Pressurised liquid extraction of polycyclic aromatic hydrocarbons from contaminated soils

The reliability and efficiency of the pressurised liquid extraction techniq ue (PLE) for extracting polycyclic aromatic hydrocarbons (PAHs) from contam inated soil has been investigated. Experimental design was used to study th e influence of seven extraction variables (sample load, solvents used, solv ent ratios, pressure, temperature, extraction time, and rinse volume). The results show that large sample loads in combination with small solvent volu mes may result in low extraction efficiency. They also indicate that the re covery 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 o f the tested variables show that PLE is an exhaustive extraction technique that generally results in high yields. In addition, extraction of a certifi ed reference material (CRM 103-100) revealed that the method is both accura te and precise. Another finding was that adding the internal standard on to p of the soil in the extraction cell causes considerable over-estimation of the concentrations when large samples are extracted with small solvent vol umes. This is because the PLE-cell resembles a chromatographic column, so c ompounds added to the top of the soil layer have a longer distance to trave l through the soil compared to the average distance of the native compounds , which are distributed evenly throughout the column. We therefore recommen d that the internal standard should be added to the extract immediately aft er the extraction or, alternatively, carefully mixed with the sample prior to extraction. (C) 2000 Elsevier Science BN. All rights reserved.