Green Determination of Urea in Moisturizers by Diffuse Reflectance Spectroscopy

Abstract

This manuscript describes the development and application of a simple, inexpensive, and green method for the quantification of urea in skin moisturizer creams, using a combined spot test/diffuse reflectance spectroscopy procedure. The method is based on the derivatization of urea with a chromogenic reagent (p-dimethylaminocinnamaldehyde) in acidic methanol, yielding a colored compound on the surface of filter paper. The reaction parameters were optimized using chemometric experimental design. Reflectometric measurements of the colored compound were performed at 530 nm, the wavelength of maximum absorption. The linear dynamic range was from 25 to 750 mg L −1. The detection and quantification limits were 6.50 and 21.65 mg L −1 , respectively. The method was successfully used for the determination of urea in skin moisturizer creams, demonstrating that it is a reliable eco-friendly alternative.

Extraction-free analysis in cosmetics by digital image colorimetry, illustrated by the quantification of urea

“… Analytical method Need for extraction step or use of organic solvent Linearity (R 2 ) Repeatability or Reproducibility CV (%) Accuracy (bias %) LOQ Source Spectrophotometry YES >0.999 (solution) 2.72 (solution) ≤ ± 1.00 (ointment ) 7.63 μg/mL (solution) [ 7 ] HPTLC-densitometry YES >0.99 (solution) 3.84 (solution) ≤ ± 3.73 (ointment) 2.77 μg/mL (solution) [ 7 ] Potentiometry (pH–enzyme electrode) YES at least 0. 6 mg urea/g formulation (ointment ) [ 9 ] Kjeldahl (official method) harsh process ≤ ± 11.0 (emulsion) [ 13 ] Hydrophylic HPLC -UV detection YES >0.997 (solution) ≤ 1.53 (emulsion) ≤ ± 3.22 (lotion) ≤ ± 3.13 (conditioner) ≤ ± 2.34 (gel) 30 μg/mL (solution) [ 10 ] Hydrophylic nanoHPLC- UV detection YES 0.999 (solution) ≤2.80 (solution) ≤ ± 4.44 (emulsion) 50 μg/mL (solution) [ 11 ] Centrifugal partition chromatography YES 0.998 (solution) 0.7 (emulsion) …”

“…The Kjeldahl method is the official method for determining urea in cosmetics and dermatological formulations, which suffers mainly from poor selectivity and long pre-treatment and procedure times [ 6 ]. A number of alternative methods have been published for quality control purposes in the dermatological and cosmetic industry, including spectrophotometry [ 7 , 8 ], electrochemistry [ 9 ], column chromatography with ultraviolet detection [ 10 , 11 ], HPTLC combined with densitometry [ 7 ], urea online derivatisation via the Ehrlich reaction in a rotor of centrifugal partition chromatography [ 12 ], or spot test coupled with diffuse reflectance spectroscopy [ 13 ]. The above alternative methods rely either on urea extraction prior to analysis and/or on the use of organic solvents, while they all employ instrumental analysis.…”