Detection and Confirmation of Ractopamine and Its Metabolites in Horse Urine After Paylean® Administration

We have investigated the detection, confirmation, and metabolism of the beta-adrenergic agonist ractopamine administered as Paylean to the horse. A Testing Components Corporation enzymelinked imunosorbent assay (ELISA) kit for ractopamine displayed linear response between 1.0 and 100 ng/mt with an 1-50 of 10 ng/mL and an effective screening limit of detection of 50 ng/mL. The kit was readily able to detect ractopamine equivalents in unhydrolyzed urine up to 24 h following a 300-rag oral dose. Gas chromatography-mass spectrometry (GC-MS) confirmation comprised glucuronidase treatment, solid-phase extraction, and trimethylsilyl derivatization, with selected-ion monitoring of ractopamine-tris(trimethylsilane)