LIQUID-CHROMATOGRAPHIC DETERMINATION OF IVERMECTIN IN FEED AT THE 2-PPM LEVEL - INTERLABORATORY STUDY

An analytical method for the determination of ivermectin in feed at th e 2 ppm level was evaluated in an interlaboratory study involving 4 la boratories. The method is based on liquid chromatographic measurement following sample preparation by adsorption chromatography on alumina a nd solid-phase extraction techniques. Each laboratory analyzed 2 compl ete, final, finished medicated feeds and the corresponding control fee ds used in their preparation. In addition, they also measured recoveri es in fortified feeds covering 50-150% of the 2 ppm ivermectin use con centration. The mean recoveries from the replicate analyses reported b y the laboratories ranged from 90 to 110% and all coefficients of vari ation (CVs) were less than 4.5%. No significant interferences were fou nd in the control feeds. The pooled distribution of all the individual analytical results (n = 100) gave a mean recovery of 100%, a recovery range of 90-111%, and an overall CV of 5.5%. Resolution of the total variance into its 2 components gave a within-laboratory CV of 4.1% and a between-laboratories CV of 3.4%. There was no significant differenc e in recoveries among laboratories, days, concentrations, and feed bas e or between fortified and medicated feeds (P > 0.2).