Silvia Prati

Gas chromatographic-mass spectrometric analysis of products arising from pyrolysis of amino acids in the presence of hexamethyldisilazane

Alpha-amino acids were pyrolysed at 600 degrees C in the presence of hexamethyldisilazane (HMDS) and the formed volatile products were analysed on line by gas chromatography-mass spectrometry (GC-MS). Glycine, alanine, valine, leucine, isoleucine, norleucine, methionine, phenylalanine yielded principally the trimethylsilyl (TMS) ester of the parent amino acid. TMS esters of carboxylic acids arising from reductive deamination were observed for serine, threonine and aspartic acid. Decarboxylation resulted in the formation of amines which represented abundant products released from tyrosine, cysteine and methionine. Cyclic compounds arising from the condensation of two amino acids were revealed as characteristic products of glycine, alanine, serine, proline and hydroxyproline. Degradation products of the side chain were released at relatively high levels from tryptophane, tyrosine and hystidine. Since each amino acid produced a characteristic distribution of TMS products, in-situ pyrolysis/silylation with HMDS may find application as a screening technique for the detection of amino acids and related materials in complex matrices. The potentiality of the procedure was tested on a dipeptide (Tyr-Leu).

In-situ pyrolysis and silylation for analysis of lipid materials used in paint layers

SummaryAlthough pyrolysis coupled with gas chromatography and mass spectrometry (Py-GC-MS) is a useful technique for the rapid characterization of the organic materials used by artists, diagnostic pyrolysis products bearing polar groups, for example carboxylic acids, require derivatization (e. g. methylation) before GC separation. In this study we propose the use of hexamethyldisilazane (HMDS) as an effective on-line derivatizing reagent to prepare the trimethylsilyl (TMS) derivatives of fatty acids released from the pyrolysis of fats. Pyrolysis in combination with HMDS has been applied to the analysis of lipid materials employed as painting media, for example siccative oils and egg.

Characterisation of Natural Resins by Pyrolysis - Silylation

SummaryMany organic materials release polar compounds containing-OH groups when subjected to pyrolysis. To improve GC detection of such polar compounds derivatisation with the silylating reagent hexamethyldisilazane (HMDS) can be achieved in situ while pyrolysing the sample (pyrolysis—silylation).In this study pyrolysis—silylation in combination with GC-MS was applied for the analysis of natural resins, utilised in artistic fields namely sandarac (Cupressaceae), Manila copal (Araucariaceae), colophony, Venice turpentine, Strasbourg turpentine (Pinaceae), dammar, mastic, and shellac.Pyrolysis-silylation of natural resins resulted in the formation of several silylated compounds characteristic of the different kinds of resins.The trimethylsilyl (TMS) ester of sandaracopimaric acid was a prominent compound released from sandarac. Pinaceae resins produced TMS esters of pimaric, isopimaric, methyl dehydroabietic and abietic acids. TMS esters of linear aliphatic and aromatic acids were generated from shellac. Distinctive though as yet unidentified silylated compounds were released from Manila copal and triterpenic resins.

Off-line pyrolysis/silylation of cellulose and chitin ☆

Abstract An off-line pyrolysis/silylation procedure followed by GC–MS analysis was utilised for the detection of anhydrosugars evolved from cellulose and chitin in the form of trimethylsilyl (TMS) derivatives. In the case of cellulose, the performance of the following four silylating reagents was evaluated: hexamethyldisilazane (HMDS), trimethylsilyldiethylamine (TMSDEA) bis(trimethylsilyl)trifluoroacetamide (BSTFA) and trimethylsilylimidazole (TMSI). With all these silylating reagents, the main derivative of levoglucosan (LG) was the corresponding 2,3,4-tris-TMS ether. The 2- and 4-monoTMS ethers of LG, previously found as dominant species from on-line Py–GC–MS with HMDS, were not produced in the off-line procedure. The 2,4-bis-TMS ether of LG was still an important product in off-line pyrolysis/silylation with HMDS and TMSDEA, but a minor derivative with TMSI and BSTFA. Yields of LG evolved in the persilylated form were calculated and best reproducibility was observed for HMDS (12±4 mg LG g −1 cellulose). Off-line pyrolysis/silylation of chitin with HMDS resulted in the predominant formation of 3,4-bis-TMS ether of 1,6-anhydro-2-acetamido-2-deoxyglucopyranose, while the 1,6-anhydro-4- O -(trimethylsilyl)-2-acetamido-2-deoxyglucopyranose was tentatively identified as an important product in the on-line procedure.

Analysis of Proteinaceous Binders by in-situ Pyrolysis and Silylation

SummaryPyrolysis coupled with gas chromatography and mass spectrometry (Py-GC-MS) is a useful technique for rapid characterization of the organic materials used in art. In the characterization of proteinaceous binders fragments from pyrolysis are of low intensity. Milk, casein, and glue, for example, are sometimes difficult to detect. In the work discussed in this paper pyrolysis-silylation, in the presence of hexamethyldisilazane, has been used for characterization of some proteinaceous binders with the objective of distinguishing between them in real samples. Piperazine-2,5-dione derivatives have been found to be pyrolytic markers of protein in the different binders. Silylated compounds arising from pyrolysis silylation of lipids and carbohydrates have been detected in egg and milk, respectively.

Molecular characterisation of organic material in air fine particles (PM10) using conventional and reactive pyrolysis-gas chromatography-mass spectrometry.

Pyrolysis-gas chromatography-mass spectrometry (Py-GC-MS) was applied to study the composition of organic constituents in air particulate matter (PM10) collected inside an industrial area. A few milligrams of sampling filters containing air particles were pyrolysed at 700 degrees C directly (conventional) or after the addition of a derivatising reagent (tetramethylammonium hydroxide, TMAH, for pyrolysis-methylation; hexamethyldisilazane, HMDS, for pyrolysis-silylation). Py-GC-MS was also applied to synthetic polymers (poly(styrene-co-isoprene), polylimonene and polypinene) and vegetation samples (coniferous pollen, bark and resin) to identify markers indicative of possible precursors. Pyrolysates of PM10 showed the same suite of compounds in all the four seasons, dominated by hydrocarbons like styrene, limonene and clusters of isomeric alkenes with 14, 15 and 16 carbon atoms. Pyrolysis products of natural origin, including furaldehyde, benzeneacetonitrile, dehydroabietin and other diterpenoids were found, while no specific markers of synthetic rubbers were detected. The principal products released from reactive pyrolysis of PM10 were methyl or trimethylsilyl (TMS) derivatives of 1,6-anhydroglucose (levoglucosan), fatty acids, dehydroabietic acid and other resin acids along with hydroxy (di)carboxylic acids. Possible sources of the detected products (e.g. pine forest, biomass combustion) are discussed.

Development of innovative embedding procedures for the analyses of paint cross sections in ATR FITR microscopy

We report the development of innovative embedding procedures for the analysis of paint cross sections by attenuated total reflection (ATR) Fourier transform IR microscopy. This technique was chosen because it is widely employed for the characterization and spatial location of organic and inorganic components in artistic samples. Moreover, the performance of the technique may be critically affected by sample preparation in terms of surface morphology and the presence of contamination. First, we evaluated the use of KBr as a barrier to contamination by the embedding synthetic medium. In this way, the sample cross section can be polished by means of a sample holder, which allows a controlled pressure to be applied to the sample, thus improving the reproducibility and quality of the surface cross section. In addition, argon ion milling was used for the polishing of samples embedded in KBr, and provided very promising results in terms of surface planarity and reduction of superficial contamination by KBr. Finally, the use of NaCl as an alternative to KBr was proposed thanks to its advantages in terms of hygroscopicity, cost, and toxicity. In addition, cross sections embedded in NaCl were characterized by greater hardness, a feature that allowed us to obtain improved contact with the ATR crystal.

Analysis of paint cross-sections: a combined multivariate approach for the interpretation of μATR-FTIR hyperspectral data arrays

The present research is aimed at introducing a suitable approach for the exploitation of the hyperspectral data obtained by μATR-FTIR analyses of paint cross-sections. The application of principal component analysis for chemical mapping is well-established, even if a very limited number of applications to μFTIR data have been reported so far in the field of analytical chemistry for cultural heritage. Moreover, in many cases, chemometric tools are under-utilized and the outcomes under-interpreted. As a consequence, results and conclusions may be considerably compromised. In an attempt to overcome such drawbacks, the present work is proposing a comprehensive and efficient procedure based on an interactive brushing approach, which combines the structural information of the score scatter plots and the spatial information of the principal component (PC) score maps. In particular, the study demonstrates not only how the multivariate approach may provide more information than the univariate one, but also how the integration of different chemometric tools may allow a more comprehensive interpretation of the results with respect to the studies up to now reported in the literature. The examination of the average spectral profile of each score cluster, jointly with the loading analysis, is functional to characterize each area investigated on the basis of its spectral features. A multivariate comparison with spectra of standard compounds, projected in the PC score space, helps in supporting the chemical identification. The approach was validated on two real case studies.

Py-GC-MS of the synthetic polymers used in past restorations on Giotto's mural paintings at the Scrovegni Chapel (Padova)

SummaryPyrolysis coupled with gas chromatography and mass spectrometry (Py/GC/MS) is a useful technique for the rapid characterisation of synthetic polymers. Many papers have been published about the application of this technique to industrial plastics and gums, but little attention has been focused on the characterisation of such group of synthetic compounds used in the artistic field. In this work pyrolysis is applied to the characterisation of consolidants and protectives used in the past restoration operations on Giottos frescoes at the Scrovegni Chapel, Padova.

DSC application in the conservation field

Differential scanning calorimetry (DSC) has been scarcely used in conservation field and in particular, in the study of painting layers. However DSC is a micro-destructive technique (sample from .05 to 1 mg can be analysed) because no pre-treatment of the sample is required and so it can give rapid and useful information. The aim of the present work is, in fact, to describe useful applications of DSC on the study of materials used in art. The analyses have been conducted under oxygen flow in order to register the thermo-oxidative reactions of the organic binder or protective. In the first part of the paper, materials of different chemical composition have been analysed to build up a data base of DSC profiles. Standard painting layers artificially degraded by treatment with ultraviolet light (365 nm) for 21 days and with NOx (5 ppm)have be en analysed. Comparing the behaviour of the treated to the reference sample, it is useful to study what kind of changes the treatments have produced on the sample. Another possible application of DSC is the study of the formation of the film; analysing the raw,liquid binders and the different naturally aged films an evolution of the chemical structure canbe registered.As a conclusion, DSC analyses on standard used in artistic field allow to investigate their thermal behaviour and to understand the effect of natural and artificial ageing.