Detection of levamisole residues in bovine liver and milk by immunobiosensor Original Research Article

Levamisole is widely used for the control of gastro-intestinal parasites in many animal species due to its broad spectrum action. A maximum residue level in liver was established at 100 μg kg−1, however no value has yet been set for residues in milk. An immuno-based screening method was developed to detect levamisole residues in liver and milk using a Biacore® biosensor. Levamisole was derivatised to aminolevamisole, conjugated to a carrier protein and the immunogen generated was used to raise a polyclonal antibody in a rabbit. The same drug derivative was immobilised onto a CM5 sensor chip. Binding of the antibody to the sensor chip surface was inhibited in the presence of levamisole. The chip surface was stable for many hundreds of regenerations. A single analytical cycle (sample injection, chip regeneration and system wash) took 6 min to complete. Sample analysis (liver and milk) included an extraction with acetonitrile. Liver samples required an additional filtration step prior to analysis. The 3σ limit of detection (LOD) was calculated as 6.8 ng g−1 for the liver assay and 0.5 ng ml−1 for the milk assay. Interassay variation was calculated as 6.7% at a concentration of 100 ng g−1 in liver and 2.4% at 2 ng ml−1 in milk. Analysis of incurred liver and milk samples showed close agreement with those determined by liquid chromatography (LC)–mass spectrometry (MS) (r2=0.998 and 0.985 for liver and milk analysis respectively).