Determination of fluoride in sodium fluoride oral solution using carbonate/bicarbonate eluent

Importance of the Topic

Maintaining the correct fluoride concentration in oral care products is critical for preventing dental caries while ensuring user safety. Reliable quantification of fluoride in sodium fluoride mouth rinse formulations supports quality control, regulatory compliance, and consumer confidence.

Objectives and Study Overview

This study aimed to develop and validate a robust ion chromatography method for determining fluoride in sodium fluoride oral solutions. By replacing the USP monograph eluent with a carbonate/bicarbonate mobile phase on a Dionex IonPac AS22 column, the method addresses long chloride retention times and achieves complete separation of fluoride, chloride, benzoate, and phosphate within a 10-minute run time.

Methodology and Instrumentation

Instrumentation and Conditions:

• IC System: Thermo Scientific Dionex ICS-6000 HPIC with conductivity detection and Dionex Chromeleon software.
• Columns: Dionex IonPac AG22 guard (4×50 mm) and AS22 analytical (4×250 mm).
• Suppressor: Dionex ADRS 600 in AutoSuppress recycle mode at 43 mA.
• Eluent: 4.5 mM Na2CO3 and 1.4 mM NaHCO3, delivered at 1.5 mL/min.
• Column Temperature: 30 °C; Injection Volume: 20 µL; Run Time: 10 min.

Reagents and Standards:

• Type 1 DI water (≥18 MΩ·cm).
• Stock solutions (1 000 mg/L) of fluoride, chloride, benzoate, and phosphate.
• Calibration range for fluoride: 0.1–1.0 mg/L.
• System suitability solution: 0.45 mg/L fluoride and 0.30 mg/L chloride.

Main Results and Discussion

Separation and Suitability:

• Baseline resolution of fluoride from the water dip and other anions in under 10 min.
• System suitability met USP criteria: resolution ≥8.8 (NLT 1.5), tailing factor 1.8 (NMT 2.0), RSD 0.33% (NMT 2%).

Linearity and Calibration:

• Calibration curve for fluoride (0.1–1.0 mg/L) yielded r2 = 0.9999.
• One-point calibration at 0.5 mg/L fluoride produced comparable results to full calibration.

Sample Analysis:

• Three commercially available mouth rinses diluted to ~0.5 mg/L fluoride.
• Measured fluoride concentrations ranged 0.49–0.55 mg/L, corresponding to 100–106% recovery of labeled values.

Accuracy and Precision:

• Recovery of spiked fluoride (20–50% levels) was 96–107%.
• Method precision over three days showed peak area RSD ≤0.33% and retention time RSD <0.15%.

Benefits and Practical Applications

This method offers rapid, precise, and accurate fluoride quantification for routine quality control of oral rinse products. The use of a carbonate/bicarbonate eluent simplifies preparation, reduces analysis time, and ensures compliance with pharmacopeial requirements.

Future Trends and Opportunities

Advances may include:

• Integration with automated sample preparation for higher throughput.
• Coupling with mass spectrometry for simultaneous multi-element analysis.
• Miniaturized flow systems for on-site monitoring of fluoride in dental clinics and manufacturing lines.

Conclusion

The validated IC method using a carbonate/bicarbonate eluent on a Dionex IonPac AS22 column provides efficient separation, high accuracy, and excellent precision for fluoride determination in sodium fluoride oral solutions. It meets USP criteria and supports quality control needs in oral care product manufacturing.

Reference

1. United States Pharmacopeia National Formulary, USP43-NF38, Sodium Fluoride Oral Solution (2023).
2. Pharmacopeia Forum 43(1) (2017), Sodium Fluoride Oral Solution.
3. Thermo Scientific Application Note 002345: Determination of fluoride in oral rinse solution.
4. Thermo Scientific Dionex IonPac AS22 Column Manual, Man-065119-EN.