Analytical grade calcium acetate (Ca(CH3COO)2IH2O),di-ammonium hydrogen phosphate ((NH4)2HPO4) and sodium fluoride (NaF) were purchased from the Beijing FTIR spectrometer (Equinox 55, Bruker, Germany) over 400¨4000 cm1 at a resolution 4 cm1. The fluoride
content was analyzed using fluoride ion electrode method(PF-1, Shanghai Leici, China).