Analysis for organic residues from aids to polymerization used to make plastics intended for food contact

Determination of silyl peroxides by ultra-performance liquid chromatography/electrospray ionisation mass spectrometry

RATIONALE

Residual initiators in polymers are a concern in the case of products that come directly into contact with the human body or food. Due to low concentrations and difficulties in the sample preparation, highly sensitive and selective methods are required.

METHODS

A series of bis-silyl- and alkyl-silyl peroxides were analysed by electrospray ionisation mass spectrometry (ESI-MS) on an ultra-performance liquid chromatography/time-of-flight (UPLC/TOF) instrument. Li, Na, K, and NH₄ acetates were used to promote the formation of [M + Me]⁺ ions. The sample preparation involved only dissolution of the polymer sample in 0.1 mL of acetonitrile, followed by precipitation with 1 mL of water. A portion of 0.1-1 μL of the solution was then analysed without further treatment by UPLC/ESI-MS.

RESULTS

Limits of detection (LODs) were in the range of 0.06-9 pmol, depending on the peroxide structure. On average, the signal intensity increased with the number of phenyl groups in a peroxide and decreased in the order Na > Li > K > NH₄ . Peroxides that did not contain phenyl groups could not be detected. Collision-induced dissociation experiments can be used for structural investigations of alkyl-silyl peroxides. It was possible to detect 2 × 10^-4 % (LOD = 7 × 10^-5 %) of unreacted Ph₃ SiOOt-Bu in the poly(methyl methacrylate) sample.

CONCLUSIONS

The method is suitable for the analysis of trace peroxide initiators in polymers and for other purposes where LODs in the pmol range are required.

On the Origins of Nitroxide Mediated Polymerization (NMP) and Reversible Addition–Fragmentation Chain Transfer (RAFT)

The early experiments on radical polymerization, which were to lead to a study of nitroxide trapping of the initiation step and the interest in defect groups, particularly the macromonomers formed by termination by disproportionation, are discussed. Results from the nitroxide trapping clearly show that the initiation step ranges from simple clean addition to the head of the monomer, to complex addition/abstraction reactions. Careful selection of the monomer/initiation system is emphasized with particular reference to two common monomers, styrene and methyl methacrylate, and two initiating radicals, t-butoxy and benzoyloxy. The discovery of nitroxide mediated polymerization (NMP) from observations made during the nitroxide trapping work is reported and the ability to have a living radical system demonstrated with numerous examples. Similarly, the study of the copolymerization of macromonomers, formed by disproportionation of the propagating chains, is discussed with the discovery of β-scission and an early form of addition–fragmentation reported. The evolution of reversible addition–fragmentation chain transfer (RAFT) to a highly versatile and commercially attractive radical system is reported and the detailed chemistry behind the discovery of this living radical system discussed. Both NMP and RAFT enable the synthesis of structures not previously possible by radical polymerization and in some cases not possible by any other process.

Identification of 2,3-dimethyl-2,3-diisobutyl succinonitrile in laser printer emissions

2,3-Dimethyl-2,3-diisobutyl succinonitrile was identified as the main volatile organic compound (>90%) emitted from laser printers during the printing process. Experiments were carried out in a large environmental chamber of 30 m3, where the printers were placed and working simulating 'real office setting' conditions. Air samples were taken on Tenax TA adsorbent cartridges in the vicinity of the printers and further analyzed by thermal desorption gas chromatography/mass spectrometry (TDGC/MS). The structure of the compound has been determined and is presented in this study. Additional data obtained by nuclear magnetic resonance (NMR), Fourier transform infrared (FTIR) spectroscopy, and liquid chromatography/tandem mass spectrometry (LC/MS/MS) support the proposed structure, with no reported CAS number, as 2,3-dimethyl-2,3-diisobutyl succinonitrile. It is a byproduct of the thermal decomposition of 2,2'-azobis(2,4-dimethyl valeronitrile), a commercially available free radical polymerization initiator used in polymerization processes during the manufacture of the toners. By means of head-space GC/MS, 15 toners used in black & white and colour printers have been investigated. Six of them contained 2,3-dimethyl-2,3-diisobutyl succinonitrile, which has also been detected in the respective processed paper.

Antioxidant content of and migration from commercial polyethylene, polypropylene, and polyvinyl chloride packages

Antioxidants commonly used in polyolefins were studied in commercial food packages made of low- and high-density polyethylene (LDPE and HDPE), polypropylene (PP), polyvinylchloride (PVC), and polyethylene terephtalate (PET) and in a LDPE film extruded at the laboratory. The phenolic antioxidants BHA, BHT, AO 2246, AO 425, Ethanox 330, Irganox 1010, and Irganox 1076 were studied together with the phosphite Irgafos 168 and their two degradation products, phosphate and DBP. Antioxidants were extracted from polyolefins using microwave energy and analyzed using high-performance liquid chromatography (HPLC) to determine the antioxidant content in the diverse commercial films. Irganox 1010 and Irganox 1076 were found in the majority of the samples generally together with the phosphite Irgafos 168 and its oxidized product (phosphate). Specific migration levels of each antioxidant were determined by HPLC after pretreatment with solid-phase extraction (SPE) in aqueous food simulants and after their dilution with tetrahydrofuran (THF) in fatty food simulant. These levels were much lower than limits allowed by legislation.

Migration from can coatings: Part 1. A size-exclusion chromatographic method for the simultaneous determination of overall migration and migrating substances below 1000 Da

The Council of Europe Resolution on coatings suggests a limit of 10 mg dm-2 for the sum of substances migrating into food simulants from an internal can coating. The Scientific Committee on Food differentiates the migrants into the substances with a molecular weight below 1000 Da, potentially being of toxicological concern, and the less toxicologically relevant species above 1000 Da. Hitherto, the determination of overall migration was based on a gravimetric method. A new method is described for the simultaneous determination of both overall migration and the migration of substances below 1000 Da based on separation by size exclusion chromatography (SEC) followed by ultraviolet detection (UVD) and evaporative light scattering detection (ELSD). The method is suitable for all volatile extraction media and simulants recommended by the European Union. For statistical comparison of both methods, the slightly modified reference method was validated in-house and extended to an additional gravimetric measurement of the migrants below 1000 Da. For the determination of the overall migration, both methods provided similar reproducibility (validated gravimetry: standard deviation (SD) = 0.16 mg dm-2; SEC-ELSD/UVD: SD = 0.12 mg dm-2) but significantly better results were obtained by the SEC-ELSD/UVD method. For migrating substances below 1000 Da, the gravimetric determination provides a poor sensitivity (limit of detection = 0.35 mg dm-2) compared with the SEC-ELSD/UVD method (limit of detection = 0.04 mg dm-2). The new method offers a lower limit of detection and higher precision as well as being less time consuming and easier to use.