Combining high-resolution mass spectrometry with migration testing to evaluate the suitability of Direct Black 19 in inks designed for food packaging Marco Albertini, Marina Santos Domino Printing Sciences PLC, Cambridge (UK) Polyazo dyes are currently the most widely employed dyes. Their relatively simple and robust synthetic pathway, comprising sequential steps of diazotisation and azo coupling, allows to finely tune the properties of the final product in terms of solubility, colour properties, and stability, among others [1]. Direct Black 19 (CI 35255, chemical structure reported in Figure 1a) is an attractive tetrakis-azo dye candidate for ink-jet formulations, mostly because of its water solubility provided by two sulphonic groups and its outstanding colour performances. In the present work, 18 different batches of Direct Black 19 from 6 different manufacturers have been analysed by uHPLC-HRMS. The chromatographic retention of the anionic species, performed in reverse-phase on a C 18 column, has been made possible using ammonium acetate as ion-pairing agent. The coupled detector, a high- resolution Orbitrap TM mass spectrometer, provided the required resolution in the mass-to-charge domain and the additional possibility of performing MS 2 experiments for univocal assignation of the detected species. This investigation revealed the presence of significant amounts of two unknown compounds. Based on HRMS and tandem mass spectrometry data, the proposed structures were the tris-azo and bis-azo species shown in Figure 1b. Their presence can be associated with the synthetic pathway of the dye itself, with the bis-azo species reported as a starting material in the manufacturing of Direct Black 19. The ratio of the three detected components varies significantly among the analysed samples, with potential adverse implications in terms of colour quality and ease of formulation. Although the physical and chemical properties of Direct Black 19 make it a desirable dye for environmentally friendly water-based ink formulations, its potential use for coding and marking of food packaging requires understanding of the migration behaviour of the dye and related compounds. With this goal in mind, we developed a methodology for the migration analysis of polyazo dyes from printed cardboard into dry foods. For this scenario, Tenax ® was selected as food simulant following the European Printing Ink Association (EuPIA) guidelines [2]. The extraction process and solvent selection were adjusted to maximise the recovery of the three azo dyes under investigation. Migration analysis of the detected polyazo species has been performed according to EuPIA guidelines on a trial formulation containing Direct Black 19. The results excluded the migration of any of the three polyazo species into the food simulant, indicating Direct Black 19 as a possible dye for inks for food packaging applications.
References 1. Benkhaya, S. et al. (2020). Helyon, (6) , e03271. 2. EuPIA Guidance on Migration Test Methods for the evaluation of substances in printing inks and varnishes for food contact materials. 2020 (2 nd amendment)
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