The Veterinary Science
Division (VSD) of AFBI Stormont's
Chemical
Surveillance Branch tests for drugs residues, and its
Agriculture Food and Environmental Science Division at AFBI Newforge tests for contaminants, using a
wide range of immunological, biological and physicochemical methods.
This
reflects the large number
and diversity of these substances which spans single chemical elements (eg cadmimum) and both natural
(eg progesterone) and synthetic (eg clenbuterol) chemicals.
Step
1: Screening Tests.
Wherever feasible, a relatively rapid,
high-volume, low-cost analysis method that can distinguish potentially positive samples (non-compliant
with European Union (EU) legislation), from the negative, (compliant) samples, is used as a screening
procedure.
Residues
of many antibiotics can be detected by their mode of action in inhibiting bacterial growth.
The
Six Plate Test developed at AFBI Stormont is used to screen for the presence of a wide range of licensed
veterinary antibiotics. It uses six combinations of bacterial species and culture conditions within
solid agar plates, to distinguish between several major classes of antibiotics by their characteristic
spectrum of activity. A small portion of the meat sample is placed onto dialysis membrane, on top of
the agar plate which has been pre-seeded with bacteria.
During an overnight incubation, any antibiotic present in the sample
will diffuse
through the membrane
into the plate, inhibiting growth immediately around the meat. The pattern of growth inhibition on the
various plates can be used to suggest that a particular class of antibiotic residues is present in the
sample.
Many of the compounds analysed at VSD of AFBI Stormont have little
or no antimicrobial
activity (e.g. ß-agonists and hormones), but may be detectable using immunological techniques.
Immunoassays require the production and use of “antibodies”, natural binding proteins that are capable of binding to the drugs of interest. The reaction between the antibody and the drug against which it is raised can be very specific, and forms the basis of many of the highly sensitive immunodiagnostic tests developed at VSD of AFBI Stormont over the years for a wide range of veterinary drug residues.
An increasing range of these immunoassays is being transferred to state-of-the-art biosensor technology,
which utilises an automated optical detection system and is relatively simple, rapid and inexpensive
to run.
The principle of real-time measurement using the biosensor
How the specific antibody-drug reaction creates an optical effect within the biosensor
Step 2: Confirmatory Tests.
All
samples that screen positive (potentially non-compliant) are confirmed using a physico-chemical technique.
These methods have the specificity and sensitivity to confirm, unambiguously the presence of a drug
in a sample and to provide an accurate measure of its concentration.
Increasingly, tandem mass-spectrometry is being used as the confirmatory technique
of choice as it can identify substances definitively by their molecules' unique mass/charge ratio. This
instrument applies an electric field to a small, vaporised sample to generate a unique trace for each
chemical compound or mixture, and can distinguish between very similar molecules. The analysis can be
combined with other physicochemical methods, such as liquid (or gas) chromatography to separate the
target compounds from other sample components.
The Mass Spectrometer
The mass spectrum for chloramphenicol
These are expensive methods, which can be time-consuming to set up, calibrate
and run for each chemical monitored. Furthermore, as a result of the complex chemical nature of food
samples such as meat or milk, the substance to be analysed usually needs to be subjected to detailed
and exacting clean-up procedures, in order to eliminate background ‘noise' arising from the sample.
Confirmatory tests provide unequivocal results. A “non-compliant” result will result
in follow up action,
which may include field investigations, exclusion of product from the food chain and prosecution in
a court of law.
