Critical comparison of spectrometric analyses of non-mineral blue dyes and pigments used in artworks † S. Kuckova, a,b * R. Hynek, a I. Němec, c M. Kodiček a and J. Jehlička d The aim of this work was to find the lowest concentrations of non-mineral blue dyes and pigments using infrared spectroscopy with Fourier transformation, Raman spectroscopy and mass spectrometry based on laser desorption/ionisation-time of flight, in fresh and artificially aged model colour layers, for which the detection of indigo, Prussian blue and copper phthalocyanine in differently prepared samples could be reliably performed. This research was motivated by the fact that the presence of a particular blue dye allows at least approximate dating of artworks and therefore may be conducive to determining their authenticity, as is shown on a Czech cubist painting in this paper. Copyright © 2012 John Wiley & Sons, Ltd. Keywords: infrared spectroscopy (FTIR); Raman spectroscopy; Laser desorption mass spectrometry; blue dyes and pigments; artworks Introduction Countless numbers of blue dyes and pigments have been used in artworks from ancient times. Fortunately, the majority of inor- ganic blues can be identified using scanning electron microscopy with electron dispersive X-rays detector (SEM/EDX) that has been routinely used for the identification of element composition, but in the case of organic materials including non-mineral blue pig- ments is unreliable because the element composition (C, H, O) is practically the same; for example, Prussian blue could not be unambiguously identified by SEM/EDX by the content of iron, because iron is contained in almost all kinds of pigments. Simi- larly, the presence of copper phthalocyanine could be confirmed neither by this method nor by the content of copper, because in its pale tints, the pigment concentration is very low; the copper cation abundantly occurs in other pigments. In this paper, we have focused on three non-mineral blues: Prussian blue, indigo and copper phthalocyanine. These blues are difficult to identify using routinely used analytical methods; however, they can serve as the most appropriate dyes for artworks age determination. The presence of Prussian blue (Fe 4 [Fe(CN) 6 ] 3 14-16H 2 O or KFe [Fe(CN) 6 ]xH 2 O) in an illuminated manuscript was determined by a combination of laser-induced breakdown spectroscopy, hyper- spectral imaging analysis and diffuse reflectance spectra. [1] In a different experiment, energy-dispersing X-ray fluorescence, optical microscopy and Raman microspectroscopy were used to identify blues in five Greek icons [2] ; only Raman microspectroscopy success- fully identified Prussian blue and indigo. Again, in another case, only Raman microspectroscopy identified Prussian blue and copper phthalocyanine in six fake Egyptian papyruses. [3] Indigo in Mayan blue was also confirmed using Raman spectroscopy. [4,5] Raman spectroscopy is the only method that can identify blues in thin, often single-layered samples, such as in the case of tapestries from the 1830s. [6] Laser desorption mass spectrometry was tested on Prussian blue and copper phthalocyanine by Grim and Allison. [7] There are also number of another analytical methods concerned with the identi fication of indigo, among them mass spectrometry techniques, [8–13] voltammetry, [14–16] thin layer chromatography, [17,18] gas chromatography, [19,20] high performance liquid chromatography [21–27] and infrared spectroscopy (FTIR). [28] In this paper the identification of non-mineral blues using FTIR, Raman spectroscopy and LDI-TOF MS (laser desorption/ionisation- time of flight mass spectrometry) in fresh and artificially aged samples is described. The paint replicas containing indigo, copper phthalocyanine and Prussian blue were prepared for this purpose. Finally, the combination of the three above-mentioned methods was applied to a sample taken from a Czech cubist painting. Material and methods Sample preparation Model coloured samples were prepared from natural indigo (product number 36000, Georg Kremer, Farbmühle, Aichstetten/ Allgäu, Germany), copper phthalocyanine (Sigma-Aldrich) and Prussian blue, which was prepared according to a traditional rec- ipe by Heinrich Diesbach [29] from 1704–1705 by mixing 300 ml of * Correspondence to: S. Kuckova, Institute of Chemical Technology, Department of Biochemistry and Microbiology, Technicka 3, 166 28 Prague 6, Czech Republic. E-mail: kuckovas@vscht.cz † Paper published as part of the ECASIA 2011 special issue. a Institute of Chemical Technology, Department of Biochemistry and Microbiology, Technicka 3, 166 28 Prague 6, Czech Republic b Charles University in Prague, Faculty of Education, Department of Chemistry and Chemical Education, M.D. Rettigove 4, 116 39 Prague 1, Czech Republic c Charles University in Prague, Faculty of Science, Department of Inorganic Chemistry, Hlavova 8, 128 40 Prague 2, Czech Republic d Charles University in Prague, Faculty of Science, Institute of Geochemistry, Mineralogy and Mineral Resources, Albertov 6, 128 40 Prague 2, Czech Republic Surf. Interface Anal. (2012) Copyright © 2012 John Wiley & Sons, Ltd. ECASIA special issue paper Received: 22 August 2011 Revised: 24 November 2011 Accepted: 6 December 2011 Published online in Wiley Online Library (wileyonlinelibrary.com) DOI 10.1002/sia.4828