Liquid chromatography/electrospray tandem mass spectrometric analysis of incurred ractopamine residues in livestock tissues.

Abstract

Ractopamine HCl is a beta-adrenergic agonist (beta-agonist) recently approved by the U.S. Food and Drug Administration, but not other governmental agencies, for use in finishing swine. For these reasons, it was important to develop and validate mass spectrometric methods for the detection and confirmation of ractopamine residues in livestock marker tissues. Incurred tissues in cattle, sheep, turkeys, and ducks were generated during 7-day ractopamine feeding (20 ppm in diets) periods. Disposition of ractopamine residues in liver and pigmented retinal epithelium was determined in animals slaughtered with withdrawal periods of 0, 3, and 7 days. Ractopamine residues, purified using solid-phase extraction, were measured using liquid chromatography (LC) and electrospray with detection by tandem mass spectrometry (MS/MS) in the multiple reaction-monitoring (MRM) mode. Total ractopamine residues (parent ractopamine + hydrolyzed conjugates) in liver were detected in all species on withdrawal day 0 (2-97 ppb) and were greatly diminished in all species by withdrawal day 7 (<1 ppb). Bovine and ovine retina had lower levels of ractopamine (0.5-3 ppb) than liver, and occular residues increased with withdrawal time, suggesting redistribution into this tissue. Lower limits of quantification were found to be approximately 0.1 ppb in liver and retina. Incurred ractopamine residues were confirmed by the precise and accurate agreement of MRM intensity ratios of diagnostic fragment ions (m/z 284, 164, and 121) from the protonated molecule between ractopamine residues in incurred samples and an authentic ractopamine standard. The limits of confirmation in liver and retina using recognized acceptance criteria were below 1 ppb. The high sensitivity and specificity for measurement and confirmation of ractopamine residues suggests this method will be applicable for regulatory residue surveillance programs.