Determination of tetracycline residues in bovine milk, serum, and urine by capillary electrophoresis.

A method was developed for simultaneous determination of oxytetracycline (OTC), chlortetracycline, tetracycline, and doxycycline levels in cow milk, serum, and urine by capillary electrophoresis (CE). The tetracyclines (TCs) were extracted specifically with a metal-chelating affinity column. Extracts were then applied to C18 cartridges treated with dimethyldichlorosilane, and the TCs were eluted with ethanol. Salt-free sample residues were run on CE with a diode array detector at 370 nm. A 50 cm x 75 micron uncoated capillary at 15 kV and 23 degrees C was used to separate the TCs. The run buffer contained 10 mM sodium dodecyl sulfate, 50 mM borate, and 50 mM phosphate, pH 8.5. TC peaks were identified by migration times, coinjection of standards, and diode array spectra. Limits of detection for TCs were 1.3-5.6 ng/mL, and limits of quantitation were 1.7-8.7 ng/mL in cow fluid samples. OTC appeared consistently at 13.3-13.4 min in milk, serum, and urine samples from a cow treated with OTC. OTC was further identified by coinjection of OTC standard and OTC diode array spectra. The CE method can be used to simultaneously determine 4 TC residues at low parts-per-billion levels.