Determination of 16 Amino Acids in Cosmetics by High Performance Liquid Chromatography with Pre-Column Derivatization

A method for the determination of 16 amino acids including glycine, leucine, histidine, valine, methionine, proline, cysteine, threonine, glutamic acid, lysine, tyrosine, arginine, isoleucine, tryptophan, phenylalanine, and alanine in cosmetics by high performance liquid chromatography with 2,4-dinitrofluorobenzene (DNFB) pre-column derivatization was proposed. 5 g of cosmetic sample was taken into a 50 mL-centrifuge tube, and 5 mL of water was added. The mixture was sonicated for 10 min and boiled for 15 min. After cooling, 15 mL of 35 g·L⁻¹ sulfosalicylic acid solution was added and the mixture was centrifuged at a rotational speed of 10 500 r·min⁻¹ for 20 min. All of the supernatant was taken and its volume was made up to 25 mL with 35 g·L⁻¹ sulfosalicylic acid solution. 1 mL of sample solution was taken, and 1 mL of 0.5 mol·L⁻¹ sodium bicarbonate solution (pH 9.0) and 1 mL of 1% (volume fraction) DNFB derivative solution were added. The mixture was heated at 60 ℃ for 10 min and cooled. Its volume was made up to 5 mL with 0.01 mol·L⁻¹ potassium dihydrogen phosphate solution (pH 7.0). 16 amino acids in the obtained solution were separated on a X Bridge C₁₈ chromatographic column, and gradient elution was performed using a mobile phase consisting of 0.05 mol·L⁻¹ sodium acetate buffer solution (pH 6.5) containing 1% (volume fraction) DMF and a mixture of acetonitrile-water at a volume ratio of 1∶1 at different volume ratios. The detection was performed at 360 nm. As shown by the results, the linear ranges of the standard curves of 16 amino acids were 0.004—40.0 mg·L⁻¹, with detection limits (3S/N) in the range of 0.1-0.2 mg·kg⁻¹. Test for recovery was made on blank sample by standard addition method at 3 concentration levels, giving results in the range of 89.0%-105%, with RSDs (n=7) of the determined values not more than 10%. This method was applied to the analysis of 11 commercially available cosmetics, and it was shown that the detected amino acid components were mostly histidine, glycine, methionine, and glutamic acid.