Comparative method validation for closantel determination in cattle and sheep milk according to European Union Volume 8 and Veterinary International Conference on Harmonization guidelines

• LC–MS/MS method for quantitative determination of closantel in bovine and ovine milk.
• Optimized extraction and clean-up using Oasis® MAX® SPE columns.
• Method validation of bovine milk according to EU Volume 8 (2005).
• Method validation of ovine milk according to EU Volume 8 (2005) and VICH GL49 (2012).
• Although both guidelines cover similar parameters, significant differences exist.

A specific and sensitive LC–MS/MS method was developed for quantitative determination of closantel in bovine and ovine colostrum and tank milk. Sample preparation consisted of extracting milk samples with acetonitrile/acetone (80/20, v/v) followed by SPE clean-up with Oasis® mixed anion exchange columns. After elution with 5% formic acid in acetonitrile and evaporation to dryness, the residue was reconstituted in acetonitrile and water. HPLC separation was achieved on a Zorbax® Eclipse Plus C18 column and a gradient elution program with 1 mM ammonium acetate in water and acetonitrile. For closantel determination in bovine milk, the method was validated according to EU Volume 8 guidelines whereas for ovine milk both EU Volume 8 and VICH GL49 criteria were applied. The linear range of the method is between 10 and 2000 μg/kg, the limit of quantification 10 μg/kg and limit of detection is 0.63 and 0.32 μg/kg for sheep colostrum and tank milk and 1.27 and 1.24 μg/kg for cattle colostrum and tank milk, respectively. Both guidelines cover a similar set of parameters (linearity, accuracy, precision, limit of detection and limit of quantification), although the acceptance criteria might differ (accuracy and precision) or no specific acceptability ranges are specified in neither guidelines (LOD and LOQ). For some parameters, only one of the guidelines indicates acceptance criteria: EU Volume 8 for applicability, practicability and susceptibility and VICH GL 49 for linearity, specificity and analyte stability.