モノフルオロリン酸リン酸塩（MFP）は、歯のエナメル質の再石灰化や歯科カリエス（虫歯）の予防によく使用されます[1]。製薬メーカーや研究所は、MFPを含む薬品や製剤の品質評価に米国薬局方および国家規準（USP-NF）を遵守するこが義務付けられています。

米国薬局方は、既存の多くのモノグラフを近代化するためのグローバルイニシアチブを開始しました。抑制導電度検出を備えたイオンクロマトグラフィ（IC）は、ナトリウムモノフルオロリン酸塩中のMFP含量を定量化するための検証された方法として、USPによって承認されています[2]。

MFPを硫酸塩から分離するためには、Metrosep A Supp 16 - 250/4.0 カラムと水酸化物勾配を使用することが可能です。USPモノグラフ「ナトリウムモノフルオロリン酸塩」のすべての受容基準が満たされ、この手順は検証されたUSP方法として承認されました[2–5]。

The system suitability solution and the standard solution are prepared from USP certified standards by dilution with ultrapure water (UPW).

The system suitability solution contains 4.0 μg/mL USP Sodium Fluoride RS, 1.4 μg/mL of USP Sodium Acetate RS, 150.0 μg/mL USP Sodium Monofluorophosphate RS, and 150.0 μg/mL USP Sodium Sulfate RS. The standard solution contains 150.0 μg/mL USP Sodium Monofluorophosphate RS.

Sample analyses were performed with customer-provided sodium monofluorophosphate (Na₂PFO₃). Of this, 1.5 g was weighed and added to a 1000 mL volumetric flask. The flask was filled up to the mark with UPW, sonicated for 15 minutes, and finally filtered through filter paper with a pore size of 0.2 μm. This sample stock solution was further diluted 1:10 with UPW. The final concentration corresponds to 150 μg/mL monofluorophosphate. No additional sample preparation is required.

System suitability solution, samples, and standard solutions were injected directly into the IC using an 858 Professional Sample Processor (Figure 1).

Baseline separation of fluoride, acetate, monofluorophosphate, and sulfate was ensured by applying a potassium hydroxide gradient (Table 2, eluent A 100 mmol/L potassium hydroxide, eluent B ultrapure water) and using the Metrosep A Supp 16 column (USP listing L91). Detection of analytes was achieved with chemically suppressed conductivity detection.

The calibration was performed by using a single 2.0 μg/mL sodium monofluorophosphate standard injected six times. The sample was analyzed in duplicate.

The IC method for the determination of monofluorophosphate content is qualified according to the USP Monograph «Sodium Monofluorophosphate» following the USP references for method validation procedures [2–5]. A chromatogram for the system suitability approval is shown in Figure 2.

The relative retention times for fluoride, acetate, monofluorophosphate, and sulfate are 0.20, 0.26, 1.00, and 1.06, respectively. These unitless values are automatically calculated with the MagIC Net software by applying the following formula:

r_G =  relative retention time, unadjusted
t_Ri = retention time peak of interest
t_Rst = retention time peak of reference peak (peak corresponding to the substance to be examined, monofluorophosphate)

All acceptance criteria for the system suitability (resolution, tailing factor, and relative standard deviation of replicate standard injections) are fulfilled (Table 3).

The results for the sample solution (Table 4) are calculated as follows:

r_U = peak response of monofluorophosphate from the sample solution
r_S = peak response of monofluorophosphate from the standard solution
C_S = concentration of USP Sodium Monofluorophosphate RS in the standard solution (µg/mL)
C_U = concentration of sodium monofluorophosphate in the sample solution (µg/mL)

The presented IC method has been successfully qualified for assessing the content of monofluorophosphate in accordance with the USP Monograph «Sodium Monofluorophosphate». This qualification strictly followed the USP validation specifications.

The system suitability met all acceptance criteria, including resolution, tailing factor, and the relative standard deviation of replicate standard injections. Furthermore, the sample analysis also fulfilled the USP requirements.

As a result, analysis with ion chromatography has been proven to be a reliable and appropriate approach for the determination of monofluorophosphate in pharmaceutical formulations. Manufacturers of sodium monofluorophosphate benefit from the high degree of automation and its ease of use.