Agriculture Reference
In-Depth Information
[88]. For example, residue variations may occur in the kidney between the
medulla and the cortex [113-115]. Therefore, it is important to take a
representative aliquot of the sample, which may require removal of several
portions throughout the composite sample to give a representative sample.
Homogenization with a blender is often advantageous for obtaining a
homogenous sample but can result in the release of enzymes, which can
degrade residues and provide inaccurate results. Liquid samples (blood,
plasma, serum, milk, bile or water) are generally easier to process than solid
samples and residues are more homogenously distributed throughout [88].
9.4 Sample Extraction
9.4.1 Free Residues and Conjugates
The residues present can vary significantly among target tissues due to the
extensive metabolism in animals after administration. The target residue for
analysis is not always the parent drug but can be comprised of the parent drug
and/or metabolites. The free parent and metabolite residues are readily
extracted by organic solvents, water or aqueous buffers. However, many
residues are present in the conjugated forms (glucuronides or sulphates) and
require liberation through enzymatic or chemical hydrolysis prior to
extraction. Hydrolysis conditions (namely pH, temperature and time) have to
be carefully optimized to ensure efficient deconjugation of residues.
Enzymatic hydrolysis commonly ensures milder conditions than acid or
alkaline hydrolysis [88, 105].
9.4.2 Bound
Residues
Residues bound through weak interactions can be easily extracted after
dialysis, proteolysis or denaturation of proteins by heat or acid treatments [88].
Analysis of bound residues is applied to a small number of drugs, namely
nitrofurans, florfenicol and triclabendazole. Nitrofuran antibiotics are rapidly
metabolised to form bound residues, which persist for many weeks after
treatment [15]. These bound metabolites pose a health risk and are used as
marker residues to monitor for nitrofurans [9]. It is proposed that binding of
residues occurs through cleavage of the nitrofuran ring by stomach acid,
leaving the specific tail group covalently bound to tissue [43]. The bound
metabolites are cleaved from tissue samples under mildly acidic conditions
before undergoing derivatisation to increase the sensitivity of detection [116].
