Determination of zinc oxide in sunscreen

Significance of the Topic

Sunscreens often rely on zinc oxide as a physical UV filter approved up to 25% by the FDA. Accurate quantification ensures product efficacy, safety, and compliance with pharmacopeial standards such as USP <591> for zinc determination.

Goals and Study Overview

This study demonstrates the development and validation of an ion chromatography method for quantifying zinc oxide in commercial sunscreens using a compact Dionex Inuvion IC system. The method follows USP <1225> and ICH guidelines to ensure reliability and regulatory compliance.

Methodology and Instrumentation

• Sample Preparation: Acid digestion of sunscreen samples as per Thermo Scientific AN72680.
• Chromatographic Conditions:
  • Column: Dionex IonPac CS5A (2 mm i.d.) with CG5A guard.
  • Eluent: 7 mM pyridine-2,6-dicarboxylic acid, 66 mM KOH, 5.6 mM K₂SO₄, 74 mM formic acid in DI water.
  • Flow Rate: 0.3 mL/min; Column Temp: 30 °C; Injection Volume: 2.5 µL.
• Post‐Column Derivatization:
  • Reagent: 0.5 mM 4-(2-pyridylazo)resorcinol in 1 M 2-dimethylaminoethanol, 0.5 M NH₄OH, 0.3 M NaHCO₃.
  • Flow Rate: 0.15 mL/min through mixing tee and reaction coil.
  • Detection: UV/Vis at 530 nm.
• Instrumentation:
  • Dionex Inuvion IC system with AXP-MS auxiliary pump (≈2000 psi).
  • Autosampler: Dionex AS-AP; Data System: Chromeleon CDS v7.3.2.

Main Results and Discussion

• Zinc eluted at ~8.11 min with baseline separation from other cations.
• Calibration curve linear from 1 to 30 µg/mL (r² = 1.000).
• Inter-day precision: <3% RSD; Accuracy: 93–105% recovery across four SPF 50 sunscreen brands.

Benefits and Practical Applications

• Rapid analysis in under 12 min with high sensitivity and reproducibility.
• Full compliance with USP <591> monograph requirements for drug and cosmetic products.
• Ideal for routine quality control in pharmaceutical and personal care manufacturing.

Future Trends and Potential Applications

• Deployment of compact IC systems for on-site quality assessment.
• Extension of method to other metal oxides and transition‐metal APIs in topical formulations.
• Automation of sample preparation and data analysis to increase laboratory throughput.

Conclusion

The validated IC method on the Dionex Inuvion system offers a robust, precise, and accurate approach for zinc oxide quantification in sunscreens, meeting pharmacopeial and regulatory standards.

Reference

• USP General Chapter <591> Zinc Determination, USP.
• Thermo Scientific Application Note 72680: Determination of zinc oxide in sunscreen using ion chromatography, 2018.
• USP General Chapter <1225> Validation of Compendial Methods, USP.
• ICH Guidelines Q2A and Q2B on Validation of Analytical Procedures.