Synthesis of carbonated hydroxyapatites: Efficiency of the substitution and critical evaluation of analytical methods

This work was aimed at evaluating the efficiency of carbonate substitution in carbonated hydroxyapatites appropriately synthesized by a modified version of old hydrothermal methods, to obtain an increase of carbonate inside the lattice. The efficiency of carbonate substitution (linearly increasing until about 10% in weight) was determined by carbon elemental analysis, IR spectroscopy and thermal analysis and the results obtained were compared. For a structural characterization, the samples were analyzed by XRD and vibrational spectroscopy, which evidenced that at increasing carbonate content, the apatitic structure becomes progressively more amorphous. XRD and IR spectroscopy revealed that the samples were prevalently constituted of B-type carbonated hydroxyapatites. The Raman full width at half-maximum of the 962cm-1 band (FWHM962) and the I1072/I962 intensity ratio can be good quantitative indexes of the carbonate content, as well as the IR E1420/E565 extinction ratio. In thermogravimetric analysis (TG), the weight loss in the 400-1200°C range was almost entirely attributed to carbonate decomposition. Both TG and IR results overestimated for most samples the carbonate content in comparison with elemental analysis, which is the most precise analytical method between those examined.