Synthetic Organic Colorants Research Articles

Beyond Artists' Colors: A Spectral Reference Database for the Identification of β-Naphthol and Triarylcarbonium Colorants by MeVSIMS.

β-Naphthol and triarylcarbonium colorants were often used by modern and contemporary artists in materials such as inks and paints. Their poor stability and ability to fade upon exposure to light make their identification in artworks crucial for planning exhibitions and preventive conservation. Secondary ion mass spectrometry with MeV primary ions (MeV SIMS) is necessary when analyzing synthetic organic colorants (SOCs) with similar molecular structures due to its advantages in high sensitivity and soft ionization, which causes a low fragmentation of organic molecules. In this work, we applied MeV SIMS with 5 MeV Si4+ to identify selected β-naphthol and triarylcarbonium colorants from the 19th/20th century Materials Collection kept at the Academy of Fine Arts Vienna. The collection contains SOCs from renowned companies, such as I.G. Farben and I.C.I., and serves as a unique source of reference materials in the analysis of artworks. Previous research on these colorants with X-ray fluorescence analysis (XRF), micro-Raman, and Fourier transform infrared (FTIR) spectroscopies failed in the case of colorant mixtures. Similar spectral features of the SOCs within one chemical class and their low concentrations in mixtures did not lead to their identification using these techniques. MeV SIMS detected molecular ions or protonated molecules in the positive-ion mode. In the negative-ion mode, the functional groups (NO2 - and SO3 -) of β-naphthol lakes/pigments and heteropolyacid species (WO3 - and MoO3 -) characteristic of triarylcarbonium toners were determined. The results demonstrate that MeV SIMS is highly effective for identifying β-naphthol and triarylcarbonium colorants in mixtures and distinguishing between pigments, toners, lakes, and dyes.

Pigments, Dyes and the Restoration History of the Painted Figurines of the Tang Dynasty from the Astana Tombs Revealed by Comprehensive Chemical Analysis

AbstractComprehensive micro‐analyses were conducted to identify different pigments and dyes used in painted figurines of the Tang Dynasty unearthed from Astana Tombs, located along the ancient Silk Road. Interestingly, a mixture of lazurite and indigo was first discovered on a painted figurine to create a special blue hue, demonstrating that ancient Gaochang craftsmen were already skilled in using mixed colorants. Moreover, superimposition techniques were as well applied in these painted figurines, such as hematite or cinnabar overlaid on red lead. Despite the lack of unambiguous restoration records in the last century, multiple synthetic organic colorants used for repainting the lokapala figurine were successfully identified by Raman spectroscopy and ultra‐performance liquid chromatography‐tandem mass spectrometry (UPLC‐MS/MS), which should be helpful for the retrospective documentation of the restoration history of these artworks. These investigations towards archaeological materials can provide precise historical facts about the artworks and fundamental knowledge for their conservation.

Determination of Carcinogenic Primary Aromatic Amines Contained as Impurities in Synthetic Organic Colorants.

Several primary aromatic amines (PAAs) have been designated carcinogenic or suspected of carcinogenicity. Several kinds of PAAs may occur either via the reduction of azo compounds or as impurities in azo colorants or other agents. An analytical method was developed and applied to determine whether certain PAAs are present as impurities in synthetic organic colorants. Target chemicals were analyzed by the ultrasound extraction of the synthetic organic colorant with a hydrochloric acid solution containing 20% methanol, followed by conversion from an acidic to alkaline solution, and then extraction using a diatomaceous earth column. We analyzed certain PAAs in 38 synthetic organic colorants, resulting in the detection of 2,4-dimethylaniline in four samples at 1.2 to 19 μg/g, o-toluidine in three samples at 1.0 to 3.4 μg/g, p-phenylazoaniline in two samples at 74 to 305 μg/g, and, in one sample each, 2,4,5-trimethylaniline (13 μg/g), 5-nitro-o-toluidine (12 μg/g), and 2-methyl-4-(2-tolylazo)aniline (13 μg/g). Nearly all PAAs were determined to be starting materials for colorant synthesis, although p-phenylazoaniline in Yellow No. 407 was apparently a byproduct formed during synthesis. For Red No. 225, in which high concentrations of p-phenylazoaniline were detected, additional samples were purchased from five companies, and p-phenylazoaniline was detected at concentrations of 88 to 370 μg/g in all samples. A method to analyze certain PAAs contained as impurities in synthetic organic colorants was developed, and the actual status of them in colorants was clarified. The analytical method developed in this study for the determination of certain PAAs contained as impurities in synthetic organic colorants may be used to improve the safety of colorants.

Recent Advances on the Analysis of Polychrome Works of Art: SERS of Synthetic Colorants and Their Mixtures With Natural Dyes.

The development and application of proper sample pretreatments is often a key step toward the successful analysis of dyes used as artists' materials by surface-enhanced Raman spectroscopy (SERS). Complexation of the organic colorants with metal ions to dye fabrics and produce lake pigments, as well as undesired interactions with other matrix components such as substrate, binding media, fillers, and extenders, are just some of the issues that typically complicate dye identification in minute samples from invaluable artworks and museum objects. These concerns may be addressed by using, prior to SERS analysis, ad-hoc sample pretreatments that, in addition to increasing the technique's sensitivity, favorably affect its selectivity toward certain molecules or molecular classes. The present work describes a newly developed sample pretreatment based on the use of nitric acid that has proven crucial for the successful detection of aniline and xanthene dyes–the first synthetic organic colorants to be used in printing and painting, among other art forms–in microscopic samples from works of art such as a 19th-century silk fabric, paper cut-outs by Henri Matisse, Vincent Van Gogh's Irises, and Japanese woodblock prints. This treatment promotes the hydrolysis of the dye-metal bond in mordant dyes or lake pigments, resulting in a more efficient adsorption of the dye molecules on the SERS-active substrate and, hence, enabling the acquisition of high-quality spectra. In the case of synthetic colorants, this method shows advantages over hydrolysis with hydrofluoric acid–a procedure previously established for the analysis of red lakes prepared from natural dyes. The nitric acid treatment presented here may be integrated into a multi-step methodology that, by exploiting differences in solubility of various dyes and lake pigments, has enabled for the first time to successfully characterize intentional mixtures of natural and synthetic colorants of the xanthene and anthraquinone molecular classes, i.e., eosin Y and carmine, in a selection of Japanese prints of the Meiji era. The present study paves the way for the systematic identification of synthetic dyes in objects of artistic and archeological interest, even when they are present in mixtures with natural organic colorants.

Mass Spectrometric Analysis of Synthetic Organic Pigments

Though synthetic organic colorants are used in various applications nowadays, there is the concern that impurities by-produced during the manufacturing and degradation products in some of these colorants are persistent organic pollutants and carcinogens. Thus, it is important to identify the synthetic organic colorants in various products, such as commercial paints, ink, cosmetics, food, textile, and plastics. Dyes, which are soluble in water and other solvents, could be analyzed by chromatographic methods. In contrast, it is difficult to analyze synthetic organic pigments by these methods because of their insolubility. This review is an overview of mass spectrometric analysis of synthetic organic pigments by various ionization methods. We highlight a recent study of textile samples by atmospheric pressure solid analysis probe MS. Furthermore, the mass spectral features of synthetic organic pigments and their separation from other components such as paint media and plasticizers are discussed.

Statistical methods and library search approaches for fast and reliable identification of dyes using surface-enhanced Raman spectroscopy (SERS)

The identification of organic colorants is of high importance in the cultural heritage field, where they are found as paint components and textile dyes, and in forensic science, because of their use in inks and paints, food colorants and textile dyes. Surface-enhanced Raman spectroscopy (SERS) has emerged as a promising technique for the detection of these materials, yet concerns over the sensitivity of SERS spectra of dyes to chemical and instrumental variables (such as pH, choice of SERS substrates and/or aggregants, and excitation wavelength) have prevented its widespread use in analytical applications. Over the last few years, the development of several microanalytical approaches has considerably increased the chances of success in the identification of minute amounts of dyes by SERS. However, the need for searchable databases is still largely to be fulfilled. In this work, we have assembled the core of a comprehensive library which contains 100 Raman and SERS reference spectra of natural and synthetic organic colorants. Experiments to classify 20 query SERS spectra of dyes from a variety of museum objects were conducted using principal component analysis (PCA) and the correlation coefficient (CC) algorithm. The effect of spectral transformations such as baseline correction, selecting a standard frequency range, normalization, smoothing, as well as carrying out the search on the second derivative of the spectra, was systematically evaluated. With this study we demonstrate that SERS spectra of organic colorants can be reliably matched against a well constructed spectral library regardless of the instrumentation and the colloids used, and of the pH conditions at which the measurements were carried out.

Nondestructive Identification of Natural and Synthetic Organic Colorants in Works of Art by Surface Enhanced Raman Scattering

We present a new method based on surface-enhanced Raman scattering (SERS) for the nondestructive identification of organic colorants in objects whose value or function precludes sampling, such as drawings, prints, historic and archeological textiles, handwritten or printed documents, and forensic evidence. A bead of a polymer hydrogel loaded with a solution containing water, an organic solvent, and a chelating agent is used to extract minimal amounts of the colorants from the work of art for SERS analysis. Using a gel as a medium for the solvent mixture confines its action only to the areas of the work of art covered by the gel bead. The gel bead is then removed from the work of art, covered with a drop of Ag colloid, and examined with a Raman microscope. Transfer of the dye from the substrate to the gel does not require removing a sample from the work of art, therefore preserving the physical integrity of the object. Spectrophotometric color measurements confirm that color change is below the limit perceivable by a human observer. Finally, the size of the polymer bead can be reduced to a fraction of a millimeter in order to further minimize any impact on the work of art, without detriment to the effectiveness of the method. The technique has been successfully used for the analysis of a mordant dye on the 15th century Netherlandish tapestry, "The Hunt for the Unicorn", and of a synthetic lake pigment on a Meiji period Japanese woodblock print.

X-ray powder diffraction investigation of green earth pigments

Laboratory X-ray powder diffraction was used to investigate mineralogical compositions of green pigments labeled by suppliers as “green earths.” It was found that glauconite and celadonite—minerals historically considered as the main ingredient of this pigment—were present only in Bohemian green earth, green earth from Thuringen (glauconite), and Bavarian green earth (celadonite). Other investigated pigments consist of mineralogical-component minerals with added synthetic organic colorants. The obtained results may be useful for scientists, restorers, and artists in proper choices of the pigments they use in their works.

Raman study of synthetic organic pigments and dyes in early lithographic inks (1890–1920)

AbstractThe colorants and additives present in lithographic inks used in a group of posters manufactured during the period between 1890 and 1920, an extremely fruitful period regarding the development of the synthetic dye and pigment industry, were identified by Raman spectroscopy. Synthetic organic pigments and dyes were detected along with inorganic pigments. Red, blue and yellow synthetic organic pigments were more frequently observed in posters manufactured by the end of the 1890–1920 period, reflecting that lithographers were actively experimenting to achieve hues not accessible using inorganic pigments alone. The water‐sensitivity of the inks containing synthetic organic colorants is discussed in relation to the potential application of aqueous conservation treatments to early posters. Copyright © 2006 John Wiley & Sons, Ltd.

Molecular Cloning and Characterization of the Gene Coding for Azoreductase from Bacillus sp. OY1-2 Isolated from Soil

Azo dyes are regarded as pollutants because they are not readily reduced under aerobic conditions. Bacillus sp. OY1-2 transforms azo dyes into colorless compounds, and this reduction is mediated by a reductase activity for the azo group in the presence of NADPH. A 1.2-kbp EcoRI fragment containing the gene that encodes azoreductase was cloned by screening the genomic library of Bacillus sp. OY1-2 with digoxigenin-labeled probe designed from the N-terminal amino acid sequence of the purified enzyme. An open reading frame encoding the azoreductase, consisting of 178 amino acids, was predicted from the nucleotide sequence. In addition, because only a Bacillus subtillis hypothetical protein was discovered in the public databases (with an amino acid identity of 52.8%), the gene encoding the azoreductase cloned in this study was predicted to be a member of a novel family of reductases. Southern blot analysis revealed that the azoreductase gene exists as a single copy gene on a chromosome. Escherichia coli-expressing recombinant azoreductase gave a ten times greater reducing activity toward azo dyes than the original Bacillus sp. OY1-2. In addition, the expressed azoreductase purified from the recombinant E. coli lysate by Red-Sepharose affinity chromatography showed a similar activity and specificity as the native enzyme. This is the first report describing the sequencing and characterization of a gene encoding the azo dye-reducing enzyme, azoreductase, from aerobic bacteria and its expression in E. coli.

Treatment and Decolorization of Dyes in an Anaerobic Baffled Reactor

Synthetic organic colorants, the majority of which are recalcitrant in nature, are used universally in many different manufacturing processes. The dyes are released into the environment in industrial effluents and are highly visible even at low concentrations (<1 mg/L). Added to this, certain dyes, dye precursors, and aromatic amines have been shown to be carcinogenic. Thus, appropriate treatment of dye wastewaters to remove color and the dye compounds is clearly an important issue. Methanogenic toxicity tests on several food dyes provided a range of toxicity results, from noninhibitory (IC50 >20 g/L) to inhibitory (IC50 0.2 mg/L). Batch biodegradability assays indicated that the dyes were not readily utilized by the anaerobic microorganisms as a sole substrate. Decolorization of the dye tartrazine was investigated in a laboratory-scale anaerobic baffled reactor at a concentration of 250 mg/L. Reduction in COD of 50–60% and color reduction of about 95% was achieved. Initially the tartrazine was not readily decolorized; however, decolorization improved with acclimation of the biomass. An industrial wastewater from a food dye manufacturer was fed to a second laboratory-scale anaerobic baffled reactor at a concentration of 5% (volume-to-volume ratio) and then increased to 10% (volume-to-volume ratio). Anaerobic degradation of the wastewater was efficient. Methanogenic activity was high; the organic content of the influent was reduced by about 70%, and color was reduced by almost 90%

Axo-dye biodegradation under anoxic conditions

Biological oxidation of azo-dyes is important for wastewater treatment. Azo-dyes are synthetic organic colorants that exhibit great structural variety. A large majority of these dyes are released into the environment. The textile industry and dyestuff manufacturing industry are two major sources of released azodyes. In the present study, we focus on the anoxic degradation of a disperse azo-dye, p-aminoazobenzene (pAAB), a simple azo-dye, by a pure culture of Bacillus subtilis, growing on a synthetic medium. Bacillus subtilis is a bacterium capable of using nitrate and/or nitrite as terminal electron acceptor under anoxic conditions. This bacterium lacks the capability for fermentation. The degradation of p-aminoazobenzene by Bacillus subtilis was examined through batch experiments in order to elucidate the mechanism of dye degradation. The results proved that Bacillus subtilis co-metabolizes p-aminoazobenzene under denitrifying conditions, in the presence of glucose as carbon source, producing aniline and p-phenylenediamine as the nitrogen-nitrogen double bond is broken.

Azo-dye biodegradation under anoxic conditions

Biological oxidation of azo-dyes is important for wastewater treatment. Azo-dyes are synthetic organic colorants that exhibit great structural variety. A large majority of these dyes are released into the environment. The textile industry and dyestuff manufacturing industry are two major sources of released azo-dyes. In the present study, we focus on the anoxic degradation of a disperse azo-dye, p-aminoazobenzene (pAAB), a simple azo-dye, by a pure culture of Bacillus subtilis, growing on a synthetic medium. Bacillus subtilis is a bacterium capable of using nitrate and/or nitrite as terminal electron acceptor under anoxic conditions. This bacterium lacks the capability for fermentation. The degradation of p-aminoazobenzene by Bacillus subtilis was examined through batch experiments in order to elucidate the mechanism of dye degradation. The results proved that Bacillus subtilis co-metabolizes p-aminoazobenzene under denitrifying conditions, in the presence of glucose as carbon source, producing aniline and p-phenylenediamine as the nitrogen-nitrogen double bond is broken.

International Regulation of Chemicals — Implications for Organic Colorants

Following the enactment of the Toxic Substances Control Act (USA), 6th Amendment (EEC) and similar legislation elsewhere, the problems of over‐regulation have become particularly serious for the dye‐making industry, with its multitude of widely‐used but usually small‐tonnage products. Apart from the heavy administrative burden imposed by these regulations, the problems are aggravated by the emotionalism which abounds concerning carcinogenic risk, and the lack of consensus on how animal tests at excessively high dose levels be interpreted in terms of risk to man from trace exposure. A further problem is posed by the incorporation of short‐term testing requirements in the laws before their relevance is fully understood.This situation leads to the imposition of ultra‐conservative safeguards against risks which are probably insignificant when viewed in the perspective of the total risk situation. The lack of harmonization in requirements between countries creates trade barriers and leads to the anomalous situation where a dye which is a permitted food additive in one country has to be labelled as a carcinogenic textile dye in another.Inadequate recognition of the importance of confidentiality to the specialised industries poses additional problems as authorities are now establishing computer data banks of ecological and toxicological data. Whilst hazards should not be concealed, the dissemination of costly data and know‐how gratis to competitors can only further penalize the innovator.These and other problems currently facing the dye‐making industry are explored and the current ecological and toxicological studies are summarized to ensure that, properly handled and used, synthetic organic colorants do not present an unreasonable risk to man or the environment.