The sample extraction method involves the use of homogenised beef tissue (5g), the addition of acetonitrile:methanol, centrifugation, evaporation and reconstitution (>40 samples can be prepared in just over 2 hours). The samples are now ready for application to the microtitre plates or the biochip platform. For the determination of ractopamine and zilpaterol, competitive immunoassays were employed. In the application to ELISA, two ELISA kits were used following manufacturer’s instructions (RT3451, ZP3483, Randox Laboratories Limited, Crumlin, UK). Measurement of the optical density was carried out at 450nm.