Free radicals in oil-derived samples: A quantitative analysis.

Free radicals have a major impact on the chemistry of several chemical processes including polymers synthesis, petroleum refining, biomass upgrading and others. In oil industries, for instance, non-catalytic thermal cracking reactions involve a free radical chemistry. Control over such free radical reactions is possible by determining the free radical concentration in solution. Unfortunately, there are limited studies on free radical quantification in chemical and petroleum processes as compared to the large body of literature in biological systems. There are several challenges faced during quantification as referred to in the book by Eaton et al. [1] and the literature does not seem to be clear on a specific procedure to follow.
In the current work, Electron Spin Resonance (ESR) spectroscopy was used to compare different calibration standards, and a new quantitative measurement of persistent free radical concentration was suggested. The quantified persistent free radical content of petroleum-derived samples was interpreted in terms of factors affecting the free radical concentration. The factors include experimental-associated errors, temperature, dilution, nature of sample and sample history.
It was observed that heavier samples contain larger amounts of free radicals. Independently of temperature, it was found that the free radical concentration depends on the matrix in which free radicals are present. For petroleum-derived samples that inherently contain paramagnetic species, the history of the petroleum fraction affects the free radical concentration. Following these observations, a mechanism of the behavior of free radicals in solution was elucidated. The free radical concentration was explained as a dynamic reaction equilibrium analogous to the ionic dissociation of water rather than the longevity of any individual radical species. The persistence was, thus, described in terms of the relative rates of the association and dissociation reactions of the radical pairs.
References:
[1] Eaton, G. R.; Eaton, S. S.; Barr, D. P.; Weber, R. T. Quantitative EPR; Springer Wien New York, 2010.