Measuring the oxidative stability of oils: Exploring chemiluminescence Charlotte Bella, Fabian Käserb, Elaine Martin OBEa, Russell Elliottc a Biopharmaceutical and Bioprocessing Technology Centre, School of Chemical Engineering & Advanced Materials, Merz Court, Newcastle University, NE1 7RU b ACL Instruments, Industriestrasse 11, Postfach 10, CH-3210 Kerzers, Switzerland c GSK, Harmire Road, Barnard Castle, DL12 8DT Measurement of oxidative stability is employed in many areas including: food shelf life prediction, biodiesel analysis, polymer degradation studies and pharmaceutical stability testing. Methods that measure oxidative stability focus on different pathways in the oxidation process and several methods are often used in combination to give a clearer picture of the stability and aging state of a substance. Accelerated stability tests involving elevated temperatures and increased gas flows, do not closely mimic storage conditions, however without these tests, the study of stability would take too long and be unfeasible. It is crucial that the extrapolation of data from accelerated conditions to normal storage conditions is reliable and this is an issue frequently reported on in the literature. (1, 2) Measurement of oxidative stability of biofuels, vegetable and fish oils using the Rancimat (Metrohm, Switzerland) as an accelerated method has been widely reported (2-4) and standard methods, ISO 6886 and EN 14112, exist for this purpose. Induction period - the time taken to the onset of oxidation – is the indicator of oxidative stability in this method and is characterised by a rapid increase in secondary oxidation products. Induction period is determined from the second derivative of the curve, which shows a maximum at the break point, alternatively it can be determined manually with tangents. (5) An alternative accelerated method is chemiluminescence, sometimes also referred to as Oxyluminescence which measures label-free photon emission occurring via the Russell mechanism (6) during the termination stages of oxidation. An excited triplet carbonyl is formed through the combination of radicals in a termination step and on relaxation to ground state, a photon is emitted. The sample can be subjected to elevated temperatures and gas flow and induction period can be determined by tangents set manually by the operator. Sample preparation is minimal, no reagents are required and measurement is continuous, thus the method has advantages over previously reported chemiluminescence methods which require several reagents (chemiluminescent markers and catalysts) and multiple measurements. (7) Within the research, a comparison will be undertaken between chemiluminescence, as measured using the single channel 110 instrument (ACL Instruments, Switzerland) and the Rancimat to determine whether a correlation exists between the induction periods determined by the two methods. The sensitivity of the technique will also be investigated along with the reliability of extrapolating the data towards ‘normal’ conditions. Corn oil will be used as a model substance as its oxidative stability is well documented in the literature. (3, 8) Additionally the full functionality of the single channel 110 chemiluminescence instrument will be explored to determine its potential for application as an accelerated oxidative stability test for pharmaceutical excipients and products. 1. Farhoosh, R., The Effect of Operational Parameters of the Rancimat Method on the Determination of the Oxidative Stability Measures and Shelf-Life Prediction of Soybean Oil, J. Am. Oil Chem. Soc. 84:205-209 (2007). 2. Méndez, E., J. Sanhueza, H. Speisky, and A. Valenuela, Validation of the Rancimat Test for Assessment of the Relative Stability of Fish Oils, J. Am. Oil Chem. Soc. 73:1033- 1037 (1996). 3. Farhoosh, R., R. Niazmand, M. Rezaei, and M. Sarabi, Kinetic Parameter Determination of Vegetable Oil Oxidation Under Rancimat Test Conditions, Eur. J. Lipid Sci. Technol. 110:587-592 (2008). 4. Mateos, R., M. Uceda, M. Aguilera, M. Escuderos, and G. Beltrán Maza, Relationship of Rancimat Method Values at Varying Temperatures for Virgin Olive Oils, Eur. Food Res. Technol. 223:246-252 (2006). 5. 873 Biodiesel Rancimat Manual 8.873.1003, Metrohm Ion Analysis 6. Russell, G.A., Deuterium-isotope Effects in the Autoxidation of Aralkyl Hydrocarbons. Mechanism of the Interaction of Peroxy Radicals, Journal of the American Chemical Society 79:3871-3877 (1957). 7. Mathäus, B., Determination of the Oxidative Stability of Vegetable Oils by Rancimat and Conductivity and Chemiluminescence Measurements, J. Am. Oil Chem. Soc. 73:1039- 1043 (1996). 8. Hasenhuettl, G., and P. Wan, Temperature Effects on the Determination of Oxidative Stability with the Metrohm Rancimat, J. Am. Oil Chem. Soc. 69:525-527 (1992).
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