Assay of florfenicol in swine feed: interlaboratory study

Nuflor (florfenicol) Premix for Swine was recently approved by the U.S. Food and Drug Administration (FDA) for control of swine respiratory disease (SRD). A simple method for the assay of florfenicol in Type C medicated swine feeds was recently evaluated as part of a 4-laboratory study. Florfenicol is extracted from ground feed with acetonitrile-water by shaking and sonication. An Envi-Carb solid-phase extraction cartridge is used to clean up the extract, retaining matrix interferences while allowing florfenicol to elute. The collected eluent is diluted and injected into a reversed-phase liquid chromatographic system. Samples are quantitated by external standard analysis versus multilevel calibration solutions. The procedure is suitable for the quantification of swine feeds in mash or pellet form medicated with 100-300 mg/kg florfenicol. The interlaboratory study was conducted according to Guidance 136 issued by the FDA Center for Veterinary Medicine. The feeds used to evaluate method performance represented different feed compositions (starter and finisher) and manufacturers. The sponsor and 3 independent laboratories obtained mean recoveries (+/-SD) from fortified swine feeds of 100.7% (+/-2.0%), 99.6% (+/-2.8%), 98.8% (+/-1.4%), and 99.3% (+/-1.7%), respectively. Excellent agreement of the results of the assay of blind samples of commercial swine mash and pelleted feeds between laboratories demonstrates that the method is rugged and reproducible.