E. René de la Rie

Analysis of fresh triterpenoid resins and aged triterpenoid varnishes by high-performance liquid chromatography–atmospheric pressure chemical ionisation (tandem) mass spectrometry

Fresh triterpenoid (dammar and mastic) resins and aged triterpenoid varnishes were analysed by on-line HPLC–MS using atmospheric pressure chemical ionisation (APCI). Twenty components were identified in the fresh resins [dammarenolic acid, ursonic acid, oleanonic acid, hydroxydammarenone (I and II), dammaradienol, oleanonic aldehyde, dammarenediol, ursonic aldehyde, oleanolic aldehyde, ursolic aldehyde, hydroxyhopanone, dammaradienone, moronic acid, (3l,8R)-3,8-dihydroxypolypoda-13E,17E,21-triene, (8R)-3-oxo-8-hydroxypolypoda-13E,17E,21-triene, masticadienonic acid, isomasticadienonic acid, 3-O-acetyl-3-epimasticadienolic acid and 3-O-acetyl-3-epiisomasticadienolic acid]. Analysis of the aged varnishes revealed the presence of some oxidised triterpenoid components [11-oxo-oleanonic acid, 11-oxo-oleanonic acid, 3-oxo-25,26,27-trinordammarano-24,20-lactone, 20,24-epoxy-25-hydroxy-3,4-seco-4(28)dammaren-3-oic acid and some ocotillone type molecules]. Most constituents of these complex samples were well resolved by reversed-phase HPLC. APCI-MS provides useful information about the molecular mass and the presence of certain functional groups. Specific marker compounds were found, which enable the discrimination between aged dammar and mastic varnishes by HPLC–APCI-MS. The fragmentation behaviour of triterpenoids under APCI conditions was compared to that under electron impact conditions. Mass spectrometric fragmentation of triterpenoids with a saturated ring system is straightforward. Triterpenoids with an unsaturation in the ring system probably give rise to double bond migration. Subsequent low energy MS–MS analysis does not provide more structural information, since mainly non-specific ring fragment ions are formed.

Use of imaging spectroscopy, fiber optic reflectance spectroscopy, and X-ray fluorescence to map and identify pigments in illuminated manuscripts

Abstract A paradigm using multispectral visible and near-infrared imaging spectroscopy is presented to semi-automatically create unbiased spectral maps that guide the site selection for in situ analytical methods (e.g. fiber optic reflectance spectroscopy and X-ray fluorescence) in order to identify and map pigments in illuminated manuscripts. This approach uses low spectral resolution imaging spectroscopy to create maps of areas having the same spectral characteristics. This paradigm is demonstrated by analysis of the illuminated manuscript leaf Christ in Majesty with Twelve Apostles (workshop of Pacino di Buonaguida, c. 1320). Using this approach the primary pigments are mapped and identified as azurite, lead-tin yellow, red lead, a red lake (likely insect-derived), a copper-containing green, brown iron oxide, and lead white. Moreover, small amounts of natural ultramarine were found to be used to enhance the blue fields around Christ, and a red lake was used to highlight different colors. These results suggest that the proposed paradigm offers an improved approach to the comprehensive study of illuminated manuscripts by comparison with site-specific analytical methods alone. The choice of broad spectral bands proves successful, given the limited palette in illuminated manuscripts, and permits operation at the low light intensity required for examination of manuscripts.

Near-Infrared Luminescence of Cadmium Pigments: In Situ Identification and Mapping in Paintings

A comprehensive study of the luminescence properties of cadmium pigments was undertaken to determine whether these properties could be used for in situ identification and mapping of the pigments in paintings. Cadmium pigments are semiconductors that show band edge luminescence in the visible range and deep trap luminescence in the red/infrared range. Emission maxima, quantum yields, and excitation spectra from the band edge and deep trap emissions were studied for sixty commercial cadmium pigments that span the color range from yellow to red (reflectance transition 470 to 660 nm). For paints containing cadmium pigments, luminescence from deep traps was more readily observable than that from the band edge, although the yield varied widely from zero to around 4.5%. Optimal excitation for emission is found to be in the visible for both pigments in powder form and mixed with a medium. The maxima of the deep trap emission shift with the band gap energy, providing a potentially useful way to assign pigment type even when used in pigment mixtures. The usefulness of the results of the study on mockups was demonstrated by the mapping of cadmium pigments of different hues with the aid of calibrated luminescence imaging spectroscopy in a painting by Edward Steichen, entitled Study for ‘Le Tournesol’ (1920). Analysis of the luminescence image cube reveals at least six unique spectral components, associated with emission from white pigments, paint binder, and cadmium red and yellow pigments. The results were compared with those from X-ray fluorescence spectrometry (XRF) and fiber-optic reflection spectroscopy (FORS) and the results obtained on paint samples containing cadmium pigments. These results show that, when present, the emission from traps can be used as an analytical tool to identify cadmium pigments, to distinguish among cadmium sulfide, cadmium zinc sulfide, and cadmium sulfoselenide, and to map cadmium pigments, even in mixtures.

Gcms Analysis of Triterpenoid Resins: In Situ Derivatization Procedures Using Quaternary Ammonium Hydroxides

Abstract Derivatization procedures using quaternary ammonium hydroxides are now frequently applied to the gas chromatography/mass spectrometry (GCMS) analysis of paint media and varnishes. This paper compares the results obtained when triterpenoid resins are analysed using different reagents and sample preparation methods. Depending on the reagent and derivatization method used, the quaternary ammonium hydroxides produced multiple derivatives and induced isomerization in a number of the components of triterpenoid resins. This has implications for selecting an appropriate derivatization technique for the analysis of triterpenoid resins such as dammar and mastic.

Exploring the Optical Properties of Picture Varnishes Using Imaging Techniques

Abstract An experimental paradigm was developed in which a varnished painting was considered as ani111aging system such as a camera: the unvarnished painting was equivalent to a scene, the varnish was equivalent to an optical system, and the observer was equivalent to the sensor of the camera. Methodologies used to evaluate image quality were employed to evaluate the quality changes that occur when a varnish is applied. An unvarnished painting was simulated by a photographic transparency in physical contact with triple-thickness window glass sandblasted on one side. Varnish resins applied to the sandblasted surface simulated a varnished paint surface. Back-illuminated images transmitted through the varnished glass were digitized using a near-colorimetricdigital camera. Differences in physical. properties between varnish resins would result in changes in image quality. These were quantified by spatial analyses (using a modulation transferfunction), colorimetricanalyses, and visualization using a reproduction of an Old Master painting. The results confirmed the utility of this approach to quantifying the effects of varnishing on the appearance if paintings.

Visible and Infrared Reflectance Imaging Spectroscopy of Paintings: Pigment Mapping and Improved Infrared Reflectography

Reflectance imaging spectroscopy, the collection of images in narrow spectral bands, has been developed for remote sensing of the Earth. In this paper we present findings on the use of imaging spectroscopy to identify and map artist pigments as well as to improve the visualization of preparatory sketches. Two novel hyperspectral cameras, one operating from the visible to near-infrared (VNIR) and the other in the shortwave infrared (SWIR), have been used to collect diffuse reflectance spectral image cubes on a variety of paintings. The resulting image cubes (VNIR 417 to 973 nm, 240 bands, and SWIR 970 to 1650 nm, 85 bands) were calibrated to reflectance and the resulting spectra compared with results from a fiber optics reflectance spectrometer (350 to 2500 nm). The results show good agreement between the spectra acquired with the hyperspectral cameras and those from the fiber reflectance spectrometer. For example, the primary blue pigments and their distribution in Picassos Harlequin Musician (1924) are identified from the reflectance spectra and agree with results from X-ray fluorescence data and dispersed sample analysis. False color infrared reflectograms, obtained from the SWIR hyperspectral images, of extensively reworked paintings such as Picassos The Tragedy (1903) are found to give improved visualization of changes made by the artist. These results show that including the NIR and SWIR spectral regions along with the visible provides for a more robust identification and mapping of artist pigments than using visible imaging spectroscopy alone.

Formation of Brown Lines in Paper : Characterization of Cellulose Degradation at the Wet-Dry Interface

Brown lines were generated at the wet-dry interface on Whatman paper No. 1 by suspending the sheet vertically in deionized water. Formic acid and acetic acid were quantified in three areas of the paper defined by the wet-dry boundary (above, below, and at the tideline) using capillary zone electrophoresis with indirect UV detection. Their concentration increased upon accelerated aging of the paper and was highest in the tideline. The hydroperoxides have been quantified using reverse phase high performance liquid chromatography with UV detection based on the determination of triphenylphosphine oxide produced from the reaction with triphenylphosphine, and their highest concentration was found in the tideline as well. For the first time, it was shown that various types of hydroperoxides were present, water-soluble and non-water-soluble, most probably in part hydroperoxide functionalized cellulose. After accelerated aging, a significant increase in hydroperoxide concentration was found in all the paper areas. The molar masses of cellulose determined using size-exclusion chromatography with multiangle light scattering detection showed that, upon aging, cellulose degraded significantly more in the tideline area than in the other areas of the paper. The area below the tideline was more degraded than the area above. A kinetic study of the degradation of cellulose allowed determining the constants for glycosidic bond breaking in each of the areas of the paper.

The Role of Varnishes in Modifying Light Reflection from Rough Surfaces - A Study of Changes in Light Scattering Caused by Variations in Varnish Topography and Development of a Drying Model

Abstract To obtain a better understanding of the variations in the appearance of paintings brought about by different varnishes, their ability to form an optically smooth film over rough surfaces was studied using laser scanning confocal microscopy and stylus profi lometry, as well as gloss and distinctness-of-image gloss measurements. Two different varnishes, one made with a low molecular weight resin and having low viscosity, and another, made with a polymer of high molecular weight and having high viscosity, were applied over glass plates that had been given surface roughness in the micrometer range. Measurement of the topography showed that the polymeric resin eliminated mid-to-high spatial frequency roughness of the ground glass but did not eliminate roughness of lower spatial frequencies. The results are discussed in terms of the surface state of the varnish, using concepts such as root-mean-square roughness, total integrated scattering and the power spectral density. These observations indicate that both varnishes reduce the wide-angle light scattering by the rough substrates, but only the varnish made with the low molecular weight resin reduces the small-angle scattering about the specular refl ection. This suggests that the smoothness of the varnished surface depends both on the molecular weight of the resin and on the spatial frequencies of the surface roughness of the surface beneath it. A model for the drying of varnishes is presented that takes the spatial frequency dependence into account.

Modification of Surface Roughness by Various Varnishes and Effect on Light Reflection

Abstract The modification of microscopically rough surfaces by commonly used varnishes and resultant changes in light reflection from these surfaces were studied using laser scanning confocal microscopy, stylus profilometry and specular gloss and distinctness-of-image gloss measurements. Natural resins as well as synthetic low molecular weight and polymeric resins were studied. The molecular weights of the resins were determined using size-exclusion chromatography. All the low molecular weight resins, with weight-average molecular weights in the range 800–6500, including the natural resins, essentially produced smooth surfaces and eliminated high-frequency as well as low-frequency roughness, while polymeric resins, with weight-average molecular weights between about 25000 and 210000, produced rougher surfaces, eliminating high-frequency roughness but not all low-frequency roughness. The spatial frequency range of the roughness of the polymeric coatings decreased with increasing molecular weight. As a result, low molecular weight varnishes reduced small-angle scattering about the specular reflection to a greater extent than the polymeric coatings. Polymeric varnishes, therefore, demonstrated lower specular gloss and, in particular, lower distinctness-of-image gloss. Distinctness-of-image gloss values showed good correlation with the weight-average molecular weights of the resins. The power spectral density of the surfaces coated with low molecular weight resins showed a sharper falloff at low spatial frequencies than the polymeric resins.

Degradation of cellulose at the wet-dry interface. II. Study of oxidation reactions and effect of antioxidants.

To better understand the degradation of cellulose upon the formation of a tideline at the wet-dry interface when paper is suspended in water, the production of chemical species involved in oxidation reactions was studied. The quantitation of hydroperoxides and hydroxyl radicals was carried out in reverse phase chromatography using triphenylphosphine and terephthalic acid, respectively, as chemical probes. Both reactive oxygen species were found in the tideline immediately after its formation, in the range of micromoles and nanomoles per gram of paper, respectively. The results indicate that hydroxyl radicals form for the most part in paper before the tideline experiment, whereas hydroperoxides appear to be produced primarily during tideline formation. Iron sulfate impregnation of the paper raised the production of hydroperoxides. After hygrothermal aging in sealed vials the hydroxyl radical content in paper increased significantly. When aged together in the same vial, tideline samples strongly influenced the degradation of samples from other areas of the paper (multi-sample aging). Different types of antioxidants were added to the paper before the tideline experiment to investigate their effect on the oxidation reactions taking place. In samples treated with iron sulfate or artificially aged, the addition of Irgafos 168 (tris(2,4-ditert-butylphenyl) phosphate) and Tinuvin 292 (bis(1,2,2,6,6-pentamethyl-4-piperidyl) sebacate and methyl 1,2,2,6,6-pentamethyl-4-piperidyl sebacate) reduced the concentration of hydroperoxides and hydroxyl radicals, respectively. Tinuvin 292 was also found to considerably lower the rate of cellulose chain scission reactions during hygrothermal aging of the paper.