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Some Cautionary Notes on ESR and ENDOR Measurements in Coal Research

Volume 36, Number 2 (April 1982) Page 187-189

Retcofsky, H.L.; Hough, M.R.; Maguire, M.M.; Clarkson, R.B.

The general nature of the free radicals in coals has been deduced via electron spin resonance (ESR) and recent reports suggest that electron nuclear double resonance (ENDOR) spectroscopy may provide additional insight into the nature of these radicals. Deductions based on spectroscopic measurements, however, require that the data be quantitatively reliable. The purpose of the present article is to alert analysts and coal researchers to possible pitfalls which may adversely affect the validity of such data. In particular, recent investigations in this laboratory have shown that (1) failure to dilute samples of high rank coals and heat-treated coals with a nonconducting medium can result in significant errors in both ESR intensities and g values, and (2) hyperfine lines in the ENDOR spectra of coals may be broadened beyond detection if the measurements are made on unevacuated samples. The objective for conducting the research was not only to ensure the validity of ESR and ENDOR data obtained for coals and treated coals in our laboratories, but also to provide the necessary experimental background for planned investigations involving the rapid heating of coals directly in the cavity of an ESR spectrometer.