Document kDvqkM8583KZMEbe7jr7kM6Qy

Using 19F Solid State Nuclear Magnetic Resonance Method for the detection of fluorinated compounds in coated paper Authors: Locations: Mike Thijs Patric Bierganns Ernest Latelas Antwerp Technical Center (ATC) Wilmington Research Center (WRC) Date: 14/12/2021 Introduction: In a recent study on PFAS used in disposable food packaging (Strakov, et al., 2021), a value of 65 mg/kg dw was found in the paper sample NL-MCD-01. The Danish Veterinary and Food Administration has introduced an indicator value of 20 mg/kg dw to assess whether organic fluorinated substances have been added intentionally to paper and board (DVFA, 2020). In this method, developed from a derivative of the European standard DIN EN ISO 10304-1 (D20), the Total Organic Fluorine (TOF) content was used as an accepted proxy for the total PFAS content. In the TOF method, the organic fluorine is determined by subtraction of the inorganic fluoride, measured by ion chromatography (IC) of the liquid extract, from the total fluoride (TF) [organic + inorganic]. The TF value is measured by ion chromatography after combustion of the sample, which converts the fluorine to hydrogen fluoride (HF) and absorption of the yielded HF in the IC eluent. The producer of the respective paper has performed additional measurements of various samples of that paper type following the same method (Eurofins, 2018). These measurements yielded results in the range of 40 - 70 mg/kg which, following the indicator value, gives the indication that the samples might contain PFAS above the unintentional background pollution. To identify the source of the unexpected high fluorine levels, the coated and uncoated papers have been measured and the coating was identified as the main constituent for the TOF value. The producer is using an alternative barrier technology to PFAS to get the desired oil/grease and moisture resistance, which is provided by a water-based barrier coating. This barrier technology is developed by Solenis with focus on more environmentally friendly barrier coatings - especially in food and beverage packaging applications - by eliminating the use of plastics like polyethylene/polypropylene, silicones and fluorochemicals. To replace these chemicals, Solenis makes use of bio-based waxes and/or special lamellar talc pigments, like in the coating product in question, TopScreenTM DS 7G. Talc and other natural minerals can contain humite group crystals in which F- anions can replace the (OH)- groups in the talc crystalline structure (L S Pangum, 1998). Because the fluorine is bound in the crystalline structure, it can not be extracted and will be measured in the TOF method as a false positive. Therefore, we want to show in this document that the source of fluorine in the paper sample NL-MCD-01 is of an inorganic, natural origin. Methods and instrumentation: To identify and prove the nature of the fluorine source, we used a 19F solid state nuclear magnetic resonance (NMR) spectrometry instrument. This NMR method is developed with the University of Delaware. It provides 19F detection of fluorine containing component(s) in a solid-state matrix, specifically for Per Fluorinated Alkyl Substances, or PFAs. By this analytical approach, fluorine components are differentiated, e.g., the presence of inorganic 19F components vs. organic 19F components can be identified and classified accordingly in a paper product or in any other solid-state matrix. The solid-state NMR system employs a 4 mm rotor and 4 mm NMR magic angle spinning (MAS) probe. A spinning rate of 15 kHz is implemented. The relaxation delay is 5 seconds, and this is not considered quantitative. These parameters are only deemed appropriate for qualitative detection of fluorine components. No 1H decoupling is necessary. The number of transients is 512. The acquisition time is ~15 ms. As well as it enhances the resolution of solid-state NMR, the MAS may lead to the presence of spinning sidebands. These are spurious signals (i.e. peaks) that result from the modulation of the magnetic field at the spinning frequency. The peaks always appear on either side of any large genuine peak at a separation of integer multiples to the spinning rate. A spin echo sequence is employed to remove background signals from the probe and rotor hardware. The tau (t) delay is predetermined at 10 ms or 0.01 s. The spin echo sequence can be considered quantitative for spin nuclei like 19F given an appropriate spin-lattice relaxation time. 19F relaxation times can vary greatly, sometimes approaching two minutes for small inorganic molecules. Inversion recovery experiments or saturation recovery experiments can be implemented to exactly determine the necessary recycle delay times, thereby providing data on a quantitative basis. All samples for solid state NMR must be a powder form. This can be accomplished using a mortar/pestle or via the use of a SPEX cryogenic grinder. Approximately 100 mg is needed for the solid-state analysis. Here, 3 grams of material was ground into a powder to have a uniform and homogenous sample. The advantage is that you use the sample as is and no extraction is needed. Any fluorine source remains in the sample and can be identified by the specific NMR signal shift. Results and discussion: The raw NMR data is processed using Bruker Topspin software. A mix of various 19F organic and 19F inorganic components is provided on the spectra in Figure 1 below. No fluorine detected (paper) PFAS detected in paper Spectrum of Pure CaF2 Fluorinated talc in NL-MCD-01 paper Fluorinated talc in TopScreen DS 7G Fluorinated talc sample Figure 1. different spectra from 19F NMR measurements. The annotations are color coded on the left. To provide a baseline, a paper sample was measured which was known to contain no traces of fluorinated compounds. Therefore, as can be seen in the black spectrum in figure 1, there are no peaks observed meaning there is no form of fluor present in the paper. For PFAs substances, the terminal CF3-R signal is typically observed between -80 ppm to -90 ppm in the 19F spectrum. The -CF2 repeat moiety is typically observed between -115 ppm to 130 ppm (Dino Camdzic, 2021). One example of a PFAs containing paper sample can be seen in Figure 1 in the yellow signal. Because of the complex matrix of PFAS molecules, different chemical shifts and interferences can be observed, resulting in a complex spectrum. As comparison, the spectra of a fluorine salt like CaF2 gives one distinctive sharp peak (as can be seen in the purple spectra in figure 1). Because salts have a defined crystal structure of the same repeating elements, there is no interference between different chemical shifts, resulting in one resonance peak. For fluorine salts, these peaks can be seen in the range -100 to -120 ppm. The small peaks observed on both sides of the large signal are the spinning sidebands as explained in the `methods and instrumentation'. For the samples at interest - the talc raw material, TopScreen DS 7G and the coated paper sample NL-MCD-01 (colored respectively in blue, red, and green in figure 1) - there are no peaks observed in the region of -80 to -90 ppm or -115 to -130 ppm in the spectra, indicating the absence of PFAs substances. The only peak that is observed is one signal in the range of 175ppm (with corresponding spinning side bands). This single sharp resonance again indicates an environment where the fluorine anion is observed in a single repeated crystal structure, namely the substitution of OH- for F- (Sharon E. Ashbrook, 2016). Conclusion In this work, we have shown that 19F NMR is a tool for total and class-specific analysis of fluorine containing materials. Characteristic chemical shifts for each tested material were determined to be useful for identification as `fingerprinting' against chemical shifts of known materials from the literature. We have qualitatively demonstrated that there are no PFAS molecules or precursors present in the paper sample NL-MCD-01 in question, our coating TopScreen DS 7G or the talc raw material. The use of the TOF method according to DIN EN ISO 10304-1 is a good first indication to screen paper and board samples used in the food packaging industry on fluorine containing compounds. However, Solenis advises against using the method as an accepted proxy, because the method cannot differentiate between organic fluorine and unsubtractable inorganic fluorine, like from a natural mineral source. We therefore hope you will reconsider your statement or add a disclaimer when using natural pigment raw materials. References Dino Camdzic, R. A. D. D. S. A., 2021. Total and class-specific analysis of per- and polyfluoroalkyl substances in environmental samples using nuclear magnetic resonance spectroscopy. Journal of Hazardous Materials Letters, Volume 2, p. 100023. DVFA, 2020. Ban on fluorinated substances in paper and board food contact materials (FCM), Fact sheet, s.l.: Danish Veterinary and Food Administration. Eurofins, 2018. DIN EN ISO/IEC 17025: 2018 DAkkS D-PL-14081-01-00. s.l.:s.n. L S Pangum, e. a., 1998. Mineralogical and metallurgical examination of fluorosilicate mineral flotation in the Ok Tedi circuit. Brisbane, Mine to Mill. Sharon E. Ashbrook, D. M. D., 2016. NMR spectroscopy of minerals and allied materials. Dans: Nuclear Magnetic Resonance: Volume 45. s.l.:s.n., pp. 1-52. Strakov, J., Schneider, J., Cingotti, N. & al., e., 2021. Throwaway Packaging, Forever Chemicals: European wide survey of PFAS. p. 54.