Determination of fluoride in sodium fluoride oral solution

Importance of the topic

Fluoride is a proven anticavity agent widely incorporated into mouth rinse formulations to prevent dental caries. Accurate determination of fluoride concentration in oral solutions is essential for product efficacy and regulatory compliance. Ion chromatography offers sensitive and selective measurement of fluoride in complex matrices and serves as the official technique in pharmacopeial monographs.

Objectives and study overview

This study aimed to adapt the United States Pharmacopeia method for sodium fluoride oral solutions by implementing a modern high performance anion exchange column. Key goals included optimizing separation of fluoride from common co-ions such as chloride, benzoate and phosphate, validating analytical performance, and comparing measured fluoride levels in commercial mouth rinses to their labeled claims.

Used instrumentation

• Thermo Scientific Dionex ICS-6000 HPIC system with ICS-6000 DP pump and DC detector with suppressed conductivity
• Dionex AS-AP autosampler with cooling and 250 µL sample syringe
• Dionex IonPac AG22 guard column and IonPac AS22 analytical column (4 × 250 mm)
• Dionex ADRS 600 anion suppressor in recycle mode at 15 mA
• Chromeleon Chromatography Data System version 7.2.9

Methodology

The mobile phase consisted of 1.42 mmol/L sodium carbonate and 1 mmol/L sodium hydroxide delivered isocratically at 1.5 mL/min. The column temperature was maintained at 30 °C and injection volume was 20 µL. Conductivity detection used a suppressor cell at 25 °C and detector cell at 35 °C. Calibration standards of fluoride were prepared across 0.1 to 1 mg/L. System suitability was verified using 0.45 mg/L fluoride and 0.30 mg/L chloride standards to confirm resolution, tailing and precision. Commercial mouth rinse samples were diluted to approximately 0.5 mg/L fluoride based on label claims.

Main results and discussion

The IonPac AS22 column provided clear separation of fluoride from the water dip and eluted common anions within a 10-min timeframe, improving throughput compared to the original column set. System suitability metrics exceeded USP criteria with resolution of 12.6, tailing factor of 1.55 and RSD under 0.3%. Linearity was excellent with r2 of 0.9999 over the 0.1–1 mg/L range. Measured fluoride in three commercial rinses agreed with labeled concentrations within 90–110% using both one-point and multi-point calibration approaches. Spike recovery experiments yielded 101–105% recovery and precision tests showed peak area RSD below 0.7% and retention time RSD below 0.1%.

Practical benefits and applications

This adapted ion chromatography method allows mouth rinse manufacturers to perform routine fluoride content analysis with improved speed and reliability. The simplified 10-min run and robust performance support quality control workflows and ensure compliance with pharmacopeial requirements.

Future trends and applications

Advances in ion chromatography include the integration of mass spectrometry detection for enhanced selectivity, development of compact and portable IC systems for on-site testing, and increased automation for high-throughput environments. Emerging stationary phases and eluent generation technologies may further shorten analysis times while maintaining separation quality.

Conclusion

The study successfully demonstrated a validated ion chromatography procedure for fluoride determination in sodium fluoride oral solutions using the Dionex IonPac AS22 column and USP mobile phase. The method delivers accurate, precise, and efficient analysis, meeting all system suitability and performance criteria. It offers a practical solution for regulatory and manufacturing laboratories tasked with fluoride monitoring.

References

• United States Pharmacopeia National Formulary USP43-NF38, Sodium Fluoride Oral Solution 2023
• Pharmacopeia Forum 43(1) Sodium Fluoride Oral Solution 2017
• Thermo Scientific Dionex IonPac AS22 column manual