On November 5 2013 the peer-reviewed scientific journal Polymer Degradation and Stability published  online the article on “Comprehensive two-dimensional gas chromatography for determining the effect of electron beam treatment of polypropylene used for food packaging” by Maurus Biedermann and colleagues from and the Official Food Control Authority of the Canton of Zürich, Switzerland and the French National Institute for Agricultural Research (INRA). The researchers compared extracts of virgin and electron beam (EB) treated polypropylene (PP) films via a comprehensive two-dimensional gas chromatography (GC x GC) setup. Identification of chemicals was carried out in GC coupled with flame ionization detection and time of flight (TOF) mass-spectrometry (MS). The scientists found the EB treatment to convert additives and polymers into numerous other reaction products and impurities (ORPI), also called non-intentionally added substances (NIAS). Irradiation by means of EB is applied to decontaminate food contact materials (FCM), pharmaceuticals and medical devices. The impact of high energy electrons alters the surface of treated materials: Ionization and formation of radicals leads through fragmentation to smaller molecules, and by crosslinking and condensation to larger NIAS’. All constituents of FCMs including the newly formed by EB treatment are covered by the basic legal requirements for human health safety (EU Regulation 1935/2004 Art. 3 and 10/2011). Hence, health relevant amounts of newly formed molecules must by obviated. But a comprehensive quantitative evaluation remains a challenge for chemical analytics. Available chemical analytics tools can detect small amounts of known substances, but not all substances (including the unknown, newly formed reaction products) at a low detection limit. The authors investigated the performance of the GC x GC analysis in detecting migrating chemicals in PP films. The setup consisted of two sequential GC columns coupled by a modulator, which cryofocuses the output form the first column for 6 seconds and then subsequently loads the second column. Biedermann and colleagues value this method for its outstanding separation power and systematic display of chemicals in the plots, thus easing chemical identification.

The scientist tested PP films produced from granulate containing different additives (Irgafos 168 (CAS 31570-04-4), Tinuvin 326 (CAS 85497-36-5), Irganox 1076 (CAS 2082-79-3), Irganox 1010 (CAS 6683-19-8)) next to samples of commercial unprinted oriented polypropylene (OPP) films. The PP film samples were exposed to either a radiation of 40 or 100 kGy, or left unexposed. Extraction for the chemical analysis was carried out for small pieces of 200 mg film immersed in 5 ml hexane for 24 h. The completeness of the extraction was further verified.

Besides polyolefin oligomeric saturated hydrocarbons (POSH) untreated film contained mainly impurities from additives as Irgafos 168 and Irganox 1010 as well as their oxidative breakdown products. Comparing GC x GC-MS plots revealed that EB treatment leads to new spots in the plots. Two new hydrocarbons, n-pentadecane (CAS 629-62-9) and n-heptadecane (CAS 629-78-7), were found. Ketones, degradation products from polymers, were only found in additive-free film implying that Irgafos 168 suppressed their formation in the rest of the samples. Most chemicals formed by radiation stem from the breakdown of the additives. 1,3-di-tert-butylbenzene (CAS 1014-60-4) was referred to as a principle degradation product. The concentrations of 1,3-di-tert-butylbenzene and phenol (CAS 108-95-2) sharply increased following the irradiation treatment. Tetra-tert-butyldiphenylether and tetra-tert-butyl-o,o’-biphenol were said to be radical reactions products from cleavage reactions of Irgafos 168. Also mass equivalents to oxidized Irgafos 168 were found, although potentially of different structures. Many batches formed through EB treatment in the plots which could not be identified. Moreover, many of the peaks found represent concentrations above 1 mg/kg film and can thereby exceed the allowed threshold. Treatments with higher energies (100 kGy) did not markedly alter the picture. The researchers found further breakdown products of Irgafos 168 and fewer oxidation products. Some chemicals as 2,4-di-tert-butylphenol (CAS 96-76-4) were found at higher intensities.  The untreated film sample with 1.5% Irganox 1076 contained 1-octadecanol (CAS 112-92-5), 7,9-di-tert-butyl-1-oxaspiro(4,5)deca-6,9-diene-2,8-dione (CAS 82304-66-3), octadecanal (CAS  638-66-4), octadecyl acetate (CAS 822-23-1) and potentially octadecyl propionate (CAS 2082-79-3). The radiated sample of the same film contained the newly generated products 2,6-di-tert-butylphenol (CAS 88-26-6) and 2,6-di-tert-butyl-4-ethylphenol (CAS 128-37-0), both exceeding concentrations of 1 mg/kg film. In a further sample containing 1.5% Irganox 1010 Biedermann and colleagues found the impurity 7,9-di-tert-butyl-1-oxaspiro(4,5)deca-6,9-diene-2,8-dione (CAS 82304-66-3) under untreated and irradiation conditions.  As for the 1.5 % Irgafos 168 sample 1,3-di-tertbutylbenzene (CAS 1012-72-2) and 2,6-di-tert-butylphenol (CAS 128-39-2) were formed by irradiation. A reference sample of commercial unprinted OPP film of mostly unknown composition was tested displaying in the untreated sample erucamide (CAS 112-84-5), stearamide (CAS 37189-35-8), 11-eicosenamide (CAS 10436-08-5), tetracosenoyl amide, 2-glyceryl monostearate (CAS 31566-31-1) and 2-glyceryl monostearate (CAS 31566-31-1). Additionally, Irgafos 168, its oxidation product and Irganox 1076 were observed. In the irradiated sample cleavage products of fatty acids and amides were found: n-C15, n-C17, 1-hexadecene (CAS 629-73-2), 1-octadecene (CAS 112-88-9), octadecadiene and 10-heneicosene (CAS 19781-72-7). Additionally, for vinyl palmitate (CAS 693-38-9) and vinyl stearate (CAS 111-63-7), N,N-dimethyl-n-hexadecylamine (CAS 112-69-6) and N,N-dimethyl-n-octadecylamine (CAS 124-28-7) corresponding mass spectra were found.

Concluding, the authors consider the applied comprehensive two-dimensional GC with FID for quantitative estimations and MS for identification as a powerful analytic tool to detect potential migrants from polyolefins, as in the present case study using PP. The clear separation of hydrocarbons from more polar chemicals bypasses an otherwise necessary pre-separation. Further advantages are the high resolution and the systematic presentation of results easing the interpretation. Limiting is, as for conventional GC, that not all chemicals up the toxicologically relevant mass of 1000 Da can be screened (the cut-off lays within a span of 800 – 1000 Da), polar substances must be derivatized to pass the GC column, and, finally, the method is unsuitable for thermally unstable chemicals and its sensitivity to overload by chemicals present at higher concentration. Hence, a comprehensive evaluation cannot be guaranteed. Comparing GC x GC to alternative analytic methods, the authors emphasize the simple applicability in a single analytic step.

The researchers demonstrated both the potential and the necessity of the presented method. They found that PP films contain large numbers of chemicals migrating into food stuff above the threshold of toxicological concern (TTC) and that the EB treatment further augments the number of chemicals in the migration extract. It must be shown that there are no chemicals among the migrants that put human health at risk at the levels of actual migration, as it is required by Article 3 of EU regulation 1935/2004, and in EU regulation 10/2011. In summary, Biedermann and colleagues state that additives help to considerably reduce the countless chemicals resulting from the EB treatment and protect the PP film. However, the breakdown of the additives themselves form many new compounds. Furthermore, it was demonstrated that the commercial films tested display a great variability urging their separate analysis.

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Biedermann, M. et al. (2013). “Comprehensive two-dimensional gas chromatography for determining the effect of electron beam treatment of polypropylene used for food packaging”. Polymer Degradation and Stability (published online November 5, 2013)