SCREENING AND CONFIRMATORY DETERMINATION OF RACTOPAMINE RESIDUES IN CALVES TREATED WITH GROWTH-PROMOTING DOSES OF THE BETA-AGONIST

Ractopamine (RCT) is a phenethanolamine member of the family of beta-a drenergic agonists (beta-agonists), This class of compounds have becom e notable for their properties of enhancing the growth rates of farm a nimal species but are not licensed for use in Europe. An ELISA procedu re employing a polyclonal antibody raised in a goat was developed to d etect RCT residues in bovine urine samples, The assay had a high sensi tivity (calibration curve mid-point of 22 pg per well), allowing the a nalysis of urine samples without the need for sample clean-up. In addi tion, an LC-MS-MS confirmatory procedure was developed which was able to act as a confirmatory procedure for the ELISA results. Four calves were orally treated with RCT (0.1 mg kg(-1) body mass for 17 d) and ur ine samples collected were assayed by both analytical procedures. It w as observed that RCT residues were excreted mainly in the form of gluc uronides and deconjugation could be achieved using two different sourc es of the enzyme beta-glucuronidase (Helix pomatia and Escherichia col i), High concentrations of RCT residues were found throughout the medi cation period (44-473 ng ml(-1); LC-MS-MS data) and remained present f or several days following removal of the drug from the diet, RCT resid ues were no longer detectable 2 weeks after withdrawal, Good agreement (r(2) = 0.73) was achieved between the ELISA and LC-MS-MS results, es pecially when sample deconjugation was applied to the urine samples fo r both sets of analyses, The results show that an effective screening and confirmatory system was devised to detect RCT residues in urine sa mples taken during treatment and close to withdrawal, However, alterna tive matrices may have to be selected to allow the illegal use of the substance to be detected following prolonged withdrawal times.