Identification of print-related contaminants in food packaging

Since the UV ink photoinitiator (PI) isopropylthioxanthone (ITX) was discovered in packaged milk, studies of print contamination have focused primarily on PIs but have also included amine synergists. Many other substances are used or formed during the print process, yet their identity and set-off properties have yet to be catalogued in food packaging. Three different techniques: direct analysis in real-time high-resolution mass spectrometry (DART-HRMS), gas chromatography-mass spectrometry (GC-MS) and ultra-high-pressure liquid chromatography electrospray ionisation/HRMS (UHPLC/ESI-HRMS) were used to detect and identify print-related molecules from the food-contact and print surfaces of three different packages with under-cured prints. This approach tentatively identified or confirmed 110 compounds, including 35 print-related molecules. The majority of compounds identified on food-contact surfaces were packaging monomers/byproducts, solvents/plasticisers, antioxidants/degradants or slip agents/lubricants. Of these, 28 showed evidence of set-off. The identities of 16 PIs, seven known scission products and five probable PI degradants were confirmed, most showing signs of set-off. Of the print-related molecules, at least five are novel print contaminants such as 4-morpholin-4-yl-benzaldehyde or 3-phenyl-2-benzofuran-1(3H)-one.