A Rapid and Green Limit Test Method for Five Synthetic Colorants in Foods Using Polyamide Thin-layer Chromatography

Abstract

Synthetic food colorants are used in very small amounts. Typically, the colorants are extracted, purified and concentrated prior to analysis; however, sample preparation is time-consuming, of high cost and environmentally unfriendly. In this work, a polyamide thin-layer chromatographic (TLC) method in combination with on-plate solid-phase extraction and backlight-assisted detection has been developed for five commonly used synthetic colorants in foods. Beverages (degassed where appropriate) were directly applied to the polyamide TLC plate. Gelatinous and solid samples were first extracted with water–ammonia aqueous solution (25 %) (99:1) assisted by ultrasonic treatment. Sample solutions were then acidified before application. Relatively large volumes of solution (5–25 μl) could be applied to the plate without significant sample diffusion. Before development, the start spots were cleaned up using ethanol (95 %)–water–acetic acid (5:5:0.1). The plate was developed using a mobile phase of ethanol (95 %)–water–ammonia aqueous solution (25 %) (75:14:1). After development, the plate was viewed in backlight mode with an LED array light. The selectivity of the method proved acceptable. The limits of detection for Ponceau 4R, Amaranth, Tartrazine, Sunset Yellow and Brilliant Blue were 10.1, 9.59, 4.19, 4.12 and 2.06 ng, respectively. The method was rapid, low-cost and sensitive, providing a green limit test method for five colorants in foods.