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2013|14 Annual Report Fraunhofer IGB

76 ELECTRON SPIN RESONANCE (ESR) FOR MEASURING RADICALS IN IRRADIATED FOODS AND MEDICAL DEVICES Dr. rer. nat. Michael Haupt Sterilization with gamma rays Gamma sterilization is increasingly being used to preserve foods or to sterilize heat-sensitive pharmaceutical products. For drugs and medical devices, the World Health Organization (WHO) explicitly recommends this sterilization, which involves products being irradiated with high-energy gamma rays from a cobalt-60 radiation source (Fig. 1). As a result of the irradia- tion, the genomes of germs and pathogenic microorganisms are destroyed and the organisms killed, which has the effect of considerably extending the shelf life of the product. Com- pared with sterilization with ethylene oxide or steam steriliza- tion, the treatment is very gentle. A further advantage of the treatment with gamma rays is, that products can be sterilized in their packaging – without any appreciable temperature in- crease or the use of chemicals. Disadvantage: radicals are formed Through the irradiation, however, chemical bonds in the products themselves are also broken which results in the gen- eration of free radicals. Radicals are atoms with an unpaired electron and are highly reactive, which means that they can react in an uncontrolled manner with their environment. As a consequence, it is possible for new, in some cases toxic, com- pounds to be formed, which may later cause adverse effects when the product, e.g. a drug, is used. The latest research results at the Fraunhofer IGB show, that the radicals created through gamma sterilization can be ex- tremely stable. Fig. 2 shows how the quantity of radicals in a gamma-irradiated antibiotic decays over time. Even several hours after exposing the product to gamma rays, most of the radicals can still be detected. In addition, continuous measure- ments show that even after weeks, the quantity of radicals no longer decays significantly. The knowledge as to whether and how many radicals have been created through the sterilization processes has made it possible to come below the threshold values for toxic compounds. Measurement principle of Electron Spin Resonance spectroscopy At the Fraunhofer IGB we have been using ESR spectroscopy for a long time for the detection of radicals, for example in order to detect the decay curves for radical density on the sur- faces of materials following plasma treatment. Because of unpaired electrons, radicals demonstrate quantum mechanical spin which in turn is linked to a magnetic mo- ment. Electron Spin Resonance spectroscopy (ESR spectrosco- py) benefits from this: By applying a directed magnetic field to a sample that contains radicals, the energy levels of unpaired electrons are split (Zeeman effect). If the sample is exposed to microwave radiation whose quantum energy corresponds to the Zeeman splitting, resonant absorption takes place. Us- ing sensitive microwave absorption measurements, the spin number, number of radicals and also the type of radical can be determined. 1 MEDICINE

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