2.4. Sample preparation
Fifteen commercially available cosmetics including moisturiz- ing creams (2), sunscreen creams (3), day creams (2), clean lotions (3), shampoos (2) and cosmetic powders (3) were obtained from commercial suppliers in Guangzhou, China. Several blank cos- metics samples were spiked with known concentrations of each analyte. After that, these spiked samples were homogenized thoroughly in a blender at high speed for 30 min and then kept in a refrigerator at 4 oC for 24 h to allow interactions between analytes and matrices perfectly. Subsequently each spiked sample was split into ten independent samples for method optimization and validation.
About 0.25 g of cosmetics samples and spiked samples were accurately weighed into 10 mL screw cap glass conical tubes and taken to volume with 2.5 mM MSA, followed by adding 1.0 mL of cyclohexane. These conical tubes were submitted to heating in an ultrasonic bath at 70 oC for 2 min, and the mixtures were then dispersed vigorously on a vibrator for 1 min and a further
sonication step was performed under 130 W ultrasonic irradiation powers with a frequency of 42 kHz for 10 min. Subsequently, aliquots of the cloudy solutions were centrifugated at 14,000 rpm for 3 min. The upper organic phases of aqueous samples were removed with syringes and the lower aqueous phases were passed through 0.45 μm filters. Twenty five microlitre of the clear filtrates were injected into ion chromatography system.