Simple off-line flash pyrolysis procedure with in situ silylation for the analysis of hydroxybenzenes in humic acids and coals

A simple device consisting of a glass pyrolysis chamber fitted for a commercial resistively heated pyrolysis probe and connected to a solvent desorption tube for air monitoring was applied to off-line pyrolysis under silylating conditions of humic acids (soil, lake) and coals. Samples were flash pyrolysed at 700 °C in the presence of excess hexamethyldisilazane, and evolved products were swept off by a nitrogen stream and trapped onto a charcoal filter from where they were desorbed with dichloromethane and analysed by gas chromatography (GC)–mass spectrometry. Humic acids afforded trimethylsilyl (TMS) ethers of phenols, 2-methoxyphenols (guaiacols), 2,6-dimethoxyphenols (syringols), and dihydroxy and trihydroxybenzenes as major products. TMSoxy benzenes were the principal products observed from pyrolysis/silylation of coals. In comparison with conventional pyrolysis, the in-situ derivatisation process enhances the levels of phenols with respect to hydrocarbons and improves the GC separation of isomers (e.g. meta- from para-cresol). With respect to tetramethylammonium hydroxide thermochemolysis, pyrolysis/silylation operates under milder conditions and permits discrimination between free and methylated hydroxy groups. The performance of the method for the quantitative determination of evolved product is described. Yields of evolved silylated mono and dihydroxybenzenes occur in the mg/g range with relative standard deviations generally between 16 and 30%.