Detection and quantification of calcium fluoride using micro-Raman spectroscopy

Investigations on the caries-preventive effect of calcium fluoride have been seriously hampered by the lack of adequate detection techniques. In this paper, the micro-Raman technique is introduced as a suitable method for CaF2 quantification with a spot size typically 5 microns. Advantages of this measuring technique and associated problems are described for CaF2 determination in the presence of large amounts of hydroxyapatite. The data show that a comparative analysis of Raman intensities of calcium fluoride and phosphate ions of hydroxyapatite can be used for quantification of CaF2 concentrations. The intensity behaviour of the CaF2 and hydroxyapatite Raman lines is explained considering the absorption-scattering cross-sections of the two components. The ratio of the Raman intensity (322 cm-1) of CaF2 to the phosphate Raman intensity (432/447 cm-1) of hydroxyapatite was found to be related linearly to the ratio of the concentration of CaF2 to that of hydroxyapatite. At present, the lower limit of CaF2 quantification is approximately 3 wt%. The technique has already been applied on fluoridated bovine enamel, and Raman emissions due to CaF2 or CaF2-like material formed on/in enamel have been detected for the first time.