EPR and ENDOR of radiation-induced CO33− radicals in human tooth enamel heated at 400 °C

Abstract

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 CO₃³⁻ to be the dominant radical. In the meantime several types of CO₃³⁻ 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 CO₃³⁻ 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 CO₃³⁻ identification are presented.