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Sodium fluoride and acidulated phosphate topical solution products are used for dental hygiene to prevent cavities on teeth. Fluoride is a mineral that helps to strengthen the enamel on teeth, making them more resistant against decay by acids and bacteria.

Pharmaceuticals such as these anticavity products require strict quality control. Therefore, it is necessary to determine any impurities in such products as well as the appropriate contents of the active substances. This Application Note outlines the fluoride IC assay as described in the USP Monograph Sodium Fluoride and Acidulated Phosphate Topical Solution [1] using the Metrosep A Supp 1 column and suppressed conductivity detection. The column equivalency study was carried out in cooperation with the U.S. Pharmacopoeia.

A dentalcare product (topical solution) with a nominal concentration of 1.23% sodium fluoride was investigated for its fluoride content. The samples were diluted in ultrapure water in two steps to obtain a nominal concentration of 0.5 μg/mL fluoride.

The system suitability solution contained 1.1 μg/mL sodium fluoride and 0.5 μg/mL of sodium chloride in ultrapure water, as described in the respective USP monograph [1]. The solution was prepared by volumetric dilution of 1000 mg/L standard solutions of fluoride and chloride using micropipettes and 100 mL volumetric flasks.

The quantification of the results was performed based on a single point calibration with a 1.1 μg/mL fluoride standard.

 Instrumental setup including a 930 Compact IC Flex with IC Conductivity Detector and a 919 IC Autosampler plus.
Figure 1. Instrumental setup including a 930 Compact IC Flex with IC Conductivity Detector and a 919 IC Autosampler plus.

The samples were injected directly into the ion chromatograph (Figure 1) without further sample preparation and analyzed using the method parameters given in the USP monograph (Table 1). Anionic components were separated isocratically on a Metrosep A Supp 1 - 250/4.0 column containing the alternative packing material L46. The conductivity signal was detected after sequential suppression.

Table 1. IC method parameters as per USP monograph «Sodium Fluoride and Acidulated Phosphate Topical Solution» [1].

Column with L46 packing Metrosep A Supp 1 - 250/4.6
Eluent 150 mg/L of anhydrous sodium carbonate and 1.0 mL/L of 1 N sodium hydroxide
Flow rate 1.0 mL/min
Column temp. 30 °C
Injection vol. 20 μL
Detection Conductivity with sequential suppression
Chromatogram   of   the   system   suitability   solution   containing  1.1  μg/mL  sodium  fluoride  and  0.5  μg/mL  sodium  chloride.
Figure 2. Chromatogram of the system suitability solution containing 1.1 μg/mL sodium fluoride and 0.5 μg/mL sodium chloride.

Fluoride and chloride were analyzed in diluted topical solutions according to the USP General Chapter Chromatography [2]. A column equivalency study was performed, and all acceptance criteria (e.g., repeatability, resolution, tailing factor, and accuracy) were fulfilled according to the USP General Chapter Validation of Compendial Procedures [3]. Resolution between the fluoride and chloride peaks was exceptional (11.6) using the Metrosep A Supp 1 - 250/4.6 column (packing material L46).

Chromatogram of a topical solution containing 1.065 μg/mL fluoride (99.8% recovery) and traces of chloride (not quantified).
Figure 3. Chromatogram of a topical solution containing 1.065 μg/mL fluoride (99.8% recovery) and traces of chloride (not quantified).

Fluoride eluted as a symmetric peak (tailing factor 1.2) with high repeatability. The relative standard deviation for the fluoride peak area in the system suitability solution was calculated at 1.2% (Table 2, Figure 2).

Figure 3 shows that the fluoride content was accurately reflected in the declared label for the analyzed samples (99.8% recovery).

Table 2. Selected performance characteristics from the system suitability study.

Performance characteristics Acceptance criteria Results
Resolution >1.5 between fluoride and chloride peaks 11.6
Tailing factor Tailing factor (asymmetry) for the fluoride ion peak is <2.0 1.2
Repeatability Relative standard deviation for the fluoride ion peak area in the standard solution is <2.0% for five replicates 1.2%

The presented IC method for Fluoride in Sodium Fluoride and Acidulated Phosphate Topical Solution with the Metrosep A Supp 1 column (packing material L46) is officially included into the USP [1]. Robustness and reliability of the method was demonstrated within the column equivalency study following the USP General Chapters for Chromatography and Validation of Compendial Procedures [2–3].

[1] U.S. Pharmacopeia. USP-NF Sodium Fluoride and Acidulated Phosphate Topical Solution. Monograph. https://doi.usp.org/USPNF/USPNF_M76520_06_01.html.

[2]〈621〉 Chromatography. DOI:10.31003/USPNF_M99380_01_01

[3]〈1225〉 Validation of Compendial Procedures; General Chapter; U.S. Pharmacopeia/National Formulary: Rockville, MD. DOI:10.31003/USPNF_M99945_04_01

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