C LEANUP AND F RACTIONATION The nonselective nature of the exhaustive extraction procedures and the complexity

7.2.3 C LEANUP AND F RACTIONATION The nonselective nature of the exhaustive extraction procedures and the complexity

of sample matrices result in complex extracts that require further purification. Despite the inherent advantages derived from partial or complete integration of this tedious and time-consuming purification with the extraction, up to now, the development in this field has been rather limited and the analytical steps involved in cleanup protocols for pesticides have usually been carried out off-line. Typical purification and fractionation procedures have been summarized in Tables 7.1 and

7.2. For biotic samples, lipid elimination should be accomplished before chromato- graphic analysis. Lipid elimination can be accomplished by destructive or non- destructive methods. Otherwise, similar protocols can be used for purification of the extracts almost irrespective of the matrix nature.

Nondestructive methods for lipid removal . Gel permeation chromatography (GPC) and adsorption chromatography on selected sorbents are nondestructive treatments applied for lipid elimination. GPC is mainly carried out either in auto- mated systems or by gravity flow columns. The current use of prepacked polysty- rene –divinylbenzene-based high-performance GPC columns has resulted in higher separation efficiencies, improved reproducibility, and lower solvent consumption as compared with manually packed columns. Satisfactory isolation of the target com- pounds from the coextracted organic material after single GPC analysis has been

achieved for samples containing limited amounts of lipids. 5 However, for more complex matrices, GPC followed by further cleanup by adsorption chromatography

may be required to remove remaining low-molecular-weight lipids, waxes, and pigments.

Silica gel, alumina, and Florisil with different degrees of activation have been widely used for lipid=pigment removal by adsorption chromatography under atmos- pheric conditions (Tables 7.1 and 7.2). Basic alumina and silica gel columns have been effective for the separation of OCs from fish lipids, although there is the possibility of minor losses due to dehydrochlorination of some OCs, for example,

p,p 0 -DDT, on the alumina. 69 Alumina and Florisil have been preferred as fat retainers because of their higher lipid-retaining capacity in procedures involving MSPD 28 or SFE. 19,41 The effectiveness of these adsorption columns depends on the mass and the water content of the adsorbent together with the polarity of the solvent. In general, 3 –8 g of absorbent is used in a 0.5–1.0 cm diameter column with silica gel or Florisil deactivated with a low percentage of water (0% –5%). Alumina and Florisil have the capacity to retain about 100 mg lipid per 10 g of adsorbent.

Destructive methods for lipid removal . Drastic treatments of the extracts are usually conducted when analytes of interest are stable in acidic or basic conditions. Lipid removal using a sulfuric acid wash or elution of the extract through sulfuric acid-impregnated silica has been described as an effective cleanup for the analysis of

OCs, such as hexachlorocyclohexanes, DDT, or chlordanes. 31,36 The dispersion of sulfuric acid onto the surface of activated silica results in a sorbent which can be

easily loaded into a column. The use of acidified silica avoids the emulsion prob- lems, reduces the sample handling and solvent consumption, and increases sample throughput. However, the acidic treatment destroys all other classes of pesticides,

Determination of Pesticides in Food of Animal Origin 191 including several other OCs, such as dieldrin. KOH-treated silica columns or KOH

wash of extracts is less effective for the removal of lipids and can be used for matrices with low lipid content. 34 Similar to acidic treatment, basic treatment also results in degradation of some analytes, for example, DDT is converted to DDE. Fractionation . For specific applications, isolation of the target analytes from other compounds present in the extract can be mandatory to avoid interferences during final determination. Due to their wide range of polarity, pesticides can be isolated in a separate fraction using classical adsorbents, such as silica gel, 70,71 while alumina and Florisil show less selectivity for pesticides. The fractionation is usually done by applying the extract in a small volume of nonpolar solvent to the adsorbent

and by eluting with various volumes of solvents with increasing polarity. 19