Micro-Raman spectroscopy of natural and synthetic indigo samples

In this work indigo samples from three different sources are studied by using Raman spectroscopy: the synthetic pigment and pigments from the woad (Isatis tinctoria) and the indigo plant (Indigofera tinctoria). 21 samples were obtained from 8 suppliers; for each sample 5 Raman spectra were recorded and used for further chemometrical analysis. Principal components analysis (PCA) was performed as data reduction method before applying hierarchical cluster analysis. Linear discriminant analysis (LDA) was implemented as a non-hierarchical supervised pattern recognition method to build a classification model. In order to avoid broad-shaped interferences from the fluorescence background, the influence of 1st and 2nd derivatives on the classification was studied by using cross-validation. Although chemically identical, it is shown that Raman spectroscopy in combination with suitable chemometric methods has the potential to discriminate between synthetic and natural indigo samples.     

Catalogue of 45 reference Raman spectra of minerals concerning research in art history or archaeology,especially on corroded metals and coloured glass

Small catalogues of reference Raman spectra of interest for analysing geomaterials or biomaterials of relevance to art history or archaeology are gradually being published by different research groups. However, except for some older catalogues, they are all concerned primarily with pigments, whether inorganic or organic. Here we present for the first time a catalogue of Raman spectra of minerals that may be found in corroded metal artworks or artefacts. At the same time we include some inorganic pigments that may be found in or on stained glass. Most of the minerals analysed came from the Gallery of Mineralogy at the Muséum National d'Histoire Naturelle and most were verified by X-ray diffraction in order to augment the confidence in the mineral identity (which is not the case with many other catalogues). A number of problems encountered with mineral terminology are discussed. Comments are made on the spectra where appropriate.     

Characterization of lipid extracts from brain tissue and tumors using Raman spectroscopy and mass spectrometry

Brain tissue is characterized by high lipid content. The amount of lipids decreases, and its composition changes in the most frequent primary brain tumor, the glioma. Scope of the current paper was to extract quantitatively lipids from porcine and human brain tissue as well as from five human gliomas using a modified protocol according to Folch. The lipid extracts were studied by Raman spectroscopy with 785 nm excitation and by mass spectrometry with electron impact ionization. Porcine and human brain tissues have similar water and lipid content and show similar Raman and mass spectra. In contrast, gliomas are characterized by increased water content and decreased lipid content. Elevated phosphatidylcholine to cholesterol ratios in lipid extracts of gliomas were indicated by Raman bands of the choline group and cholesterol. Due to its higher sensitivity, mass spectrometry detected increased levels of cholesterol ester relative to cholesterol in lipid extracts of gliomas. For comparison, thin tissue sections were prepared from the glioma specimens before lipid extraction; infrared spectroscopic images were recorded and analyzed by a supervised classification model. This study demonstrates how to improve the analysis of brain tumors and to complement the diagnosis of brain pathologies using a multimodal approach.     

Raman spectroscopic analysis of the Maya wall paintings in Ek'Balam, Mexico

Raman spectroscopy has been applied to the examination of wall painting fragments from the archaeological site of Ek'Balam (Yucatán, Mexico). Thirty-three samples have been studied, all originating from room 23 of the Acropolis, and being representative of the painting technique at Ek'Balam during the late Classic Maya period. Several pigments such as haematite, calcite, carbon, cinnabar and indigo were identified in these samples. The latter pigment was presumed to be present as 'Maya blue', which is an intercalation product of indigo and palygorskite clay. The observed Raman spectra are reported and some band assignments have been made. This survey is the first Raman spectroscopic examination of a whole set of pigments in archaeological Maya wall painting fragments.     

Fragmentation trees for the structural characterisation of metabolites

Metabolite identification plays a crucial role in the interpretation of metabolomics research results. Due to its sensitivity and widespread implementation, a favourite analytical method used in metabolomics is electrospray mass spectrometry. In this paper, we demonstrate our results in attempting to incorporate the potentials of multistage mass spectrometry into the metabolite identification routine. New software tools were developed and implemented which facilitate the analysis of multistage mass spectra and allow for efficient removal of spectral artefacts. The pre‐processed fragmentation patterns are saved as fragmentation trees. Fragmentation trees are characteristic of molecular structure. We demonstrate the reproducibility and robustness of the acquisition of such trees on a model compound. The specificity of fragmentation trees allows for distinguishing structural isomers, as shown on a pair of isomeric prostaglandins. This approach to the analysis of the multistage mass spectral characterisation of compounds is an important step towards formulating a generic metabolite identification method. Copyright © 2012 John Wiley & Sons, Ltd.     

Semi-quantitative analysis of indigo by surface enhanced resonance Raman spectroscopy (SERRS) using silver colloids

In this paper we report for the first time semi-quantitative analysis of indigo using surface enhanced Raman spectroscopy (SERS) and surface enhance resonance Raman spectroscopy (SERRS). Indigo, a dye widely used today in the textile industry, has been used, historically, both as a dye and as a pigment; the latter in both paintings and in printed material. The molecule is uncharged and largely insoluble in most solvents. The application of SERS/SERRS to the semi-quantitative analysis of indigo has been examined using aggregated citrate-reduced silver colloids with appropriate modifications to experimental protocols to both obtain and maximise SERRS signal intensities. Good linear correlations are observed for the dependence of the intensities of the SERRS band at 1151 cm(-1) using laser exciting wavelengths of 514.5 nm (R=0.9985) and 632.8 nm (R=0.9963) on the indigo concentration over the range 10(-7)-10(-5) and 10(-8)-10(-5) mol dm(-3), respectively. Band intensities were normalised against an internal standard (silver sol band at 243 cm(-1)). Resonance Raman spectra (RRS) of aqueous solutions of indigo could not be collected because of its low solubility and the presence of strong fluorescence. It was, however, possible to obtain RS and RRS spectra of the solid at each laser excitation wavelength. The limits of detection (L.O.D.) of indigo by SERS and SERRS using 514.5 and 632.8 nm were 9 ppm at both exciting wavelengths. Signal enhancement by SERS and SERRS was highly pH dependent due to the formation of singly protonated and possibly doubly protonated forms of the molecule at acidic pH. The SERS and SERRS data provide evidence to suggest that an excess of monolayer coverage of the dye at the surface of silver colloids is observed at concentrations greater than 7.85x10(-6) mol dm(-3) for each exciting wavelength. The data reported herein also strongly suggest the presence of multiple species of the indigo molecule.     

亚甲基富勒烯衍生物[6,6]-苯基-C61丁酸甲酯的密度泛函研究

运用密度泛函理论和含时密度泛函理论研究了亚甲基富勒烯衍生物[6,6]-苯基-C61丁酸甲酯(PCBM)的几种物理化学性质, 包括几何结构、电子结构、电荷布居与成键, 以及IR、Raman和电子吸收光谱. 自然键轨道方法的结果表明, 大约有0.11个电子通过成键由分子的一部分苯基和丁酸甲酯基团(电子给体)转移到富勒烯笼(电子受体). 最强的IR和Raman谱峰来自于不同的振动模式, 分别位于1773和1492 cm-1处. 计算的各向同性极化率、极化率各向异性不变量以及超极化率分别是577.7、96.9、-22.8 a.u.. 基于含时密度泛函理论计算并分析了PCBM的电子吸收谱, 在349 nm处的吸收峰与实验结果符合很好.     

Differentiation of stereoisomers with electron impact mass spectrometry: (E)- and (Z)-α-hydroxyoximes

The electron impact mass spectra of 5,8-diethyl-7-hydroxydodecan-6-one (Z)-oxime and (E)-oxime provide a basis for distinguishing these stereoisomers.     

亚甲基富勒烯衍生物[6，6]-苯基-C61丁酸甲酯的密度泛函研究

运用密度泛函理论和含时密度泛函理论研究了亚甲基富勒烯衍生物[6.6]-苯基-C61丁酸甲酯(PCBM)的几种物理化学性质,包括几何结构、电子结构、电荷布居与成键,以及IR、Raman和电子吸收光谱.自然键轨道方法的结果表明,大约有0.11个电子通过成键由分子的一部分苯基和丁酸甲酯基团(电子给体)转移到富勒烯笼(电子受体).最强的IR和Raman谱峰来自于不同的振动模式,分别位于1773和1492 cm-1处.计算的各向同性极化率、极化率各向异性不变量以及超极化率分别是577.7、96.9、-22.8 a.u..基于含时密度泛函理论计算并分析了PCBM的电子吸收谱,在349 nm处的吸收峰与实验结果符合很好.     

Kombination von Feldionen- und Elektronestoß-Massenspektren zur Struckturbestimmung,Demonstriert am Beispiel von Monoterpenen

The field ionization and electron impact mass spectra of some monoterpenes are compared. Some general rules applicable also to other substances are derived, summarizing the new structural information that can be obtained by combination of field ionization and electron impact data. The general conclusions are: (1) By comparison of FI and EI mass spectra one can recognize which of the most intense peaks in the EI mass spectra are originating from rearrangement or multistip dissociation processes on the one hand, or from simple direct bond rupture on the other hand. (2) The molecular weight of each substance containing C, H, O, N atoms (or some of them) can be determined unambiguously by field ionization, even if there is no parent peak detectable by electron impact. (3) Extremely strong metastable peaks in the FI mass spectrum are indicative of the splitting off of a polar group from a highly branched carbon atom, under rearrangement of the molecular ion. (4) Ions of about double the molecular weight are often found in the FI mass spectra if the molecule contains carbonyl or hydroxyl groups, or conjugated multiple bonds in aliphatic compounds.     

Electron enhanced Raman scattering and its applications in solution chemistry.

The present review describes a new enhancement technique for Raman scattering in aqueous solutions. Raman scattering spectroscopy has an inherent ability to distinguish between molecules with great similarity and provides useful information on local physical and chemical environments at their functional groups' level. Since the Raman scattering signals from water molecules are quite weak, Raman spectroscopy has great advantage for detection or discrimination of a trace amount of analytes in aqueous environments. However, Raman scattering cross-sections are inherently small and it generally requires high power excitation and long acquisition times to obtain high-quality Raman spectra. These conditions create disadvantages for the analyses for living cells and real-time monitoring for environmental analyses. Here, I describe a new Raman enhancement technique, namely "electron enhanced Raman scattering (EERS)", where artificially generated electrons additionally affect the polarizability of target molecular systems and enhance their inherent Raman cross-section. Principles of the EERS and its applications to aqueous solutions are presented.     

Identification of luminescent europium beta-diketonates using high-resolution emission spectrometry and direct-inlet electron ionization mass spectrometry.

The high-resolution emission spectra measured using a microscope Raman spectrometer and direct-inlet electron ionization mass spectrometry (DI-EIMS) were studied for the rapid analysis of a trace amount of various europium beta-diketonates in order to utilize them as fluorescent markers. Five kinds of europium beta-diketonates were distinguished from the emission spectra obtained using the Raman spectrometer. For DI-EIMS, the electron ionization mass spectra of the europium beta-diketonates showed the characteristic fragment ion peaks due to the isotope effect of 151Eu and 153Eu. The DI-EIMS was very useful for identifying the compounds compared with spectrofluorometry. The scan and selected ion-monitoring modes of DI-EIMS showed the detection limits of the samples at levels of hundreds and tens of nanograms (ng), respectively.     

A new instrument adapted to in situ Raman analysis of objects of art

The analysis of precious artefacts and antiquities demands care in order to minimise the risk of accidental damage during measurement. Mobile fibre-optic-based Raman instruments offer a means to avoid destructive sampling and eliminate the need to transport artefacts for spectrochemical analysis. In this work we present a new mobile instrument developed and optimised for the in situ Raman investigation of objects of art and antiquities. The instrument is controlled by a portable computer. Selected mounts cover many types of artefacts. Newly written software routines organise spectra together with measurement parameters and facilitate calibration of the instrument. The present paper describes this new Raman instrument and discusses some challenges in the transition from a laboratory environment to in situ investigations in museums.     

DFT Study on Methanofullerene Derivative [6,6]-Phenyl-C61 Butyric Acid Methyl Ester

Density functional theory (DFT) and time-dependent density functional theory (TDDFT) with hybrid functional B3LYP were used to investigate several physical and chemical properties of [6,6]-phenyl-C₆₁ butyric acid methyl ester (PCBM), including the geometry, electron structure, charge population, bond properties, as well as IR, Raman and electronic absorption spectra. The analysis of the natural bond orbital (NBO) suggested that there were about 0.11 electrons transferred from moiety phenyl and butyric acid methyl ester group of PCBM to fullerene cage. The strongest IR and Raman peaks came from different modes with the frequencies of 1773 and 1492 cm⁻¹, respectively. The calculated isotropic polarizability, polarizability anisotropy invariant, and hyperpolarizability were 577.7, 96.9, and −22.8 a.u., respectively. Based on TDDFT, the electronic absorption spectra of PCBM were calculated and analyzed. The calculated absorption band near 349 nm agreed well with the experimental measurement.     

Distinguishing by principal component analysis of o-xylene, m-xylene, p-xylene and ethylbenzene using electron ionization mass spectrometry

The possibility of distinguishing between o-, m-, p-xylene and ethylbenzene on the basis of only their 70 eV electron ionization (EI) mass spectra has been investigated. These four isomers were distinguished by principal component analysis (PCA) of respective EI mass spectra (recorded under identical conditions). Considered mass spectra contained either eleven or five m/z values, which had intensity greater than 5% or 10% of base peak, respectively.     

The effect of light and dye composition on the color of dyeings with indigo, 6-bromoindigo,and 6,6′-dibromoindigo,components of Tyrian purple

Quantitative HPLC and colorimetry are used to study color variations in dyeings with indigo, 6-bromoindigo, and 6,6′-dibromoindigo, the main components of the historic dye Tyrian purple. For the first time, visible light is identified conclusively as a cause of debromination of the leuco form of 6-bromoindigo. A dyeing run using 6-bromoindigo alone is found to yield a dyed fabric containing large amounts of indigo, when the vat is exposed to visible light. The extent of debromination is dependent upon the pH of the dye bath and also the source of the visible light. This information allowed development of a dyeing procedure which is demonstrated to give consistent colors through two passes. Quantitative HPLC analysis of extracts from the dyed fabrics indicates that the leuco form of 6-bromoindigo vs. the leuco forms of indigo and 6,6′-dibromoindigo has the strongest affinity for wool fabric. This is postulated to be due to attractive electrostatic interactions between the leuco form of 6-bromoindigo and wool.

     

Analysis of artist’s palette on a 16th century wood panel painting by portable and laboratory Raman instruments

Non-destructive analysis of the artist’s palette of ancient wooden panel paintings is a difficult task and studies are rare. Here we compare different methods of analysis of a wooden panel painting, dated to the early sixteenth century, mainly by Raman and infrared spectroscopies. Raman spectra were recorded on collected/sampled micrometric fragments using portable Raman instruments with laser excitation lines at 532 and 785 nm and transportable Raman instruments at 532, 633 and 785 nm; a fixed 1064 nm Raman spectrometer was also used. Infrared analyses were performed in Attenuated Total Reflection (ATR-FTIR) mode. Using the portable instrument, the Raman spectra evidenced white lead, calcite and vermilion only. Raman spectra recorded by transportable and fixed instruments enabled the identification of most of the artist’s palette: (i) white lead, calcite, gypsum and cerussite for white colour; (ii) vermilion, red lead, litharge, haematite for red; (iii) azurite, indigo and lapis lazuli for blue. IR spectra gave information on the organic binding media. XRF analysis on a brown pigment suggested an heterogeneous mixture of a red pigment (such as haematite and/or minium) and a green one as malachite. GC-MS analysis allowed identifying terpenic resin in the composition of the outer protective layer.     

Electron impact and chemical ionization mass spectra of aryl ureas

The electron impact and chemical ionization mass spectra of a series of N,N′, -diaryl ureas have been compared. The electron impact mass spectra indicate rearrangements leading to two pairs of aromatic amines and isocyanates, either as ions or molecules. The chemical ionization mass spectra showed the formation of protonated amines and isocyanates via rearrangement.     

Positive and negative‐mode laser desorption/ionization‐mass spectrometry (LDI‐MS) for the detection of indigoids in archaeological purple

Laser‐based ionization techniques have demonstrated to be a valuable analytical tool to study organic pigments by mass spectrometric analyses. Though laser‐based ionization techniques have identified several natural and synthetic organic dyes and pigments, they have never been used in the characterization of purple. In this work, positive and negative‐mode laser desorption/ionization mass spectrometry (LDI‐MS) was used for the first time to detect indigoids in shellfish purple. The method was used to study organic residues collected from archaeological ceramic fragments that were known to contain purple, as determined by a classical high‐performance liquid chromatography‐based procedure. LDI‐MS provides a mass spectral fingerprint of shellfish purple, and it was found to be a rapid and successful tool for the identification of purple. In addition, a comparison between positive and negative mode ionization highlighted the complementarity of the two ionization modes. On the one hand, the negative‐ion mode LDI‐MS showed a better selectivity and sensitivity to brominated molecules, such as 6,6'‐dibromoindigo, 6‐monobromoindigo, 6,6'‐dibromoindirubin, 6‐ and 6’‐monobromoindirubin, thanks to their electronegativity, and produced simpler mass spectra. On the other hand, negative‐ion mode LDI‐MS was found to have a lower sensitivity to non‐brominated compounds, such as indigo and indirubin, whose presence can be established in any case by collecting the complementary positive‐ion LDI mass spectrum. Copyright © 2013 John Wiley & Sons, Ltd.     

Raman spectroscopy of archaeological and ancient resins: problems with database construction for applications in conservation and historical provenancing

The adoption of Raman spectroscopy as a screening technique for the presence of organic resins on diverse substrates is now being advocated for the first pass non-destructive examination of potential sites for limited sampling using other analytical techniques. The characterisation of ancient resins in art work and specimens recovered from archaeological excavations is critically dependent upon the analytical capability of Raman spectroscopy using different wavelengths of excitation from the visible to the near infrared and the interpretation of the data illustrates the advantages and limitations of the technique. Resin specimens from art works and artefacts span a period of about 7000 years of recorded history and the influence of factors such as the environmental degradation, burial deposition, reaction with associated substrates and mineral pigments on the observed Raman spectra have been assessed. The key molecular Raman spectral features that are definitive for the discrimination between contemporary resins are considered in respect of these factors and thereby illustrative of the difficulties posed for the creation of a Raman spectral database of ancient resins, in contrast with the extensive and definitive literature equivalents that are available for their mineral pigment and organic dye analogues.