A Validated HPTLC Method for the Determination of Illegal Dyes in Spices and Spice Mixtures

For screening large numbers of samples of spice and spice mixtures for the presence of illegal dyes a rapid high performance thin layer chromatography (HPTLC) method was developed and validated. From paprika, chili, or curry powder, dyes are extracted with acetonitrile. An aliquot of the raw extract is filtered and treated with iron(III) chloride to oxidize the natural dyes. After evaporation to dryness, the residue is taken up with basic dichloromethane, cleaned through a solid phase extraction cartridge containing silica gel, and evaporated again. The purified extract is dissolved in acetonitrile, separated on a reversed phase (RP18) HPTLC plate, and then evaluated visually and by scanning densitometry. The screening method was validated regarding precision and recovery. It has been routinely used by the Official Food Control Authority of the Canton of Zurich since 2007.