Fourier transform infrared spectroscopy of the solution-mediated conversion of amorphous calcium phosphate to hydroxyapatite: new correlations between X-ray diffraction and infrared data.

Fourier Transform infrared spectroscopic analysis of maturing, poorly crystalline hydroxyapatite (HA) formed from the conversion of amorphous calcium phosphate (ACP) at constant pH or variable pH show only subtle changes in the v1, v3 phosphate absorption region (900 cm-1-1200 cm-1). This region is of interest because it can be detected by analysis of mineralized tissue sections using FT-IR microscopy. To evaluate the subtle spectral changes occurring during the maturation, second derivatives of the spectra were calculated. HA formed at constant pH showed little or no variation in the second derivative peak positions with bands occurring at 960 cm-1, 985 cm-1, 1030 cm-1, 1055 cm-1, 1075 cm-1, 1096 cm-1, 1116 cm-1, and 1145 cm-1. These bands can be assigned to molecular vibrations of the phosphate (PO4(3-)) moiety in an apatitic/stoichiometric environment of HA. In contrast, during the early stages of maturation of the HA formed at variable pH, second derivative peak positions occurring at 958 cm-1, 985 cm-1, 1020 cm-1, 1038 cm-1, 1112 cm-1, and 1127 cm-1 shifted in position with maturation, indicating that the environment of the phosphate species is changing as the crystals mature. Peaks at 1020 cm-1, 1038 cm-1, 1112 cm-1, and 1127 cm-1 were attributable to nonstoichiometry and/or the presence of acid phosphate-containing species. This concept was supported by the lower Ca:P molar ratios measured by chemical analysis of the synthetic material made at variable pH. Using the second derivative peak positions as initial input parameters, the v1, v3 phosphate region of the synthetic HAs prepared at constant pH were curve fit. X-ray diffraction patterns of these same materials were also curve fit to calculate the changes in crystallinty (size/perfection) in the c-axis 002 reflection as well as the 102, 210, 211, 112, 300, 202, and 301 planes. Linear regression analysis showed that the changes in the percent area of the underlying bands at 982 cm-1, 999 cm-1, 1030 cm-1, 1075 cm-1, 1096 cm-1, 1116 cm-1, and 1145 cm-1 were correlated with changes in crystallinity in one or more of the reflection planes. It is suggested that a combination of second-derivative and curve-fitting analysis of the v1, v3 phosphate contour allows the most reproducible evaluation of these spectra.