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Volodymyr Pauk ´ Petr Bartak Karel Lemr Faculty of Science, Department of Analytical Chemistry, Regional Centre of Advanced Technologies and Materials, Palacky´ University in Olomouc, Olomouc, Czech Republic Received June 15, 2014 Revised August 24, 2014 Accepted September 9, 2014

Review Article

Characterization of natural organic colorants in historical and art objects by high-performance liquid chromatography High-performance liquid chromatography plays an important role in analysis of historical organic colorants. A number of papers have been published in this field over the last 30 years. Classification of the most commonly used natural dyes and an overview of highperformance liquid chromatography methods with main focus on recent works (2008 to the beginning of 2014) are provided. The review deals with an entire analytical protocol covering sample preparation, chromatographic separation, and suitable detection (UV/visible and fluorescent spectroscopy and mass spectrometric techniques). High-performance liquid chromatography has been successfully used in the complete characterization of some organic dyestuffs present in historical and art objects. The possibilities and difficulties for identification of natural sources of historical colorants are also discussed. Keywords: Anthraquinoids / Flavonoids / Indigoids / Lake pigments / Tannins DOI 10.1002/jssc.201400650



Additional supporting information may be found in the online version of this article at the publisher’s web-site

1 Introduction The history of the usage of natural colorants in art dates back to the Stone Age [1]. In prehistoric times, plant dyes were mainly used for body painting and hair dyeing, but as recent findings show [2], colors had already been applied to fibers as long as 30 000 years ago. Natural colorants also functioned as pigments for murals, frescos, paintings, drawings, and for decoration of other materials and objects such as wood, leather, sculptures, vessels, and even for ritual treatment [3]. Up to the 19th century, a “chemical palette” of organic dyes was formed by natural species and remained nearly unchanged. In 1856, Perkins synthesized mauveine [4], and a new era of synthetic organic colorants was initiated. Afterwards, natural colorants were almost completely replaced by synthetic ones. The identification of historical dyestuffs provides crucial information for conservators and restorers to choose appropriate technique for preservation and restoration of art objects. The analytical data give useful hints on the dyeing technique of a craftsman or an artist but also on household activity and the culture of particular historical period. Since Correspondence: Karel Lemr, Faculty of Science, Department of Analytical Chemistry, Regional Centre of Advanced Technologies and Materials, Palacky´ University in Olomouc, 17. listopadu 12, 771 46 Olomouc, Czech Republic E-mail: [email protected] Fax: +420585634433

Abbreviations: MeOH, methanol; MSA, methanesulfonic acid; PCA, principal component analysis  C 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

certain dyes were used in different times or geographic locations, their identification assists in dating or localizing of the origin of historical artifacts. Minor components or degradation products of dyes [5–7] can confirm or disprove the authenticity of the evaluated object of interest.

1.1 Methods of dye analysis The available amount of a sample from an art object is often extremely small, but information about main and minor components, corresponding degradation products, as well as a quantitative ratio of substances can be required to reach a suitable conclusion. This issue can be solved by the use of modern and highly sensitive and selective analytical methods. There are many analytical tools potentially suitable for analysis of art objects [8]. Techniques such as UV/Vis [9, 10], IR [9], Raman [3, 11–14], fluorescence [15], X-ray fluorescence [9, 13], or NMR [13] spectroscopy are virtually nondestructive, require no or little sample preparation, and most of them can be performed in situ. Nowadays, multitechnique spectroscopic protocols are preferred [16, 17]. Since historical dyes are often complex mixtures of organic compounds, the spectroscopic techniques do have some limitations. UV/Vis spectroscopy is not as selective as other techniques and gives limited structural information. Fluorescence or spectral overlap of binders, pigments, and other components can interfere in Raman and IR spectra. Raman spectroscopy can fail to discriminate dyes of very similar chemical structure. Fluorescence spectroscopy is useful only for www.jss-journal.com

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fluorescent active compounds. X-ray fluorescence requires the presence of metals in a sample and is unsuitable for the identification of organic compounds. NMR spectroscopy is an excellent tool but can be insufficient for very small amounts of sample and is unsuitable for analysis of trace components. On the contrary, the hyphenation of separation and MS (CE– MS, GC–MS, and HPLC–MS) is sufficiently sensitive and selective, provides structural information, and allows identification of unknown compounds; however, sampling of an object must be performed [18]. Although CE has been successfully used to identify natural dyes in historical objects, its detection limits (even using MS detection) were substantially higher in comparison with HPLC with diode array detection (DAD) and HPLC–MS [19, 20]. GC–MS has not been widely used in the study of natural dyes, due to the relatively high molecular mass and polarity of the analytes. Derivatization of the target analytes is a compulsory requirement for GC– MS [7, 21–24]. The vast majority of the recent works dealing with identification of colorants has used HPLC. Starting from 1975 when HPLC provided data for investigation of binding media in paintings [25] and, later, for dyestuffs [26], it is the most preferred method for analysis of natural dyes. In 2008, Surowiec published a paper delineating high-performance separation techniques used for analysis of archaeological objects [18]. In that work, HPLC of natural dyes was briefly mentioned. In the same year, Rosenberg reviewed LC–MS methods for the characterization of historical organic dyestuffs [27]. He focused more on MS detection rather than chromatographic separation. In 2009, Degano published an extensive review on analytical methods used for the analysis of artworks and historical textiles [28].Various approaches using spectrometric techniques, direct mass spectrometric methods, LC, and CE were described. This paper primarily surveys recent analytical protocols for the identification of historical organic dyes by HPLC. Deliberations on the operational and instrumental features indicate broad spectrum of information that can be obtained from HPLC data.

2 Classification of organic colorants Colorants are commonly divided into two groups: dyes and pigments [4]. Dyes are generally organic substances at least temporary soluble in the vehicle. Pigments are usually inorganic compounds (metal oxides, sulfides, and other minerals) insoluble in the paint medium. Nevertheless, organic colorants can be used as pigments like indigo in oil paintings [29] and drawings [1], or cochineal that after treatment with mordant, e.g. alum (KAl(SO4 )2 ), and addition of metal salt, such as chalk (CaCO3 ), produces an insoluble Ca–Al– colorant complex called the carmine lake pigment [1]. A dye penetrates into the substrate, usually fibers of textile, whereas a pigment is bonded to the surface of the substrate and is usually applied as a suspension in a suitable binding medium (oils, proteins, and resins).  C 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

J. Sep. Sci. 2014, 37, 3393–3410

A dye itself can be a complex mixture of substances. Compounds containing chromophore groups are called coloring matters or coloring principles. Some dyes can contain different amounts of the same coloring matters. For example, the main component of kermes dye is kermesic acid, which is also present in cochineal (see Supporting Information Table S1). Moreover, the ratio of coloring matters in the same dyes derived from different species can vary greatly (Supporting Information Table S1). A dyestuff can also contain minor components characteristic of the particular species, like in case of sea snails. Some of these molluscs (Bolinus brandaris L., Stramonita haemastoma L.) produce dyes composed entirely of brominated indigo-derivatives, and the others (Hexaplex trunculus L.) with some amount of nonbrominated indigo [30, 31] (see Supporting Information Table S2). Dyes can also contain degradation products and mineral impurities useful for identification purposes. Colorants can be classified according to their source, application method, or by color and corresponding chemical structure. Most natural colorants are of plant origin but animals are used for their production too. They are traditionally extracted from roots (madder, turmeric, bedstraw), berries (buckthorn), flower heads (saffron), bark (oak), leaves (indigo, henna), and even from lichens and mushrooms [1]. Besides being available, dyes also have to be sufficiently stable. For example, chlorophyll (green parts of plants), carotene (carrot), and betanine (red beet) degrade too fast, especially in light. Sometimes the consumption of sources is enormous. The production of only 1.4 g of Tyrian purple dye requires 12 000 snails [30]. Moreover, the dye itself is not present in the shells. It is derived from precursor compounds, also called chromogens, present in hypobranchial gland secretions of molluscs by means of subsequent fermentation, reduction, and photoreactions [32, 33]. Harvesting the red dyes of cochineal requires planting cacti, the manual collection of insects, and their subsequent drying and extraction [34]. The most important historical dyestuffs, their chemical constituents, and application methods are listed in Supporting Information Table S1. Three types of dyes are distinguished in historical textile dyeing: direct, mordant, and vat. Direct dyeing uses soluble dyes with some affinity to the fibers. It does not require any mordants or metallic salts for fixing. As a result, the products exhibit poor color-fastness to washing due to weak chemical interactions. Most of the natural dyes bind to fibers through the application of a mordant. They become more resistant to light and washing. Acid mordants have been obtained from tannin-rich sources such as oak gallnuts or bark. Basic mordants are salts of different metals, such as aluminum, iron, copper, tin, or zinc, that form coordination complexes between molecules of dyes and textile substrate such as wool or silk (Fig. 1). The color of the resulting dye is dependent on the mordant used. Iron salts produce darker shades while alum intensifies color brilliance [1]. Vat dyes are insoluble in common solvents. They are reduced in alkaline medium to form a colorless water-soluble substance called leuco dye (Fig. 2). Fibers or textiles are www.jss-journal.com

Liquid Chromatography

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Extracts are separated on a suitable HPLC column; gradient elution is usually applied. Various additives to the mobile phase improve separation. Chromatography is followed by diode array, mass spectrometer, or other common detection systems. Methods capable of detection and quantitation of a number of colorants belonging to different chemical groups were developed to enable tracing of their natural sources [37–48]. Essential information on many of the methods devoted to analysis of organic colorants in historical and art objects described in the literature is summarized in Table 1. Some comments on the methods and sample preparation useful for their adoption are presented in Supporting Information Tables S3–S5.

3.1 Sample preparation 3.1.1 Sampling

Figure 1. Formation of wool–mordant–dye complex.

immersed into a prepared leuco-bath. After oxidation, the dye restores its original color and properties. There are only three natural vat dyes: indigo, woad, and Tyrian purple. These dyes are problematic for HPLC due to their negligible solubility in common mobile phases. The fundamental property of a dye or a pigment is its color. Often dyes of a similar color have very similar chemical structure (see Supporting Information Table S1 and Figs. S1–S12). Sampling and analysis of historical colorants is usually conducted according to their color. Natural dyes are classified into four color groups: blue/purple, red/pink/orange, yellow, and black/brown. There are very few green dyes, and in most cases they are obtained by mixing blue and yellow. True purple was too expensive and was imitated by double-dyeing with blue (indigo) and red (madder). The resultant color was called “poor people’s purple” [35]. Sometimes this name refers to purple orchil dye [36]. Therefore, what we actually see can be a mixture of a few coloring species.

Since the physical integrity of art and historical artifacts has to be maintained, the amount of sample available is often highly limited. A 0.3 to 1.0 cm long (0.5–1 mg weight) piece of textile fiber of a certain color is usually taken [6, 40–44, 49–52] and coloring components are then extracted. In a recent work, a polychromic yarn (

Characterization of natural organic colorants in historical and art objects by high-performance liquid chromatography.

High-performance liquid chromatography plays an important role in analysis of historical organic colorants. A number of papers have been published in ...
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