Determination of quinolones in milk samples using a combination of magnetic solid‐phase extraction and capillary electrophoresis

A magnetic solid‐phase extraction ( MSPE ) method combined with capillary electrophoresis for the simultaneous determination of seven quinolones ( QN s) (danofloxacin, ciprofloxacin, marbofloxacin, enrofloxacin, difloxacin, oxolinic acid, and flumequine), using (S)‐(+)‐6‐methoxy‐α‐methyl‐2‐naphthaleneacetic acid as internal standard, in milk samples was developed. The variables involved in the preconcentration magnetic procedure were: the composition of the magnetic support composition, the sample pH, and the weight of magnetic adsorbent used. The variables were optimized using a simplex‐lattice design. Different magnetite covered with octyl‐phenyl silica adsorbents were synthesized by varying the molar ratio of phenyltrimethylsilane and octyltrimethoxysilane; the solids were evaluated for QN preconcentration. Under optimal conditions, a linear range was obtained from 27 to 1000 μg L −1 with limits of detection ranging from 9 to 12 μg L −1 for the seven QN s. The absolute recoveries of the seven QN s at three different spiked levels (40, 150, and 400 μg L −1 ) ranged from 74% to 98% with a relative standard deviation less than 10% in all cases. The proposed method was applied to analyze 20 whole milk samples of different brands. All samples were positive for the presence of QN residues; in some cases, extract dilution was required. The concentrations found are in the range from 31.1 to 5047.3 μg L −1 . Marbofloxacin was the most frequently found. The method proposed offers advantages in terms of simplicity, sensitivity, efficiency, cost, and analysis time making it an alternative for the analysis of QN s in whole milk samples.