Determination of isopropyl-9H-thioxanthen-9-one in packaged beverages by solid-phase extraction clean-up and liquid chromatography with tandem mass spectrometry detection

A simple method was developed and validated for the trace determination of 2-isopropylthioxanthone (ITX) in packaged drinks. Samples were extracted from the food matrix using acetonitrile:water (60:40, v/v), and further subjected to clean-up and preconcentration using solid-phase extraction prior to analysis by liquid chromatography–tandem mass spectrometry using multiple reaction monitoring (MRM) mode. The use of 2-isopropyl-[2H7]thioxanthen-9-one was incorporated into the method as an internal standard. Excellent 3-day interday precision data (RSD 0.72%, n = 10), and intraday precision data (RSD 0.52%, n = 10) were obtained on a 0.10 μg/L standard solution. Spiked samples (n = 8) were used to gauge the accuracy of the method at the concentration levels of 2.5, 100, and 500 μg/kg in food; recoveries ranged from 97.0 to 103.0%. These excellent validation data suggest the exciting possibility of using this method for the determination of low levels of ITX migrating from printed food packaging materials into beverages with a method quantitation limit of 0.50 μg/kg. For the first time, analysis on a range of milk, juice, tea and yoghurt drinks, as well as their respective food packaging materials were performed for comparative studies on their ITX content.