The effect of carbonate content and drying temperature on the ESR-spectrum near g=2 of carbonated calciumapatites synthesized from aqueous media

Summary

The ESR spectrum of X-irradiated carbonated apatites precipitated from aqueous solutions was studied as a function of their carbonate content and drying temperature. When the latter increases from 25 to 400°C, the ESR spectrum is gradually modified and becomes similar to the spectrum of carbonated apatites, synthesized at high temperatures by solid state reactions. The latter ESR spectrum is dominated by CO ³⁻₃ -contributions whereas the spectrum of precipitated samples dried at 25°C can mainly be interpreted in terms of CO ⁻₂ , CO ⁻₃ , and O⁻ ions. The behavior of these earlierreported CO ⁻₂ , CO ⁻₃ , and O⁻ centers is now studied as a function of drying temperature. In addition, the Spin Hamiltonian parameters of the CO ³⁻₃ centers are determined and some other new paramagnetic radicals are discussed. It is shown that a CO ²⁻₃ ion at a phosphate lattice site (B-type substitution) may give rise to either a CO ⁻₂ , CO ⁻₃ , or CO ³⁻₃ radical on X-irradiation, depending on the sample preparation conditions. A surface CO ²⁻₃ ion may cause a surface CO ⁻₂ , CO ⁻₃ , or O⁻ radical. From the reported results it is not unambiguously clear whether the CO ³⁻₃ ion detected in the samples with the relatively lowest carbonate content should be located on the surface or on a hydroxyl lattice site (A-type substitution). An important result is that the absolute concentration of the B-type CO ³⁻₃ ion increases with increasing carbonate content as was also the case for the earlier reported B-type radicals (isotropic CO ⁻₂ and CO ⁻₃ ). On the other hand, the absolute concentration of the surface radicals decreases with increasing carbonate content. The reported results show that similar deconvolution techniques can be applied in the future for the study of ESR spectra of calcified tissues. This will allow a more efficient phenomenological investigation of the latter.