Fast, easy and green method for the first quantification of niacinamide in serums and creams by digital image analysis using iron (III) chloride.

A green digital image photometry method for the quantification of niacinamide in commercial cosmetic serums and creams is described. The method is based on the formation of an orange complex between niacinamide and iron (III) chloride in microplates under LED backlighting. The analytical signal was defined as (255 - B), where B corresponds to the blue channel intensity in the RGB colour model. Calibration was performed using microplate-based standards, and the method was applied to commercial cosmetic samples. Relative errors compared to HPLC-DAD were lower than 10.0%, and an elliptical joint confidence region test indicated no significant differences between slopes and intercepts and the ideal values of 1 and 0 at the 95% confidence level. Paired Student's t-test and F-test confirmed no significant differences in agreement and precision between methods. Deviations from labelled concentrations were generally below 6.0%, with a maximum value of 14.0%. Precision, evaluated over two months, showed relative standard deviations below 9.0%. Recovery values ranged from 84 to 114%. UV-Vis spectrophotometry was used to support the validation of the colorimetric response of the niacinamide-FeCl₃ system. The proposed approach was applied to rapid screening of niacinamide in cosmetic products and compared with a reference HPLC-DAD method.