Phenol and methylphenol isomers determination in soils by in-situ microwave-assisted extraction and derivatisation

A simultaneous microwave-assisted extraction-derivatisation procedure was developed and optimised for phenol and methyl phenol isomers from soil samples. Both spiked and real soil samples (carbon content of 18%) were irradiated with microwaves in a closed-vessel system while immersed in hexane that had been previously doped with acetic anhydride and pyridine so as to effect an in situ catalytic acetylation of the target phenolic compounds. Spiked samples were prepared at least 20 days before treatment to simulate natural weathering processes and allow for the formation of analyte-matrix interactions. Optimization of the method was achieved by using a factorial design approach on parameters such as the volume of solvent, the quantity of acetic anhydride and the extraction temperature. Comparison with ultrasonic extraction procedures indicated that microwave-assisted methods gave superior recoveries (ca. two-fold) and greater precision in addition to being characterised by shorter extraction times. The procedure using microwaves yielded samples that could be analyzed directly on a GC-MS system without any preliminary clean-up or concentration steps. The detection limit of this novel method was found to be in the low-ppb range.