Mild extraction methods for organic colorant analysis^Beta

Extraction methods for the HPLC characterisation of organic colorants in cultural heritage objects:

Creation of an online searchable bibliographic reference database.

The unique nature and typically small size of samples, the effects of time and past treatment and the diverse range of potential biological sources make identification of the natural organic colorant source for dyes and pigments in cultural heritage contexts highly challenging. Selection of the most appropriate extraction method to solubilise the constituents of the colorant for HPLC analysis is a further problem. For samples from cultural heritage objects, extraction must not only dissolve the molecular constituents, but typically must free the colorant components from the fibre or dye–mordant complex (for textiles and other dyed materials), or from the pigment substrate and frequently also from an additional matrix (such as the paint binding medium).

A wide range of extraction methods have been proposed in the literature and applied to such samples: some have been optimised for particular colorant classes, or to allow highly quantitative extraction from tiny samples, or are designed for particular sample types (e.g. dyed textiles but not paint), etc. Natural organic colorants are complex mixtures of components, including both the basic colorant components (aglycones) and sugar derivatives of these components (glycosides). There is growing interest in so-called ‘soft’ extraction methods that preserve the glycoside content or other chemically labile components which can be highly informative of the biological source and/or dyeing or pigment-making technologies. Such approaches also allow a wider range of colorant classes to be explored. There is, however, a lack of knowledge about the advantages, limitations and wider applicability of the different extraction methods.

Within the FP7 European project CHARISMA (Cultural Heritage Advanced Research Infrastructures: Synergy for a Multidisciplinary Approach to Conservation/Restoration), five laboratories undertaking colorant research have reviewed the literature relating to the extraction of organic colorants in cultural heritage objects.

To permit other researchers undertaking HPLC analysis of samples from cultural heritage objects to benefit from the review when selecting the most appropriate extraction procedure, this online searchable bibliographic reference database containing standardised descriptions of the extraction protocols and details of the types of samples or colorant classes the methods have been applied to has been created. This database is offered as a resource to help other researchers to select the most appropriate procedure to maximise the information that can be obtained by HPLC analysis of samples from unique cultural heritage objects. The online database will continue to be developed, expanded and improved but it currently contains near 150 articles.

On the basis of this review, work is on-going within the CHARISMA project to explore the advantages, limitations and applicability of the different extraction methods with the aim of developing a 'universal' extraction procedure (or sequence of steps) optimised for samples from the cultural heritage field or to provide a number of optimised methods suitable for a range of sample types which will

• avoid conversion of glycosides into the aglycones
• allow extraction of unstable colorants (brasilein, turmeric, etc)
• allow extraction of hydrophobic colorants
• achieve a maximum response for all colorant components present
• allow investigation of the widest range of colorant classes and mixtures of colorants
• be suitable for investigation of colorants applied to a range of supports/sample types and in a range of matrices or complexes

As they are developed within the framework of the CHARISMA project, additional resources for researchers in the scientific and heritage community undertaking analysis of organic colorants will be made available via this website.

To begin your search click here.

Related Posters:

• T. Devièse, C. Higgitt, I. Karapanagiotis, J. Kirby, M. van Bommel and I. Vanden Berghe, Review of extraction methods for the characterisation by HPLC of organic colorants in textiles and pigments in cultural heritage objects, poster presented at the 29th Meeting of Dyes in History and Archaeology, Lisbon (Portugal), 2010.
• N. Bosscher, T. Devièse, J. Padfield, C. Higgitt, A. Ness Proaño Gaibor, H. Schraven and M. van Bommel, Extraction methods for the HPLC characterisation of organic colorants in cultural heritage objects: creations of an online searchable bibliographic reference database, poster presented at the 30th Dyes in History and Archaeology conference, Derby (UK), 2011.