Development and validation of a solid-phase extraction and gas chromatography–tandem mass spectrometry method for the determination of isopropyl-9H-thioxanthen-9-one in carton packaged milk

A simple and efficient method for the determination of isopropyl-9H-thioxanten-9-one (ITX) in different fat content milk samples and baby milk samples stored in packaged cartons was developed and validated. Samples were extracted using solid-phase extraction (SPE) and analysed by gas chromatography–tandem mass spectrometry operated in selected reaction monitoring mode (SRM). Validation was carried out in terms of limit of detection (LOD), limit of quantitation (LOQ), linearity, precision and trueness. LOD and LOQ values in the low μg/L were achieved, whereas linearity was established within 0.5–500 μg/L range. Good precision was obtained both in terms of intra-day repeatability and inter-day precision on two concentration levels (RSD% lower than 2%). Good percentage recoveries were obtained (92.0–102.0%) even in the presence of high amount of fat. Finally, the developed method was successfully applied to analyse a number of commercial milk samples with different fat content and baby milk samples.