as compared to those less aged, while some persisted, such as permethrin. We also noted changes in the occurrence of long-chain linear alkanes (C₂₀-C₂₅) used as preservative solvents providing insights into the nature of wood decay as a result of long-term environmental exposure. Finally, the method provided elution profiles of wood extractives, i.e., products of native wood polymer (lignin) pyrolytic decomposition. The observed wood decay did not show a conclusive relationship with the level of wood extractives but occurred along with the gradual depletion of hydrocarbons (treatment solvent), most likely due to restriction in preservatives' mobility within the wood.