Solid-phase microextraction followed by gas chromatography–mass spectrometry for the determination of ink photo-initiators in packed milk

A novel, single step method for the determination of seven ink photo-initiators in carton packed milk samples is described. Solid-phase microextraction (SPME) and gas chromatography (GC), combined with mass spectrometry (MS), were used as sample preparation and determination techniques, respectively. Parameters affecting the performance of the microextraction process were thoroughly evaluated using uni- and multivariate optimization strategies, based on the use of experimental factorial designs. The coating of the SPME fibre, together with the sampling mode and the temperature were the factors playing a major influence on the efficiency of the extraction. Under final conditions, 1.5 mL of milk and 8.5 mL of ultrapure water were poured in a glass vessel, which was closed and immersed in a water boiling bath. A poly(dimethylsiloxane)-divinylbenzene (PDMS–DVB) coated fibre was exposed directly to the diluted sample for 40 min. After that, the fibre was desorbed in the injector of the GC–MS system for 3 min. The optimized method provided limits of quantification (LOQs) between 0.2 and 1 μg L−1 and a good linearity in the range between 1 and 250 μg L−1. The inter-day precision remained below 15% for all compounds in spiked whole milk. The efficiency of the extraction changed for whole, semi-skimmed and skimmed milk; however, no differences were noticed among the relative recoveries achieved for milk samples, from different brands, with the same fat content.