Kinetic spectrophotometric assay for the determination of vitamin C in cosmetics following ultrasound-assisted emulsification

In this work, a new analytical approach based on ultrasound-assisted emulsification followed by a photoreaction with methylene blue (MB) and kinetic analysis by UV-vis spectrophotometry has been developed for the determination of l-ascorbic acid (AA) in cosmetic samples. The emulsification of cosmetic samples results in a transparent solution that allows an easy and rapid quantitation by UV-vis spectrophotometry. The emulsified sample is mixed with a MB aqueous solution and this mixture is subjected to irradiation with a tungsten lamp for 5 min (fixed-time kinetic assay). A reduction in the MB absorbance intensity at 664 nm occurs as the concentration of AA increases. The observed change in absorbance intensity was used for calibration and further quantitation using the relationship of absorbance logarithm vs. AA concentration (mu g mL(-1)). In order to achieve an optimal response, different parameters involved in the reaction between AA and MB were fully investigated. Under optimal conditions, the limits of detection and quantification were 0.04 mu g mL(-1) and 0.15 mu g mL(-1), respectively. Repeatability and reproducibility, expressed as relative standard deviation, were in the range of 0.4-0.6% and 0.6-1.5%, respectively. Finally, the proposed method was applied to the analysis of 15 cosmetic samples, namely, (i) 12 samples without AA, which were used to carry out recovery studies, obtaining results in the range of 97.5-100.7%; (ii) 3 serum samples containing pure AA among their ingredients, which were used for AA stability studies.

Automated summary

A new analytical approach combining ultrasound-assisted emulsification, photoreaction with methylene blue, and UV-vis spectrophotometry has been developed to determine l-ascorbic acid (AA) in cosmetic samples. The emulsification process produces a transparent solution that is easily measured by UV-vis spectrophotometry. The reaction between the emulsified sample and methylene blue is irradiated with a tungsten lamp for 5 minutes, resulting in a decrease in absorbance intensity as the AA concentration increases. The proposed method has high sensitivity, with limits of detection and quantification of 0.04 μg mL(-1) and 0.15 μg mL(-1), respectively.