Tsotsou GE. Extraction-free analysis in cosmetics by digital image colorimetry, illustrated by the quantification of urea.
Heliyon 2024;
10:e25503. [PMID:
38333856 PMCID:
PMC10850965 DOI:
10.1016/j.heliyon.2024.e25503]
[Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/11/2024] [Revised: 01/28/2024] [Accepted: 01/29/2024] [Indexed: 02/10/2024] Open
Abstract
An extraction-free methodology is proposed for quantifying urea in cosmetics, which relies on urea-mediated decrease of methyl red decoloration by sodium hypochlorite. The method is applied directly to the cosmetic formulation and the resulting color intensity is captured by a smartphone camera. We demonstrate a linear relationship between color intensity and urea concentration in O/W emulsions and a shampoo. This quantification methodology is fully validated by determining its technical characteristics in an O/W cosmetic emulsion: The standard curve is linear over 2.5-30.0 % w/w urea (R2 ≥ 0.985). The coefficient of variation (CV %) on all quality control levels is ≤ 12.54 % for intermediate precision, indicating acceptable precision. Bias is up to ±4.93 % in the emulsion, indicating acceptable accuracy and a countable matrix effect. The proposed analysis setup in combination with a standard addition methodology is applied to verify urea content in purpose-made emulsions: bias is ≤±10.9 %, even in the presence of interfering ammonia. We finally demonstrate that the camera-captured color intensity of an O/W emulsion is proportional to different colorant concentrations in the formulation. This opens the route for further applications of the proposed setup to other ingredients capable of generating a colored product upon suitable reaction inside the formulation matrix.
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