The extraction process from solid samples will almost inevitably lead to the co-extraction of other compounds. This would include not only low-molecular-mass compounds (perhaps other pollutants), but also high-molecular-mass materials. The term 'lipid' is often used for such naturally occurring high-molecular-mass organic species which can be extracted into organic solvents. Clean-up is vital before chromatographic analysis is carried out. As well as the techniques discussed earlier in Chapter 4 (e.g. column chromatography and solid-phase extraction), additional techniques can be used for lipid removal. Gel permeation chromatography separates compounds according is their molecular size. The equipment used is either a classical low-pressure column or a preparative-scale high-pressure liquid chromatograph. The method can be used for a broad range of pollutants, regardless of any knowledge of their detailed chemical structures. You should contrast this with adsorption columns (see Sections 4.2.2 and 4.2.3 above), which need a prior knowledge of the polarity of the compounds to determine suitable separation conditions. Chemical methods which destroy the lipid by saponification (a typical reagent is 20% potassium hydroxide in ethanol), or by dehydration or oxidation (concentrated sulfuric acid), are often too harsh for many pollutants (e.g. organophosphorus pesticides), but can be used for some organochlorine pollutants.
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