Daguerreotype images consist of mercury-silver or mercury-silver-gold amalgam nanoparticles on si... more Daguerreotype images consist of mercury-silver or mercury-silver-gold amalgam nanoparticles on silver-coated copper substrates that are par-ticularly sensitive to environmental pollutants such as hydrogen sulfide and chlorine-containing compounds. Typically, silver along with copper and lead foils are used in the standard Oddy test at cultural institutions to asses the suitability of materials for the display and storage of silver objects. In this study, it was observed that silver foil does not reflect the sensitivity of a daguerreotype image under the same testing conditions. Therefore, the use of vapor deposited silver films with a thickness opti-mized to best follow the reactivity of a daguerreotype sample in a modi-fied Oddy test setup is proposed. Results are reported for a group of materials chosen for testing because they are either used for housing daguerreotypes at several institutions or could be good replacement options. The new procedure allows for facile visual evaluat...
Artists at the turn of the 20th century experimented with a wide range of photographic techniques... more Artists at the turn of the 20th century experimented with a wide range of photographic techniques to create complex and unique prints by processes that are difficult to fully identify. FTIR, both in transmission and ATR, Raman spectroscopy and XRF were applied to characterize and identify the materials and processes used in photographs from the period in which pigment-based techniques were employed alone or superimposed to others such as platinum. FTIR in the transmission mode and ATR acquired in the high image density areas of the images allow to characterize the organic components in the photographic colloid, and the analysis of the low image density areas gives vibrational information about the organic compounds used for sizing the paper substrates. Raman spectroscopy allows the identification of the pigments, while the chromium-containing compounds, such as chrome alum used to size the paper substrates and the compound/s that chromium forms with the oxidized organic component in gum print images, were found to be below the detection limit of the technique both in samples prepared strictly following historic recipes and in hundred year old works of art. The advantages and limitations of the multitechnique approach used to identify and differentiate gum prints are discussed.
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, 2008
The characterization of the binding media and pigments in modern and contemporary paintings is im... more The characterization of the binding media and pigments in modern and contemporary paintings is important for designing safe conservation treatments, as well as for determining suitable environmental conditions for display, storage and transport. Raman spectroscopy is a suitable technique for the in situ non-destructive identification of synthetic organic pigments in the presence of the complex binding media characteristic of synthetic resin paints or colour lithographic inks. The precise identification of a pigment by comparing its spectrum to that of a reference is necessary when conservation treatments with aqueous solutions or organic solvents are being considered for a work of art, since solubility properties can sometimes vary within the same pigment group. The Raman spectra of 21 yellow synthetic organic pigments, belonging to the monoazo, monoazo lakes, diarylide, disazo condensation, benzimidazolone, bisacetoacetarylide, azo-methine metal complex, isoindolinone and isoindoline groups are presented. Since modern artists frequently mixed paint developed for other applications, in addition to colorants developed as artists' paints, other synthetic organic pigments were included in the spectral database. Two monoazo pigments, Pigment Yellow 1 and Pigment Yellow 3, a benzimidazolone, Pigment Yellow 154 and a phthalocynanine, Pigment Green 7, were identified in sample cross-sections from four modern and contemporary paintings in the collection of The Museum of Modern Art in Ljubljana, Slovenia.
Artists' acrylic paint surfaces can accumulate surface grime. Aqueous surface cleaning of th... more Artists' acrylic paint surfaces can accumulate surface grime. Aqueous surface cleaning of these paints can be particularly problematic because they swell upon exposure to water and because of the presence of water extractable additives. Research to determine the parameters of ...
Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 2005
The elemental composition and depth profiles obtained by IBA techniques on some gilded-copper fra... more The elemental composition and depth profiles obtained by IBA techniques on some gilded-copper fragments from the Moche site of Loma Negra, in the Piura Valley, on the Northern Coast of Perú are presented in this article. A previous radiocarbon dating of a wooden fragment indicated that Loma Negra was occupied around 295 AD. A PIXE analysis using a 2.6MeV external
Dramatic condition changes were noted in a group of daguerreotypes during a 10 month‐long exhibit... more Dramatic condition changes were noted in a group of daguerreotypes during a 10 month‐long exhibition featuring masterworks of the prominent Boston‐based studio of Southworth & Hawes. The changes took the form of a hazy, extended white surface layer and of white spots that partially obscured the images after between four weeks and 10 months of exhibition. Raman spectroscopy, complemented by in situ scanning electron microscopy–energy dispersive X‐ray spectrometry (SEM–EDS), was used to nondestructively characterize this form of image deterioration in eight representative daguerreotypes from the studio. Raman proved to be a sensitive technique to noninvasively identify the compounds on the plates due to the fact that their Ag surfaces behave as surface‐enhanced Raman scattering (SERS) substrates. A band at ca 242 cm−1 assigned to the AgCl stretching mode was observed in the areas where a white surface layer or white spots are present. In some of the areas probed, substituted aromatic...
The complexes between Al(III) and hematein, the main coloring matter in alum logwood inks, were c... more The complexes between Al(III) and hematein, the main coloring matter in alum logwood inks, were characterized by Raman and 27 Al NMR (nuclear magnetic resonance) spectroscopies. Raman spectra of the crystallized complexes and of the compounds applied on a paper substrate are presented and assigned based on published data for the parent compounds. These Raman spectra show that the coordination of the hematein to the Al(III) ions takes place in both cases through the carbonyl and hydroxyl groups in the molecule, and that the aromatic rings are also involved in the interaction. The Raman spectra of the pure hematein-Al(III) complexes were found to be consistent with those recorded for a logwood ink prepared following a late 19th century recipe, using logwood chips instead of pure hematein, and applied on a paper substrate. These spectra can be used as references for the noninvasive identification of the compounds in works of art. 27 Al solid-state NMR showed that the coordination of the Al(III) atoms in the crystallized powder is predominantly octahedral, while when applied on a paper substrate the colorant is present mainly as a tetrahedral complex, with an octahedral coordination also present in a smaller proportion. The fact that the predominant coordinations for the complexes in the crystallized material and for the ones present on the paper substrate are different is relevant for the study of the lightfastness and thermal stability of works of art bearing these media.
... Font Size: Decrease Font Size Increase Font Size. ... Karina I. Gobbato a , 1 , Christiane Le... more ... Font Size: Decrease Font Size Increase Font Size. ... Karina I. Gobbato a , 1 , Christiane Leibold a , Silvia Centeno b , Carlos O. Della Védova b , Hans-Georg Mack a and Heinz Oberhammer a , Corresponding Author Contact Information. ...
... Permissions & Reprints. Surface enhanced Raman scattering (SERS) and FTIR characterizatio... more ... Permissions & Reprints. Surface enhanced Raman scattering (SERS) and FTIR characterization of the sepia melanin pigment used in works of art. Silvia A. Centeno a , Corresponding Author Contact Information , E-mail The Corresponding Author and Jacob Shamir a , b. ...
X-ray diffraction (XRD) complemented by Raman spectroscopy analyses of synthetic organic pigments... more X-ray diffraction (XRD) complemented by Raman spectroscopy analyses of synthetic organic pigments in powder samples, layered paint systems, and commercial artists' paints bound in acrylic, alkyd, and oil media are presented. The potential and limitations of the techniques to identify and characterize mixtures of these pigments, along with inorganic extenders, in works of art are exemplified and discussed. Stratified model paint systems that mimic the layering structure typically found in modern paintings are used to evaluate the effect of the µXRD experimental parameters, as well as extenders or fillers commonly found in modern artists' paint formulations, on the quality of the patterns recorded in microsamples of paint. XRD is demonstrated for the first time to be an effective tool for the specific identification of synthetic organic pigment mixtures and fillers in acrylic and alkyd bound artists' paints, while the identification of these pigments by XRD in oil bound paints appears problematic. Detailed crystallographic information provided by XRD is shown to be complementary to molecular information provided by Raman analysis. The combined use of these techniques allows for more frequent unambiguous compound identification than would be possible using one technique alone.
Proceedings of the National Academy of Sciences, 2019
Significance Daguerreotypes, among the earliest photographs of the 19th century, owe their incred... more Significance Daguerreotypes, among the earliest photographs of the 19th century, owe their incredible optical properties, image resolution, and dynamic range to light scattering produced by metallic nanostructures on their surface. Here we provide a detailed experimental and theoretical analysis on how the material composition, morphology, and dimensions of these nanostructures determine the characteristics of the daguerreotype image. Our results provide a scientific understanding of the unique optical effects of these artworks and therefore, in addition to providing valuable insight for developing preservation protocols, can inspire additional approaches for color printing, where nanostructures are directly manufactured by light.
Identification of pyroxene minerals used as black pigments in painted human bones excavated in no... more Identification of pyroxene minerals used as black pigments in painted human bones excavated in northern patagonia by raman spectroscopy and xrd,
The natural sepia pigment is based on eumelanin, the most prevalent type of melanin, and can be o... more The natural sepia pigment is based on eumelanin, the most prevalent type of melanin, and can be obtained from the ink sac of different members of the Cephalopoda class, such as cuttlefish and squid. The main components of natural sepia are indole derivatives, but pigments prepared following historical recipes are generally more heterogeneous as they may also contain proteins, polysaccharides, and lipids. Sepia is difficult to identify in works of art owing to its heterogeneity, insolubility in most organic solvents, interference by the binding media, and its poorly defined spectroscopic properties. In the present study, three commercial natural sepia pigments, along with a sample extracted from cuttlefish following a historical recipe in an artists' manual, a relatively more pure commercial melanin, and a synthetic eumelanin obtained by the oxidation of dopamine were characterized by normal Raman and surface-enhanced Raman scattering (SERS), complemented by Fourier transform infrared, pyrolysis-gas chromatography-mass spectrometry, and X-ray fluorescence, in order to obtain vibrational signatures for the identification of the pigment in works of art. Pyrolysis-gas chromatography-mass spectrometry and X-ray fluorescence analysis showed that the pigment composition is strongly influenced by the extraction procedure used. Lipids were identified in the chromatograms of all the natural samples analyzed, and the majority of these also showed the presence of cholesterol derivatives. Additional components resulting from the decomposition of polysaccharides were found to be present in the sepia extracted from cuttlefish in our laboratories. Normal Raman and SERS were also used to study two sepia inks applied on paper and subjected to accelerated aging. In the SERS measurements, a hydroxylamine-reduced silver colloid was used as this substrate is suitable for the identification of other natural heterogeneous black-brown pigments. The SERS methodology developed was applied to identify sepia in Waiting for Aaron, a drawing by the 19th century artist Robert Frederick Blum.
ABSTRACTA combination of FTIR, normal Raman, SERS, Py-GC/MS and XRF was used to analyze commercia... more ABSTRACTA combination of FTIR, normal Raman, SERS, Py-GC/MS and XRF was used to analyze commercial bistre samples to determine specific biomarkers that will allow for a rapid identification of the pigment in works of art. The results of the XRF analysis showed that potassium, calcium and iron are the main elements present. Characteristic bands belonging to phenolic components of lignin were observed in the FTIR spectra. The SERS analysis provided a fingerprint that may originate in the polymerization of the phenolic components catalyzed by the presence of the nanostructured silver surface under alkaline conditions. The Py-GC/MS analysis revealed the presence of lignin and cellulose biomarkers and a series of polycyclic aromatic hydrocarbons. The similarities observed between the commercial samples studied suggest that both originate in the same raw material, hardwood. The results demonstrate the potential of the multi-technique approach used for the characterization of this complex ...
... Silvia A. Centeno a , Corresponding Author Contact Information , E-mail The Corresponding Aut... more ... Silvia A. Centeno a , Corresponding Author Contact Information , E-mail The Corresponding Author , Veronica Williams b , Nicole C. Little c , Robert J. Speakman c. a, The Metropolitan Museum of Art, Department of Scientific Research. ...
Adsorption of phenol on a polycrystalline gold electrode has been studied by capacitance and modu... more Adsorption of phenol on a polycrystalline gold electrode has been studied by capacitance and modulated reflectance measurements. Two main processes were detected. First, an adsorption-desorption peak was observed at negative charge on the electrode which was related to a flat configuration of the molecule bonded to the surface through its ir system. In this case the interactions between adsorbed phenol and the electrode are not strong enough to modify the intrinsic optical properties of the metal. A second capacitive peak, detected over a region of positive charge on the metal, was found to show hysteresis and, besides, it distorts the electroreflectance effect of gold to such an extent that separation of the optical effects of charge and coverage was not possible. This behavior, typical of the formation of strong covalent bonds, was associated with the reorientation of the molecule from flat to vertically attached to the metal by means of its oxygen atom.
Methyl viologen (MV 2+) has been studied at a thin mercury film electrode in the presence of iodi... more Methyl viologen (MV 2+) has been studied at a thin mercury film electrode in the presence of iodide ions. Conventional electrochemistry and in-situ optical probes have been applied to investigate processes not previously observed in fluoride solutions. As the potential is made negative, iodide desorption is seen attended by an intense increase in reflectivity. The MV 2+ first reduction coincides with the formation of the (MV •+-I-) species on the surface, at the monolayer level, showing strong adsorbate-adsorbate interactions and a flat orientation of the pyridyl groups. The ac current response accompanying the appearance of the (MV •+-I-) film was found to be similar to that of a phase transition (condensation) in the adsorbed state. The dissociation of (MV •+-I-) clusters, to yield MV 0 (solid) + I-(aq), at the onset of the second reduction wave of MV 2+ , leads to a reflectivity enhancement in the in-phase optical signal paralleled by a sharp doublet in the imaginary current response. The splitting of the reduction peak of (MV •+-I-) may arise from different orientations of similar clusters or from distinct oligomer sizes.
Daguerreotype images consist of mercury-silver or mercury-silver-gold amalgam nanoparticles on si... more Daguerreotype images consist of mercury-silver or mercury-silver-gold amalgam nanoparticles on silver-coated copper substrates that are par-ticularly sensitive to environmental pollutants such as hydrogen sulfide and chlorine-containing compounds. Typically, silver along with copper and lead foils are used in the standard Oddy test at cultural institutions to asses the suitability of materials for the display and storage of silver objects. In this study, it was observed that silver foil does not reflect the sensitivity of a daguerreotype image under the same testing conditions. Therefore, the use of vapor deposited silver films with a thickness opti-mized to best follow the reactivity of a daguerreotype sample in a modi-fied Oddy test setup is proposed. Results are reported for a group of materials chosen for testing because they are either used for housing daguerreotypes at several institutions or could be good replacement options. The new procedure allows for facile visual evaluat...
Artists at the turn of the 20th century experimented with a wide range of photographic techniques... more Artists at the turn of the 20th century experimented with a wide range of photographic techniques to create complex and unique prints by processes that are difficult to fully identify. FTIR, both in transmission and ATR, Raman spectroscopy and XRF were applied to characterize and identify the materials and processes used in photographs from the period in which pigment-based techniques were employed alone or superimposed to others such as platinum. FTIR in the transmission mode and ATR acquired in the high image density areas of the images allow to characterize the organic components in the photographic colloid, and the analysis of the low image density areas gives vibrational information about the organic compounds used for sizing the paper substrates. Raman spectroscopy allows the identification of the pigments, while the chromium-containing compounds, such as chrome alum used to size the paper substrates and the compound/s that chromium forms with the oxidized organic component in gum print images, were found to be below the detection limit of the technique both in samples prepared strictly following historic recipes and in hundred year old works of art. The advantages and limitations of the multitechnique approach used to identify and differentiate gum prints are discussed.
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, 2008
The characterization of the binding media and pigments in modern and contemporary paintings is im... more The characterization of the binding media and pigments in modern and contemporary paintings is important for designing safe conservation treatments, as well as for determining suitable environmental conditions for display, storage and transport. Raman spectroscopy is a suitable technique for the in situ non-destructive identification of synthetic organic pigments in the presence of the complex binding media characteristic of synthetic resin paints or colour lithographic inks. The precise identification of a pigment by comparing its spectrum to that of a reference is necessary when conservation treatments with aqueous solutions or organic solvents are being considered for a work of art, since solubility properties can sometimes vary within the same pigment group. The Raman spectra of 21 yellow synthetic organic pigments, belonging to the monoazo, monoazo lakes, diarylide, disazo condensation, benzimidazolone, bisacetoacetarylide, azo-methine metal complex, isoindolinone and isoindoline groups are presented. Since modern artists frequently mixed paint developed for other applications, in addition to colorants developed as artists' paints, other synthetic organic pigments were included in the spectral database. Two monoazo pigments, Pigment Yellow 1 and Pigment Yellow 3, a benzimidazolone, Pigment Yellow 154 and a phthalocynanine, Pigment Green 7, were identified in sample cross-sections from four modern and contemporary paintings in the collection of The Museum of Modern Art in Ljubljana, Slovenia.
Artists' acrylic paint surfaces can accumulate surface grime. Aqueous surface cleaning of th... more Artists' acrylic paint surfaces can accumulate surface grime. Aqueous surface cleaning of these paints can be particularly problematic because they swell upon exposure to water and because of the presence of water extractable additives. Research to determine the parameters of ...
Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 2005
The elemental composition and depth profiles obtained by IBA techniques on some gilded-copper fra... more The elemental composition and depth profiles obtained by IBA techniques on some gilded-copper fragments from the Moche site of Loma Negra, in the Piura Valley, on the Northern Coast of Perú are presented in this article. A previous radiocarbon dating of a wooden fragment indicated that Loma Negra was occupied around 295 AD. A PIXE analysis using a 2.6MeV external
Dramatic condition changes were noted in a group of daguerreotypes during a 10 month‐long exhibit... more Dramatic condition changes were noted in a group of daguerreotypes during a 10 month‐long exhibition featuring masterworks of the prominent Boston‐based studio of Southworth & Hawes. The changes took the form of a hazy, extended white surface layer and of white spots that partially obscured the images after between four weeks and 10 months of exhibition. Raman spectroscopy, complemented by in situ scanning electron microscopy–energy dispersive X‐ray spectrometry (SEM–EDS), was used to nondestructively characterize this form of image deterioration in eight representative daguerreotypes from the studio. Raman proved to be a sensitive technique to noninvasively identify the compounds on the plates due to the fact that their Ag surfaces behave as surface‐enhanced Raman scattering (SERS) substrates. A band at ca 242 cm−1 assigned to the AgCl stretching mode was observed in the areas where a white surface layer or white spots are present. In some of the areas probed, substituted aromatic...
The complexes between Al(III) and hematein, the main coloring matter in alum logwood inks, were c... more The complexes between Al(III) and hematein, the main coloring matter in alum logwood inks, were characterized by Raman and 27 Al NMR (nuclear magnetic resonance) spectroscopies. Raman spectra of the crystallized complexes and of the compounds applied on a paper substrate are presented and assigned based on published data for the parent compounds. These Raman spectra show that the coordination of the hematein to the Al(III) ions takes place in both cases through the carbonyl and hydroxyl groups in the molecule, and that the aromatic rings are also involved in the interaction. The Raman spectra of the pure hematein-Al(III) complexes were found to be consistent with those recorded for a logwood ink prepared following a late 19th century recipe, using logwood chips instead of pure hematein, and applied on a paper substrate. These spectra can be used as references for the noninvasive identification of the compounds in works of art. 27 Al solid-state NMR showed that the coordination of the Al(III) atoms in the crystallized powder is predominantly octahedral, while when applied on a paper substrate the colorant is present mainly as a tetrahedral complex, with an octahedral coordination also present in a smaller proportion. The fact that the predominant coordinations for the complexes in the crystallized material and for the ones present on the paper substrate are different is relevant for the study of the lightfastness and thermal stability of works of art bearing these media.
... Font Size: Decrease Font Size Increase Font Size. ... Karina I. Gobbato a , 1 , Christiane Le... more ... Font Size: Decrease Font Size Increase Font Size. ... Karina I. Gobbato a , 1 , Christiane Leibold a , Silvia Centeno b , Carlos O. Della Védova b , Hans-Georg Mack a and Heinz Oberhammer a , Corresponding Author Contact Information. ...
... Permissions & Reprints. Surface enhanced Raman scattering (SERS) and FTIR characterizatio... more ... Permissions & Reprints. Surface enhanced Raman scattering (SERS) and FTIR characterization of the sepia melanin pigment used in works of art. Silvia A. Centeno a , Corresponding Author Contact Information , E-mail The Corresponding Author and Jacob Shamir a , b. ...
X-ray diffraction (XRD) complemented by Raman spectroscopy analyses of synthetic organic pigments... more X-ray diffraction (XRD) complemented by Raman spectroscopy analyses of synthetic organic pigments in powder samples, layered paint systems, and commercial artists' paints bound in acrylic, alkyd, and oil media are presented. The potential and limitations of the techniques to identify and characterize mixtures of these pigments, along with inorganic extenders, in works of art are exemplified and discussed. Stratified model paint systems that mimic the layering structure typically found in modern paintings are used to evaluate the effect of the µXRD experimental parameters, as well as extenders or fillers commonly found in modern artists' paint formulations, on the quality of the patterns recorded in microsamples of paint. XRD is demonstrated for the first time to be an effective tool for the specific identification of synthetic organic pigment mixtures and fillers in acrylic and alkyd bound artists' paints, while the identification of these pigments by XRD in oil bound paints appears problematic. Detailed crystallographic information provided by XRD is shown to be complementary to molecular information provided by Raman analysis. The combined use of these techniques allows for more frequent unambiguous compound identification than would be possible using one technique alone.
Proceedings of the National Academy of Sciences, 2019
Significance Daguerreotypes, among the earliest photographs of the 19th century, owe their incred... more Significance Daguerreotypes, among the earliest photographs of the 19th century, owe their incredible optical properties, image resolution, and dynamic range to light scattering produced by metallic nanostructures on their surface. Here we provide a detailed experimental and theoretical analysis on how the material composition, morphology, and dimensions of these nanostructures determine the characteristics of the daguerreotype image. Our results provide a scientific understanding of the unique optical effects of these artworks and therefore, in addition to providing valuable insight for developing preservation protocols, can inspire additional approaches for color printing, where nanostructures are directly manufactured by light.
Identification of pyroxene minerals used as black pigments in painted human bones excavated in no... more Identification of pyroxene minerals used as black pigments in painted human bones excavated in northern patagonia by raman spectroscopy and xrd,
The natural sepia pigment is based on eumelanin, the most prevalent type of melanin, and can be o... more The natural sepia pigment is based on eumelanin, the most prevalent type of melanin, and can be obtained from the ink sac of different members of the Cephalopoda class, such as cuttlefish and squid. The main components of natural sepia are indole derivatives, but pigments prepared following historical recipes are generally more heterogeneous as they may also contain proteins, polysaccharides, and lipids. Sepia is difficult to identify in works of art owing to its heterogeneity, insolubility in most organic solvents, interference by the binding media, and its poorly defined spectroscopic properties. In the present study, three commercial natural sepia pigments, along with a sample extracted from cuttlefish following a historical recipe in an artists' manual, a relatively more pure commercial melanin, and a synthetic eumelanin obtained by the oxidation of dopamine were characterized by normal Raman and surface-enhanced Raman scattering (SERS), complemented by Fourier transform infrared, pyrolysis-gas chromatography-mass spectrometry, and X-ray fluorescence, in order to obtain vibrational signatures for the identification of the pigment in works of art. Pyrolysis-gas chromatography-mass spectrometry and X-ray fluorescence analysis showed that the pigment composition is strongly influenced by the extraction procedure used. Lipids were identified in the chromatograms of all the natural samples analyzed, and the majority of these also showed the presence of cholesterol derivatives. Additional components resulting from the decomposition of polysaccharides were found to be present in the sepia extracted from cuttlefish in our laboratories. Normal Raman and SERS were also used to study two sepia inks applied on paper and subjected to accelerated aging. In the SERS measurements, a hydroxylamine-reduced silver colloid was used as this substrate is suitable for the identification of other natural heterogeneous black-brown pigments. The SERS methodology developed was applied to identify sepia in Waiting for Aaron, a drawing by the 19th century artist Robert Frederick Blum.
ABSTRACTA combination of FTIR, normal Raman, SERS, Py-GC/MS and XRF was used to analyze commercia... more ABSTRACTA combination of FTIR, normal Raman, SERS, Py-GC/MS and XRF was used to analyze commercial bistre samples to determine specific biomarkers that will allow for a rapid identification of the pigment in works of art. The results of the XRF analysis showed that potassium, calcium and iron are the main elements present. Characteristic bands belonging to phenolic components of lignin were observed in the FTIR spectra. The SERS analysis provided a fingerprint that may originate in the polymerization of the phenolic components catalyzed by the presence of the nanostructured silver surface under alkaline conditions. The Py-GC/MS analysis revealed the presence of lignin and cellulose biomarkers and a series of polycyclic aromatic hydrocarbons. The similarities observed between the commercial samples studied suggest that both originate in the same raw material, hardwood. The results demonstrate the potential of the multi-technique approach used for the characterization of this complex ...
... Silvia A. Centeno a , Corresponding Author Contact Information , E-mail The Corresponding Aut... more ... Silvia A. Centeno a , Corresponding Author Contact Information , E-mail The Corresponding Author , Veronica Williams b , Nicole C. Little c , Robert J. Speakman c. a, The Metropolitan Museum of Art, Department of Scientific Research. ...
Adsorption of phenol on a polycrystalline gold electrode has been studied by capacitance and modu... more Adsorption of phenol on a polycrystalline gold electrode has been studied by capacitance and modulated reflectance measurements. Two main processes were detected. First, an adsorption-desorption peak was observed at negative charge on the electrode which was related to a flat configuration of the molecule bonded to the surface through its ir system. In this case the interactions between adsorbed phenol and the electrode are not strong enough to modify the intrinsic optical properties of the metal. A second capacitive peak, detected over a region of positive charge on the metal, was found to show hysteresis and, besides, it distorts the electroreflectance effect of gold to such an extent that separation of the optical effects of charge and coverage was not possible. This behavior, typical of the formation of strong covalent bonds, was associated with the reorientation of the molecule from flat to vertically attached to the metal by means of its oxygen atom.
Methyl viologen (MV 2+) has been studied at a thin mercury film electrode in the presence of iodi... more Methyl viologen (MV 2+) has been studied at a thin mercury film electrode in the presence of iodide ions. Conventional electrochemistry and in-situ optical probes have been applied to investigate processes not previously observed in fluoride solutions. As the potential is made negative, iodide desorption is seen attended by an intense increase in reflectivity. The MV 2+ first reduction coincides with the formation of the (MV •+-I-) species on the surface, at the monolayer level, showing strong adsorbate-adsorbate interactions and a flat orientation of the pyridyl groups. The ac current response accompanying the appearance of the (MV •+-I-) film was found to be similar to that of a phase transition (condensation) in the adsorbed state. The dissociation of (MV •+-I-) clusters, to yield MV 0 (solid) + I-(aq), at the onset of the second reduction wave of MV 2+ , leads to a reflectivity enhancement in the in-phase optical signal paralleled by a sharp doublet in the imaginary current response. The splitting of the reduction peak of (MV •+-I-) may arise from different orientations of similar clusters or from distinct oligomer sizes.
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