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The construction of immunochemical inhibition assays for beta-agonist and hormone residues have previously been described. In the present work the beta-agonist assay was further optimised for application to biological samples, using urine as the main model matrix. Matrix interferences with the antigen-antibody interaction and non-specific binding (NSB) of matrix components to the sensor surface were systematically studied. A full factorial design experiment was employed for evaluating the effects of assay buffer composition. In addition, the influence of antibody concentration and sample dilution on the matrix background was investigated. NSB from urine was highly affected by buffer pH and salt concentration, while buffer composition had little effect on matrix interferences with the antigen-antibody interaction. Ultra-filtration efficiently prevented NSB from urine and serum samples. Increased antibody dilution reduced the matrix background while sample dilution had an opposite effect.

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