Summary of Methods Used in the Study of Pesticide Biotransformation and Disposition - Pesticide Biotransformation and Disposition - page 10

migrates up the plate, and the toxicant and other constituents move with the solvent;
differential rates of movement result in separation. The compounds can be scraped from
the plate and eluted from the adsorbent with suitable solvents ( Hodgson, 2010 ).
Column Adsorption: Hydrophobic and Affinity
A large number of adsorbents are available to the analyst. The adsorbent can be activated
charcoal, aluminum oxide, Florisil, silica, silicic acid, or mixed adsorbents. The charac-
teristics of the toxicant determine the choice of adsorbent. When choosing an adsor-
bent, select conditions that either bind the coextractives to it, allowing the compound
of interest to elute, or vice versa. The efficiency of separation depends on the flow rate
of solvent through the column (cartridge) and the capacity of the adsorbent to handle
the extract placed on it. This amount depends on the type and quantity of adsorbent,
the capacity factor and concentration of sample components, and the type and strength
of the solvents used to elute the compound of interest ( Hodgson, 2010 ).
Cartridge technologies are improving, however, to allow similar concentrations
of sample to be added, which results in a less expensive and more rapid analysis. A
number of SPE apparatuses have been introduced since the early 1980s. Most con-
tain 0.5 to 2.0 g of the adsorbent in a plastic tube with fitted ends. The columns can
be attached to standard syringes. Other companies have designed vacuum manifolds
that hold the collecting device. The column is placed on the apparatus, a vacuum is
applied, and the solvent is drawn through the column. Some advantages of these sys-
tems include preweighed amounts of adsorbent for uniformity, easy disposal of the
coextractives remaining in the cartridge, no breakage, and decreased cost of the analysis
because less solvent and adsorbent are used. Affinity chromatography is a potent tool
for biologically active macromolecules that can be used for purifying small molecules,
such as pesticides ( Hennion, 2003; Stoks et al., 1999 ). It depends on the affinity of an
enzyme for a substrate (or substrate analog) that has been incorporated into a column
matrix or the affinity of a receptor for a ligand.
Size-Exclusion Chromatography
Also referred to as gel-permeation chromatography (GPC), this technique is primarily
used during the analysis of biological samples. When tissue samples are extracted with
a nonpolar solvent, significant amounts of lipids are also extracted. GPC columns are
packed with a cross-linked polymer material that is very porous. Cross-linked dextrans
such as Sephadex or agarose are commonly used materials. Small molecules can get
into the pores and are thus retained for longer periods of time on the column, whereas
large molecules (e.g., lipids) cannot and therefore pass through the column very
quickly. The GPC material is available in varying pore sizes depending on the applica-
tion for which it will be used. When this type of cleanup is used, the lipid fraction is