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Reconstruction Procedure for 3D Micro X-ray Absorption Fine Structure Lars Lühl,*,†,‡ Ioanna Mantouvalou,‡ Wolfgang Malzer,‡ Ina Schaumann,§ Carla Vogt,§ Oliver Hahn,† and Birgit Kanngießer‡ †

Bundesanstalt für Materialforschung und -prüfung, Berlin, Germany Technische Universität Berlin, Berlin, Germany § Leibniz Universität Hannover, Hannover, Germany ‡

ABSTRACT: A new approach for chemical speciation in stratified systems using 3D Micro-XAFS spectroscopy is developed by combining 3D Micro X-ray Fluorescence Spectroscopy (3D Micro-XRF) and conventional X-ray Absorption Fine Structure Spectroscopy (XAFS). A prominent field of application is stratified materials within which depthresolved chemical speciation is required. Measurements are collected in fluorescence mode which in general lead to distorted spectra due to absorption effects. Developing a reliable reconstruction algorithm for obtaining undistorted spectra for superficial and in-depth layers is proposed and validated. The developed algorithm calculates the attenuation coefficients of the analyte for the successive layers facilitating a new spectroscopic tool for three-dimensionally resolved nondestructive chemical speciation.

3D Micro X-ray absorption fine structure spectroscopy (3D Micro-XAFS) combines conventional XAFS with a confocal setup hence making three-dimensional microscale measurements possible. The confocal setup has first been developed in the realm of Micro X-ray fluorescence analysis facilitating a three-dimensional elemental analysis (3D Micro-XRF).1−3 It uses two polycapillary halflenses: one in front of the excitation beam and the other in front of an energy dispersive X-ray detector, creating a probing volume in which the excitation beam is focused as well as the field of view of the detector is restricted. Moving the probing volume laterally and vertically through a specimen allows obtaining three-dimensional information on the elemental distribution (Figure 1). A

detailed description of the confocal setup is explicitly described by Malzer and Kanngiesser (2005) and Mantouvalou et al. (2008).4,5 X-ray Absorption Fine Structure (XAFS), in its two parts, Xray Absorption Near Edge Spectroscopy (XANES) and Extended X-ray Absorption Fine Structure spectroscopy (EXAFS), is considered a powerful tool for nondestructive chemical speciation6 and is carried out on a routine basis at many synchrotron radiation facilities. XAFS is an energy dependent technique which requires tuning a stable high intensity and monochromatic radiation across a characteristic absorption edge of the analyte, a reason for which synchrotron radiation is needed. Its provides information about the local structure and unoccupied electronic states. Synchrotron radiation 3D Micro-XAFS offers the possibility to obtain nondestructively depth resolved information on the chemical state of an analyte within a stratified material. By placing the probing volume at certain depth of the specimen and varying the excitation energy across an absorption edge, 3D Micro-XAFS spectra can be detected. However, absorption effects may distort the detected 3D Micro-XAFS spectra resulting in misinterpretation. The challenge then becomes developing an adequate reliable reconstruction procedure that takes all relevant absorbing effects into account. The absence of such a procedure or Received: October 18, 2011 Accepted: January 18, 2012 Published: January 18, 2012

Figure 1. Confocal setup for 3D Micro-XRF and 3D Micro-XAFS. © 2012 American Chemical Society

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dx.doi.org/10.1021/ac202285d | Anal. Chem. 2012, 84, 1907−1914

Analytical Chemistry

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diluted or very thin. In this case, it is not possible to gain depthresolved information, but on the other hand the detected spectra are not distorted by self-absorption effects and therefore are called ‘undistorted spectra’. In fluorescence mode, the analyte’s fluorescence intensity as a function of the monochromatic excitation energy is measured with an energy-dispersive detector. XAFS spectra detected in fluorescence mode are proportional to the photoionization cross section and the absorption of the excitation and the fluorescence beams. For measurements in fluorescence mode, there are no restrictions for specimen properties. The main disadvantage of this mode is the fact that, due to the selfabsorption effects in the specimen, the measured spectra are distorted. Spectral distortion for homogeneous specimen analyzed by conventional XAFS in fluorescence mode is well-known.13−15 The overall effect of self-absorption in conventional XAFS spectra is a ‘damping effect’ of the fluorescence intensity as it is detected in an integral manner over the information depth. For 3D Micro-XAFS analysis of homogeneous specimen the ‘damping effect’ depends on the depth of the probing volume. The deeper the probing volume inside the specimen, the more distinct the distortions are. If in a stratified specimen each layer contains the analyte in a different chemical state as sketched in Figure 2, then the

algorithm explains the few literature found regarding technique’s applications. For instance, in comparison to the wide applications of 3D Micro-XRF or XRF Microtomography which has quantification possibilities,7−9 XAFS 3D Microspectroscopy (both XANES and EXAFS) are far more hampered up until now due to the lack of proper reconstruction method. Only a few applications of 3D Micro-XANES have been published. Denecke et al. (2005) published results of investigations of uranium speciation in a tertiary sediment from a waste disposal natural analogue site.10 Distortion of the detected 3D Micro-XANES spectra due to absorption effects was not discussed. Silversmit et al. published (2009) and (2010) investigations of microscopic mineral iron inclusions in ‘ultradeep’ diamonds.11,12 With conventional XANES this investigation would not have been possible due to strong scattering in the light matrix of the diamond leading to a high background in the spectra. In the latter publication the surface of the specimen was polished down close to the inclusions to avoid strong absorption effects. For applications in the field of cultural heritage, polishing the specimen is not desired and sometimes impossible. On the other hand, there are many questions in the field of cultural heritage where 3D Micro-XAFS might help to get more insight into manufacturing techniques or to find adequate restoration methods. For this particular application, a reliable reconstruction procedure is therefore needed to render this method into a trustworthy nondestructive analytical tool. This paper presents a reconstruction procedure and its validation for depth resolved XANES applied onto layered samples. In principle, the procedure is applicable for depth resolved EXAFS measurements as well. The procedure is based on Mantouvalou’s et al. (2009)8 quantification method designed for 3D Micro-XRF. In previous studies we developed a calibration procedure for the 3D Micro-XRF setup4 in combination with a quantification procedure for stratified materials5 based on an expression for the primary fluorescence intensity using monochromatic excitation. This work discusses the absorption artifacts encountered in 3D Micro-XAFS spectra, which substantially differ from selfabsorption effects known for conventional XAFS measurements in fluorescence mode. Equations for the primary fluorescence intensity of the analyte are developed for homogeneous and stratified material. Based on these equations the reconstruction procedure is explained. Its validation is shown with the help of self-produced reference material containing various copper compounds as the analyte. Discussion of the validation concentrates on the reconstruction of XANES spectra. In the last section current restrictions of the reconstruction procedure are discussed, and perspectives for further developments are given.

Figure 2. A stratified specimen with N layers Ln and thicknesses dx. Each layer contains a specific chemical state ((τj)n).

product of the photoionization cross section τ and the jump factor j of the investigated edge represents the XAFS signal of the chemical state ((τj)n). The stratified specimen presented in Figure 2 consists of N layers; each layer n has the thickness Dn = dn − dn−1. The excitation intensity I0 is undistorted for the first upper most layer (I0 = I1) but distorted by transmission through the successive layers beneath (n > 1). In is the incoming intensity for layer n, IFn′ is the generated signal in this layer, and IFn is the signal after assembled absorption from n − 1 layers in the detection channel. In conventional XAFS the measured intensity is the sum of all IFn, while with 3D MicroXAFS the individual IFn is obtained. Thus, depending on thickness, density, and composition of layers, conventional XAFS-spectra reflect an assembled signal that may contain more than one chemical state of the analyte. Therefore, a spatial reconstruction of the different chemical states using conventional XAFS is not possible. On the other hand, 3D Micro-XAFS is capable of detecting the distorted XAFS spectra for each individual layer, if the probing volume is placed within only one layer. These detected depth-resolved signals are influenced by the absorption effects from upper layers and within the investigated layer.

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ABSORPTION EFFECTS IN CONVENTIONAL- AND 3D MICRO-XAFS Conventional XAFS is performed in two different modes: transmission and fluorescence. In transmission mode, the absorbance of an analyte is obtained by measuring the incoming and transmitted intensities, both detected by an ionization chamber. The negative natural logarithm of the ratio of these intensities as a function of the monochromatic excitation energy forms the XAFS-spectrum which is proportional to the attenuation. In order to obtain reasonable signal for measurements in transmission mode, the specimen has to be either 1908



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excitation (E0) and fluorescence (EF) intensities in a certain geometry μE ⎤ ⎡ μ E0 F μlin + ⎥ρ ̅ , E0 = ⎢ ⎣ cos(α) cos(β) ⎦ (2)

For a complete analysis of the detected 3D Micro-XAFS signals, both above-mentioned effects, the absorption in upper layers and the ‘damping’ due to self-absorption inside the investigated layer have to be taken into account. Only the first layer can be measured without influence from other chemical states; this can be achieved by placing the probing volume only in this layer. For the following layers, the absorption of excitation and fluorescence radiation in upper layers has to be known in order to get reliable XAFS results. This necessitates measuring the XAFS-spectra of all layers down to the layer of interest. The following section presents a reconstruction procedure, which takes into account all absorption effects as well as the extension of the probing volume.

where α and β are the angles between excitation radiation and surface normal and between detected radiation and surface normal, respectively. The mass attenuation coefficient μE0 of the excitation energy can be split into two terms: the mass attenuation coefficient μi,E0 of the analyte i and the mass attenuation coefficient μM,E0 of the Matrix M μ E = μi , E + μM , E (3)

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0

THEORY AND ALGORITHMS FOR RECONSTRUCTION The reconstruction algorithm for 3D Micro-XAFS is based on our quantification procedure for 3D Micro-XRF.5 First, the intensity equations used for the calibration and reconstruction are introduced followed by a description of both calibration and reconstruction procedures. The calibration is carried out in an analogue way to the one for 3D Micro-XRF. The procedure has been adapted to the variation of the excitation energy which is necessary for 3D Micro-XAFS. The reconstruction procedure is then elaborated for homogeneous and stratified specimen. Intensity Equations. The reconstruction procedure of undistorted 3D Micro-XAFS spectra relies on an expression of the primary fluorescence intensity (I(x,E0)) as a function of the probing depth (x). The basic equation of the reconstruction procedure is given in eq 1. For a single homogeneous layer with thickness D = dn − dn−1 and layer boundaries at dn−1 and dn, the intensity of a specific X-ray emission line as a function of the probing position and the excitation energy (E0) in a confocal setup can be written as I (x , E 0 ) = e

0

0

μi , E = wi[(τijK )XAFS , E0 + (τi(1 − jK ))at , E0 0 + σscat , i , E0] μM , E = 0

∑

wmμm , E

mεM

2 ⎛d ⎞⎤ ̅ , E0σ x − x ⎟⎥ n − 1 + μlin ⎜ − erf⎜ ⎟⎥ 2 σx ⎝ ⎠⎥⎦

(1)

(5)

⎛ n−1 ⎞ −μ · D ⎜ ̅ lin I (x , E 0 ) L n = ⎜ ∏ (e )l ⎟⎟ ·I(x , E0)n ⎝ l=0 ⎠ n ∈ 1, 2, 3, ...

x⎞ ⎟ ⎟ ⎠

0

The undistorted XAFS-signal is reflected in the photoionization cross section of the investigated edge (τijK)XAFS,E0. jK is the jump factor at the investigated edge K. The term (τi(1 − jK))at,E0 stands for the atomic attenuation due to the photoionization of the edges with lower energies, and σscat,i,E0 is the attenuation due to scattering. For a stratified specimen, the generalized equation of the primary fluorescence intensity (I(x,E0)Ln) for each layer takes into account the absorption of upper layers and is written as

ηwρg ω −μlin ̅ , E0(dn − 1− x) × I · (τj) 0 XAFS , E0 ·

2 ⎡ ⎛ 2 d + μlin ̅ , E0σ x − (μ̅ , E σx)2 /2 ⎢ ⎜ n 0 × e lin × ⎢erf⎜ 2 σx ⎢⎣ ⎝

(4)

(6)

The term I(x,E0)n is calculated from eq 1 and represents the intensity of a specific X-ray emission line for the nth layer as a homogeneous specimen. Note that the absorptions of upper layers (e −μ̅ l in ·D ) l include the undistorted XAFS signals (τijK)XAFS,E0 of chemical states i ∈ {1,...,n − 1}. For n = 1 eq 6 is equal to eq 1. Equation 6 may be used if contributions from neighboring layers to the detected 3D Micro-XAFS spectrum are negligible. In order to meet this precondition, layers must be at least as thick as the complete probing volume. The complete probing volume is defined as 95% of the Gaussian distribution, which defines the probing volume normal to the sample surface. This percentage corresponds to 4 times the width of the probing volume (±2 · σx). Calibration Procedure. For the 3D Micro-XRF calibration procedure two element specific parameters are determined by means of multielement standards: the integral sensitivity η and the width of the probing volume σx normal to specimen’s surface. It is sufficient to calibrate the setup for one excitation energy. We adapted this procedure for the requirements of 3D Micro-XAFS. Where a major difference is that the excitation energy is varied across an absorption edge and the fluorescence intensity of only one element is detected. Hence, it is necessary to determine the calibration parameters for the various excitation energies in the region of the investigated edge. In

where I0 is the intensity of the excitation radiation with energy E0. The mass concentration of the analyte is w, while ρ is the density of the sample. The fluorescence yield is ω, and g is the transition probability. The integral sensitivity η and the width of the probing volume σx in scanning direction are the calibration parameters characterizing the confocal setup. Both parameters depend on the excitation energy and the fluorescence energy of the analyte. The product of the photoionization cross section and the jump factor (τj)XAFS,E0 is equivalent to the undistorted XAFS signal at excitation energy E0. The product is suffixed with XAFS, to be distinguished from atomic values. It is the contribution of (τj)XAFS,E0 to the effective linear mass attenuation coefficient μ̅lin which complicates the reconstruction of the undistorted XAFS-signal. The effective linear mass attenuation coefficient comprises the attenuation of the 1909



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BESSY II. A detailed description of this beamline is given by Erko et al.17 The confocal setup described by Mantouvalou et al.5 for 3D Micro-XRF at the μSpot beamline is the same setup used here for 3D Micro-XAFS experiments. A polycapillary halflens in the excitation channel produces a spot size of 30 μm fwhmE in the specimen for the energy region around the copper K-edge (∼9 keV). For the polycapillary halflens in the detection channel, the fwhmD of the spot size in that energy region is about 18 μm. After careful alignment to overlap both spots the width of the probing volume normal to the sample surface was determined by the calibration procedure to be σx = (11.4 ± 0.2) μm, this corresponds to a fwhmx of (26.8 ± 0.5) μm. Technically, a seven-element Si(Li) detector was used, and a polycapillary halflens was placed in front of one of the crystals. A sufficient spectral resolution of the excitation synchrotron radiation was provided by a Si (311) double crystal monochromator with an approximate energy resolution of E/ ΔE ≈ 25,000. Calibration. A thick pure copper standard was used for the calibration procedure of the confocal setup, yielding the integral sensitivity η scaled with a conversion constant k and the width of the probing volume normal to the specimen’s surface σx. Both parameters have to be determined by depth scans for energies around the Cu K-edge used in this case. The conversion constant k relates the incoming flux (Φ0) to the measured current (I0) of a calibrated ionization chamber according to the following equation: Φ0 = kI0. Figure 3 shows the result of the calibration procedure for six excitation energies (9.03, 9.04, 9.14, 9.30, 9.45, and 9.60 keV,

addition, it is sufficient to use pure element standards for calibration. The width of the probing volume σx is composed of the spot sizes of the two polycapillary halflenses in the excitation (σE) and detection channel (σD). For 3D Micro-XAFS, σE is the part changing reciprocally with the excitation energy. It decreases with excitation energy increase due to the decrease of the critical angle of total reflection.16 The integral sensitivity η depends on the spot size of both polycapillary lenses σE and σD as well as on the associated global transmissions TE and TD for excitation and detection channels, respectively. The transmission T of a polycapillary halflens shows a stronger energy dependency than its spot size.16 Thus, the main influence on the energy-dependent integral sensitivity η in 3D Micro-XAFS stems from the transmission of the lens in the excitation channel TE. Reconstruction Procedure. For the reconstruction of 3D Micro-XAFS spectra, the composition and thicknesses of layers have to be known. Therefore, a preliminary investigation using 3D Micro-XRF precedes the 3D Micro-XANES measurements where the quantification procedure developed by Mantouvalou et al.5 provides such needed information. After calibration and quantification of 3D Micro-XRF, the only unknown parameter left is (τj)XAFS,E0. The layers are investigated in sequence from the top of the specimen by placing the probing volume completely inside each layer and detecting a 3D Micro-XAFS spectrum in each layer. For top layers or homogeneous specimen, eq 1 is used for the reconstruction of the 3D Micro-XAFS spectra. Equation 1 is analytically not solvable for (τj)XAFS,E0. Hence, a numerical tool has to be introduced. We found the Newton method well-suited for the reconstruction procedure. The Newton method finds roots of a continuously differentiable function. For each starting parameter the root of the derivation is calculated as a better approximation and is iteratively taken as the next starting parameter. Starting parameters are the corresponding atomic fundamental parameters. For spectral reconstruction of buried layers, eq 6 is used, and so the value of μ̅lin for layer 1 (L1) is then needed to be calculated. This is obtained using eqs 2- 5 where for μi,E0 and μM,E0 the values of (τ(1 − jK))at,1,E0, σscat,1,E0, and μm,E0 are obtained from the literature, while XANES signal (τj)XAFS,1,E0 for L1 is already measured and reconstructed as described in the homogeneous specimen spectral reconstruction above. However, the absolute intensity of L1’s reconstructed signal is influenced by uncertainties in layer’s composition, density, and calibration values. This is minimized by normalizing the high energy part of the signal, where XAFS oscillations become small, to the atomic photoionization cross section of the investigated edge which can be obtained from the literature. For reconstructing second layer’s signal, the effective linear mass attenuation coefficient and the thickness of the first layer ((e−μ̅linD)1) are inserted in eq 6, then the equation is solved for the second layer’s signal in an analogous way for homogeneous specimen. For each following layer this procedure has to be repeated utilizing the respective effective linear mass attenuation coefficients and thicknesses of each layer above.

Figure 3. Calculated integrated sensitivity η (gray squares) and a linear fit (gray line), left axis; measured width of the probing volume σx (black triangles) and mean value (black line), right axis.

respectively) close to the Cu K-edge. For each energy the stepsize into depth was 5 μm, and acquisition times range from 10 to 60 s. The deduced integral sensitivity η multiplied by the conversion constant k is plotted with its linear fit in Figure 3 (in gray). η increases with increasing energy due to an increasing transmission of the lens in the excitation channel. The width of the probing volume σx does not change in the considered energy range, see black triangles in Figure 3. The black line is the mean value of 11.44 μm for the experimental width of the probing volume σx. Reference Material. For validation, an ideal stratified reference material would be a light matrix containing a specific transition metal in various chemical compositions and distributed homogeneously in layers. A light matrix guarantees

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EXPERIMENTAL SECTION The experimental validation of our reconstruction procedure for homogeneous and stratified specimen was carried out at the μSpot beamline of Berlin’s synchrotron radiation facility, 1910



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Figure 4. (a): 3D Micro-XAFS spectrum of the homogeneous specimen composed of copper(II) chloride at 13 μm depth (backside of reference material A) (black) in comparison with a reference spectrum from a XAFS database18 (green) and the reconstructed spectrum (red); (b): Residua of the measured (black) and the reconstructed (red) XAFS spetra with respect to the reference material spectrum.

Figure 5. (a): Reconstructed (red) and gathered (gray) spectra from copper(II) chloride pellet of reference material A, compared to reference spectrum from a XAFS database (green),18 normalized absorption of the upper copper foil gathered in transmission mode (black); (b): Residua of Cu(II)Cl measured and reconstructed XAFS spectra with respect to the reference spectrum; (c): Sketch ofthe multilayered structure designed as reference material A.

In total, eight different stratified reference materials were produced. Reference material B consists of two layers with 10 wt % of copper as copper(I) oxide and one layer of 5 wt % of copper as copper(II) oxide in between. The thickness of layers in reference material B is about 50 μm. Another reference material (A) was produced by compressing a thick pellet of CuCl2 then on top a kapton foil (50 μm) as spacer and a thin copper foil (2 μm) were mechanically fixed. Reference materials differ in their layers’ thickness, copper concentration, and in sequence of the copper compounds. The

sufficient penetration depth for the excitation radiation. As adequate reference materials were not available, they were manufactured at the University of Hanover according to validation requirements needed for the present model. Stratified reference materials with various compounds of copper in different layers with UV-cured lacquer as a matrix were produced. As mentioned above, layers’ thicknesses should be at least 95% of the complete probing volume. For copper layers, thicknesses of approximately 46 μm are required. 1911



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Figure 6. (a): Reconstructed spectra gathered at reference material B with layer 1 (blue), layer 2 (black), and layer 3 (red), and th corresponding reference spectra (green); (b): Residua of the reconstructed XAFS spectra of the three layers of reference material B with respect to the reference spectra; (c): Sketch of the multilayered structure designed as reference material B.

thickness of various layers is in the range of (25−130) μm as determined by light microscopy. The composition of the dispersions of liquid lacquer and copper compounds was analyzed with Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES). A detailed description of the manufacturing and characterization of reference materials will be published separately. Twenty-six 3D Micro-XANES measurements of the various systems with alternating copper compositions and layer thicknesses were performed. Due to the tight beamtime, measurements were carried out with adequate energy step size in the XANES region (0.15−0.30 eV), while for pre-edge and EXAFS regions the step size was just sufficient for normalization purposes (5−10 eV). The aquisition times changed from 15 to 40 s before the absorption edge to 15 to 25 s for the following regions. A whole 3D Micro-XANES scan lasts for about 3 h for each layer for a primary beam intensity of about 1 × 1010 to 2 × 1010 photons per second impinging the first polycapillary halflens.

Homogeneous Specimen. The bottom layer of reference material A was used to test the reconstruction procedure for homogeneous specimen. To calculate the position of the surface with the quantification procedure of 3D Micro-XRF, depth scans were performed with an excitation energy of 9.1 keV. By knowing the spatial coordinates for the surface and those for measurements, the depth of the probing volume inside the specimen is calculated. With the known composition of the reference material, eq 1 is solved for the XANES signal. The result of the reconstruction procedure for the 3D MicroXANES measurement of the Cu(II)Cl-pellet is shown in Figure 4(a) (red) together with the measured spectrum (black) and a reference material spectrum from the XAFS database (green). The measured 3D Micro-XANES spectrum was gathered at the maximal intensity of the depth scan which corresponds to a calculated depth of 13 μm. The difference between measured and reference spectra and reconstructed and reference spectra are shown in Figure 4(b) in black and red, respectively. The reconstructed and the reference spectrum are in good agreement, especially regarding the structure shortly above the edge (region 1) and the white line (region 2). At lower energies, the deviation of the reconstruction from the reference can be interpreted as a hint for the occurrence of a different Cu-state in minor or trace concentration. Stratified Material. As mentioned above, the absorption of upper layers in stratified material can significantly distort the detected 3D Micro-XANES spectra of buried layers. The system of reference material A (Figure 5(c)) was designed to show strong distortion effects.

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RESULTS OF VALIDATION - RECONSTRUCTION OF 3D MICRO-XANES SPECTRA In this section, the results of the reconstruction procedure are presented for homogeneous specimen (bottom layer of reference material A) and stratified specimen (reference materials A and B). By means of spectra from reference material A, the dependency of the undistorted spectra and the absorption of upper layers on the detected 3D Micro-XANES signal is discussed in detail. Using spectra from reference material B, the reconstruction procedure is further validated for buried layers. 1912



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It is also possible to reduce the required layer thickness to its half by placing the probing volume at boundaries. In this case, the detected spectrum is not only influenced by the absorption of upper layers but also the undistorted signal of one layer and the undistorted signal of a second neighboring layer. In general, the thickness of the layers and the location of the surface and the interlayer boundaries are obtained with our 3D Micro-XRF reconstruction procedure, which has always to precede the 3D Micro-XAFS reconstruction. Thus, the whole procedure does not require any additional, possibly destructive, investigation method. Nevertheless it is always beneficial to use independent information from other analytical methods if available and feasible. The self-produced reference materials represent the ideal stratified material for 3D Micro-XAFS investigations in terms of defined boundaries and composition, i.e. light matrix with heavier elements as analytes. The results of the reconstruction procedure of the presented XANES spectra are thus the best one may obtain. Many specimen of interest are not ideal samples and often are not composed of well-defined layers’ boundaries such as objects from cultural heritage, for which 3D Micro-XAFS offers a new and unique possibility. Applying the method on unknown specimen, which is expected to be more complex, results in larger uncertainties. One major point is the quantification procedure for 3D Micro-XRF, which yields to higher uncertainties in the composition of the layers and the position of the boundaries for complex samples. These uncertainties influence the quality of the reconstructed XAFS spectra. Furthermore, in some analytical applications it might not be possible to determine the layer boundaries with 3D Micro-XRF. This is the case if the concentration of the element of interest in different layers is the same, and only its chemical bonding is different. For example, if corrosion processes are investigated, the composition detectable with 3D Micro-XRF may not change, but the chemical bonding does change. If the chemical bonding can be restricted to a few expected cases, the methodological restriction might be overcome by carrying out depth scans at certain characteristic energies, where the intensities of XAFS spectra of the various chemical states differ. In conclusion, with the presented reconstruction procedure for 3D Micro-XAFS nondestructive, depth resolved chemical speciation becomes possible. Especially, the feasibility of chemical speciation in buried interlayers of stratified materials with layer thicknesses in the micrometer regime is an interesting analytical option. We consider it an important tool for chemical speciation of stratified material for which a destructive analysis is not suitable or even excluded, such as objects from cultural heritage.

Figure 5(a) shows the results of XANES spectra of reference material A. In this stratified material with copper foil on top, the measured spectrum of the Cu(II)Cl-pellet (gray) differs significantly from the reference spectrum (green). The main reason for the complete change of the shape of the spectrum is the absorption in the upper copper foil. For example, in region 1 the absorption in the copper foil (black) increases to a maximum between 9.02 and 9.03 keV. The reference spectrum (green) remains steady, but the measured one (gray) decreases due to less intensity impinging onto the Cu(II)Cl-pellet. At higher excitation energy, the absorption of the copper foil (black) decreases while the undistorted spectrum (green) remains steady, but the measured spectrum (gray) increases due to higher intensity impinging on the Cu(II)Cl-pellet. For the reconstruction of 3D Micro-XANES spectrum of copper(II) chloride pellet in the stratified reference material A, eq 6 is used, and the resulting spectrum is shown in Figure 5(a) (red). The difference between the reconstructed and measured spectra is instantly observed, while the agreement of the reconstructed with the reference spectrum is satisfying. This is also emphasized in Figure 5(b), where the respective residua are shown. The current reconstruction model makes it possible to resolve buried layers containing the same analyte as the layers above. Reference material B is a three-layer system designed to validate the current case (Figure 6(c)). Figure 6(a) shows the reconstructed spectra for the three layers: 10% Cu as Cu2O (blue), 5% Cu as CuO (black), and 10% Cu as Cu2O (red), in sequence from top to bottom. Both CuO and Cu2O reference spectra are added for comparison. The measured spectra are not included here for the sake of clarity. The added spectra for both references show excellent agreement with reconstructed spectra. This can also be seen in Figure 6(b) where the respective residua for layer 1 (blue), 2 (black), and 3 (red) are shown. It was possible to reconstruct the buried layer without contributions of the neighboring layers. Hence, the reconstruction procedure is also appropriate for high Z elements in buried layers with a light matrix and thicknesses of few tens of μm.

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DISCUSSION AND PERSPECTIVE A reconstruction procedure for 3D Micro-XAFS spectra of homogeneous and stratified material has been developed and validated. The validation was carried out with self-produced stratified reference materials consisting of layers with various thicknesses and well-defined boundaries and composition. The layers consist of a light matrix containing copper with various chemical bonding. We have chosen copper to be the analyt for the reference samples because it is wildly common in cultural heritage material such as bronze objects and copper-based pigments. For example, the corrosion process of copper green pigments in glass reverse paintings is not fully understood yet, and the 3D Micro-XAFS may help in this front. The validation showed that the reconstruction procedure is capable to reconstruct undistorted XANES spectra of stratified material compensating for absorption artifacts. This holds, as long as the layers are thicker than the complete probing volume, which corresponds to 95% of the Gaussian distribution (±2σx).

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected], [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

The authors acknowledge the support of Ivo Zizak from the μSpot beamline at BESSY during beamtimes. 1913



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REFERENCES

(1) Kanngiesser, B.; Malzer, W.; Reiche, I. Nucl. Instrum. Methods Phys. Res., Sect. B 2003, 211, 259−264. (2) Vincze, L.; Vekemans, B.; Brenker, F.; Falkenberg, G.; Rickers, K.; Somogyi, A.; Kersten, M.; Adams, F. Anal. Chem. 2004, 76, 6786− 6791. (3) Tsuji, K.; Nakano, K.; Ding, X. Spectrochim. Acta, Part B 2007, 62, 549−553. (4) Malzer, W.; Kanngiesser, B. Spectrochim. Acta, Part B 2005, 60, 1334−1341. (5) Mantouvalou, I.; Malzer, W.; Schaumann, I.; Lühl, L.; Dargel, R.; Vogt, C.; Kanngiesser, B. Anal. Chem. 2008, 80, 819−826. (6) Aksenov, V.; Koval’chuk, M.; Kuz’min, A.; Purans, Y.; Tyutyunnikov, S. Crystallogr. Rep. 2006, 51, 908−935. (7) Chukalina, M.; Simionovici, A.; Zaitsev, S.; Vanegas, C. Spectrochim. Acta, Part B 2007, 62, 544−548. (8) Mantouvalou, I. Ph.D. Thesis, Universitätsbibliothek TU Berlin, 2009. (9) de Jonge, M.; Vogt, S. Curr. Opin. Struct. Biol. 2010, 20, 606−614. (10) Denecke, M.; Janssens, K.; Proost, K.; Rothe, J.; Noseck, U. Environ. Sci. Technol. 2005, 39, 2049−2058. (11) Silversmit, G.; Vekemans, B.; Nikitenko, S.; Tirez, K.; Bras, W.; Brenker, F.; Vincze, L. Polycapillary based μ-XAS and confocal μXANES at a bending magnet source of the ESRF. J. Phys.: Conf. Ser. 2009; p 012136. (12) Silversmit, G.; Vekemans, B.; Nikitenko, S.; Schmitz, S.; Schoonjans, T.; Brenker, F.; Vincze, L. Phys. Chem. Chem. Phys. 2010, 12, 5653−5659. (13) Eisebitt, S.; Böske, T.; Rubensson, J.; Eberhardt, W. Phys. Rev. B 1993, 47, 14103−14109 , Selbstabsorptionskorrektur Anhand von Messungen unter verschiedener Geometrie. (14) Pfalzer, P.; Urbach, J.; Klemm, M.; Horn, S.; Denboer, M.; Frenkel, A.; Kirkland, J. Phys. Rev. B 1999, 60, 9335−9339 , Selbstabsorptionskorrektur Anhand von Messungen unter verschiedener Geometrie. (15) Iida, A.; Noma, T. Jpn. J. Appl. Phys. 1993, 32, 2899−2902 , Benutzte Selbstabsorptionskorrektur. (16) Wolff, T.; Mantouvalou, I.; Malzer, W.; Nissen, J.; Berger, D.; Zizak, I.; Sokaras, D.; Karydas, A.; Grlj, N.; Pelicon, P.; Kanngiesser, B. J. Anal. At. Spectrom. 2009, 24, 669−675. (17) Erko, A.; Zizak, I. Spectrochim. Acta, Part B 2009, 64, 833−848. (18) http://xafsdb.iit.edu/database (accessed on 11/2008).

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