Determination of fluoride in tooth gel containing sodium fluoride

Importance of the Topic

Fluoride is widely used in dental care products to strengthen tooth enamel and prevent decay. Accurate measurement of fluoride concentration in tooth gels ensures product quality, compliance with pharmacopeial standards, and patient safety. Recent pharmacopeial revisions propose ion chromatography as a modern, precise alternative to ion-selective electrode titration for this analysis.

Study Objectives and Overview

This study evaluates an IC method using a Thermo Scientific Dionex IonPac AS15-5 µm column and suppressed conductivity detection to quantify fluoride in sodium fluoride tooth gel. The aim is to meet or exceed United States Pharmacopeia monograph requirements, replacing the existing potentiometric titration procedure, and to assess method ruggedness on two separate columns.

Methodology and Instrumentation

• Chromatographic system: Dionex ICS-5000+ platform with EG III KOH eluent generator and AERS 500 suppressor in recycle mode.
• Column setup: IonPac AS15-5 µm analytical column (3.0 × 150 mm) with AG15 guard column (3.0 × 30 mm).
• Eluent: 5 mM KOH generated electrolytically; flow rate adjusted to 0.5 mL/min; column temperature 30 °C; injection volume 20 µL.
• Detection: suppressed conductivity, 7 mA, baseline noise 0.2–0.3 nS.
• Standards and reagents: USP sodium fluoride and sodium acetate reference standards; calibration mix at 2 µg/mL fluoride and 1 µg/mL acetate.
• Sample preparation: ~83 mg of 0.24% w/w NaF gel dissolved in water, vortexed, filtered through 0.2 µm prior to injection.

Main Results and Discussion

• Fluoride eluted at 7.42 min and acetate at 9.74 min, achieving resolution >6 and tailing factor ~1.1, well above USP thresholds (resolution ≥1.5, tailing ≤2).
• Flow rate adjustment followed USP <621> guidelines, yielding equivalent performance on smaller particles.
• System suitability across two columns showed retention time RSD <0.05% and area RSD <1%, with consistent relative retention times (fluoride = 1.00; acetate ≈1.32).
• Assay of the commercial tooth gel sample returned 96.8–97.1% of label claim, within the USP acceptance range of 90–110%.

Benefits and Practical Applications

The IC method provides a fast, precise, and automatable alternative for routine fluoride analysis in dental gels. It replaces manual titration, enhances laboratory throughput, and resolves potential anionic interferences more effectively than ion-selective electrodes. Compliance with USP monograph criteria ensures regulatory acceptance.

Future Trends and Potential Applications

Advances in column packing materials, detector sensitivity, and automated eluent generation are expected to lower detection limits and shorten analysis times. The approach may be extended to other fluorinated dental formulations and complex matrices, and integrated into high-throughput quality control workflows.

Conclusion

The Dionex IonPac AS15-5 µm column with suppressed conductivity detection meets USP monograph performance criteria for fluoride assay in sodium fluoride gels. The method demonstrates excellent precision, accuracy, and robustness across different columns, offering a validated ion chromatography alternative to potentiometric titration.

References

• United States Pharmacopeia General Chapter <621> Physical Tests.
• United States Pharmacopeia Pharmacopeial Forum, 44(4): Proposed revision for Sodium Fluoride Gel monograph.
• Borissova R. et al. Titrimetric determination of phosphates and monofluorophosphates in toothpastes. Fresenius J Anal Chem, 1993;347:63–66.