Study of radiation-induced stable radicals in synthetic octacalcium phosphate by pulsed EPR

Created at room temperature by X-ray irradiation at the dose of about 5 kGy radiation-induced centers in stable octacalcium phosphate (OCP) powders obtained by double transformation of α-tricalcium phosphate (TCP) are studied by means of pulsed electron paramagnetic resonance (EPR). No EPR signals within the sensitivity of the used equipment in the non-irradiated samples were detected. In the irradiated species complex EPR spectrum signals appeared. Three types of the paramagnetic centers are decomposed in the investigated samples. Their spectroscopic parameters (components of g-factors and hyperfine constants) are defined. Based on the extracted parameters two of them are ascribed to CO2− and NO32− radicals while the nature of the third one is still questionable. The spectroscopic parameters of NO32− stable radical is found to be differ than these for TCP or hydroxyapatite. The obtained results could be used for the tracing of the mineralization processes from its initiation to the completion of the final product and for the identification of the OCP phase.