Automated analysis of sun protection factor (SPF) in sunscreen using UV-visible spectroscopy

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Introduction

The increasing awareness of the risks of skin cancer with sun exposure requires that sunscreen products be appropriately tested and labeled. The numerous possible sunscreen formulations demands that a simple and robust analysis method be available.

Sunscreens work by either reflecting or absorbing the ultraviolet (UV) radiation before it reaches the skin. The UVA and UVB region (400 – 290 nm) is the spectral region that must be blocked for effectiveness of the sunscreen. Whether organic or inorganic, the active ingredient that shields the skin from the sun must be present in sufficient quantity and uniformity to ensure skin protection.

The traditional method for sunscreen analysis is based on a quantitative analysis of a diluted sample. A series of standards based on different concentrations of the active ingredient are measured and a quantitative method developed based on Beer’s Law.

Figure 1. Sunscreen spectral scans.

By contrast, the in-vitro method developed by Diffey and Robson uses a UV transparent substrate, 3M Transpore surgical tape to examine the final sunscreen formulation. No dilution and no preparation of standards is necessary. Simply spread the sunscreen onto the substrate, place the sample in the V-530 spectrophotometer and measure the spectrum.

Result

V-530 Spectrophotometer

Spectra of sunscreen formulations were collected over the spectral range 400 – 290 nm with a 0.5 nm data point resolution using a V-530 UV-Vis spectrophotometer. A fixed SBW of 2 nm was used with a ‘Fast’ response and scan speed of 400 nm/min. Sunscreen formulations were spread onto a 20 cm2 piece of 3M Transpore tape, the sunscreen covered substrate was then placed into the sample compartment and several spectra collected from different areas. Representative spectra of two different sunscreen formulations are presented as Figure 1.

An average sunscreen spectrum is calculated from the multiple sample spectra and the average submitted to a Windows Excel macro. The macro calculates the SPF rating and other relevant data for the sunscreen sample (Figure 2) which is printed as a single page report. The results can be used during formula development or for quality control.

Conclusion

The Diffey-Robson in-vitro SPF calculation is a simple, rapid and universal analysis method for the determination of SPF values of sunscreens containing either inorganic or organic screening components. The versatile JASCO V-530 spectrophotometer provides SPF analysis in addition to a full range of UV-Visible analyses for QA/QC.

About the Author

Leah Pandiscia received her PhD from Drexel University where she studied Biophysical Chemistry. She is a Spectroscopy Applications Scientist at JASCO.