Use of solid-phase extraction in the determination of benzene, toluene, ethylbenzene, xylene and cumene in spiked soil and investigation of soil spiking methods

Abstract

A method has been developed for the determination of benzene, toluene, ethylbenzene, xylene and cumene (BTEXC) in soils, based on methanol extraction, solid-phase extraction (SPE) of the diluted extract and gas chromatography. Quantitative recoveries of BTEXC were obtained from methanol extracts provided the solvent composition was adjusted to methanol–water (50 + 50) prior to SPE, and care was taken to avoid the development of headspace into which analytes could partition. Cartridge (500 mg) load volumes of up to 20 ml of methanol–water extract were possible for all the analytes, except benzene (7 ml), without significant loss due to volatilization. The minimum elution volume for 100% removal of the analytes from the SPE cartridge was 1.5 ml of dichloromethane. It was possible to recover >90% of analytes added as a concentrated methanolic solution to a dry, clay soil, but the recoveries decreased if field-moist soil was used and if the soil was spiked with petrol. Recoveries were also reduced if the soil and spiking solution were left in contact for extended periods (as would occur in the event of a real contaminant spillage). Over a 17 d period, more than 30% of the BTEXC added to a soil as a dilute solution in methanol–water (50 + 50) became too tightly bound for removal by a single aliquot of extractant. When the method of vapour fortification was used to produce performance evaluation materials, both uptake of BTEXC and stability of the analyte concentrations after spiking were found to depend strongly on the soil type.