Determination and Confirmation of Pirlimycin Residue in Bovine Milk and Liver by Liquid Chromatography/Thermospray Mass Spectrometry: Interlaboratory Study

Abstract

Three laboratories participated in trials of methods to determine and confirm pirlimycin residue in bovine milk and liver. The methods used liquid chromatography/thermospray mass spectrometry (LC/MS) with an internal standard to measure residue concentration. The internal standard was isopirlimycin, a stereoisomer of pirlimycin, which was resolved chromatographically. Determinative procedures were validated by replicate analyses of negative control, fortified control, and residue-incurred milk. For the milk method, average corrected recoveries (and coefficients of variation, CVs) were 83-113% (CV, 7.5-15.4%) at 0.2 ppm, 91-98% (CV, 3.4-18.5%) at 0.4 ppm, and 89-102% (CV, 8.8-22.9%) at 0.8 ppm. For the liver method, average corrected recoveries were 94-103% (CV, 2.2-7.1%) at 0.25 ppm, 87-94% (CV, 4.8-10.3%) at 0.5 ppm, and 96-101% (CV, 5.5-6.9%) at 1.0 ppm. There were no interferences in control samples of either matrix. Pirlimycin was confirmed by matching the retention time and relative abundances of 4 ions from sample extracts to corresponding values obtained for pirlimycin standard. Pirlimycin was confirmed in all residue-incurred samples and all samples fortified at regulatory tolerances (0.4 ppm in milk and 0.5 ppm in liver) by 2 of the 3 laboratories and in most samples by the third laboratory.