FLUORIDE UPTAKE
Each sample was individually weighed, immersed lifted by a nylon wire and stored in individual plastic containers with 10 mL of 1 M solution of potassium hydroxide (Merck reagent P.A.; Merck, Kenilworth, NJ, USA) under constant shaking for one week. After this period, each sample was weighed again to determine the fluoride uptake for each specimen. The analysis of fluoride ions was conducted by using 0.5 mL of the stored solutions. The free fluoride determination was measured using ion chromatography (Dionex DX-100; Thermo Fisher Scientific, Waltham, MA, USA) with suppressed conductivity. The instrument was fitted with an IonPac AS10 analytical column (Thermo Fisher Scientific, Waltham, MA, USA). Each stored solution was injected onto the injection loop of the instrument and a flow rate of 1.0 mL/min was adopted. Free fluoride ions have a well-defined retention time and the peak corresponding to fluoride can be readily determined from the chromatogram. The peak area was used to determine fluoride concentrations by linear interpolation between standard solution concentrations that were slightly higher and lower than that of the test solution