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An HPLC method with diode-array detection at 210 nm is described for the routine determination of amphoretic surfactants with a betaine structure in cosmetic cleansing products. In a strongly acidic pH range cocamido-propylbetaine and alkylbetaine are successfully separated from non-ionic and anionic matrix components on a cation-exchange column (Nucleosil 100-5 SA, 5 μm, 250 × 4 mm I.D.). Chromatographic separation was carried out under isocratic conditions at a flow-rate of 1.0 mm/min. The mobile phase consisted of 70% acetonitrile and 30% 0.05 M lithium hydroxide in water, adjusted to pH 1.6 with phosphoric acid (v/v). As sample preparation the products and standards were simply diluted and subsequently filtered before injection. The method is precise, robust, independent of the shampoo matrix and thus suitable for routine use, such as product quality control. |
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