Commercial toothpastes typically contain 1000-1500 ppm fluoride; however, the concentration of fluoride ion available for tooth brushing (bioavailable-F) decreases over time. Some of the reduction in bioavailable-F results from chemical interactions with dentifrice ingredients including abrasives and detergents, while some ingredients may preserve bioavailable-F. Objectives: Determine the change of bioavailable-F in NaF-containing laboratory-made dentifrices that use silica abrasives as a function of time. Methods: Dentifrices were made by thoroughly mixing bentonite clay(15g), glycerine(10mL), silica(15g), NaF(150mg, 310mg) and H2O(100mL) and letting stand overnight. Two batches were ~500ppmF (low-F) and two were ~1000ppmF (high-F). A standardized method that determines the total-F and bioavailable-F was used to analyze the four batches of dentifrices made at different times. Analyses: vigorously mix 1.0g dentifrice and 3mL distilled water for 60s. Total-F: acidify 1mL aliquot of slurry at least 8h (2mL of 1mol/L HCl). Bioavailable-F: centrifuge 1mL of the slurry 2min at >12,000rpm, acidify 0.1mL supernatant (0.5mL of 1mol/L HCl) for at least 8h then neutralize (0.5mL of 1mol/L NaOH). Analyze all samples using fluoride ion-selective electrode methods.
Results:
Sample | Bioavailable F (ppm) | Total F (ppm) | Age (days) |
1 (Low-F) | 312(20.2) | 429(39.8) | 63 |
1 (Low-F) | 185(18.8) | 431(17.8) | 105 |
2 (Low-F) | 230(31.0) | 439(24.5) | 1 |
2 (Low-F) | 167(10.4) | 430(14.9) | 43 |
3 (High-F) | 578(25.5) | 963(19.0) | 29 |
4 (High-F) | 587(10.8) | 949(25.4) | 2 |
Values in the table are mean(standard deviation), n=3.
All bioavailable-F are significantly lower than their total-F (p<0.05; paired-test).
Bioavailable-F values for samples 1 and 2 were significantly lower over time, (p<0.05; student's-t test). Conclusions: Based on these preliminary results, there is a significant decrease in the bioavailable fluoride in both low and high level [F] laboratory-made dentifrice as a function of time. Based on these results, further studies designed to explain these effects, and to investigate the role of other ingredients on bioavailable fluoride are planned.