henderson 2008

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CHEMICAL ANALYSIS TECHNIQUES

Julian Henderson, University of Nottingham,

Nottingham, UK

2008 Elsevier Inc. All rights reserved.

Glossary

electron probe microanalysis (EPMA) A method used for

determining the elemental composition of materials, based on the

X-rays emitted by different elements.

mass spectrometry An analytical technique used to measure

the mass-to-charge ratio of ions. It is most generally used to find

the composition of a physical sample by generating a mass

spectrum representing the masses of sample components.

neutron activation analysis A nuclear testing method that

determines elemental content regardless of oxidation state,

chemical composition, or physical location.

particle accelerator A device that uses electric fields to propel

electrically charged particles to high speeds and to contain them.

scanning electron microscopy A method for high-resolution

imaging of surfaces.

synchrotron-induced radiation A form of very high radiation

produced using an accelerator. High intensity is accompanied by

fine beam dimensions and this allows it to be used for a range of

applications using a range of analytical techniques.

Introduction

The investigation of archaeological materials usingscientific techniques has a long history, dating backas far as Renaissance Italy. Following on from George

IIIs assay master, Mr. Alchorn, who analyzed IrishBronze Age swords, in 1798 Klaproth published theanalytical results of Roman glass and bronze mirrors.Later, the eminent scientists Michael Faraday (17911867) and Humphrey Davy (17781829) analyzedarchaeological materials, Davy analyzing Egyptianblue and a material labeled red enamel. These earlyinvestigations were enquiries into the technologicalcapabilities of the ancients.

Since then, the analysis of archaeological materialsand the interpretation of the results obtained havedeveloped in various ways. The application of a range

of scientific techniques for the first time and the initialassemblage of data showing chronological contrastsin the developments of technologies occurred. Thiscontinued in the 1960s and by this time scientifictechniques were being used to investigate provenancein order to contribute to models of productionand exchange. The chemical analysis of obsidian inparticular produced results that were especially use-ful for provenance. At this time it was thereforedemonstrated that scientific analysis could contrib-ute in important ways to mainstream archaeology.

It represented an important step change in archaeolog-ical science. Confidence had developed in the accuracyand precision of specific applications of the techni-

ques used. Given the possible range of organic andinorganic materials used and made in past societies,there has always been a degree of unpredictabilityabout the results obtained, which created an addedincentive for those involved. Whereas the analystand the archaeologist have tended to collaborate asindependent specialists, and they still do, the twoaspects are increasingly understood and the resultscombined by the same person. Since the 1970s,research using scientific analysis has continued tocontribute to mainstream archaeology in a range ofimportant ways and has started to show how it canshed light on the study of embedded technologies,thereby contributing to social, economic, and ritualaspects of materials in society.

Scientific analysis can be destructive, microdestruc-tive, or nondestructive. The use of a particular tech-nique will be determined by the research questionsto be answered, the availability of techniques, theexpertise of those involved, the availability of fund-ing, and the extent to which one or more artifacts canbe sampled or surfaces prepared for analysis (seeArtifacts, Overview). The scientific investigation ofweathering clearly has a role to play in archaeologicaland museum environments. Scientific analysis can

contribute to an understanding of what is depleted,what has replaced it, the rate at which weathering hasoccurred, and under what conditions it occurred.

Microdestructive techniques have become increas-ingly more acceptable. Although there may be a ques-tion over how much sample is representative of thewhole artifact, microsampling of a homogeneous(single phase) material such as glass or obsidian isoften adequate (see Sampling Methods, Theory andPraxis). Moreover, by analyzing a sufficient numberof artifacts of a specific type and date, it can provideevidence of the raw materials used, their composi-

tional variations (including impurities), workshopcompositions, regional compositions, the decorativetechniques employed provenance and trade. If micro-samples are unweathered, it is possible to producequantitative results that are directly comparable withother unweathered samples. Standard materials ofknown chemical composition, which have been sub-ject to scrupulous scientific analysis using a range ofanalytical techniques, are used to highlight instrumen-tal errors, as a means of calibrating analytical systemsand for monitoring any analytical drift. Repeated

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analysis of the same standard material producesresults for accuracy. The use of standard materials isequally applicable to the use of destructive techniques,where all of the samples are dissolved in a solution.Microdestructive techniques may also involve thesolution of the (small) sample, but moreover the sam-

ples may be mounted for analysis and stored so thatthey can be reanalyzed using the same or a differenttechnique.

Destructive Techniques

Arc emission spectrometry (AES), introduced in the1920s, involved the removal and dissolution of apowdered sample and was a time-consuming tech-nique. A graphite electrode was positioned so thatan arc occurred between electrode and sample caus-ing the sample to be volatilized and light to be emit-ted. Each element emitted a specific wavelength that

was recorded on a photographic plate. The relativeintensities of each line were measured and convertedinto relative concentrations in the sample. This wassemi-quantitative and it was used for early work onobsidian sourcing. Around 1970, a more accurate andless destructive wet chemical technique started toreplace AES: atomic absorption spectroscopy (AAS).It involves similar principles to those involved inAES although analysis involves the use of a flameto atomize the sample at temperatures c. 2000 C.A sample of c. 10 g is dissolved and injected intoflames; the kind of gas being burnt depends on the

element being analyzed. When a particular chemicalelement is vaporized in the flame, specific wave-lengths of light characteristic of the element areabsorbed, and others emitted. These absorptions oflight are measured. As with AES, AAS records theelements present, each with its own characteristicwavelength, usually one at a time. AAS is a quantita-tive technique. When a sample with an unknownlevel of that element is analyzed, its concentrationcan be plotted on a calibration curve. Detection levelsare between 1 and 100 ppm, though this dependson the element sought, the element absorption lineconcerned, and the conditions of analysis.

With the introduction of the graphite furnace, theflame no longer plays a part in AAS, with the samplesolution being injected into a chamber that is heatedin a controlled way. Computer automation of thisprocess has provided a much faster analysis time.Interference between two or more elements can occurin the sample. In this case, solutions are made up inorder to assess the extent to which the quantitativeresults are affected. The technique has been one ofthe principal techniques used for the bulk analysis

of ancient inorganic materials, especially pottery andmetal, and has contributed in a significant way.

Inductively coupled plasma emission spectroscopy(ICPS) is also destructive. It was introduced later thanAAS and involves the dissolution of the sample. Thesample is dispersed at temperatures above 8000 C in

a plasma torch in which argon is combusted. Thesample is injected into the flame and breaks up intoits constituent parts. Since the temperatures involvedare much higher than those used in AAS, theoreticallythis should make the technique more sensitive andreduce the amount of interference between elements.It is also much faster. After calibrating the system, thetechnique allows the operator to analyze 20 elementssimultaneously. Inductively coupled plasma massspectrometry (ICP-MS) can provide both chemicaland isotopic analysis (for laser ablation ICP (LA-ICP),see microdestructive analysis).

Thin-section petrology is used to analyze pottery,

crucibles, bricks, stone (e.g., stone axes and flint),slags, and minerals (see Pottery Analysis: Petrologyand Thin-Section Analysis). Thin sections of pot ofc. 30mm in thickness are prepared, examined with apetrographic microscope, and polarized light interactswith minerals in the pottery producing characteristiccolors characteristic of the minerals and their orienta-tions. The light patterns produced, often of differentcolors, are called interference figures. The use ofpolarized light allows the analyst to identify the aniso-tropic minerals present. These minerals have physicalproperties which vary in different directions, as op-

posed to isotropic minerals which have the same opti-cal properties in all directions. The colors observedfor mineral crystals are basically due to the differencesbetween the largest and the smallest refractive indexes.It is one of the cheaper techniques, but to be success-ful it needs experience. There are many examplesof very successful studies of archaeological ceramicsusing thin-section petrology. By combining thin-section petrology and X-ray diffraction spectrometry(XRD; see below), it is possible to identify crystalsand measure the distribution of each crystal leadingto quantitative or semi-quantitative results. When thematerials being examined do not contain diagnosticminerals, it may be possible to characterize theceramic by counting the relative size and occurrenceof grains.

Neutron Activation Analysis

When samples are powdered, this technique is de-structive with sample sizes of between 50 and 200 mg.However small artifacts, such as beads, can be ana-lyzed, although their geometry may make it difficultto produce quantitative results. It is a sensitive

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technique (see Neutron Activation Analysis), butothers such as LA-ICP do not require access to anatomic pile or create radioactive materials so theyare starting to take the place of neutron activationanalysis (NAA). The samples are irradiated in an atom-ic pile for a defined period of time. The radioactive

samples then decay according to the half-lives of eachelement in the sample, those with shortest half-livesdecaying fastest. The number of counts detected isdirectly relatable to the concentration of the elementin the material being analyzed. The gamma radiationproduced is directly relatable to the activity of theatomic flux at the time of the radiation. The inclusionof standard materials is vital so that a measure of thereaction of standard and sample to the flux can bemade. Normally, the standard will be made of a similarmaterial to that being analyzed. The peak wavelengthsare characteristic of the elements present; it may benecessary to strip out interference peaks.

NAA can be a sensitive technique with potentialdetection limits down to parts per million for 3035elements simultaneously, though it is relatively insen-sitive for the detection of major components. Thetechnique has been used especially for the analysis ofceramics for a full range of trace elements, the struc-ture of the data being further analyzed using multi-variate statistics. Much of the pottery work has beenfocused on the relationships between chemical char-acterization, distribution patterns, the clays used,other raw materials used, and the location of kiln sites.

Mass Spectrometry

The technique is becoming increasingly important inthe study of ancient materials. In the investigation oforganic materials, it can be used as a means of recon-structing ancient diets and charting the movementsof populations through the analysis of bones andteeth. Alternatively, it can be used to investigate theprovenance of inorganic materials such as pottery,metals, and glasses.

The main technique that has sufficient accuracy inthe determination of isotopes in ancient materials hasbeen thermal ion mass spectrometry (TIMS) incorpor-ating spectrometers of the magnetic sector type. Avariety of isotopes can be analyzed in this way, includ-ing lead, strontium, rubidium, neodymium, oxygen,samarium, hydrogen, carbon, and nitrogen. More re-cently, multiple collector plasma mass spectrometry(MC-ICPMS) systems have also been used to pro-duce data of acceptable levels of accuracy. Quadrupolemass spectrometers can scan the mass spectrumvery rapidly, though a sacrifice may be the massresolution.

Mass spectrometry relies on the principle that it ispossible to separate electrically charged atoms of anelement according to their atomic masses (e.g., 87Srand 86Sr). This separation is achieved by using mag-netic or electrical fields. Simple mass spectrometershave magnetic or electrostatic deflector systems.

Since the determination of lead isotopes has one ofthe longest histories in archaeological research, it isworth describing what factors may (or may not) leadto its successful application. Lead is a mixture ofprimordial lead and radiogenic lead. Radiogenic leadwas produced as a result of the radioactive decayof uranium and thorium. Primordial lead consists offour isotopes (labeled according to their differentatomic masses): lead-204, lead-206, lead-207, andlead-208 with abundances of 1.3%, 26.3%, 20.8%,and 51.5%, respectively. Lead-204 is the only typewhich has not been formed as a result of radioactivedecay, so there is a constant amount in the earths crust.

Lead isotopes are continuously accumulating by theradioactive decay of uranium-238, uranium-235, andthorium-232. To characterize lead-containing depos-its, a comparison of the relative amounts of stable andradiogenic isotopes is made. Thus, clearly the leadores themselves need to be characterized sufficientlyand the degree of mixing that may have occurredneeds to be established. Lead can occur as part ofan ore body such as galena or in association withother minerals. An important assumption involvedin lead isotope analysis is that, even if the lead ispresent at varying concentrations in an ore body, as

long as it has been formed under the same conditionsand that it has undergone the same geological pro-cesses, the ratios between the lead isotopes presentwill be the same. Another important characteristic oflead is that the isotopes do not fractionate (i.e., asystematic change in the abundance ratios of twoisotopes). A result is that if an ore contains both leadand copper, the lead isotope ratios in one metal willbe the same in the other. However, remobilization of alead-containing ore as a result of a postdepositionalgeochemical process can produce heterogeneity lead-ing to a large range of lead isotope ratios, even withina single mineral type. This would therefore generate asevere problem in attempting to characterize the oreand the metal objects made from the ore in prove-nance studies. Another possible complicating factor isthat different lead ores with correspondingly differentformation times (and therefore isotopic ratios) can belaid down very close together. If used as a source ofmetal in the same production center, objects havingthe same or similar chemical compositions may havevery different isotopic compositions. Moreover, someore bodies, such as copper-bearing ones, can contain


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low lead concentrations with a spread of lead isotopecompositions due to the uranium present continuingto decay after the copper ore has been formed.A detailed understanding of the geological history oflead ore deposits is therefore essential. With theseconsiderations in mind, it is generally agreed that

20 samples of ore from a deposit should be sufficientto characterize it. The fractionation of lead isotoperatios occurs during smelting and corrosion, but thereis a general consensus that this is not a problem.Mixing of metals produced using materials contain-ing lead with different lead isotope signatures canpose a problem when it comes to characterizationand provenance. However, even here if the end mem-bers can be defined, the mixing lines can be usedadvantageously. This assessment of mixing can alsobe applied to glasses and ceramics. The use of massspectrometry to determine the age and type of rawmaterials used to make glasses is a relatively recent

development. Here, strontium isotope ratios arechemically linked to the source calcium and neodym-ium to the source of silica. Lime can be introduced inthe plant ash or in the seashell fragments in sand usedto make glass. Calcium carbonate often occurs inpottery. With glass and ceramics, as with other prov-enance studies using mass spectrometry, it is impor-tant to define the variations of these isotopes inpotential raw material sources across the landscapeand then to compare such variations with both mate-rial from primary production sites and with otherartifacts. Clearly, as for the application of mass spec-

trometry to the determination of lead isotopes, thetechnique depends on there being sufficient geologi-cal contrast in the source of the raw materials usedto be successful. The same principles apply to theuse mass spectrometry to the study of populationmovements.

Carbon-13 and nitrogen-15 are stable isotopeswhich are absorbed into the human body as a resultof being metabolised. Their presence is a reflection ofancient diets. During the process of photosynthesis,plants take up carbon dioxide from the atmosphere.The uptake of carbon in this process makes it possibleto distinguish between two different plant typeswhich are important in human diets: C3 and C4.These plant types have distinct ranges of carbon-13values depending on whether C3 or C4 plants havebeen consumed. C3 plants such as rye, oats, wheat, andbarley typically grow in temperate climates, whereasC4 plants such as maize, sorghum, tropical grasses,millet, and sugar grow in dryer, warmer climates.A relative enrichment of carbon-13 can also be amean of distinguishing between the intake of differentsources of dietary protein. Bicarbonates in seawater

is the principle source of carbon dioxide for marineplants. In the seawater, it is enriched in 13C by 7%relative to the atmospheric carbon dioxide that isphotosynthesized by land plants. Marine plant (andprotein) resources are therefore enriched in thisway compared to terrestrial sources and are com-

parable to C4plants. In the assumed absence of sub-tropical plants the determination of 13C in humanbone collagen can be used to estimate the relativeproportions of terrestrial and marine foods in humandiets. d15N values of marine plants are enriched by 4%relative to terrestrial plants and this can be seen notonly in the difference between maritime and terrestrialfood chains but also in the particular trophic levelconsidered. A gaseous sample is used for the determi-nation of these stable isotopes for mass spectrometry.Standards are used as an essential comparison.

When mass spectrometry is used to analyze organicmaterials, complex mixtures of biochemical compo-

nents are identified. These may be overcooked foodresidues, organic binders, and the organic fractions ofpigments. Hydrolysis, oxidation, and polymerizationcan further degrade organic materials such as oils andfats. Mass spectrometry can provide detailed infor-mation about the structures and compound distribu-tions of complex mixtures. The reason that it can beused in such a way is that chromatographic techni-ques such as gas chromatography (GC) and high-performance liquid chromatography (HPLC) can becoupled to mass spectrometers, leading to the separa-tion of individual molecular species and the quantifi-

cation of trace levels. The kinds of materials that canbe investigated using these techniques are leaf waxes,beeswax, fats, oils, resins, tar, human remains, andplant remains.

Microdestructive Techniques

Nondestructive and (Other) Microdestructive

Techniques

The principles of X-ray fluorescence spectrometry(XRF) form the basis for microanalysis in the contextof electron probe microanalysis and spectrometersattached to scanning electron microscopes. The prin-ciples of XRF therefore will described first.

XRF analysis can be a totally nondestructive tech-nique. It is a surface technique of spectroscopic ana-lysis which relies on the interaction of primary X-rayswith the sample generating, among other particles,a range of secondary X-rays which have energiescharacteristic of each of the elements in the sample.It produces a spectrum of energies in the same way thatAES does. The primary energy source can either be a


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radioactive material which will generate g-rays; whenfired at the sample in a solid geometry, the interactionof the g-rays with the sample will generate secondaryX-rays. A much more common source of X-raysis an X-ray tube which, when configured within acommercially produced system, will be located in a

stable position, allowing the same analytical geo-metry to be repeated each time (the same analyticalgeometry is important if quantitative analysis isattempted).

The primary X-rays interact with each of the ele-ments in the sample surface. During this process,secondary X-rays are emitted at the takeoff angle,escaping from the sample at the same angle at whichthe primary X-rays were fired at the sample. A varietyof energy transitions occur between inner atomicelectron shells in each atom, and these lead to thegeneration of the secondary X-rays. The secondaryX-rays hit a detector (typically silicon drifted with

lithium) with an analog-to-digital converter attachedto it. Conversion of discrete pulses of secondaryX-ray energies into electrical pulses occurs dependingon the atomic weight of the element concerned. Theelectron pulses are then fed into a multi-channel ana-lyzer which displays the maxima in the spectrum ofX-ray energies as a series of peaks above the back-ground (see Figure 1 for an X-ray spectrum). Thepeaks are a Gaussian shape because the process bywhich the peaks are formed is one dependent oncounting probability, whereby the maximum numberof events which fall at the exact center of the peak

produce the maximum peak height. The tails of thepeak are due to a smaller number of events at energiesabove and below the peak centroid.

The depth to which the primary X-rays penetratethe sample is mainly dependent on the energy of theprimary X-rays, the angle at which the X-rays are

fired at the sample, and the matrix composition ofthe sample. With a material which contains a relativelyhigh proportion of a heavy element, like lead, withan atomic number of 82, secondary X-rays for arelatively light element, like potassium, also in thematerial, will be derived from a shallower maximumdepth than a material which on average has a lightermatrix. The greatest depth from which elementalX-rays are derived is therefore an important consid-eration because it partly determines the sensitivity ofeach element to X-rays and therefore the numberof X-ray photons that are detected per unit time. Ifthe count rate is low, the length of the count time

in order to produce acceptable statistics needs tobe increased.

There are two principal types of XRF: energy-dispersive and wavelength-dispersive spectrometry.Energy-dispersive spectrometry operates by collectingdata from the detector, distinguishing it accordingto its energy and displaying it in spectral form.Wavelength-dispersive spectrometry, on the otherhand, relies on a different means of operation. Inthis case, the spectrometer relies on the presence ofcrystals causing the secondary X-rays to be diffractedat a particular angle, according to its atomic number.

Figure 1 An X-ray spectrumof an opaque yellow lead-silica glass which is opacified with lead-tin oxide. The X-axis displays the relative

number of counts for each element detected above the background. The Y-axis displays the X-ray emission energies for each of the

elements. Note that some elements are more sensitive to X-rays than others so the relative peak heights are nota reflection of the relative

quantities of each. Tin (Sn) and iron (Fe) have a number of X-ray peaks. These result from the interaction of electrons (in this case) with

different orbitals in the atoms. (Source: author.)


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The secondary X-rays are detected electronically. Thedispersion of the secondary X-ray is greater than withenergy-dispersive X-ray spectrometry which makesit possible to separate the X-rays peaks more com-pletely and, in general, also makes it possible todetect elements at lower levels. Wavelength-disper-

sive spectrometry is normally slower simply becausethe spectrometer angle needs to be changed, and fromtime to time also the crystals in the spectrometer,depending on what elements are sought.

Typically wavelength-dispersive X-ray spectrom-etry is used to analyze material that has been pow-dered and made into a silicon borate glass bead. Thebead is cast so that it lower surface is flat and of theappropriate diameter for the beam of primary X-raysused for its analysis. Energy-dispersive spectrometry,on the other hand, can more readily be used as atotally nondestructive technique and is thereforemore appropriate to a museum environment. How-

ever, if the sample has an irregular surface and/or isweathered/or depleted in any way, it is difficult toproduce a quantitative result. The obvious use forenergy dispersive (ED) analysis in a museum is toprovide an initial identification of a material, thoughit must never be forgotten that the technique onlyprovides an analysis of the surface. The depth towhich the exciting energy (X-ray org-ray) penetratesthe sample is dependent on, in the case of an X-raytube, the voltage used and also the matrix composi-tion of the material: for a light matrix the depthfrom which the heaviest secondary X-ray may escape

to be detected isc. 40 mm, whereas for the analysis ofmaterials with heavy matrices the maximum depthis more typically 15 mm.

The successful quantification of the results fromXRF depends on a number of factors. An ideal sampleis one which is polished flat and compositionallyhomogeneous. The factors which affect the analysisinclude X-ray tube voltage, the composition of theanode which is used in the X-ray tube, the geometryof the analytical system, the use of a collimator, theroughness of the sample surface, the extent of inter-ference between elements in the X-ray spectrum,whether the X-ray emission peaks are successfullydeconvoluted, and the composition of the samplebeing analyzed (a greater absorption of light ele-ments, such as sodium or magnesium occurs in amatrix which consists of a heavier averageZ (atomicnumber). In addition, the use of a reliable set ofstandards of known composition is absolutely essen-tial, as it is for almost any analytical technique.Obtaining a reliable set of standards can be difficultand is one way of checking the quality of the analyses.Finally, the kind of quantification program employed

can affect the results. In the case of XRF, the use offundamental parameters or the influence coefficientscan be used.

Electron Probe Microanalysis

A microdestructive technique which produces high-

quality results is electron microprobe analysis. Thisfollowed on from the milliprobe and its early use inanalysis, becoming commonly used, especially forresearch in mineral sciences, in the 1970s. As thetechnique suggests, microsamples as small as 0.5 mmcan be mounted and analyzed. The technique involvesthe use of a microbeam of electrons which are focusedon the sample surface using a series of magnetic lenses.The electrons themselves are generated using an elec-tron gun (in Figure 2). The interaction of the electronswith the sample generates secondary X-rays which arecharacteristic to the chemical elements in the material.

Figure 2 A Jeol JSM 8200 electron microprobe in the Depart-

ment of Archaeology, University of Nottingham. An energy-

dispersive spectrometer is located on the left-hand side of the

machine, the electron gun in the center (mounted vertically) and

one of four wavelength-dispersive spectrometers is on the right-

hand side of the machine with a cathodoluminescence facility

mounted behind it. In this case the samples can be imaged in

both secondary- and backscattered electron modes. The screen

shows a secondary-electron image. Samples are located beneath

the electron gun. (Source: courtesy of Edward Faber.)


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This technique provides an analysis of a shallowerlayer of material than with XRF (35 mm comparedto c. 3050mm), and by sampling and preparingthe sample carefully the quality of the results whencompared to open geometry ED X-ray analysis is farhigher. The samples are normally embedded in epoxy

resin and polished flat so that the geometry of theanalysis is repeated exactly each time. In the processof doing this, any weathered material can be removed.

The electron beam can be focused or defocuseddepending on the intended area of analysis; it mayalso be essential to defocus the beam in order to mini-mize or eradicate the possibility of volatalizing the sam-ple surface, causing elements like sodium to be boiledoff. In practice, the minimum diameter of a focusedelectron beam during the analysis of a metal or evenaceramicis1mm on the sample surface, which spreadsout in the metal itself to c. 2 mm; for glass, the beamneeds to be deliberately defocused toc. 5080mm.

The electron probe was introduced in c. 1975 pri-marily as a tool used by geologists. As a result, someof the early models used were ideal for the analysis ofsilicates, part of which to analyze chemically individ-ual crystals in ceramic materials. Apart from beingmicrodestructive, one of the other advantages of thetechnique is that it is possible to locate the electronbeam precisely on the area of the sample to be ana-lyzed with the use of a microscope attached to thesystem; if compositional heterogeneity is suspected,an energy-dispersive detector attached to the systemcan be used to carry out qualitative point anal-

yses before the quantitative analyses are performed.A scanner attached to the machine can also providean image of the sample. It would be possible to quan-tify the ED results from the system but the levels ofprecision and detection achieved with the wavelengthdispersive probe are of a far higher quality. This samecomparison is also true when the results from an EDspectrometer attached to a scanning electron micro-scope and those from a wavelength-dispersive systemin an electron microprobe are compared.

The technique is a quantitative one, and given thesmall beam size it is normal to analyze three to fivespots of a homogeneous material like glass, and aver-age the results. The system must be calibrated beforechemical analysis is attempted with the use of stand-ards, preferably of the material being analyzed for themajor elements, so as to reproduce the matrix condi-tions of differential absorption of secondary X-rays.Geological standards may be used for pure elementsoccurring at minor or trace levels in the unknown. Aswith any analytical technique, the cross-analysis of amultielement standard not used in calibrating thesystem and of proven reliability at the start and end

of the analysis will provide two things: the determi-nation of relative analytical accuracy and a means ofmonitoring any drift in the system. EPMA can be usedto distinguish between different production zones(seeFigure 3).

Scanning Electron Microscopy

As mentioned above, scanning electron microscopy(SEM) can be used for microanalysis, and it is possi-ble to attach both energy-dispersive and wavelength-dispersive spectrometers to the instrument, but thequality of the analyses will not be as high as for adedicated electron microprobe because in the contextof an SEM it is not possible to reproduce the sameanalytical geometry as for a microprobe. Indeed, thesystems should not be confused they are differentand should be referred to as an electron microprobeand an analytical SEM, respectively.

The SEM is primarily used for imaging structurallyor compositionally heterogeneous organic and inor-ganic materials. The systems are often fitted with anenergy-dispersive spectrometer, a secondary electrondetector, and a backscattered electron detector(see Figure 4). The secondary electron detector pro-vides images of the surface texture of materials,whereas the use if a back-scattered detector mainlyprovides images of variations in composition. Suchdetectors can also be attached to the EPMA.

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Weight % soda (Na2O)

Weight%

potassimoxid

e(K2O)

London rawLondon others

Amsterdam rawAmsterdam others

Venice cristalloVenice vitrum blancum

Figure 3 The relative levels of sodium and potassium oxides in

late sixteenth and seventeenth century raw and vessel glassesfrom factory and other sites in London, Amsterdam and Venice

showing distinctions in the chemical compositions of glasses from

each zone. Note that three samples of raw glass from Amsterdam

fit with the compositions of vitrum blancumglass from Venice and

were therefore imported from Venice.


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The source of energy for an SEM is an electron gun.Instead of the electrons being focused as a point as inthe electron microprobe they are focused in a par-ticular plane, but also scanned across the materialusing scanning coils so as to build up an image oftheir interaction with the material. Because backscat-tered electrons travel in almost straight lines and the

detector is located to one side of a specimen, a sha-dowing effect is produced. A secondary-electronimage is dependent on the angle between the beamand the specimen surface (see Figure 1). Differences ingray level are, as a result, a reflection of the angle ofthe surface to the detector: the roughness of the sur-face and the shape of the sample can be seen clearly inthis image of the surface of glass, including character-istic conchoidal fractures. Backscattered electronimages, on the other hand, provide pictures of com-positional heterogeneity by recording the number ofbackscattered electrons from the sample, a measureof the relative average atomic number in the area

being analyzed. The secondary electron images there-fore provide highly magnified images of the surfacetextures of materials, whereas backscattered electronimages are built up from variations in composition.The two techniques can of course be also used inconjunction with each other: if a small area of deco-ration has been applied to a metal surface which is ofa different composition from the body of the artefact,the SEM may provide evidence for both the way inwhich the decoration has been attached and also howthe materials used are different.

Whereas with an electron microprobe an initialexamination of a thin section of a pottery samplecan provide an unambiguous identification of thecrystal, which can then be pinpointed and analyzedchemically by using a photograph, the SEM can pro-vide clear images of how a particular compositionalfamily of crystals (e.g., alkaline feldspars) might vary

in composition and texture within the same sample orbetween samples from different sources. In addition,of course, the SEM can be used to show great compo-sitional contrasts between crystals and the matrixmaterial they are sitting in, between layered materialsand between depleted/ corroded surfaces and the par-ent material. By examining cross-sections throughmaterials, it is possible to relate structural corrosionto changes in chemical composition which can, inturn, be recorded photographically.

For successful imaging using a backscattered detec-tor, it is appropriate to use a sample that has beenmounted and polished flat in the same way as the

sample was prepared using the electron microprobe.This procedure is of course inappropriate if the sam-ple is being examined for its surface texture, in whichcase it should be carefully attached to a stub withinthe SEM and can be rotated to examine differentareas of interest.

In order to examine materials like metals whichconduct electrons, the sample can be attached to thestub within the system. If the material is glass, glaze,or obsidian, for example, and does not conduct elec-trons, then it needs to be coated so as to prevent

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Figure 4 A backscattered electron micrographof a thick sectionof Ottoman ceramics. The image is formed fromcontrasting greylevels

which are dependant on the variations of average Z (atomic number) in each component. The grey angular grains are silica, the whitematerial in the middle is calcium phosphate (bone ash) and the pale grey matrix materials that surrounds many of the silica crystals is a

soda-rich glass. (Source: courtesy of Martin Roe.)


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distortion and deflection of the electron beam; this isalso true for electron microprobe analysis. The sam-ple size that can be examined under the SEM is deter-mined by the size of the sample chamber. Sincesamples are examined under vacuum, open geometryis not possible, as it can be with XRF spectrometry.

A range of SEM systems are manufactured and rela-tively large objects can be accommodated, of up toc. 5cm3 cm in dimensions.

Particle-Induced X-Ray Emission

The feature that characterizes particle-induced X-rayemission (PIXE) is the cost and size of the instru-ment! The analytical technique requires a tandemvan der Graaf accelerator in order to generate parti-cles which are accelerated at high speeds toward thesample where they collide and penetrate the sampleat great speeds. A particle accelerator can cost up to500 000. The same size of sample prepared in thesame way as for the SEM and electron probe micro-analysis (EPMA) can be used and is presented to thebeam in a fixed geometry. The system can be operatedin both an open geometry and in a sample chamber, sothe sample size can, theoretically, be infinite. With anopen geometry system, it is also possible to analyzematerials containing light elements by bathing thesample in helium so as to prevent the light secondaryX-rays produced from being absorbed in the airbefore being detected.

Analytically, the most significant difference fromEPMA is that the backgrounds produced by PIXE

are a factor of 10 lower (see Figure 5). The reasonfor this is that the particles bombarding the sampleenter the sample at such a great speed that less scat-tering of the particles occurs than when electrons orX-rays are used as the primary exciting energy. The

net result is that the background of the X-ray spec-trum is significantly lower, allowing far lower con-centration of components to be detected. Some PIXEsystems are fitted with scanning coils so that thedistribution of elements through the surface of thematerial can be mapped, including at very low con-

centrations. The same detector system and multichan-nel analyzer is often used as for EPMA and SEMsystems, a lithium-drifted silicon detector.

The depth of analysis for PIXE is in the same mag-nitude of order as for stand-alone ED-XRF analysis,but also, as with XRF, is dependent on the voltagethat the system is operated at, which in the case ofPIXE is typically 1.5 or 2 MeV. The depth of penetra-tion by the analyzing beam can be up to 50 mm ina material with a light matrix and only c. 15 mm in aheavy matrix. If the substance being analyzed con-tains crystals which are 10mm below the surface thereis no way, at present, of separating the contribution

made by the crystal to the analysis from that made bythe matrix of the material. SEM and EPMA systemsare more appropriate analytical techniques in theanalysis of crystalline materials.

In addition to being able to detect elements at verylow concentrations, by using PIXE it is also possibleto map elemental distributions through the thicknessof a sample by analyzing its surface This technique isknown as Rutherford backscattering and relies onbackscattering of the protons from crystallites orother particles within the sample being analyzed.

X-ray Diffraction Spectrometry

This technique can be used to identify unambiguouslythe type of crystals present in minerals, stone, metals,pottery, opaque glasses, opaque enamels, and opaqueglazes, or it can be used to assess the degree of

Li isotopes (3) Na (284) Mg (26) Al (126) Si (77)

K (250) Fe/CaO (40) Pb isotopes (167) PbSbO2 (77)

Field of view: 78 78m2

Sb isotopes (?)

Figure 5 A series of compositional maps produced from time-of-flight secondary ion mass spectrometry. Each image is for different

ions in the same sample of opaque turquoise glass. This is similar to those produced using a scanning-electron microscope. However the

levels of detection can be an order to 1001000 lower. The opacity is due to the presence of calcium antimonate crystals. By using this

technique, however, it can be seen that iron and lithium impurities are co-located in the crystals. No other technique has such a shallow

depth of penetration combined with great analytical sensitivity (especially for light elements) combined with high special resolution.


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crystalinity in a material. It is destructive in that anobject must be sampled, but the sample can be smalland it is not destroyed in the same way that it is withtruly destructive techniques. The technique involvesfiring radiation of a particular wavelength (monochro-matic) at the crystalline material which is mounted

at a specific angle to the incoming energy. The inter-action of the radiation with the crystal(s) producesan X-ray pattern which is characteristic of the struc-ture of the crystal. Crystals are composed of latticesbuilt up in a regular pattern; their size and spacing ischaracteristic of the crystal species. The technique pro-duces spectra which sometimes include several peaksfor a single crystal providing the crystal identity. Thus,while it is possible to determine the chemical composi-tion of calcium antimonate, for example, it is onlywith XRD that it is possible to distinguish betweenCa2Sb2O7and Ca2Sb2O6. In some cases, different spe-cies of crystals are formed at different temperatures, so

XRD can help to determine what temperature regimeswere involved in the production process. For example,a-quartz is the normal form of silica which occurs innature. When it is heated to 5735 C it is convertedto b-quartz. At 867C, b-quartz is converted intoanother form of silica, tridymite; at 1250 C, cristoba-lite is formed. These reactions can be slow, a featurewhich may be of interest to the archaeological scientist,because it shows that the material was held at thetemperature before the transformations occurred. Inany case, since all three crystal types have the samechemical composition (silica), only by using XRD is it

possible to identify the species involved.The original XRD camera was used to take photo-

graphs of patterns produced by d-spacings in one ormore crystal samples in the material analysed. Morerecent developments incorporating fully automatedequipment has involved a thin slurry of the mate-rial deposited on a slide. The resulting radiation ismeasured and the spectra fed electronically into acomputer which has a library of d-spacings makingit possible to match the unknown pattern to thosestored in the computer. Quantitative assessments ofthe relative proportions of different crystal speciesin the sample are possible by examining the relativeintensities of the peak heights. This needs to be car-ried out on several subsamples of the materialbeing analyzed to produce a representative picture.Thin-section petrology is a more effective means ofcarrying out such quantitative work.

Other Techniques

A number of techniques which are used less common-ly or have only recently been used to investigate ar-chaeological materials need to be discussed. These

include synchrotron-induced (SR) radiation, X-rayphotoelectron spectroscopy (XPS), auger-electronspectroscopy (AES), particle-induced gamma emis-sion (PIGE), time of flight secondary ion mass spec-trometry (ToF-SIMS) and transmission electronmicroscopy (TEM).

Synchrotron radiation is produced by an instru-ment known as a synchrotron. The system consistsof an accelerator, which produces high energy elec-trons, a booster accelerator and a storage ring. Thetechnique produces intense radiation of all frequen-cies in the electromagnetic spectrum. The high inten-sity is accompanied by fine beam dimensions and thisallows it to be used for a range of applications usinga range of analytical techniques. For example SRcan be used for SR-induced X-ray micro-diffraction,SR-induced X-ray fluorescence and SR-induced neu-tron diffraction. SR has been used to chemically char-acterize a range of materials including pigments,

glass, luster glazes, the crystals in bones, ink, ironand phase transformations in pottery. Most recentlya synchrotron source at Cornell has been used inconjunction with confocal X-ray fluorescence to ana-lyze non-invasively separate layers of pigments inpictures. It has been possible to create a detaileddepth profile through the layers of pigments. Studieshave also been carried out using SR in order to inves-tigate deterioration and in conservation science ofboth inorganic and organic materials. With theachievement of a one micron beam diameter at thenew Diamond facility at Didcot in the UK, the time is

ripe for a wide range of archaeological and conserva-tion science applications using SR.

X-ray photoelectron spectroscopy (XPS) and augerelectron spectroscopy (AES) both have roles to play inthe investigation of archaeological materials. XPSessentially provides information about the chemicalenvironment of the elemental species induced by asuitable radiation such as Al Ka or a monochroma-tized synchotron radiation (see above). The resultinginformation is useful because it can detect the pres-ence of different bonding states for the same element.Photoelectrons have low kinetic energies and areabsorbed if they are produced below a depth of2050 A, so the technique provides informationabout surfaces. An X-ray photoelectron spectrum isa plot of binding energies versus the number of elec-trons detected. The different binding energies corre-spond to peaks in the spectrum. The technique hasbeen used to investigate glass, pottery, stone, metals,dyes, pigments, paintings, paper, ink and stone. Thedegradation of materials can obviously benefit frombeing investigated by using XPS. Auger electrons area kind of particle that is produced when a material isbombarded with electrons (such as from an electron


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gun). They also have very low kinetic energies andderive from the surface two or three atomic layers.Like XPS, Auger electron spectroscopy (AES) can beused for examining the chemical state of the atomfrom which they derive. The reason why this is possi-ble is because Auger electrons are emitted from the

outer orbitals of atoms. The outer orbitals are ofteninvolved in chemical bonding so the Auger electronscharacterize the state of the atom.

With particle-induced gamma ray emission (PIGE)instead of the more commonly measured X-rays emis-sions being measured, gamma rays are measuredinstead. Typically this type of analysis is carried outat the same time at PIXE (see above) by bombardingthe sample with protons. Although not commonlyused (partly because accelerators are expensive) thetechnique, which is especially sensitive to the detec-tion of light elements such as lithium and fluorine,can be used quantitatively.

Time-of flight secondary ion mass spectrometry(ToF-SIMS) is a technique which has only very recent-ly been used to examine archaeological materials.SIMS involves sputtering a material with a beam ofpositively charged ions in an ultra high vacuum cham-ber. The fragments of the surface that are ejected(sputtered) range from atomic species to large ensem-bles of atoms (clusters). These secondary ions areextracted into a mass spectrometer for mass analysis.In the case of ToF-SIMS a short pulse (about 1 ns) ofprimary ions hits the surface and the resulting second-ary ions are accelerated to a constant energy before

entering a field-free drift tube. Ions of different massmust have different velocities so they are separated byflight-time in the drift tube (heavy ions travel moreslowly). Their arrival time at a detector is registeredand this produces an intensity versus time spectrum.The spectrum is built up from the addition of manysuch pulses. Both positively and negatively chargedfragments are sputtered yielding both positive andnegative ion mass spectra. The sputtered fragments,typically, can only escape from a shallow depth(about 95% of the signal comes from the top twomonolayers of atoms). The technique is especiallyuseful for detecting light elements such as boronand lithium and in principle it can be used in aquantitative mode. It is the only technique which

can be used for both mapping crystallites and analyz-ing their impurities, down to ppb. Isotopic informa-tion is also produced. So far it has apparently onlybeen used for the analysis of ancient opaque glass.Another sensitive technique which is used infrequent-ly for examining archaeological materials is transmis-

sion electron microscopy (TEM). In this case a thinsection of the sample is prepared and the electronstravel through the sample. It is especially useful forthe detection of small crystallites in archaeologicalmaterials, identifying pigments and for the investiga-tion of bone and tooth diagenesis.

See also:Archaeometry; Artifacts, Overview; NeutronActivation Analysis; Pottery Analysis: Chemical; Petrol-ogy and Thin-Section Analysis; Sampling Methods,Theory and Praxis; Stable Isotope Analysis; VitreousMaterials Analysis.

Further Reading

Blackman MJ and Bishop RL (2007) The SmithsonianNIST part-

nership: the application of instrumental neutron activation anal-

ysis to archaeology. Archaeometry49: 321341.Ciliberto E and Spoto G (eds.) (2000)Modern Analytical Methods

in Art and Archaeology.New York: Wiley.Hatcher H, Tite MS, and Walsh JN (1995) A comparison of

inductively-coupled plasma emission spectrometry and atomic

absorption spectrometry analysis on standard reference silicate

materials and ceramics. Archaeometry37: 8394.Henderson J (2000)The Science and Archaeology of Materials: an

Investigation of Inorganic Materials. London and New York:Routeledge.

Li B-P, Zhao J-X, Greig A, Collerson KD,Feng Y-X, SunX-M,Guo

M-S, and Zhuo Z-X (2006) Characterisation of Chinese Tang

Sancai from Gongxian and Yaozhou kilns using ICP-MS trace

element and TIMS Sr-Nd isotope analysis. Journal of Archaeo-logical Science33: 5662.

Pollard AM and Heron C (1996) Archaeological Chemistry.Cambridge: The Royal Society of Chemistry.

Pollard AM, Batt C, Stern B, and Young S (2007)Analytical Chem-istry in Archaeology. Cambridge Manuals in Archaeology.Cambridge: Cambridge University Press.

Rutten FJM, Roe MJ, Henderson J, and Briggs D (2006) Surface

analysis of ancient glass artefacts with ToF-SIMS: A novel tool

for provenancing.Applied Surface Science252: 71247127.Whitbread IK (1995) Greek Transport Amphorae A Petrological

and Archaeological Study.Fitch Laboratory Occasional Paper 4.Athens: The British School at Athens.

Chiefdoms, Rise of See:Political Complexity, Rise of.


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