Stock standard solutions of amaranth (1.0 mg mL−1), Ponceau 4R (1.0 mg mL−1), sunset yellow (0.5 mg mL−1) and allure red (1.0 mg mL−1) were prepared in ultrapure water and stored at 4 °C in a fridge. The working standard solution was prepared freshly by dilution with hydrochloric acid water solution at certain pH values.
Three food samples including powdered beverage, lollipop, and juice were purchased from local retail markets. The two solid samples were homogenized before weighing. The amount of 0.1 g (powdered beverage) and 0.08 g (lollipop) of the samples were accurately weighted and then dissolved in 20 mL ultrapure water. 16 g of juice weighted directly was degassed by ultrasonication. All sample solutions were diluted to about 100 mL with ultrapure water, and then adjusted to pH 2.0 with 1 M hydrochloric acid.