Identification of the Coloring Constituents of Four Natural Indigoid Dyes

Abstract

A high performance liquid chromatography (HPLC) method combined with spectrophotometric UV‐Vis detection is developed for the separation and identification of seven indigoid coloring compounds: indigotin, which is commercially available, and indirubin, 6‐bromoindigotin, 6′‐bromoindirubin, 6‐bromoindirubin, 6,6′‐dibromoindigotin, and 6,6′‐dibromoindirubin, which are synthesized to be used as reference compounds in the HPLC analysis. The chromatographic method is employed for the identification of the blue/purple coloring compounds in samples extracted from four mollusks, which have been used for the production of Tyrian Purple since antiquity: Hexaplex trunculus, Murex brandaris, Nucella lapillus, and Thais haemastoma. The composition of the analyzed samples, with respect to the reference materials, is compared and discussed. All seven indigoids are identified in Hexaplex trunculus and some of them are identified in the other three purple extracts.

Keywords:

• HPLC
• Dye
• Tyrian Purple
• Indigo
• Art analysis

Acknowledgment

This work has been partially developed within the framework of MED‐COLOUR‐TECH project, which is supported by the European Commission, contract No: FP6‐2003‐INCO‐MPC‐2‐015406.