On-line preconcentration and determination of tetracycline residues in milk using solid-phase extraction in conjunction with flow injection spectrophotometry

A simple, cheap and highly sensitive system with on-line preconcentration using solid- phase extraction in conjunction with flow injection spectrophotometry for the determination of tetracycline residues in milk samples is described. C18 was used as packing material in a designed minicolumn used for preconcentration of tetracyclines. Tetracycline standard or sample solutions were dissolved in a mixed buffer solution of pH 4.0 containing boric acid, citric acid and sodium phosphate, then loaded to the minicolumn for 6 min followed by elution with a solution containing methanol : mixed buffer solution (40:60 by volume) of pH 6.5 The absorbance of the eluate was measured at 370 nm. The calibration graph was linear in the range of 0.20-1.00, 0.20-4.00, and 0.20-1.00 mg L -1 for tetracycline (TC), oxytetracycline (OTC), and chlortetracycline (CTC) respectively. The limits of detection were 0.08, 0.10, and 0.09mg L -1 for TC, OTC, and CTC respectively. Relative standard deviations for 20 replicated determinations of 0.20, 0.40, and 0.60 mg L -1 of TC were 7.03, 7.23, and 6.55 % respectively. Per cent recoveries for four commercial types of milk: U.H.T., pasteurised, raw, and sterilised milk were in the range of 86-109 (TC), 90-109 (OTC), and 89-108 (CTC). The sample throughput was 6 h -1 .