3.4.1 Extraction of PAHs from solid environmental matrices
To realize the extraction of PAHs from solid environmental matrices, the method used generally were soxhlet extraction, alkaline saponification, ultrasonic extraction and accelerated solvent extraction (Wilcke, 2007). Investigation of several types of extraction procedures is usually done to reduce the time of operation and the use of large amount of solvent. The most commonly used technique for the extraction of PAHs from soil samples is soxhlet extraction, other techniques more recently applied are soxthern, supercritical fluid extraction and microwave extraction (Graham et al., 2006).Shaking methods and ultrasonic extraction were applied since a long time but were considered to be less effective than the other type of extraction methods (Jonker et al., 2002). Comparing the use of soxhlet extraction, microwave-assisted extraction and pressurized fluid extraction, one study demonstrated that the best repeatability and recoveries were obtained with PFE (Pressurized Fluid Extraction) in the first position, then MAE (Microwave Assisted Extraction) and finally Soxhlet (Itoh et al.,
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2008). This was confirmed by a similar study comparing the three methods and showing the best extraction efficiency for PFE compared to MAE and soxhlet (Wang et al., 2007). It was concluded that PFE and MAE were more suitable methods for the extraction of PAHs from soils. SFE was also shown to be a good technique to extract PAHs from soils compared to methods like soxhlet, ultrasonic and shaking. Methanolic saponification demonstrated very good PAHs recoveries but variations were observed for the low molecular weights PAHs (Berset et al., 1999).
3.4.2 Pressurized fluid extraction
The principal components of a PFE system are a source of organic solvent, a pump to circulate the solvent, a sample cell into which is placed the sample, an oven in which the sample cell is heated and its set temperature monitored, a serie of valves that allows pressure to be measured and generated within the sample cell, and an outlet point (Figure 3.1) (Dean, 2009).This method allows reaching very high temperature of extraction without evaporating the solvent. This is explained by the fact that pressure is applied on the cell where the sample is placed, so when the solvent is mixed with the sample, temperature above boiling points can be reached, improving the extraction efficiency (Fidalgo-Used et al., 2007).
Figure 3.1: Schematic of the principle of Pressurized Fluid Extraction (Cyberlipidcenter, website)
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3.4.3 Clean-up methods
Generally, column chromatography or solid phase extraction were used with florisil, silica gel, aluminium oxide, or a combination of them, in different mesh sizes, with various types of solvents to elute PAHs resulting from the extraction of solid environmental matrices (Wilcke, 2007). Other types of sorbents, or tools such as filtration, are obviously available and depend on the type and structure of the compounds studied and the interference that needs to be removed (oil, food, etc), and also on the matrix. The US Environmental Protection Agency has approved a list of protocols for the application of the clean-up with the same type of sorbent such as alumina, florisil, and silica gel for the purification of organic extracts from solid environmental matrices (Method 3610B, 1996; Method 3620 B, 1996; Method 3630 C, 1996; Method 3660, 1996). Generally, each of the method considers the clean-up using column chromatography or solid phase extraction. The former uses larger amounts of solvent and adsorbent but has a greater clean-up capacity (Method 3620 B, 1996).
A new type of clean-up that is being used for various solid environmental and non- environmental matrices combine the extraction and the purification in only one simultaneous step and is called on-line, in cell (for PFE), in-situ or selective clean-up (Gomez-Ariza et al., 2002; Bjorklund et al., 2006; Fidalgo-Used et al., 2007; Hussen et al., 2007; Westbom et al., 2008). It consists in realizing the clean-up simultaneously with the extraction, by adding the sorbent inside the cell, in the case of PFE. Few publications have been dealing with this integrated form of clean-up and generally involve the PFE and SFE techniques (Fidalgo-Used et al., 2007). The objective of this one-step clean-up is to reduce (1) extraction and purification procedure time, (2) cost, and (3) use of large volume of solvent. The method has shown efficiency for the extraction and purification of PAHs from soil samples (Kim et al., 2003). The difference in recoveries between the off-line and on-line clean-up, in this study, was not significant (less than 5%), and the extract was very clear and contained less
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impurities using the latter method (Kim et al., 2003) The comparison of the on-line and off-line procedure shows clearly that the former method is faster, easily automated and consumes less solvents (Gomez-Ariza et al., 2002; Fidalgo-Used et al., 2007).
3.4.4 Clean-up using copper powder
According to the literature, copper is used to reduce the sulphur content in soils (Berset et al., 1999; Notar et al., 2000). This clean-up is realized because high sulphur content in soil can involve bleeding and deterioration of the GC-MS column. Moreover, recently, the copper powder has been used directly into the cell of the PFE, as described before for the clean-up with sorbents such as florisil or alumina (Notar et al., 2000). However, one study demonstrated no effect on the sulphur removal with copper addition using a selective PFE (Rodil et al., 2008).
3.4.5 Pre-concentration step
When finalizing the preparation of the samples for injection in the instrument, pre- concentrations of the solutions can be done in order to improve signal sensitivity. The most common approach of pre-concentration step are gas blow down, Kuderna- Danish evaporative concentration, the automated evaporative concentration system (EVACS), or rotary evaporation (Dean, 2009). Care should be taken when doing the evaporation, firstly because compounds can be lost due to their high volatility or low ebullition point, and secondly contamination due to the glassware can appear. Sometimes solutions can be evaporated directly to dryness and reconstituted with a small known quantity of solvent before injection into the analytical instrument.