Agriculture Reference
In-Depth Information
9.1 Sample Selection
Several edible tissues from food producing animals can be collected for
residue control including muscle, liver, kidney, skin and fat, which are usually
taken at slaughter houses. In addition, further sample matrix types can be taken
on-farm or at production sites, including milk, honey, eggs and fish [88, 105].
The approach normally adopted in residue control is to target the matrix
where residues are most persistent for banned substances and at their highest
concentration for licensed veterinary drugs. Sample matrix selection for
imported foods is limited to traded commodities such as muscle, honey, milk
and eggs. Muscle is a particularly advantageous tissue for residue control
because it is the main consumed tissue and can be used to analyse both
imported and domestic samples, thereby reducing laboratory validation
requirements. However, muscle can present analytical difficulties because of
variability in residue distribution [106-108], particularly in the area
surrounding injection sites [109-111]. There is also the concern of lower
probability of finding non-compliant samples compared to matrices such as
liver and kidney [86].
9.2 Sample Storage and Preservation
Sample storage is an important step, because of the lag time between
sample collection and analysis. Both physico-chemical factors (oxidation,
proteolysis and precipitation) and biological factors (microbiological and
enzymatic reactions) need to be considered when storing samples [88, 105].
EU validation criteria describe guidelines for stability studies to be carried out
during method validation [112]. Stability ought to be determined for the
analytes in matrix and in solution at various stages of the sample preparation
process. Incurred tissue should be used, whenever possible, otherwise matrix
fortified material is used. A practical approach is to run a test to see how long
a sample and/or analyte can be held without degradation and then to complete
the analysis within that time.
9.3 Sample Pre-Treatment
The variation of residues within a single organ or tissue is an important
feature to consider prior to sample preparation, although it is oftenly ignored
