In-situ imaging of chemical contaminants in recycled paper and board
Résumé
High levels of contamination by mineral oil hydrocarbons (MOH) from recycled materials and from printing inks have been frequently reported in the literature [1]. These contaminations are of high concern because they are associated to possibly carcinogenic and disrupting substances. To prevent MOH from entering in the recycling loop, the French decree 2020-1725 of the new French anti-waste law N°2020-105 will restrict severely the occurrence of MOAH (aromatic MOH) and MOSH (saturated MOH) in all packaging, printing for the public including leaflets, catalogues, etc. As reviewed in [2], the limits of detection and quantification in real food are usually far above the maximum acceptable level in food. LC-GC-FID gold standard for MOSH-MOAH analysis requires a prior complete extraction and possibly a sample cleanup before analysis. This process is known to generate potential false-positive due to the presence of natural interfering compounds present in the matrix. The critical study [3] suggests a more root-cause analysis to confirm MOH contamination. In this perspective, direct chemical imaging on solid samples with preparation may resolve many of the complications met with chromatographic techniques: high dilution, nonspecific detection, complex mixture of compounds, time-consuming experiments requiring complex setup and calibration. The collaborative project FoodSafeBioPack (ref. ANR-20-CE21-0009, 2020-2024), funded by the French National Research Agency, explores the potentialities of confocal chemical imaging to measure local concentrations and identify contamination pathways through cellulosic materials and foods. The vibrational signature of unknown substances is collected either by fluorescence emission or inelastic scattering (Raman) with the help of an imaging spectrometer or a focal-plane array. The signal of interest is separated from the background (e.g., autofluorescence) by choosing laser lines (e.g., 780 or 785 nm) that interact little with the cellulosic support. Additionally, polarization spectroscopy allows us separating molecules adsorbed or crystallized on surfaces. The 3D spatial organization of MOH contaminants will be presented in real and model cellulosic materials (blotting and Kraft papers) as well as in dry and fatty foods.