Elemental speciation: where do we come from? where do we go?{

Freddy C. Adams

University of Antwerp, Micro and Trace Analysis Centre, Universiteitsplein 1,

B-2610 Wilrijk, Belgium. E-mail: freddy.adams@ua.ac.be

Received 13th January 2004, Accepted 24th February 2004

First published as an Advance Article on the web 19th April 2004

The present day status of elemental speciation is largely based on the use of hyphenated techniques in

particular in the combination of various types of chromatography with inductively coupled plasma mass

spectrometry. The present status of environmental speciation with the hyphenated techniques is reviewed. There

is a growing need for more comprehensive approaches including the direct analysis of solid samples especially

for the elemental speciation of heterogeneous materials with microscopic methods of analysis. Methods for

solid-state speciation analysis with several types of beam methods of analysis are critically discussed and are

illustrated with two examples of our laboratory experience.

Introduction

The development of elemental speciation analysis started in theearly 1980s with the introduction of methodologies based onthe hyphenation of different chromatographic techniques toseveral kinds of elemental detectors. Other early applicationsinvolved the speciation of elements over different oxidationstates in aqueous solutions. At present there exists a plethora ofmethods for the determination of a wide range of differentcompounds in matrices such as the air, various solutions andbiological materials that are all based on the use of a com-bination (hyphenation) of various types of chromatography forseparation and sensitive tools for detection of metals withspectrometric detectors.1

Elemental speciation analysis has been able to help in anumber of pollution incidents, e.g. the Minamata Bay poison-ing with methyl mercury compounds,2 the global pollution withorganolead compounds as a result of the widespread use ofantiknock agents in gasoline3 and the problems arising inoyster farming due to the use of organotin (tributyl) anti-fouling agents that was originally detected in Arcachon Bay,France, and later appeared in various other locations.4 Thesocial and economic burden was devastating in all these casesand led to the development of analytical methods andprompted reactions for more control and effective legislation.

It is not a coincidence that at all these examples concern thepollution of the environment with organometallic compounds.In the past differences in toxicology or differences in thebehavior in the environment of elemental species gave rise tothe most important environmental problems. Despite nowmore than two decades of continuous development on thedetermination of simple organometallic species such asalkylated Hg, Pb and Sn compounds and similar species ofAs and Se, there remain today many problems in their accuratedetermination.

Despite the seriousness of many of such problems mostregulations as formulated today still tend to continue to putemphasis on the total levels of toxic metals, for instance, theEU legislation often mentions rather vaguely concepts such as‘‘elements and their compounds’’ in various regulations.Scientists have emphasised for quite some time the need toimprove the specificity of legislation with respect to the specia-tion of elements and also promoted the transfer of fundamentalknowledge to industry and governmental regulatory bodies. In

Europe, the BCR and succeeding programmes of the EuropeanCommission recognised the need for actions and providedfunding for a wide series of research projects in order toimprove the state-of-the-art of measurements and chemicalmetrology for chemical species of environmental or health con-cern (including those of the elements As, Cr, Hg, Pb, Se, Sn).More recently, the European scientific community providedfunding to enhance multidisciplinary collaborations and toboost communication of research results to decision-makersand end-users (e.g. legislators, industrialists, and routinecontrol laboratories). Recently, the EU approved the EuropeanVirtual Institute for Speciation Measurements (EVISA) toprovide industry and policy makers with access to the widearray of expertise concerning speciation measurements throughthe inventorisation and dissemination of the existing knowl-edge on metal species determinations.5 An earlier, lessambitious initiative was the Thematic Network ‘‘Specia-tion’21’’ which was funded by the EC Standards, Measure-ments and Testing Programme over the period 1998–2000.6

The solution of many problems of elemental speciationrequires the determination of the chemical and physicalbehaviour and their interactions in different biogeochemicalsystems in addition to determinations in one particular phase ofthe environment. Especially determinations in solid matricesare becoming of growing importance for environmental andhealth application and for various applications in materialsscience.

Hyphenated techniques for speciation analysis

Despite the development of various hyphenated approachesfor elemental speciation, routine application of environmentalspeciation analysis has not kept pace with the development.Routine measurements require the application of robustanalytical procedures and simple small and reliable instrumen-tation, e.g. those based on capillary columns and miniaturizedplasmas, but up to now the analytical instrumentation marketdoes not seem to follow development efforts. Analytical instru-ment manufacturers are lagging behind in providing suitableinstrumentation. One example is the development in aEuropean project of an automated speciation analyzer fororganometallic species (ASA)7 that 3 years after its successfuldevelopment has not yet led to market introduction.

There are persistent difficulties in present day proceduresthat derive mostly from steps in the analytical procedure priorto the measurement of elemental species: the recovery of the

{ Presented at the 2004 Winter Conference on Plasma Spectro-chemistry, Fort Lauderdale, FL, USA on January 5–10, 2004.D

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analyte from a solid e.g. biological, the analyte stability in thesample, the stability of the compounds used for calibration orvalidation, various forms of species transformation during theanalytical process. These problems are now actively studiedmass spectrometrically with isotopically labeled compoundsthrough accurate measurements of isotopic ratios.1,8 Massspectrometric isotope dilution analysis with a species unspecificspiking mode is now applied for the accurate quantification ofspecies with unknown chemical composition or compounds forwhich pure standard compounds are not available. Thesystematic use of isotope dilution mass spectrometry shouldincrease the accuracy of organometal determinations and helpin the elaboration of more reliable reference materials forvalidation. All this should help in providing more accurateresults and in the development of methods that are sufficientlyrobust to be used in forthcoming regulation and legislationissues.

The high sensitivity obtained with plasma based methods ledto the overwhelming popularity of ICPMS as an ultrasensitivedevice and this exposed simultaneously the weakness of theother partner in the hyphenation process, chromatographicseparations and its ability to structurally identify a number ofunknown compounds that are identified by mass spectrometryas metal containing species. It is now possible to detect manypreviously unknown metal-containing species whose identity isnot or insufficiently elucidated. The purity of many chromato-graphic peaks in biological materials must be insured by the useof several chromatographic separations on the same samplestogether with bi- or multi-dimensional HPLC or combinedHPLC-CZE separations. Unambiguous identification of com-plex metal containing biological molecules (peptides, carbohy-drates…) is now actively pursued with a range of massspectrometric techniques based on MALDI TOF MS andelectro-spray ionization (ESI MS and ESI tandem MS (triplequad and Q-TOF) but such methods lack the sensitivityrequired in many applications.9,10 The determination of metalcontaining biomolecules is one area of development in whichelemental speciation joins the fields of biotechnology andproteomics.9

Real breakthroughs for the solution of many problems canbe achieved with methods that provide simultaneously theelemental and structural information of various chromato-graphic effluents through the use of modulated plasma sources.In development are also combinations (hybrids) of massspectrometric techniques combining the advantages of differenttypes of mass spectrometric instruments.11

Driven by current developments in organic analysis, thehyphenation concept is in rapid evolution too. In essence, forelemental speciation the advantages of a separation bychromatography with the advantages (especially the sensitivity)of an elemental spectrometric detector should be optimallycombined. The structural weakness of chromatography as anidentification tool should be solved. New, so-called ‘‘hyperna-tion’’ techniques are now in full development in which severalspectrometric tools are combined (e.g. Fourier transforminfrared, FTIR for structural analysis and atomic emissionspectrometry or inductively coupled plasma mass spectro-metry, ICPMS for elemental analysis) to provide additionalstructural information on the species measured after chroma-tographic separation.12

Elemental speciation in solids

Despite all the breakthroughs and advances, it appears thatmany environmental problems cannot be solved with analyticaltechnology as presently available, as it is too much centredon methods that start with the analysis of aqueous samples.Potentially labile species cannot easily be separated from allsolid matrices. Hence, there are at present many situations

where a radically different approach is urgently needed. We willgive two examples but there are many more that could be cited.

One example where present technology cannot solve theproblem at hand is the widespread pollution of ground waterwith arsenic in large areas of the world (resulting from the useof arsenic containing ground water for human consumption).The arsenic contamination of the Lower Ganga Plain of WestBengal in India and Bangladesh is the most often citedexample.13 A thorough understanding of why a given watersupply is not toxic (As concentration below safe drinking waterstandard) while another is heavily toxic, requires more detailedknowledge of the interaction of the element in its 2 valencestates with a number of rocks and minerals present in thewater supply. The detailed analysis including speciation of theelement in the various minerals and its reactivity and bondingstrength to minerals is therefore needed. Several laboratoriesare working on this issue using direct non-destructive methodssuch as the ones described later in this article.14

Another example consists in finding a solution of globaleffects such as the sources and sinks of Hg in the polarenvironment15 that cannot be studied effectively with methodsavailable presently. Especially considering the global changeissue, it is very important to be able to predict sources and sinksof gaseous elemental mercury (the overwhelming emissionsource of the element) in the cold polar environment whereit tends to deposit. An intriguing recent observation theoccurrence of mercury depletion events (MDE) in the polarenvironment at the start of the solar season and highlycorrelated with the ozone concentration in the air, is presentlynot understood at all.16,17 Present methods for the analysis ofthe element are insufficient to resolve problems such as theMDE and other phenomena in the remote polar environmentas only a few analytical methods are presently available for thedetermination of Hg species (the determination of elementalmercury and monomethyl mercury in the air, snow andbiological samples). Some of the available methods are notalways reliable and often fall under the denominator of genericor analytical total indices.18 Only through a detailed analysisof mercury species in the particulate fraction of the air itwill eventually be possible to explain this and other naturalphenomena.

These two examples, but there are many more that could becited, indicate that there is presently a clear need for alternativeapproaches in several areas of research and application. Formany environmental applications of elemental speciation itbecomes gradually apparent that the study of many problemsrequires elemental speciation analysis simultaneously in thevarious compartments of the bio- and geosphere, i.e. elementalspeciation needs to be pursued simultaneously in the atmo-sphere, the hydrosphere and various inorganic or organicdispersed solid materials. Also for technological applicationsthere is presently a rapid and continuous development ofmethods of analysis capable of determining quantitative struc-tural and molecular as well as elemental composition of com-plex materials with increasing levels of heterogeneity.

There is at present a continuing research on methods ofanalysis capable of determining quantitative structural andmolecular as well as elemental composition of complex solidmaterials with increasing levels of heterogeneity such asshallow surface layers, heterogeneities on the microscopic(a few micrometers), mesoscopic (ca. 100 nm) and nanoscale(v100 nm) level and present advances are overall spectacular.In the first place the driving force of development is thatmodern industrial material design (nanotechnlogy, materialsscience) is often hampered by the lack of possibilities for in situcharacterisation because the dimensions of the current materialtechnology, specifically for the use of nanomaterials and nano-electronics, are shrinking faster than the resolution attainableby state-of-the-art analytical methodologies. This discrepancybecomes a general problem for a wide range of industrial

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activities, ranging from the life science and biotechnology tothe development of new materials, molecular electronics andprocessing techniques.

In many environmental and geochemical problems the needarises to be able to do analysis on the level of heterogeneityencountered in nature; this is very often on the micrometer orsub-micrometer spatial level or for surface phenomena on thenm scale. In addition it is necessary to preserve the samplein its natural (often moist) condition and to avoid modifica-tions during analysis (photooxydation/reduction, migration,amorphisation…). Some elements are often present at veryhigh concentration level (e.g. Fe) hence specificity is often veryimportant.

The major approach to do such work is to use so-called beammethods of analysis based on spatially confined interactionof ions, electrons or electromagnetic radiation with the sampleand measuring various types of interaction phenomena.Current beam methods, together with confined scanningprobe techniques, are necessary for understanding compositionand properties of nanoscale materials. In what follows wewill discuss only a few of them, namely some of those used inour laboratory.

Strictly speaking, speciation of a pure compound can bebased on the first information level, i.e. the measurement ofrelative elemental abundances. Such methods are often success-ful, e.g. the use of scanning electron microscopy combined withX-ray emission (SEM-EDX) can provide information on thepresence of specific compounds from the interrelation ofimages of different elements on the result of X-ray emission.Chemometrics, e.g., principal component analysis can beemployed to derive the information efficiently. However,such approaches become often irrelevant in mixtures. More-over, it is never certain that if an element E is associated with amineralogical matrix M that E is necessary bound to M. Asecond level of speciation is attained by methods capable ofdetermining the oxidation state by bond-specific information,e.g. X-ray photoelectron spectroscopy (XPS), infrared andRaman spectrometry. However, also in this case the problem ofidentification of specific compounds in mixtures persists.Therefore, detection of signals referring to the individualanalyte molecules as a whole or molecular speciation is ulti-mately required. This implies, in practice, that the methodenables to (1) set free individual molecules from the solid bybreaking the intermolecular bindings but without affecting theintramolecular ones and (2) obtain the molecular weight andspecific composing structural fragments or elements. This taskis inherently suited to mass spectrometry with techniques fordirect soft ionisation of solids, on the basis of, e.g., laser beams,fast atom or ion bombardment with primary ions in the keVrange or fission particles in the MeV range.

Solids mass spectrometry with ion and laser beams

Mass spectrometry (MS) excels in delivering full molecularinformation on organic and inorganic analytes. It is only ratherrecently that the traditionally separated fields of organicand inorganic analysis became merged by the use of primaryion bombardment ionisation in time-of-flight (TOF) staticsecondary ion mass spectrometry (S-SIMS) or with UV lasermicroprobe irradiation of solids (in laser microprobe massspectrometry, LMMS). Both techniques operate at the inter-face between traditional molecular (organic) and atomic(inorganic) MS. In S-SIMS sputtering is limited to a fractionof the upper monolayer which maximizes the contribution ofions representative for the molecular form of the element. Todistinguish between numerous analytes that are locally presentin the sample, it is important to obtain high mass resolu-tion spectra that provide the high specificity required forunambiguous identification of species. TOF MS allows high

resolution SIMS with resolution 10,000 while Fourier trans-form (FT) MS is able to go considerably beyond this limit.

When full molecular information is needed, state-of-the-artMS for solids offer an information depth of 1–10 nm but thelateral resolution (at full molecular information) is limited atpresent to 0.5–10 mm. At this moment, the technology for moretightly focused primary ion or photon beams is, however,available down to the 50 nm level. The lateral resolution isprimarily limited by the yield (detection sensitivity) of themolecular (adduct) ions and structural fragments. At presentthe Ga1 liquid metal ion source is most often used as it can befocused to very small and bright spots with a current densityof 1 A cm22 and spot diameters of the order of 20 to a fewhundred nm.

The alternative SIMS technique, dynamic SIMS, is con-siderably more destructive as extensive atom relocationdestroys molecular structure. It is typically used for depthprofiling the elemental concentrations. Hence, it shows littlepotential for elemental speciation. Limited valence typespeciation information is possible, e.g. to characterise thedegree of oxidation of semiconductor surfaces. Low energyprimary ions are used for improved depth resolution to thesub-nm level.

By now, many prospects of the development of SIMS areconnected with the use of poly-atomic (cluster) ions as primaryprojectiles. They tend to increase ion yields by more than afactor of 10.19,20 As compared to the atomic ion bombardment,cluster ion bombardment thus results in an increase of both thedetection sensitivity of the element analysed and the efficiencyof generating large cluster ions, hence providing more reliablespeciation information. The cluster projectile leads also to thesubstantial improvement of depth resolution and reduces theaccumulation of primary beam-induced damage. Therefore,the development and application of cluster ion sources isimportant for further improvement of SIMS analysis. Along ofvarious types of poly-atomic sources the most attractive areSF5

1, caesium and gold (Au31) sputter cluster sources.

The EU 6th Framework programme recently funded anetwork of excellence of 13 laboratories including 2 instrumentmanufacturers for the development of efficient instrumentationfor measurement, analysis and manufacture at the nano-scalewith a resolution for atoms and molecules at or below 10 nmwith static and dynamic SIMS, Auger emission spectrometryand transmission electron microscopy.

Fourier transform LMMS (FT LMMS) is a complementarytechnique to S-SIMS. The lateral resolution is of the order of5 mm and the information depth is ca 10 nm (corresponding to100 monolayers). Mass resolution is very high (100,000–4,000,000), hence mass accuracy is better than 1 ppm, thusproviding unique possibilities for identification of unknownanalytes. The energy per single event is only a few eV comparedto 10–20 keV in S-SIMS, a major advantage for desorptionionisation, which leads to more specific molecular informationin the resulting mass spectra. Up to now FT LMMS is the onlymicroprobe techniqe that is able to do molecular microanalysiswith high mass resolution.21 Detection limits are of the orderof 106 to 107 molecules in the analysed microvolume providingspectra for a single laser pulse, i.e. very close to the detectionlimits reached with S-SIMS. Our laboratory exploits instru-ments for dynamic and static SIMS and FT LMMS and ishence, well equipped to compare the different techniquesregarding elemental speciation. It appears that they providecomplementary information.

Up to now, the literature contains only a few discrete studieson inorganic speciation analysis with different instrumentsand experimental conditions, e.g. with respect to the ion dose.For instance with S-SIMS, sulfates/sulfites and nitrate/nitritesof sodium and/or silver have been studied in the so-called lowdamage regime while nitrates and nitrites were analysedwith different projectiles such as (CsI)n?Cs1 and ReO4

2.

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Spectral libraries are limited to about 30 compounds, most ofwhich are related to specific technological applications; theyare not directly useful to describe the MS behavior of salts ingeneral.

Therefore, our laboratory made a systematic study ofinorganic compounds, selected as a function of their anti-cipated MS behaviour. A comprehensive database of positiveand negative mass spectra of over 50 binary salts, oxysalts (e.g.

carbonates, sulfates, nitrates) and oxides was established. Ingeneral, the fingerprint spectra provide diagnostic informationand are built up of molecular ions (formed by the removal oraddition of one or more electrons from an intact molecule),mono- and polymeric adduct ions (combinations of ionswith one or more neutral species) and fragment ions (break-down products that preserve part of the original structure).Systematic trends could be established in the relationshipbetween the detected signals and molecular composition of theanalyte. The prediction of mass spectra from a given analytemust account for the charge state of the ions in the salt, theformation of oxide-type neutrals from oxysalts and theoccurrence of oxidation-reduction processes.22,23

One example of speciation research at the microscopic levelis an attempt in our laboratory to obtain molecular informa-tion with S-SIMS from the inorganic components at the surfaceof single ambient aerosol particles.24 Collection of the aerosolon a flat silicon wafer and a low particle loading are essentialrequirements. Furthermore, pre-cleaning of the sample isrequired by rastering the primary beam a few times over thearea of interest as the very first scans only yield informationabout the surface coverage by water (H1, H2, OH2 ions). Ithas been demonstrated that pre-cleaning does not destroy theinorganic molecular information in the aerosol. Highly specific

adduct ions were imaged in aerosol particles of 1–4 mm, whichto the best of our knowledge has never been achieved before.The characterisation of large particle populations (e.g. 1000particles) to obtain statistically valid conclusions, however,poses major problems in S-SIMS. In our experience, the massspectra reconstructed from selected particles are usually lessinformative than the ion images themselves. The compositionalinterpretation of the data is to be achieved by correlating thefeatures seen in images of different ions. Obviously, theknowledge from the speciation database24,25 is essential toallow components to be identified in the locally heterogeneousmixtures. The analysis remains qualitative in nature becausethe matrix effect on the ion yield and the topography effectscannot be accounted for properly. Results provide a realbreakthrough in image analysis when it is realised thatmolecule-specific information in a given pixel originates fromonly 10,000–40,000 molecules at the surface. Figure 1 showssecondary ion images recorded from an urban aerosol samplein the positive and negative mode. The images providespeciation information that is often more easily interpretablethan the individual mass spectra.

The study of the heterogeneous reactions at the surface ofCaCO3 particles exposed to HNO3 and H2SO4 demonstratedthe capabilities of S-SIMS to yield detailed molecular informa-tion on the inorganic surface components by direct imaging.The transformation of CaCO3 into CaSO4 and the presenceof HSO4

2 on the hydrated surface could be detected.23 Thus,S-SIMS allows a significant step to be achieved towards theold dream of looking directly to chemical composition on themicroscopic scale. Knowing that the information directly refersto the molecules present (and not to its constituting elements),such an achievement becomes even more significant.24

Fig. 1 Secondary ion images recorded from an urban aerosol sample in the positive mode (upper two series) and negative mode (lower 2 series).Applied ion dose: 1.6 1011 ions cm22. The notation M refers to the m/z of the ions. Images from positive ions are given for NH4

1 (m/z 18), Na1 (m/z23), Mg1 (m/z 24), Si1 (m/z 28), Ca1 (m/z 40), CaO1 (m/z 56), Na2O?H1 (m/z 63), NaNO2?Na1 (m/z 92), NaNO3?Na1 (m/z 108) and Na2SO4?Na1

(m/z 165). Images from negative ions are given for O2 (m/z 16), CN2 (m/z 26), S2 1 O22 (m/z 32), Cl2 (m/z 35), NO3

2 (m/z 62), SO32 (m/z 80),

NaNO3?NO22 (m/z 131), NaNO3?NO3

2 (m/z 147), Mg(NO3)2?NO22 (m/z 194) and Mg(NO3)2?NO3

2 (m/z 210).

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Micro and nano analysis using synchrotron radiation

Third generation synchrotron radiation (SR) sources can pro-vide intense X-ray beams which are eminently suitable forcharacterizing the local elemental composition, chemical bond-ing, structure and electronic and magnetic properties ofheterogeneous systems, thus combining sensitive elementalanalysis with elemental speciation information in one single,albeit complex and expensive instrument.26

The choice of X-rays as an analytical probe can be motivatedby many considerations but is often related to propertiessuch as their relatively high penetrative power, the similaritybetween the energy of the photons and typical core-levelelectronic states, and photon wavelengths which are compar-able with typical inter-atomic spacing.

Microscopic XRF analysis (m-XRF) is based on the localisedexcitation and analysis of a microscopically small area on thesurface of a larger sample.27 It provides accurate informationon the distribution of many major, minor and trace elements inheterogeneous materials or can be used for the analysis ofobjects of reduced dimensions. m-XRF is currently alsoexploited with laboratory X-ray sources but is considerablymore powerful when applied with X-rays emitted from a SRsource. Due to their high intensity and directionality SRsources are ideal for the generation of microscopically confinedX-ray beams. The polarised nature of the SR beam can be usedto reduce the relative contribution of scattered radiation reach-ing the detector and thus to enhance considerably the signal-to-background ratio of fluorescence spectra, decreasing detectionlimits by up to two orders of magnitude in optimum conditions.SR based m-XRF offers a number of advantages compared toother microprobe techniques: it combines high spatial resolu-tion with high sensitivity, can be used in atmospheric con-ditions and is relatively insensitive to beam damage to thesample. The simplicity of the method and the quite goodunderstanding of the physics of the processes provide quan-titative elemental data at a high level of accuracy.28,29

Systematic scanning of the sample in the X-ray beam provides2D images of the repartition of elements in complex objects.The availability of high intensity SR sources has given rise tothe development of instruments available to exploit the methodwith constantly improving lateral resolution, already exceeding100 nm in some applications now.

A typical instrumental arrangement is illustrated in Fig. 2.27

Elemental imaging analysis in 2 dimensions is possible byscanning the sample in the path of the X-ray beam.

One other possibility is computed X-ray fluorescence micro-tomography (XFCT) (Fig. 3). The method is based on thesystematic measurement of the beam as it is impinging on thesample that is rotated in the beam path.30 For obtaining 3Dinformation XFCT exploits one of the weaknesses of m-XRF,namely the rather penetrative nature of the impinging X-rays.For XFCT measurements the sample, in addition to beingrastered, is also rotated over 360u through the incorporation of

a sample rotation stage. Using a reconstruction procedurebased on filtered back projection a 2D elemental distributionmap in the horizontal sample plane x z (z being the direction ofthe impinging X-ray beam) is thus obtained. Systematicrepetition of the measurement process at other planes (shiftingthe sample up or down in the y direction) eventually allows theelemental analysis of the entire 3D object. Imaging can be donewith the white spectrum as well as with monochromaticradiation.31 It is now possible to carry out XFCT with spatialresolution as low as about 1 mm3 using more than 109

individual volume elements (voxels) each of them containing amulti-elemental X-ray spectrum. This provides possibilities fornon-destructive observation of the interior of the sample forthe study of shape, density and composition, e.g. non-destructive measurements in inclusions, interior pore struc-tures, buried phases or other specific features within thesample.32,33 The technique is time consuming but shortcuts areavailable through the use of confocal fluorescence measure-ments in which the analysed area is confined to an interestingpart of the object through focusing the impinging beam, theoutgoing radiation or both.34

Major applications are for the analysis of inorganic matrixcomposites, transport phenomena in porous media, the studyof calcified tissues and fatigue cracks in materials.33

Techniques are available to speed up the measurementprocess, combining fast detector systems, high-speed datanetworks and parallel computing systems to a few minutes.35

Another possibility illustrated in Fig. 2 is micro X-raydiffraction (XRD). The punctual measurement of X-raydiffraction pattern over a complex sample provides informa-tion on the variation of its crystallographic structure. MicroX-ray diffraction is now common in many X-ray micro-probes.36 Micro XRD maps showing the repartition at everyimpact point of several crystallographic states can be obtainedtogether with the maps of elemental information and can assistconsiderably in the characterisation of complex samples.

Elemental speciation information can be derived from X-rayabsorption measurements. In the applications of X-ray absorp-tion spectrometry (XAS, also called X-ray absorption finestructure spectroscopy, XAFS) the energy dependence of theinner shell photoelectric cross is exploited to increase either thespecificity of elemental analysis or to obtain information onthe chemical environment. Extended X-ray absorption finestructure analysis (EXAFS) provides information on thenumber, the atomic number and the distance of neighbouringatoms. The technique is based on irradiation with a highlymonochromatic X-ray beam of tunable energy and scanningover an absorption edge of an element of interest whilerecording either the absorption of the beam (absorption XAS),the fluorescence radiation produced (fluorescence XAS) oranother shell dependent phenomenon (Fig. 4). X-ray absorp-tion near edge structure spectrometry (XANES) measures theposition of the edge and characterisation can be achieved byexploiting specific near edge features of the X-ray absorptionspectrum. Recently, the combination of m-XRF with spatially

Fig. 2 Schematic of SR X-ray methods for elemental, speciation andcrystallographic analysis.

Fig. 3 Schematic of (a) X-ray fluorescence tomography set up whichallows the collection of (b) elemental distributions for a singlehorizontal slice of a sample.

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resolved XAS became an important tool for speciation inenvironmental and geological materials and for the study ofprocesses in chemical species transformation. Most m-XASapplications are performed in the XANES fluorescence detec-tion mode used as a ‘fingerprinting’ technique. The complexityof XAS can be diminished by obtaining the fluorescence orabsorption information with two closely spaced excitationenergies that are characteristic for the valence states of differentions in the sample.37

Recently there were a large number of demonstrations on theuse of spatially resolved speciation to distinguish valencestates of the elements, e.g. Cr, Mn,38 Fe,39 Zn,40 U and Pu41

etc., in geological, cosmological and environmental studies, forthe determination of redox state, solution complex formation,sorption on mineral phases or natural organic components,finally the bio-availability of metal compounds. Such informa-tion is essential for risk assessment, management and reductionof hazards associated with the elemental release of contami-nants. Bertsch and Hunter42 cover the literature on the subjectuntil 2000. Exploitation for the identification of chemical formsin situ in molecular toxicology are scarce up to now. One recentapplication of XAS at the Hg LIII edge concerns the deter-mination of the chemical form of Hg compounds (CH3HgCl,methylmercury cysteine…) in different fish samples.43

Still other possibilities to obtain speciation informationexist in instruments available at SR sources, such as low energyX-ray microscopy (LEEM), X-ray photoelectron spectro-scopy (XPS) and X-ray photoelectron electron microscopy(XPEEM).

Examples of solid state elemental speciation

In what follows we will briefly review two different examplesfrom our own laboratory’s experience as an illustration of whatcan be achieved with available methodologies today. The firstexample is concerned with elemental speciation of corrosionlayers on the surface of ancient bronze objects, the second withthe non-destructive structural and elemental identification ofspecific inclusions within a particular diamond crystal. Bothexamples show that it is necessary to do the measurements onthe local level and that the combination of different methods isnecessary to derive definite conclusions on particular molecularidentities.

Corrosion of ancient bronze objects

It is well known that corrosion of ancient bronzes is charac-terized by their chemical and metallurgical structure. Toaccomplish a complete characterization of the corrosion layer,it is necessary to combine several complementary analyticaltechniques.44 Commonly used techniques are optical micro-scopy (OM) to describe corrosion visually and scanningelectron microscopy with energy dispersive X-ray detection

(SEM-EDX) to obtain information about the elementaldistributions in the corrosion layers. Techniques capable ofproviding molecule specific information include XRD, FTIR,XPS and Raman spectroscopy.45–47 The main problem in usingtechniques such as XRD, FTIR and XPS as macroscopiccharacterisation tools (i.e. on powdered material) is that localor spatial information is lost. Microbeam analysis on specificparts of the objects prevents this disadvantage and also givesrise to simpler spectra.

The combination of OM, SEM-EDX with techniques cap-able of molecular speciation, such as S-SIMS, synchrotronradiation FTIR and XRD (SR-FTIR and SR-XRD) wasapplied for elemental speciation of small fragments of 2 heavilycorroded bronze fibula dating from the 3rd millennium BCand originating from the ancient Syrian excavation site ofTell Beydar.48,49 Additionally XANES was applied to deter-mine the oxidation states of copper throughout the sampleand shows, not unexpectedly, that divalent, monovalent andelemental copper are present from the surface to the interior ofthe sample.

The combination of OM, SEM-EDX and SR-XRD providesa detailed characterization of corrosion layers on these ancientmetal objects.50 Preliminary OM and SEM-EDX are used tolocate various corrosion layers through colour and elementalcomposition, while SR-XRD, XANES and FTIR providesinformation on the mineralogy of the detected compounds. TheXRD spectra proved to be particularly helpful when aided bythe results obtained with OM and SEM-EDX. The diffractionpatterns obtained with SR XRD (Fig. 5) allow the identifica-tion of 7 different corrosion compounds, i.e. cuprite (Cu2O).

Fig. 4 Set up for X-ray absorption measurements (XAS, EXAFS orXANES) with a double crystal Si 111 monochromator and poly-capillary focusing of X-rays.

Fig. 5 X-ray diffractograms of corrosion layer on ancient bronzeobject from outer layer (1) to interphase layer with bulk uncorrodedmetal (4). Q: quartz, Cal: calcite, M: malachite, C: cuprile, N: nantokite(ref. 48).

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nantokite (CuCl), brochantite (Cu4SO4(OH)6), calcite (CaCO3),quartz (SiO2), paratacamite (Cu2(OH)3Cl) and cassiterite(SnO2). The interpretation of the diffraction patterns wasfacilitated by the results obtained with S-SIMS and FTIR;without this extra information the interpretation wouldhave been considerably more difficult. The small beam sizeof 100 mm 6 100 mm allow the location of the compounds inthe different corrosion layers with a high degree of confidence.

Inclusion in diamond

Inclusions in minerals embedded inside diamonds allow thestudy of Earth’s composition deep in the mantle. A particularinclusion was studied below a polished surface in a well-studieddiamond by X-ray elemental analysis performed in theconfocal XFCT mode with the ID18F microprobe of ESRFillustrated in Fig. 2. Analysis of the data consisting of 27,000individual volume elements (voxels) by principal componentanalysis provided elemental information on three distinctfeatures in the 100 mm inclusion (Fig. 6). Raman mappingwas also done with step-by-step analysis over the inclusion andidentified two of the phases recognised by XFCT as Ca-silicateslarnite (b-Ca2 SiO4) and walstromite (CaSi2O5-titanate) bycomparison with Raman reference spectra.51 The smallestphase, less than 10 mm in size, remains as yet of an unknownmineralogical structure.52

The example shows the power of the combination of elemen-tal analysis with a molecular spectroscopic technique such asmicro Raman for the non-destructive in situ elemental specia-tion of such small features inside a an object. In fact thecombination of X-ray analysis and Raman spectroscopy is apotentially a powerful tool for speciation in the laboratoryenvironment. Our laboratory now coordinates a EU fundedproject (PRAXIS, 2002–2005) for the development of a com-bined XRF/Raman instrument in a collaboration of several EUlaboratories and 2 instrument manufacturers.53

Conclusion

Future elemental speciation should refine and improve existingtechnologies based on hyphenated techniques of analysis. Errorsources should be effectively studied, especially the possibilitiesoffered by isotope ratio measurements should be exploited.For future work it will be necessary to further refine existingmethods for direct elemental speciation in solid samples. This isthe paragon of elemental speciation for the time to come.

With the analytical methodology of today it is alreadypossible to address a number of problems. Although S-SIMShas been proven to produce ions with high structural speci-ficity, speciation applications are hampered by the relativelylow intensity of the diagnostically most useful signals. In thisrespect, the use of poly-atomic projectiles offers in principlesignificant potential to increase the molecular information. In

addition, such cluster ions allow signals to be detected from theupper mono-layer of the sample while also imaging capabilitiesare available that are a considerable help in the identification.FT LMMS, on the other hand, is a point analysis techniquewith yields molecular information with high mass resolutionwith an information depth of 10 nm. SR methods are thrivingon the development of storage ring technology and newpossibilities to focus X-rays to fine very intense beams. Theyoffer simultaneous potential in one single instrument forelemental, structural and molecular analysis of heterogeneoussamples.

Acknowledgements

The author wants to thank A. Dommergue of the University ofGrenoble, France and the following members of the MicroTrace Analysis Centre of the University of Antwerp fordiscussions or the use of illustrative material: L. Van Vaeck,A. Adriaens, R. Van Ham, I. De Ryck, P. Jitaru, B. Vekemansand L. Vincze.

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