FT-IR spectroscopy of fluoro-substituted hydroxyapatite: strengths and limitations

Abstract

Fluoro substituted hydroxyapatite (FHAp) samples were prepared by a cyclic pH method. Both calcined and uncalcined samples were subjected to elemental analysis (F, Ca, P) and X-ray diffraction (XRD) analysis to verify composition and phase purity. Good correlation between a-axis parameters and fluoride ion content was found for calcined samples, however, for uncalcined samples the fluoride ion content was higher than estimated from the a-axis values. Fourier transform infra red (FT-IR) spectroscopy analysis of the calcined samples showed OH band shifts and splitting in accordance with F-HO interactions affecting the OH vibration. We conclude that the OH libration (620–780 cm^-1 range) is more suited for estimation of fluoride ion content than the OH stretching. In contrast, uncalcined samples all displayed FT-IR spectra similar to that of hydroxyapatite (HAp) despite the presence of fluoride ions (18–73%). FT-IR emission spectroscopy was used to probe the changes occurring in the FT-IR spectra of HAp and FHAp samples upon heating. Interpretation of the spectral changes occurring during heating to 1,000 °C and subsequent cooling is given. Room temperature spectra of samples heated to various temperatures was used to determine the temperature necessary to produce FT-IR spectra displaying the expected OH bands. A model accounting for the combined observations is proposed.