| A method has been developed to analyze tetracyclines (TCs) using high-performance liquid chromatography with a coulometric electrode array system (HPLC-ECD). The optimal condition for analysis was found through the research of the method of pretreating samples, the condition of the mobile phase and the potentials of electrodes. Under this condition, the analysis of oxytetracycline-HCl tablet, tetracycline-HCl tablet and the residue of TCs in ovine mile was carried out, respectively. The chromatography was performed using a C18 column (150mm×4mm i.d. and 5μm) with a mobile phase: sodium phosphate monobasic dihydrate (pH 2.2, 0.05M)-acetonitrile (78:22,v/v). The flow rate of mobile phase was kept constantly at 1ml/min. Four electrodes in series were used for quantification and the potentials of electrodes were set at 400, 660, 680 and 700mV, respectively. Oxytetracycline-HCl tablet and tetracycline-HCl tablet were dissolved by 0.01M hydrochloric acid and passed through a 0.22μm filter. The samples would be directly injected into the HPLC-ECD system. The milk samples adjusted to pH 4 with 1M hydrochloric acid and were pretreated using liquid-liquid extraction based on acetonitrile and hexane. After passing through a 0.22μm filter, the samples would be injected in the HPLC-ECD system. Under the optimal condition, the qualitative analysis to the pretreated samples was accurate, simple, and economical in cost and time. In quantitative analysis, the calibration curves showed good linearity and relative standard deviations (RSD) were low. Moreover, when this method was used to analyze the residue of TCs in ovine milk, the detection limits were below maximum residue limits (MRLs), which were regulated by some Europe countries and USA. Therefore, this method can be used to routine monitor of milk were more than 88% and the less than 4.3%. This method was reliable, sensitive, economical and suited for routine monitoring of TC residues in ovine dairy milk.
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