Determination of Anions in Toothpaste by Ion Chromatography

Importance of the Topic

The accurate measurement of fluoride and monofluorophosphate (MFP) in toothpaste is essential for ensuring dental products deliver effective decay prevention. Fluoride derived from compounds such as sodium fluoride or MFP can form insoluble complexes or hydrolyze during storage, reducing its bioavailability. A reliable analytical approach is therefore critical for quality control and formulation stability assessment.

Objectives and Overview

This study aims to develop and validate a direct ion chromatography method with reagent-free eluent generation for simultaneous quantification of active anions (fluoride and MFP) and common inactive anionic ingredients in toothpaste formulations. Two gradient methods using different high-capacity anion-exchange columns are evaluated to optimize separation and throughput.

Methodology and Instrumentation

The procedure involves dissolving 1 g of toothpaste in 100 mL of deionized water, vortex mixing, and filtration through a 0.45 µm membrane. Calibration standards were prepared from 1000 mg/L stock solutions of target anions with six concentration levels covering expected assay ranges. Two gradient elution methods were applied:

• Method A (IonPac AS18 column, 4×250 mm) with KOH gradient: 22–30 mM (7–8 min), 30–40 mM (12–13 min), 40–60 mM (16–20 min), flow 1.0 mL/min, 30 °C.
• Method B (IonPac AS17 column, 4×250 mm) with KOH gradient: 3 mM (0–5 min), 3–15 mM (5–14 min), 15–40 mM (14–18 min), flow 1.5 mL/min, 30 °C.

Suppressed conductivity detection with an ASRS ULTRA suppressor in recycle mode provided sensitive measurement. Calibration linearity exceeded r²=0.999 for all analytes.

Instrumentation Used

The analysis was conducted on a Dionex ICS-2000 reagent-free IC system equipped with an Eluent Generator, column heater, pump degas, EGC-KOH cartridge, CR-ATC module, AS50 autosampler, and Chromeleon software. Both IonPac AG18/AS18 and AG17/AS17 guard and analytical columns were employed.

Main Results and Discussion

Standard mixtures of fluoride, chloride, MFP, sulfate, benzoate, and phosphate were baseline separated on the AS18 column within 30 min. Calibration ranges spanned:

• Fluoride: 0.25–25 mg/L
• Chloride: 0.10–10 mg/L
• MFP: 1–100 mg/L
• Sulfate: 0.50–50 mg/L
• Benzoate: 0.25–25 mg/L
• Phosphate: 1–100 mg/L

Correlation coefficients were ≥0.9991. Three commercial toothpastes were analyzed in replicate (n=10): two containing MFP and one containing sodium fluoride. Active ingredient contents matched label claims: MFP formulations had ~0.65–0.81% MFP and ≤0.02% free fluoride, while the sodium fluoride product contained ~0.08% fluoride. Pyrophosphate tartar control was detected in one sample using the AS17 method. Retention time RSDs were <0.2% and peak area RSDs <0.9%, indicating excellent precision.

Benefits and Practical Applications

• Direct, simultaneous determination of multiple anionic species without derivatization or laborious hydrolysis.
• High reproducibility enabled by on-line eluent generation.
• Scalable to routine quality control in dental formulation and manufacturing laboratories.

Future Trends and Opportunities

Further developments may include coupling ion chromatography with mass spectrometry for enhanced selectivity, miniaturized and high-throughput systems for rapid screening, automated sample preparation, and expanded anion panels to cover complex formulations such as polyphosphates and novel bioactive additives.

Conclusion

The reagent-free ion chromatography approach presented here offers a robust, accurate, and efficient solution for quantifying fluoride, MFP, and related anionic ingredients in toothpaste. Its precision and simplicity make it well suited to support product development, stability studies, and quality assurance workflows.

References

1. McCoy MC. Approval of decay-preventing compounds. EN. 2001;42.
2. Wang P, Li SFY, Lee HK. Determination of fluoride and phosphate in toothpaste. J Chromatogr A. 1997;765:353–359.
3. Biemer TA, Asral N, Sippy AJ. Phosphate hydrolysis in dental formulations. J Chromatogr A. 1997;771:355–359.
4. Official Journal of the European Communities. Determination of fluoride in toothpaste. 1983;L291:37–40.
5. Borissova R, Debouki A, Nikolov T. Ion chromatography of toothpaste anions. Fresenius J Anal Chem. 1993;347:63–66.
6. Hattab J. Colorimetric methods for fluoride. J Dent. 1989;17:77–83.
7. Farcas F, Chaussadent T, Fiaud C, Mabille I. Electrolytic eluent generation for toothpaste analysis. Anal Chim Acta. 2002;472:37–43.
8. Murawski DJ. Analysis of polyphosphates by IC. J Chromatogr A. 1991;546:351–367.