Development and Application of LC-MS/MS Method for the Detection of Naturally Occurring Thiouracil in Milk Samples

The liquid chromatography tandem mass spectrometry method to determine five thyreostats in cow’s milk has been developed and validated according to 2002/657/EC criteria. Thyreostats were extracted with methanol from the milk after the precipitation of the protein. Then, the derivatisation of analytes with 3-iodobenzylbromide was performed and derivatives were analysed on a Poroshell C18 column with triple quadrupole MS detection. For all compounds, good recoveries were obtained; the decision limit CCα and the detection capability CCβ of five thyreostats do not exceed 2.2 and 3.8 μg L−1, respectively. Thirty-four samples of milk were tested for the presence of thyreostats; in 13, thiouracil was found above the decision limit. The highest concentration in raw milk reached a value of 4.2 μg L−1. To the authors’ knowledge, this study is the first to report the presence of naturally occurring thiouracil in cow’s milk.