Ion Chromatography: Setting New Standards for Accuracy in Fluoride Analysis

Fluoride’s ability to fight tooth decay by enhancing resistance to plaque-causing bacteria makes it an important ingredient in toothpastes, tooth gels and mouth rinses. Most toothpastes and tooth gels sold over the counter in the U.S. contain 0.24% w/w sodium fluoride, while those prescribed by dentists typically contain sodium fluoride at levels of up to 1.1% w/w. To ensure the stability and safety of these products, accurate determination of fluoride is essential.

Fluoride determination: Protecting the quality and safety of dental products

Several techniques are available for determining fluoride in dental products, including gas chromatography (GC), ion chromatography (IC), colorimetry, and potentiometric methods. In the U.S., the use of these techniques is driven by standards set by the United States Pharmacopeia National Formulary (USP-NF), with the monograph for sodium fluoride gels the key method for analyzing toothpastes and tooth gels.

Until recently, the procedure described by the USP-NF monograph for sodium fluoride gels was based on a potentiometric titration using ion-selective electrodes (ISEs). However, while ISE methods are effective for fluoride determinations, they tend to be less quantitative than techniques such as IC, and can be subject to interferences that lead to inaccurate results.

Monograph modernization makes way for IC

The USP-NF seeks to ensure monographs reflect modern laboratory methods and regularly revises outdated and/or less accurate techniques. Ongoing advances in IC technologies have given analytical laboratories access to highly selective methods for fluoride analysis. Alongside improvements in instrument design, novel polymer chemistries have led to the development of innovative strong anion-exchange columns that offer enhanced separation efficiency. The Thermo Scientific Dionex IonPac AS15-5 µm column, for example, is a hydroxide-selective column consisting of cross-linked ethylvinylbenzene and divinylbenzene grafted with smaller particles containing alkanol quaternary ammonium ions, which delivers superior resolution of early-eluting anions like fluoride.

Under the USP-NF’s monograph modernization initiative, the IES procedure for assaying fluoride present in sodium fluoride gel is set to be replaced by an IC method. The new technique aligned to this proposed monograph will ensure both manufacturers and consumers benefit from more accurate results, and therefore improved product safety.

Determining sodium fluoride in toothpastes and tooth gels using IC

The IC method described in the revised monograph uses a 4.6 µm L91 packing column. In a recent app note, we showed that the Dionex IonPac AS15-5 µm column can be used as an alternative separation system for this assay.

Using the Dionex IonPac AS15-5 µm column on a Thermo Scientific Dionex ICS-5000⁺ system, (currently available as Thermo Scientific Dionex ICS-6000) we analyzed a USP reference standard mix containing 2 µg/mL sodium fluoride and 1 µg/mL sodium acetate. The relative retention times for the fluoride and acetate ions were 1.0 and 1.32, respectively, consistent with the values of 1.0 and 1.3 specified in the revised USP monograph. The total run time for the method was 23 minutes, meeting the system suitability requirement for a minimum run time of three times the retention time of fluoride (7.42 minutes).

To evaluate the ruggedness of this method, we analyzed an over-the-counter tooth gel containing 0.24% sodium fluoride using two separate Dionex IonPac AS15-5 µm columns (Figure 1). The retention times on the two columns were found to be in excellent agreement, differing by just 0.7%. Both columns passed the system suitability requirements set in the USP monograph and delivered results showing the fluoride tooth gel sample passed the assay acceptance criteria of 90–110%.

Pushing the limits of accuracy in toothpaste and tooth gel fluoride analysis

Advances in strong anion-exchange chromatography are setting new standards for accuracy and reliability in fluoride analysis. Thermo Fisher IC instruments and columns are aligned with the new requirements set out in the revised USP-NF monograph, and our dedicated experts can help advise and demonstrate ways to meet these new methods.

You can read more about this method for determining fluoride in toothpastes and tooth gels in this app note.

Alternatively, for more on the high-performance capabilities of modern IC methods, check out our blog on the use of IC-MS for the determination of polar pesticides in drinking water.