フッ化物は、水や一部の食品に自然に含まれるミネラルです。歯のエナメル質を強化し、腐食から保護することが証明されています[1]。ただし、過剰なフッ化物にさらされると、歯の外観に影響を与える歯のフルオローシスという状態を引き起こす可能性があります。そのため、ジェルや歯磨き粉などの歯科ケア製品中のフッ化物の量を監視することが重要です。
ナトリウムフッ化物ジェルは、虫歯（カリエス）を効果的に予防する有益な製品です。米国薬局方・国家規準（USP-NF）の権威あるモノグラフ「ナトリウムフッ化物ジェル」[2]で指定されているように、抑制導電度検出を備えたイオンクロマトグラフィ（IC）は、ナトリウムフッ化物ジェル中のフッ化物および不純物を測定するための信頼性の高い方法です。
本研究では、米国薬局方・国家規準の基準を満たすMetrosep A Supp 16 - 250/4.0 カラムと水酸化物溶出剤を使用したIC法を検証しています。フッ化物は、ジェル歯磨き粉中の塩化物や他の不純物から高い精度と正確さで分離されます。IC法は、米国薬局方一般章<621>クロマトグラフィ[3]および<1225>薬局方手続きの検証[4]に従って検証されています。
Commercial gel toothpaste was diluted to a known concentration of approximately 2 μg/mL sodium fluoride (NaF). Here, 1.585 g of a gel toothpaste containing 331.5 mg NaF/100 g was diluted in 500 mL ultrapure water (UPW). The solution was sonicated for 10 minutes and further diluted 1:8.8 with UPW. Afterwards, the diluted solution was filtered using 0.2 μm pore size filters. The nominal sodium fluoride concentration for these samples was 1.19 μg/mL.
No additional sample preparation is required.
The standard solutions and the system suitability solutions are prepared from the respective 1000 μg/mL certified standards by dilution with UPW.
For the assay, the standard solution is obtained by diluting a sodium fluoride solution to 2 μg/mL. The system suitability solution contains 2 μg/mL sodium fluoride and 1 μg/mL sodium acetate. For the impurity test, the standard solution consists of 0.2 μg/mL sodium chloride in UPW. The system suitability solution for the impurity test contains 1 mg/mL sodium fluoride and 1 μg/mL sodium chloride in UPW.
Samples and standard solutions were directly injected into the IC using a 919 IC Autosampler plus (Figure 1).
Fluoride was separated from acetate and chloride by using a potassium hydroxide eluent and the Metrosep A Supp 16 column (column material, Table 1). The analytes were quantified by evaluating their conductivity signal after chemical suppression.
The calibration was performed using a single 2.0 μg/mL sodium fluoride standard injected six times. The sample was analyzed in duplicate.
Table 1. Requirements for IC method as per USP Monograph «Sodium Fluoride Gel» [2].
| Column with L91 packing | Metrosep A Supp 16 - 250/4.0 |
|---|---|
| Eluent | 15 mmol/L potassium hydroxide |
| Flow rate | 1.0 mL/min |
| Temperature | 40 °C |
| Injection volume | 20 μL |
| Detection | Conductivity with suppression |
The IC assay for fluoride content was validated according to the USP Monograph «Sodium Fluoride Gel» [2]. Suitability requirements for resolution, tailing factor, and relative standard deviation were fulfilled (Table 2).
Table 2. Suitability requirements for the fluoride assay.
| Parameter (assay) | Actual | USP requirement | Status |
|---|---|---|---|
| Resolution F- /acetate |
5.9 | NLT 1.5 | Pass |
| Tailing factor | 1.1 | NMT 2.0 | Pass |
| RSD fluoride (%, n=5) | 0.52 | NMT 0.73 | Pass |
Commercial gel toothpaste samples were analyzed for their sodium fluoride content and the results showed the concentration at 104% of the label claim (Figure 2). The recovery of fluoride for the sample analysis was within the USP acceptance criteria of 90–110%.
When performing the impurity tests for potential contamination with chloride, the IC method showed excellent compliance with the USP requirements (Table 3).
Table 3. Suitability requirements for the chloride impurity in sodium fluoride gel.
| Parameter (impurity) | Actual | USP requirement | Status |
|---|---|---|---|
| Resolution F- /Cl- | 7.7 | NLT 4 | Pass |
| RSD fluoride (%, n=5) | 4.2 | NMT 5 | Pass |
| S/N ratio Cl- | >740 | NLT 20 | Pass |
The presented IC method complies with the USP General Chapters <621> and <1225> [3,4]. It is suitable to determine sodium fluoride in gels containing sodium fluoride according to the USP Monograph «Sodium Fluoride Gel» [2].
- Yeung, C. A. A Systematic Review of the Efficacy and Safety of Fluoridation. Evid Based Dent 2008, 9 (2), 39–43. https://doi.org/10.1038/sj.ebd.6400578.
- Sodium Fluoride Gel; Monograph; U.S. Pharmacopeia/National Formulary: Rockville, MD. https://doi.org/10.31003/USPNF_M3947_02_01.
- 〈621〉 Chromatography; General Chapter; U.S. Pharmacopeia/National Formulary: Rockville, MD. https://doi.org/10.31003/USPNF_M99380_01_01.
- 〈1225〉 Validation of Compendial Procedures; General Chapter; U.S. Pharmacopeia/National Formulary: Rockville, MD. https://doi.org/10.31003/USPNF_M99945_04_01.
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