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A new and simple HPLC-MS method was developed for monitoring iodinated casein in feed premixes. In this method, feed premixes were hydrolyzed, and the iodotyrosines thus released were analyzed. Sample pretreatment included precipitation of transition metals ions with Na(2)S, hydrolysis with sodium hydroxide and cleaning up with an Oasis SAX cartridge. Gradient elution was carried out on a C(18) column with water (containing 0.1% formic acid)and acetonitrile. Ion detection was performed using ESI positive SIM at m/z 262, 308, 388, and 434. Iodinated casein levels were monitored by qualitative analysis of the iodotyrosines released upon sample hydrolysis and by quantifying the 3,5-diiodotyrosine released. The validation data demonstrated that the method was selective and sensitive (<= 0.2 mg g(-1)) for iodinated casein and had acceptable accuracy (recoveries: 81.3-106.7%) and precision (RSD: 1.7-16.0%). (C) 2009 Elsevier B.V. All rights reserved.