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Monofluorophosphate (MFP) is often used for tooth enamel remineralization and to prevent dental caries (cavities) [1]. Pharmaceutical manufacturers and laboratories are obliged to use the Monographs from the United States Pharmacopeia and National Formulary (USP-NF) for the assessment of drug and formulation quality, including MFP.

The USP has initiated a global initiative to modernize many of their existing Monographs. Ion chromatography (IC) with suppressed conductivity detection has been approved by the USP as a validated method to quantify the MFP content in sodium monofluorophosphate [2].

The required separation of MFP from sulfate is possible by using the Metrosep A Supp 16 - 250/4.0 column and a hydroxide gradient. All acceptance criteria for the USP Monograph «Sodium Monofluorophosphate» are fulfilled and the procedure was approved as a validated USP method [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).

Figure 1. Instrumental setup including a 940 Professional IC Vario, 858 Professional Sample Processor, and an 800 Dosino for Dosino regeneration of the MSM (Metrohm Suppressor Module).

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.

Table 1. Requirements for the IC method as per USP Monograph «Sodium Monofluorophosphate» [2].

Column with L91 packing Metrosep A Supp 16 - 250/4.0
Flow rate 1.0 mL/min
Eluent Eluent A: 100.0 mmol/L Potassium hydroxide
Eluent B: Ultrapure water
Temperature 40 °C
Injection volume 10 μL
Detection Suppressed conductivity

Table 2. Binary gradient program for the USP Monograph «Sodium Monofluorophosphate» [2].

Time (minutes) Eluent A (%) Eluent B (%)
0.0 15 85
20.0 15 85
30.0 30 70
35.0 60 40
45.0 60 40
45.1 15 85
50.0 15 85

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.

Figure 2. Chromatogram of the system suitability solution. The sodium fluoride concentration corresponds to 4.0 μg/mL, sodium acetate 1.4 μg/mL, sodium monofluorophosphate 150.0 μg/mL, and sodium sulfate 150.0 μg/mL.

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:

rG =  relative retention time, unadjusted
tRi = retention time peak of interest
tRst = 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).

Table 3. System suitability requirements as per USP.

Parameter Actual USP required Status
Resolution monofluorophosphate / sulfate 1.84 NLT 1.5 Pass
Tailing factor 1.02 NMT 2.5 Pass
*RSD (%); n=6 0.38 NMT 2.0 Pass

* Relative standard deviation of six replicate injections of the standard solution

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

rU = peak response of monofluorophosphate from the sample solution
rS = peak response of monofluorophosphate from the standard solution
CS = concentration of USP Sodium Monofluorophosphate RS in the standard solution (µg/mL)
CU = concentration of sodium monofluorophosphate in the sample solution (µg/mL)


Table 4. Sodium monofluorophosphate sample analysis and requirements as per USP.

Analyte Actual USP requirement Status
Sodium MFP [%] 95.56 91.7–100.5 Pass

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. 

  1. Vogel, G. L.; Mao, Y.; Chow, L. C.; et al. Fluoride in Plaque Fluid, Plaque, and Saliva Measured for 2 Hours after a Sodium Fluoride Monofluorophosphate Rinse. Caries Research 2000, 34 (5), 404–411.
  2. U. S. Pharmacopeia/National Formulary. USP Monographs, Sodium Monofluorophosphate; USP/NF, Rockville, MD, USA.
  3. U. S. Pharmacopeia/National Formulary. <621> Chromatography. In General Chapter; USP/NF, Rockville, MD, USA.
  4. U. S. Pharmacopeia/National Formulary. General Chapter, <1065> Ion Chromatography; USP-NF: Rockville, MD, USA, 2023.
  5. U. S. Pharmacopeia/National Formulary. General Chapter, <1225> Validation of Compendial Procedures; USP-NF: Rockville, MD, USA, 2023.

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