Pressurised liquid extraction of polycyclic aromatic hydrocarbons from contaminated soils

The reliability and efficiency of the pressurised liquid extraction technique (PLE) for extracting polycyclic aromatic hydrocarbons (PAHs) from contaminated soil has been investigated. Experimental design was used to study the influence of seven extraction variables (sample load, solvents used, solvent ratios, pressure, temperature, extraction time, and rinse volume). The results show that large sample loads in combination with small solvent volumes may result in low extraction efficiency. They also indicate that the recovery of low-molecular-mass PAHs is reduced by low extraction temperatures. The exact settings of the other variables are, however, less significant for the extraction efficiency. Repeated extractions at optimised settings of the tested variables show that PLE is an exhaustive extraction technique that generally results in high yields. In addition, extraction of a certified reference material (CRM 103-100) revealed that the method is both accurate and precise. Another finding was that adding the internal standard on top of the soil in the extraction cell causes considerable over-estimation of the concentrations when large samples are extracted with small solvent volumes. This is because the PLE-cell resembles a chromatographic column, so compounds added to the top of the soil layer have a longer distance to travel through the soil compared to the average distance of the native compounds, which are distributed evenly throughout the column. We therefore recommend that the internal standard should be added to the extract immediately after the extraction or, alternatively, carefully mixed with the sample prior to extraction.