The collected samples of de‐ and remineralization buffers from days 1, 2, 3, and 8 were also analyzed for fluoride content. On each day (1, 2, 3, and 8), duplicate samples were analyzed in batches. One milliliter of the samples and standard fluoride solutions were vigorously mixed with 200 μl of 1 M nitric acid (HNO3) and 400 μl toluene reagent [17 (link)] in 1.8 ml autosampler vials (Brown Chromatography).
Gas chromatography was operated in split mode and was equipped with a cooled auto‐injector (AOC‐20i; Shimadzu), a backflow system, and a flame ionization detector. The separation took place in two RTX‐1 columns (15 and 30 m, 0.32 mm internal diameter, 0.5 μm film thickness) at a gradient temperature of 35 to 50°C (with an increase of 2°C/min). Calibration curves were used to determine the fluoride concentration in the de‐ and remineralization buffers based on the internal standard method.
Gas chromatography was operated in split mode and was equipped with a cooled auto‐injector (AOC‐20i; Shimadzu), a backflow system, and a flame ionization detector. The separation took place in two RTX‐1 columns (15 and 30 m, 0.32 mm internal diameter, 0.5 μm film thickness) at a gradient temperature of 35 to 50°C (with an increase of 2°C/min). Calibration curves were used to determine the fluoride concentration in the de‐ and remineralization buffers based on the internal standard method.