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How Much Zinc Oxide Do You Use in Sunscreen?

Sunscreens are classified as over-the-counter (OTC) drug products in the USA. Zinc oxide is one of the active ingredients approved (up to 25%) by the U.S. Food and Drug Administration (FDA) for use in sunscreens. Zinc oxide is a powdered mineral that sits on the outermost layer of the skin scattering, reflecting, and absorbing ultraviolet radiation. Zinc oxide is unique among sunscreen ingredients in that it is a broad-spectrum blocker, protecting from both UVA and UVB radiation.

The United States Pharmacopeia (USP) is engaged in the ongoing challenge of obtaining the information needed to create and sustain up-to-date quality drug product monographs for FDA approved drugs, including OTC drugs. The USP also embarked on a global initiative to modernize many of the existing monographs across all compendia. As part of this USP effort, an ion chromatography (IC) method has been proposed for addition to General Chapter <591>, Zinc Determination. This IC method will replace existing titration-based assays in the zinc-containing drug product monographs (such as Zinc Oxide, Zinc Oxide Neutral, and Zinc Sulfate Ophthalmic Solution).

Thermo Scientific™ Dionex™ IonPac™ CS5A ion-exchange columns were designed for the separation of transition and lanthanide metals. The transition metal (e.g. Fe3+, Cu2+, Ni2+, Zn2+, Co2+, Cd2+, Mn2+, and Fe2+) separation uses a pyridine-2,6-dicarboxylate (PDCA) eluent. After separation, transition metals are detected using a post column reagent (4-(2-pyridylazo) resorcinol) (PAR)) and absorbance detection at 530 nm. The Dionex IonPac CS5A column (USP L100) is proposed for the determination of zinc.

The analysis begins with sample preparation where a samples are weighed, then dissolved in HCl with a certain concentration, then diluted with DI water and filtered. The dilution is repeated if the sample has a high concentration until the calculated concentration is in the range of 15 µg/mL.

The zinc oxide content of a sunscreen product was determined using the Dionex ICS-5000+ HPIC system with a Dionex IonPac CS5A ion-exchange column. The determination of zinc is achieved by the formation of an anionic complex with a chromatographic eluent that contains pyridine-2,6-dicarboxylic acid (PDCA), separation of that complex, a subsequent post column complexation with 4-(2-pyridylazo)resorcinol (PAR), and detection of that complex by its absorbance at 530 nm. This IC method is selective for zinc as it is well separated from other transition metals (Fe3+, Cu2+, Ni2+, Zn2+, Co2+, Cd2+, Mn2+, and Fe2+) with these conditions.

sunscreen analysis

Four purchased sunscreen samples were tested. Figure shows the chromatogram of a standard prepared from the Zinc Oxide USP reference standard and chromatograms of the sunscreen samples. The retention time of the zinc complex is at about 7.9 min using the first of two columns used for this study. The IC method was validated following the guidelines outlined in USP General Chapter <1225>, alidation of Compendial Methods. The method is linear, accurate, precise, and robust for the determination of zinc oxide in sunscreen lotions.

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