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Zinc oxide is an important active ingredient in sunscreen, providing protection from potentially damaging ultraviolet (UV) radiation by scattering, reflecting and absorbing sunlight. Zinc oxide is unique among sunscreen components as it shields the skin against both UVA and UVB radiation, which are associated with skin aging and skin burning, respectively.

 

The U.S. Food and Drug Administration (FDA) classifies sunscreens as over-the-counter drugs, and allows zinc oxide to be included at concentrations of up to 25%. To ensure sunscreens are safe and provide the level of UV protection consumers expect, the use of accurate and reliable methods for determining zinc oxide content in these products is essential.

 

Zinc oxide analysis: Protecting the quality and safety of sunscreen

 

A variety of techniques exist for determining zinc content in FDA-approved drug products, including titration-based assays and ion chromatography. The use of these methods is guided by standards set out by the U.S. Pharmacopeia (USP). Currently, the recommended procedure for determining zinc oxide in sunscreen is a titration-based assay. However, titrations are typically less quantitative than ion chromatography methods and are vulnerable to interferences that can lead to inaccurate results.

 

Recent years have seen significant advances in ion chromatography, resulting in impressive improvements in analytical reliability. In particular, ongoing innovations in polymer science have led to the development of highly efficient ion exchange columns for the separation of transition and lanthanide metals. The Thermo Scientific™ Dionex™ IonPac CS5A column, for example, is a high-resolution column that provides a sensitive and selective method for the determination of transition metals, such as zinc, in a wide range of sample matrices.

 

The USP is engaged in a global initiative to modernize many of the existing drug product monographs by revising less accurate or outdated techniques. As part of these efforts, an ion chromatography method employing the Dionex™ IonPac CS5A column has been proposed for addition to the USP General Chapter on zinc determination. This new method will ensure consumers and manufacturers benefit from more accurate zinc oxide analyses, resulting in higher standards of product safety and quality.

 

Determining zinc oxide in sunscreen using ion chromatography

 

To test the performance of the Dionex™ IonPac CS5A column in the proposed method, the Thermo Scientific™ Dionex™ ICS-5000+ HPIC system (currently available as the Thermo Scientific™ Dionex™ ICS-6000 HPIC system) was used to determine zinc oxide in four commercial sunscreen products, as reported in this application note.

 

Using a mobile phase containing 7.0 mM pyridine-2,6-dicarboxylic acid (also known as dipicolinic acid), 66.0 M potassium hydroxide, 5.6 mM potassium sulfate and 74.0 mM formic acid in deionized water, the 12-minute method resulted in a retention time for the zinc complex of approximately 7.9 minutes. Chromatograms of a zinc oxide USP reference standard and the four sunscreen samples are shown in Figure 1.

 

To measure the limit of detection (LOD) and limit of quantification (LOQ), baseline noise was determined in a representative 1-minute segment of the chromatogram for seven injections of a 0.2 µg/mL Zn2+ reference sample. Using this approach, the method was found to be highly sensitive, with a LOD of 0.1 µg/mL and LOQ of 0.3 µg/mL.

 

The method accuracy was then assessed by comparing the measured zinc oxide content for the four commercial sunscreen samples against the labeled values. The samples were prepared for injection using acetonitrile to disperse the sunscreen, followed by the addition of hydrochloric acid. The results were in good agreement with the labeled values (96–101%), highlighting the accuracy of the method.

 

Method robustness also was evaluated by measuring the impact of small changes in flow rate, eluent concentration and column temperature on the retention time, peak asymmetry and peak area RSD. A standard solution containing 15 μg/L Zn2+ was injected three times across a range of concentrations using two separate Dionex™ IonPac CS5A columns. Although small changes in retention time were observed, peak asymmetry remained within 2%. The peak area RSD ranged from 0.15–0.71%, meeting the system suitability requirements set in the monograph and demonstrating the robustness of the method.

 

Enhancing sensitivity, accuracy, and robustness in sunscreen analysis

 

Advances in ion chromatography have led to highly efficient columns designed for selective separation of transition and lanthanide metals. The proposed ion chromatography method for zinc oxide determination in sunscreen, using the Dionex™ IonPac CS5A column delivers highly sensitive, accurate and robust analysis, supporting improved confidence in results.

Read more about this method for determining zinc oxide in sunscreen in this application note.

 

Figure 1. Chromatograms of a zinc oxide standard and commercial sunscreen samples.Figure 1. Chromatograms of a zinc oxide standard and commercial sunscreen samples.