Vanhaelewyn, G. C. A. M., Sadlo, J., Matthys, P. F. A. E. and Callens, F. J. Comparative X- and Q-Band EPR Study of Radiation-Induced Radicals in Tooth Enamel. Radiat. Res. 158, 615–625 (2002).Human tooth enamel blocks and powders that were either unheated or heated prior to X irradiation at room temperature were investigated by means of Q-band electron paramagnetic resonance (EPR). It was found that the EPR spectra of unheated human tooth enamel consist mainly of two different anisotropic CO2− signals, as was suggested previously from an X-band study of analogous samples. In the present study, the two CO2− radical contributions could be differentiated convincingly by comparing the anisotropic Q-band spectra of heated and unheated enamel blocks. One type of CO2− is probably located in the bulk of the apatitic microcrystallites that constitute the enamel, and it appears in both heated and unheated samples. The other type is presumably located in an intercrystallite position and appears mainly in the unheated samples. Clear differences between g values in the Q-band spectra of heated and unheated enamel suggest that the CO2− radicals in the bulk exhibit larger g anisotropy than those in intercrystallite positions. Isotropic CO2− signals and contributions that may be from CO33− and CO− radicals have also been detected. However, the present work focuses mainly on the CO2− signals and discusses potential and/or real difficulties that may be encountered in applications of EPR dosimetry using calcified tissues.

Upon irradiation human tooth enamel exhibits the well-known “asymmetric EPR singlet near g = 2”. In recent years it has been convincingly demonstrated that the main radical contributing to this spectrum is CO₂ ⁻ although EPR studies in the seventies very often erroneously claimed CO₃ ³⁻ to be the dominant radical. In the meantime several types of CO₃ ³⁻ radicals [carbonate replacing phosphate (B-site) or hydroxy groups (A-site)] have been identified in synthetic apatites prepared at high temperatures or in apatites precipitated from solution and dried at high temperature (typically 400 °C). In the present study human tooth enamel was heated at 400 °C and subsequently irradiated with X-rays at 77 K. It is shown that warming up to about 150 K yields a simple isolated CO₃ ³⁻ spectrum with contributions from exactly the same two radicals identified in synthetic apatites. Under certain conditions strong spectra from phosphate derived radicals are present. The CO₂ ⁻ signal which is dominant after room temperature irradiation of unheated enamel is very weak, however gaining importance after storing the low temperature irradiated sample at room temperature. Also ENDOR results supporting the CO₃ ³⁻ identification are presented.