Determination of tetracycline residues in shrimp and whole milk using liquid chromatography with ultraviolet detection and residue confirmation by mass spectrometry

Two methods have been developed for the simultaneous determination of tetracycline, oxytetracycline, and chlortetracycline in shrimp and in whole milk. These methods were designed to simplify sample extraction and clean-up steps and to be fast and convenient for routine testing in a regulatory environment. Both methods rely on a simple extraction of the shrimp or milk matrix with succinic acid followed by isolation on a copolymeric solid phase extraction column. Chromatographic separation was achieved using a polar end-capped C8 column with an isocratic mobile phase consisting of organic acid, acetonitrile, and methanol, where 0.1% formic acid or 0.01 M oxalic acid was used as the acid. Formic acid allowed direct confirmation of the three residues by liquid chromatography-tandem mass spectrometry (LC-MS-MS). LC with ultraviolet absorbance at 370 nm resulted in the quantitation of all three tetracycline residues from shrimp and milk samples fortified at 50, 100, 200, 300, and 400 ng g−1. Average recoveries were greater than 75% with R.S.D. values less than 10%. All three tetracycline residues were confirmed in shrimp (25-400 ng g−1) and milk (50-300 ng g−1) samples by LC-MS-MS.