An off-line supercritical fluid extraction method has been developed for the determination of polychlorinated biphenyls and organochlorine pesticides at the part-per-trillion level in marine sediments. Four different extraction conditions were evaluated: pure carbon dioxide at 60 °C/220 atm and 100 °C/350 atm; addition of methanol prior to the static extraction at 80 °C/350 atm; addition of methanol prior to the static extraction step and continuously during the dynamic extraction step at 80 °C/350 atm. Only the latter condition gave extraction recoveries that were comparable and for some compounds higher than Soxhlet recoveries. The significance of differences between the two extraction methodes was determined in an F- and t-test. The method was validated by analysis of a reference material from an international intercomparison exercise.
The use of a Dewar condenser placed on top of the supercritical fluid extraction off-line collection vial reduces analyte losses significantly. Five groups of analytes (EPA-PAHs, alkanes, nitro-PAHs, fatty acid esters, and chlorinated hydrocarbons), all of them of environmental concern, were investigated and quantified by GC-FID/GC-ECD. Collection losses were reduced to about 2%, with the condenser kept at -25 °C. Supercritical fluid extraction (SFE) of environmentally relevant compounds from different matrices has become a common technique in analytical chemistry.1-5 The extraction of analytes from a solid matrix by means of a supercritical fluid (SF) can be divided into four steps. The first step is the diffusion of the SF into the particle pores of the matrix. The second step is the overcoming of the analyte-matrix interaction and the solvation of the analyte. In the third step, the analyte is transported to the trapping device, where it is collected (fourth step).The development of an extraction method requires efficient trapping of analytes. Different trapping techniques are used in off-line SFE.6 The simplest technique reported is the expansion of the SF into an empty vial.7 891011Precipitation of the analytes takes place after rapid depressurization due to the low solubility of the analytes in gases and the cooling effect of the expanding SF. More common techniques are the collection in cryo traps and adsorbent traps, where the analytes precipitate on cold surfaces or are collected on sorbent resins.8-11 Solvent trapping is another method used frequently in off-line SFE, where the SF is depressurized directly into an organic solvent.12-14 Combinations of these methods, such as a cooled adsorbent or liquid-solid trapping, have been reported.15-17
The applicability of SFE for trace analysis was expanded by an intensive cleaning procedure of the SFE equipment and development of an adsorbing device for commercial carbon dioxide. For a selection of chlorinated hydrocarbons, the limit of detection was reduced to ≤20 pg/g of solid matrix. The procedure can be transferred to other analytical SF delivery systems for off- and on-line SFE, and also to an SFC apparatus. Also, the blank value for the analysis of nonchlorinated aliphatic and aromatic compounds was improved significantly. The purity of carbon dioxide is no longer the limiting factor for trace analysis with SFE.
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