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Cite this: Anal. Methods, 2018, 10,5463

Received 25th October 2018

DOI: 10.1039/c8ay90151a

rsc.li/methods

This journal is © The Royal Society of C

tical pyrolysis in cultural heritage

Analytical Methods Committee, AMCTB No. 85

Analytical pyrolysis (Py), especially when

coupled with gas chromatography and mass

spectrometry (Py-GC-MS), is a powerful

technique for the characterisation and iden-

tification of organicmaterials used in artwork.

The thermal degradation of macromolecules

(for example, resins, lacquers, proteins, poly-

saccharides, oils, modern synthetic polymers,

etc.) using heat (thermal energy) generates

smaller molecules (pyrolysis products) which

are easier to identify and study. Some of these

pyrolysis products are molecular markers

allowing the identification of a specific

material despite the sample’s complexity. A

micro-sample (50–100 mg) is destroyed

during the analysis but the lack of sample

preparation makes Py-GC-MS a very attrac-

tive techniquewith amuch reduced analytical

time and cost compared to other chromato-

graphic methods.

Introduction

Pyrolysis is the decomposition of a mate-rial at high temperature in the absence ofoxygen. Its use in combination with gaschromatography rst appeared in 1960.In the 1970s and 1980s it was used tocharacterise natural and synthetic poly-mers of industrial importance. The late1990s saw the rst applications to

hemistry 2018

artwork materials. Initially, macromole-cules such as polysaccharides andproteins were the main targets but resins,waxes, lacquer, wood and synthetic poly-mers were also studied soon aer. In thelast twenty years there has been a signi-cant increase in the application ofanalytical pyrolysis to the character-isation of organic materials used inartwork. It is now used as a screeningtechnique to study complex mixtures andto investigate particular properties ofnatural and synthetic polymers, such astheir chemical degradation.

What is analyticalpyrolysis good for?

In the cultural heritage eld, the mainuse of Py-GC-MS is the identication oforganic materials in composite samples.For example:

Paint samples

Binding media (oils, proteins and gums)and coating materials, such as varnishes(resins and waxes), can be identied andsometimes differentiated.

Lacquer

Different types of both Asian and Euro-pean lacquer can be characterised.

Modern materials

Synthetic formulations, such as alkyd,acryl and vinyl resins, other synthetic

polymers (plastics) and many syntheticorganic pigments, are also identied.

Fibres

Cellulose-based bres (cotton and ax)can be distinguished from proteinaceousbres (silk and keratin).

Lignocellulosic materials

Wood and wood products can be identi-ed, although not at the species level.

Degradation of materials

In addition to the identication of mate-rials, analytical pyrolysis is also useful forthe study of their degradation. Most ofsuch applications study synthetic poly-mers and lignocellulosic materials, inparticular wood.

How does it work?

Pyrolysis is a process in which heat(thermal energy) is used to break themolecular bonds in a large molecule (amacromolecule like a protein or a polymerlike a plastic) and to form the so-calledpyrolysis products. The idea behind thisis to reduce the complexity of the originalmolecule and study some of its features bylooking at the simpler and smaller pyrol-ysis products (Fig. 1).

Temperatures between 400 �C and800 �C are generally used. To avoidcombustion the entire process occurs inthe absence of oxygen.

Anal. Methods, 2018, 10, 5463–5467 | 5463

Fig. 1 Simplified and hypothetical scheme of the pyrolysis process of a general macromolecule, showing the formation and gas chromato-graphic separation of the pyrolysis products.

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Types of pyrolyser andinstrumentalconfigurations

Two main types of pyrolyser are used inanalytical work.

Filament pyrolysers are equipped withametal coil at the end of a probe. A quartztube containing the sample can beinserted in it. The probe is introducedinto a pyrolysis chamber and is electri-cally heated to the desired temperature(Fig. 2a and b).

Furnace or micro-furnace pyrolysershave a pre-heated pyrolysis chamber.The sample is loaded at the top of thechamber, usually using a stainless steelcup, and then dropped inside bya releasing mechanism. The transfer of

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the sample into the cup and its weighingare easier than the process involvinga quartz pyrolysis tube and betterreproducibility has been reported(Fig. 2c).

Pyrolysers can be directly coupledwith a detector, usually a mass spec-trometer. This conguration is calledPy-MS or DE-MS (direct exposure massspectrometry). Gas chromatographicseparation is oen interposed betweenthe pyrolyser and the mass spectrometer(Py-GC-MS). In the absence of a chro-matographic separation, the result is anoverall mass spectrum of all the pyrol-ysis products (Fig. 3a). However, certainprominent masses can stand out and beused to identify molecules, or chemo-metric methods can be applied to

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highlight information. In Py-GC-MS allpyrolysis products are separated in theGC column and can therefore be iden-tied one by one, making it easier toidentify markers and interpret data(Fig. 3b).

Recent instrumental developmentshave led to other pyrolysis modes. It ispossible to program the pyrolysistemperature as a ramp rather thana single temperature. This producesa slow pyrolysis of the sample atincreasing temperatures and the pyrol-ysis products, directly transferred to themass spectrometer, can be studied asa function of the temperature. Thismode is referred to as evolved gas anal-ysis mass spectrometry (EGA-MS).Alternatively, the same sample can

is journal is © The Royal Society of Chemistry 2018

Fig. 2 (a) The probe of a filament pyrolyser, showing the coil and the quartz tube; (b) the samefilament pyrolyser with the probe inserted into the pyrolysis chamber; (c) a micro-furnacepyrolyser.

Fig. 3 (a) DE-MS mass spectrum of a hardwood: each number represents the mass ofa pyrolysis product (or a fragment of it); (b) Py-GC-MS profile of a hardwood: each peak isa single pyrolysis product with its ownmass spectrum–H-holocellulose pyrolysis products; G-guaiacyl lignin pyrolysis products; S-syringyl lignin pyrolysis products.

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undergo more than one pyrolysis shot atdifferent temperatures and GC-MS canfollow each shot. If two temperaturesare chosen, the technique takes the

This journal is © The Royal Society of Chemistry 2018

name of ‘double-shot Py-GC-MS’. Ifmore than two temperatures are chosen,the technique is referred to as ‘multi-shot Py-GC-MS’.

Advantages andlimitations

The main advantage of analytical pyrol-ysis over other chromatographic tech-niques is that no sample preparation isrequired: the sample is analysed in itssolid state without undergoing any wetchemical treatment. This drasticallyreduces the analysis time and the costs ofsolvents and equipment used in samplepre-treatment. Even in the case ofcomplex mixtures no prior chemicalseparation is necessary as the specicmolecular markers are produced in thepyrolysis process and are sufficient toidentify the materials present in thesample.

Moreover, some natural and syntheticpolymers do not respond well to chemicaltreatments: the Py-GC-MS method isclearly invaluable in obtaining detailedmolecular information in these cases.

Among the limitations of the tech-nique there is the requirement of thepyrolysis products to be sufficiently vola-tile to undergo gas chromatography. Thisproblem can be overcome by the use ofderivatising agents such as tetramethy-lammonium hydroxide (TMAH), forthe process called methylation, andhexamethyl-disilazane (HMDS) for silyla-tion. These agents are added in situdirectly in the sample holder togetherwith the solid sample. Their function is tosubstitute polar OH groups therebyreducing the polarity of the pyrolysisproducts and enhancing their volatility.

Another limitation is that Py-GC-MS isnot a quantitative technique, so whilematerials can be qualitatively identied,it is difficult to assess how much of eachis present in a mixture. This is becauseeach material has its own pyrolysis yield,an intrinsic property that is difficult topredict, especially because other mate-rials and inorganic compounds that maybe present can affect the pyrolytic path-ways and thus the pyrolysis yield. Inaddition, one material can generate somany pyrolysis products that it is virtuallyimpossible to obtain a calibration curvefor each. Nevertheless, pyrolysis data canbe used in a semi-quantitative way whenthe chromatographic areas of pyrolysisproducts are integrated, summed and

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expressed as percentages. Such percent-ages are not necessarily related to abso-lute concentrations, but can giveinformation on the degradation of thematerial itself. These comparisons arecorrect only if the same experimentalconditions are used within a singlelaboratory.

Finally, the interpretation of a pyro-gram can be a challenging and time-consuming process, requiring a highlevel of expertise in chromatographicanalysis and in the interpretation of massspectra.

Case study: Py-GC-MSanalysis of archaeologicalwood

Wood is a complex natural material. It ismainly composed of three interlinkedbiopolymers, cellulose, hemicelluloseand lignin. Archaeological wood is foundin particular burial conditions such aswaterlogged, very dry or frozen ground.Before applying any conservation treat-ment, usually needed to guarantee thestructural stability of wood, it is impor-tant to evaluate its state of chemicaldegradation. This can be estimated bymeasuring the residual cellulose, hemi-cellulose and lignin in the sample andcomparing the results with those froma sample of sound reference wood of thesame species.

Fig. 4 An example of the distribution of lignina soundwood sample and in archaeological samp

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The pyrogram of a wood samplemainly contains pyrolysis products fromthe three polymers. The rst step istherefore to identify the products andassign them to the right polymer.Cellulose and hemicellulose produce thesame pyrolysis products and cannot bedistinguished: the sum of cellulose andhemicellulose is thus called holocellulose.By contrast, p-hydroxyphenyl-lignin,guaiacyl-lignin and syringyl-lignin can bedistinguished through their pyrolysisproducts. As p-hydroxyphenyl-lignin ispredominant in grasses, guaiacyl-lignin inconifers (sowoods) and syringyl-lignin inbroad-leaved trees (hardwoods), thesetypes of plants can be distinguished,though the exact plant species cannot beidentied by pyrolysis and anatomicalobservations are still the best way toobtain this information.

Once holocellulose (H) and lignin (L)pyrolysis products are identied, initialinformation about the degradation stateof the wood is given by the so-called H/Lratio. Percentage chromatographic areasof each pyrolysis product are calculatedand those of the pyrolysis products fromholocellulose and lignin are summedseparately. The ratio between these twovalues provides the pyrolytic H/L ratio.When this ratio is compared betweena sound reference wood (H/Lref) and anarchaeological wood (H/Larch) of the samespecies, an estimation of the prevalentloss of holocellulose or lignin in the

pyrolysis products divided into categories inles showing different preservation conditions.

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archaeological sample is obtained. Inparticular:

- if H/Larch < H/Lref, holocellulose ismostly lost;

- if H/Larch > H/Lref, lignin is mostlylost.

However, the H/L ratio has some limi-tations, as it does not provide informationabout the degradation of individual woodcomponents (holocellulose and lignin). Toprovide this information, the hol-ocellulose and lignin pyrolysis productscan be further categorised according totheir molecular structure and pyrolyticformation, as exemplied in Fig. 4.

Lignin oxidation, depolymerisation anddemethylation can be seen, as well ascellulose depolymerisation. Such informa-tion can be related to a particular cause ofdegradation and inform about the besttreatments to be used to preserve the wood.

Further reading

1 S. C. Moldoveanu, Pyrolysis of organicmolecules with applications to healthand environment, Elsevier, Oxford(UK), 2010.

2 I. Bonaduce and A. Andreotti, Py-GC/MS of Organic Paint Binders in OrganicMass Spectrometry in Art andArcheology, ed. M. P. Colombini andF. Modugno, Wiley, Chichester (UK),2009, pp. 303–326, DOI: 10.1002/9780470741917.ch11.

3 I. Degano, F. Modugno, I. Bonaduce,E. Ribechini and M. P. Colombini,Recent Advances in AnalyticalPyrolysis to Investigate OrganicMaterials in Heritage Science, Angew.Chem., Int. Ed., 2018, 57, 7313–7323,DOI: 10.1002/anie.201713404.

4 A. P. Pinder, I. Panter, G. D. Abbottand B. J. Keely, Deterioration of theHanson Logboat: chemical andimaging assessment with removal ofpolyethylene glycol conserving agent,Sci. Rep., 2017, 7, 13697, DOI:10.1038/s41598-017-14057-w.

5 S. Wei, V. Pintus and M. Schreiner,Photochemical degradation study ofpolyvinyl acetate paints used inartworks by Py–GC/MS, J. Anal. Appl.Pyrolysis, 2012, 97, 158–163, DOI:10.1016/j.jaap.2012.05.004.

6 D. Tamburini, J. J. Łucejko,E. Ribechini and M. P. Colombini,

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New markers of natural andanthropogenic chemical alterationof archaeological lignin revealedby in situ pyrolysis/silylation-gaschromatography-mass spectrometry,

This journal is © The Royal Society of Chemistry 2018

J. Anal. Appl. Pyrolysis, 2016, 118, 249–258, DOI: 10.1016/j.jaap.2016.02.008.

Dr Diego Tamburini, The BritishMuseum.

This Technical Brief was prepared by theHeritage Science Subcommittee andapproved by the Analytical MethodsCommittee on 05/10/2018.

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