Measuring the oxidative stability of oils: Exploring chemiluminescence by 2X0Z9Gt

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									             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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