Double Dispersive X-Ray Fluorescence (D2XRF) based on an Energy Dispersive pnCCD detector for the detection of platinum in gold

Microchemical Journal 125 (2016) 56–61

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Double Dispersive X-Ray Fluorescence (D2XRF) based on an Energy Dispersive pnCCD detector for the detection of platinum in gold☆ Martin Radtke a,⁎, Günter Buzanich a, Ana Guilherme a, Uwe Reinholz a, Heinrich Riesemeier a, Oliver Scharf b, Philipp Scholz a,c, Maria F. Guerra d a

Bundesanstalt für Materialforschung und –prüfung (BAM), Richard-Willstaetter-Strasse 11, 12489 Berlin, Germany IfG-Institute for Scientific Instruments GmbH, Rudower Chaussee 29/31, 12489 Berlin, Germany Humboldt University, Department of Chemistry, Brook-Taylor-Strasse 2, 12489 Berlin, Germany d CNRS, UMR8096-ArchAm, MAE, 24 Allée de l’Université 92023 Nanterre, France b c

a r t i c l e

i n f o

Article history: Received 20 July 2015 Received in revised form 29 October 2015 Accepted 30 October 2015 Available online 10 November 2015 Keywords: Gold Synchrotron XRF, Platinum Wavelength dispersive D2XRF

a b s t r a c t With the aim of improving limits of detection (LOD) of trace elements in a matrix with adjacent fluorescence energies, a simple double dispersive X-ray fluorescence detection system (D2XRF) was constructed to operate at the beamlines BAMline and the mySpot @ BESSY II. This system is based on the combination of a crystal analyzer with an energy resolving single photon counting pnCCD. Without further collimators, the efficient suppression of the background by the pnCCD and the good energy resolution of the crystal results in improved LOD. In first order reflections, an energy resolution of 13 eV for Cu Kα was reached, and an energy range of 1 keV was covered in one shot. This new system was applied to the detection of platinum (Pt) in gold leaves with a LOD of 0.9 mg/kg, which is the lowest attained by totally non-destructive methods nowadays. The presence of Pt in gilded objects from Abydos and Byzantine mosaics provides vital information, as it indicates the alluvial origin of the gold for these examples. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Quantitative analyses of trace and major elements with adjacent fluorescent lines are always a challenge for energy dispersive X-ray Fluorescence (EDXRF) spectrometry. If the trace element is heavier than the major element the detector is saturated by the latter. If the trace element is lighter than the major element, an excitation energy below the major element edge prevents the excitation of the major element. In this case, resonant X-ray Raman scattering (XRRS) is inevitably induced, which is very likely to overlap with the desired fluorescence signal. Therefore, an accurate choice of the excitation energy of a monochromatic beam is necessary. One example of this difficult experimental situation is the determination of trace amounts of platinum in the mg/kg range in gold alloys that have more than 80% gold in weight in most cases. However the presence of Pt in ancient gold objects is one of the most reliable criteria for the identification of the alluvial origin of gold. The average amount of

☆ Selected papers presented at TECHNART 2015 Conference, Catania (Italy), April 27-30, 2015. ⁎ Corresponding author at: BAM Federal Institute for Materials Research and Testing, Richard-Willstaetter-Strasse 11, 12489 Berlin, Germany. E-mail address: [email protected] (M. Radtke).

http://dx.doi.org/10.1016/j.microc.2015.10.039 0026-265X/© 2015 Elsevier B.V. All rights reserved.

Pt in gold alloys gives information on the circulation of the metal in the past [1]. In the case of precious, rare and small museum objects made from gold alloys, the determination of Pt in a non-destructive way becomes essential. In order to overcome the difficulties connected to the determination of small amounts of Pt in gold alloys several trials were carried out, notably with Ion Beam Analysis (IBA) techniques [2], but the LOD attained (about 80 mg/kg) was insufficient. Other authors used wavelength dispersive X-ray fluorescence (WDXRF) for the analysis of ancient gold coins (flat and solid large objects), but the LOD attained was 20 mg/kg only [3]. In order to decrease the LOD of Pt in gold for objects of any form and size, three different approaches were followed based on an energy dispersive (ED) setup developed at the BAMline. The first trial consisted on the measurement of the fluorescence signal with two excitation energies, one just above and another just below the Pt L-edge. The difference between these two measurements was owed to the presence of Pt. A detection limit of 22 mg/kg was reached [4]. The second trial, predicated on the direct use of the K-lines of Pt and Au at high excitation energies, achieves a LOD of 40 mg/kg [5]; The third trial consisted on the acquisition of the fluorescence spectra of pure elements that were summed up to fit the spectra obtained for the samples. With this approach a LOD of 3 mg/kg was reached, considering the uncertainty of calculating the difference between two high values [6].

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Taking the above into account a new system based on a wavelength dispersive detector was developed and is presented here. This system allows the determination of low contents of trace elements in thin layers of gold. On one hand the main features of using WD systems for XRF analyses are better energy resolution, that allows discerning adjacent elements like Au and Pt [7] and straightforward calculation of peak overlaps, which can occur for elements such as As (K lines) and Pb (L lines). On the other hand the detector is not saturated by undesired information as scatter background or unrequested peaks of other elements. Some drawbacks can however be pointed out. Because each element must be measured separately, the acquisition time is obvious higher than for an ED system. The proposed set-up combines the positive characteristics of both approaches. The crystal used provides high-energy resolution and the energy-dispersive pnCCD chip allows the simultaneous measurement of an energy range, so that not every element has to be measured separately. In this work the case study of determining Pt in gold leaves is tested for ancient Egyptian gilded objects (gilding by leaf had also been used for metal objects, papyrus, etc. [8,9]) and gold Byzantine mosaics.

simultaneous analyses of all the X-ray lines of interest. Typical distances are 13 cm for both sample to crystal and crystal to detector. In the presented case for the analysis of Pt using an LiF 200 crystal, this corresponds to an angle ranging from 17.3° to 19.7° and an energy window of 1400 eV around the Pt-Lα. The advantage of this setup is the combination of a single photon counting, energy dispersive detector, which suppresses scattered radiation and an analyser crystal with high-energy resolution. Scattered and other unwanted radiation can be suppressed by the detector. From their position at the CCD chip, the incoming photon energy can be determined and photons outside the energetic region of interest can be rejected. No slit is necessary between the sample and the pnCCD. The first experiments were carried out at the beamlines BAMline [11] and mySpot [12]. These two experimental stations are the high energy beam lines for XRF measurements at the synchrotron BESSY II at the Helmholtz-Zentrum Berlin. At these beamlines the primary radiation source is a 7 T wavelength shifter. The energy of the X-ray beam was tuned by a double-crystal Si(111) monochromator (DCM) and a double multilayer monochromator (DMM) or a mirror for suppression of higher harmonics.

2. Experimental Setup And Sample Description

2.1.1. The crystal A flat crystal was chosen to achieve the simplest geometry. Two parameters must be considered for the selection of the crystal type. The first is the reflectivity that determines the achievable count rate and the second, the most important, is the d-spacing that strongly influences the geometry and the achievable energy resolution. A LiF(200) crystal with 2d = 4.027 Å was chosen. LiF(200) is described as the best general crystal, which combines high intensity and high dispersion [13].

2.1. D2XRF setup description A newly developed double dispersive setup, called D2XRF, which combines a flat crystal geometry with an ED pnCCD [10]chip for detection, is presented. A pencil beam, achieved by a slit system in front of the sample, excites the sample and the characteristic X-ray lines emitted by the sample are dispersed by a crystal towards the area detector. Fig. 1a shows a sketch of the setup, where all lines can be seen, including trace element (blue), major (orange) and respective XRRS (light orange). A picture of the setup is presented as well (Fig. 1b). Depending on the geometry of sample-crystal-detector a certain bandwidth of Bragg angles is reflected. This corresponds to an energy window according to the Bragg equation: Emin ¼

hc hc and Emax ¼ 2  d  sinΘmax 2  d  sinΘmin

The crystal is mounted on a linear stage and the detector is moved in 2Θ position automatically. The choice of the distance crystal to detector is based on the relevant energy region that corresponds to the

a)

2.1.2. Energy Dispersive CCD detector A silicon-based energy dispersive pnCCD chip is the main component of the D2XRF setup. This chip is the main unit of the Color X-ray Camera (CXC) installed at the BAMline. The standard operation of the CXC allows two-dimensional mappings of elements [14]. The sensitive area of the pnCCD chip is 12.7 × 12.7 mm2, consisting of 264 × 264 pixels of 48 × 48 μm2. Further information can be found elsewhere [15]. 2.1.3. Data processing To achieve the final WD spectra, a two-steps data processing is necessary. Experimental data are saved in a 264 x 264 x 1000 channels hyper spectral data cube, were 264 x 264 is the pixel size of the CCD chip and 1000 the number of channels for each pixel of the ED spectra.

b) CXC under 2Θ

LiF (200) crystal analyzer under Θ

Sample Incident X-ray beam Height = 2 mm, width = 100 µm Fig. 1. a) Sketch for D2XRF measurements, with the contribution of all lines: trace element (blue), major element (orange) and the respective XRRS (light orange); b) picture of the developed setup.

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The first step consists on the addition of all spectra in one column, which results in a matrix of 264 spectra of 1000 channels. Each of the 264 columns represents one energy of the wavelength dispersive spectrum. To extract the necessary information from the ED spectra, the events in the matching energy range of the pnCCD energy dispersive spectra are added. This means, if the analysed line is e.g. the Cu-Kα, than only the ROI around 8.048 keV is taken into account. All events with othe energies are dismissed, because they are attribute to the background. The definition of an adequate ROI is fundamental to obtain reliable data. If the ROI is too big the background rises, if it is too small significant counts are dismissed. When the chosen ROI corresponds to the whole spectra, the extracted wavelength dispersive spectra approximates measurements with a standard CCD chip, without single photon counting capacity. To improve the determination of the background, it is as well possible to fit the characteristic lines in the sum spectra for each column.

2.2. Sample description The analyzed samples are Ancient Egyptian gilded objects and goldleaf tesserae from Byzantine mosaics. Gold-leaf tesserae consist of a thin gold leaf applied on a glass support and covered with a glass layer. Gilded objects are covered with foils and leaves of gold and gold alloys that may reach thicknesses below 1 μm. The presence of Pt in ancient gold is characteristic of its alluvial origin and can be used as an indicator of particular workshop productions particularly when primary gold is also available [12]. It must be reminded, that the presence of inclusions of Platinum Group Elements (PGE) has been so far the only criteria for the identification of the alluvial origin of the gold used in the fabrication of Ancient Egyptian solid jewellery items [16]. Some big inclusions could be mapped by using fast mapping with newly developed IBA setups on the surface of objects dated to the New Kingdom (ca. 1550 -1070 B.C.) [2]. However, if one single publication focusing on the analysis of three gilded wood samples containing Pt (detected by SEM-EDS) from the tomb of Tutankhamen is excepted [17], PGEs could never be detected in gold foils and leaves [18], which could indicate a different origin for the gold that is beaten into foils in Ancient Egypt. In fact, scenes of the beating of gold can be found in quite a few Egyptian tombs dated to the 6th Dynasty (ca. 2345-2181 BC) onwards [19] and they seem to indicate that gold beating is made by specialized crafts [20] and not goldsmiths, in workshops dedicated to this function and not to a general work of gold. One specialized craft is clearly cited in the 14th c. BC papyrus of Neferrenpet, where Neferrenpet is said “chief of the makers of thin gold” [21]. It is even more challenging to determine low contents of Pt in thin gold leaves applied on a glassy material. The use of an incident beam that excites the elements entering in the composition of both the gold

Table 1 The influence of the beam size on energy resolution and count rate. Beam size [mm] 1 0.5 0.2 0.1 0.05 0.02

Counts [s-1]

FWHM [eV]

Time [hh:mm:ss]

Count rate [s-1]

1342512 1192599 646780 868352 649709 411778

18.4 16.8 13. 6 12. 4 12. 4 12. 4

00:01:01 00:01:01 00:01:01 00:03:01 00:05:00 00:08:02

22375 19877 10780 4824 2166 858

leaf and the glass increases the spectra background and, in consequence, the LOD of the trace elements to be determined. This is the case of the golden tesserae used in ancient Byzantine mosaics [22,23]. The gold leaves applied on the colorless or slightly colored transparent glass regularly reach a thickness of 0.2 to 0.4 μm. It is impossible to date the fabrication of the golden tesserae based on the composition of the glass (its composition is constant during too large periods), but several authors suggested the reuse of Byzantine coins to fabricate the tesserae [23,24]. The Byzantine gold is characterised by the presence of Pt, and the Byzantine coins contain amounts of Pt that depend on their date of issue [24]. Pt could never be determined in gold Byzantine tesserae, but if its quantifications was possible in the thin gold leaves, the relationship between the amounts of major elements and Pt in the Byzantine coins and in the golden tesserae could provide unique dating criteria for the mosaics and distinguish between Islamic and Byzantine productions [25]. 3. Results And Discussion 3.1. Energy resolution and range The first tests were performed using a 1 mm thick pure Cu target, in order to determine the achievable energy resolution under different conditions. The influence of the beam size was tested by the measurement of the Cu Kα1 (8047 eV) and the Kα2 (8027 eV) lines. These lines have an energy difference of 20 eV and cannot be separated by an ED detector. Fig. 2a and 2b show the D2XRF spectra achieved with horizontal beam sizes of 0.1 mm and 1 mm, respectively. Table 1 summarizes the Cu Kα1 intensity, counts per second and the Full Width at Half Maximum (FWHM) in the different spectra. The FWHM is nearly constant for a beam size between 0.005 mm and 0.2 mm. For larger beams the resolution is slightly worse, but a reasonable result is still obtained for the 1 mm beam with a FWHM of 18.4 eV. The FWHM for second order Cu Kα1 is 7.7 eV (Fig. 3). The Cu Kα1 and Cu Kα2 peaks are clearly separated.

Fig. 2. Comparison of the Cu Kα peaks with a beam size of a) 0.1 mm and b) 1 mm. The degradation of the energy resolution from 12.6 eV to 18.4 eV FWHM is clearly visible.

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Fig. 3. The Cu Kα peaks obtained for a 1 mm beam size using second order reflection. The 7.7 eV FWHM energy resolution allows the complete separation of peaks whose energy differ by 20 eV.

Fig. 5. Same measurement as in Fig. 4, but taking advantage of the energy resolution of the pnCCD. Different expected lines from Pt and Au as well as the XRRS contribution can be properly distinguished.

3.2. LOD for platinum in gold

the energy of the M absorption edge of gold EAu-M and the excitation energy E0:

As previously stated, the main objective of the D2XRF setup is the improvement of the LOD. As an example, the measurement of trace amounts of Pt in Au alloys demonstrates the capability of the setup. Measurements with a monochromatic beam with a size of 0.1 mm x 2 mm and no higher harmonics suppression were performed at the BAMline. The normally undesired higher harmonics weakly excite AuL fluorescence radiation. The L-lines are used to calibrate the setup. In the particular case of determination of the Pt contents, the choice of the optimal excitation energy E0 was crucial. If the lower E0 limit is the L3 absorption edge of Pt (11564 eV) the upper limit is restricted by the energy of the XRRS, which in order to assess the best situation, must be completely separated from the Pt fluorescence line. The maximum energy of the XRRS ER is given by the difference between

Fig. 4. The intensity distribution for NA-Au-30 measured with the pnCCD detector without applying the energy resolution of the pnCCD chip. The excitation energy was a few eV above the Pt L-edge, but with a significant contribution of higher harmonics due to the Si(111) DCM. Therefore the Au L-lines contributed to the signal. The background is caused by the unpreventable scattered radiation from the sample and the free air path. As can be seen in Fig. 5 it can be completely removed by using the energy dispersion of the pnCCD and the correct ROI. The curving is due to the geometry of the set-up and is mainly a projection of the annular collimator in front of the pnCCD.

ER ¼ E0 ‐ EAu‐M The LOD for Pt in gold was determined with the fine gold reference RM8058 from the Royal Canadian Mint, containing a certified Pt concentration of 40.8 mg/kg. Two fine gold bars fabricated by Aurubis AG, references NA-Au-30 and NA-Au-31[26], with Pt contents of respectively 58 mg/kg and 1152 mg/kg, served as control samples. Calculations were carried out by IUPAC definition. Fig. 4 shows the raw image, obtained with the pnCCD for the reference material NA-Au-30 without applying the energy resolution of the pnCCD. Adding up the total charge per columns leads to the D2XRF spectrum showed as solid line. While the Au lines are visible, the background is too high to identify the Pt and XRRS contribution. Fig. 5 illustrates the impact on the energy resolution of the pnCCD. Adding up the columns to gain the D2XRF spectra (solid line in Fig. 5) made it possible to identify and separate the Au-L-lines, Pt-Lα1-line and Au-XRRS. The spectra obtained with the pnCCD clearly demonstrate the potential of the D2XRF setup for the analysis of trace elements adjacent to major elements.

Fig. 6. D2XRF spectrum of NA-Au-30 with 58 mg/kg Pt obtained at the mySpot beam line with an acquisition time of 1000s. A LOD of 0.9 mg/kg was achieved.

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RM 8058 Rus Abydos 2_3 Abydos 2_2

140

Pt Lα1

Counts 70

Fig. 7. Some of the gilded wood objects from tomb 381 at the southern edge of the socalled ‘North Cemetery’ at Abydos, now in the Garstang Museum of Archaeology at the University of Liverpool.

Further measurements were executed at the mySpot beamline. At this beamline the higher harmonics of the DCM are suppressed by a focusing mirror for energies above 10 keV. Fig. 6 shows the D2XRF spectra for NA-Au-30. The Au-L-lines are minimized. Additionally the ROI of the each ED spectra was adjusted to the energy interval of the corresponding pixel of the pnCCD to suppress the unavoidable XRRS. As result the LOD could be lowered down to 0.9 mg/kg, which is nowadays to our knowledge the best LOD for totally non-destructive analysis of Pt in Au. 3.3. Pt in gold leaves Two thin gold leaves (Abydos 2.2 and 2.3) from gilded wood objects discovered in one Middle Kingdom (ca. 2055–1650 BC) burial from the cemetery of Abydos in Upper Egypt (Fig. 7) and two golden tesserae (Hierapolis 1 and 2) belonging to one Byzantine mosaic (Fig. 8) from the basilica of St. Philips the Apostle in Hierapolis (present Turkey), were analyzed in this work by D2XRF to check whether Pt can be accurately determined in the gold alloys. The Pt contents in the gold leaves were calculated with the rule of proportion using gold reference material RM8058 (FAU8) from the

0 9200

Raman

9250

9300

9350

9400

9450

9500

Energy (eV) Fig. 9. Spectra obtained by D2XRF for the two Egyptian gold leaves Abydos 2-2, and Abydos 2-3, the gold standard RM8058 containing 40 ppm Pt and the control sample Rus with known Pt content.

Royal Canadian Mint with 40 mg/kg of Pt. Signals were normalized prior to calculations to the total intensity of Raman scattering in the energy range from 9200 eV to 9350 eV. The Pt signal was summed up in the energy range from 9420 eV to 9460 eV. The presence of Pt could be identified and quantified in the two analyzed Egyptian foils, concentration of 339 - 340 μg/g, and in the two analyzed Byzantine tesserae, concentrations of 103 - 143 mg/kg. The control sample Rus has a known concentration measured by LA-ICP_MS (between 192 314 mg/kg) and by proton activation analysis (286 mg/kg) [1]. The D2XRF measurements yielded 242 mg/kg. The measured spectra are shown in Fig. 9, the results are summarized in Table 2. The presence of Pt in the Egyptian and Byzantine gold leaves, identified with a reached LOD of 15 mg/kg, shows that Pt can be used as criteria for provenancing and dating ancient productions, even in the case of very thin golden layers. Concerning the Egyptian gold, we can assume that in spite of the expected separation of gold beater workshops from other crafts producing gold objects, the origin of the raw material is the same. Workshops producing both the leaves and the jewellery have the same suppliers. Concerning the Byzantine golden tesserae, the determination of Pt in such small and thin leaves opens a new opportunity for dating ancient mosaics. The amounts of Pt in the gold leaves of a statistically significant group of tesserae can be related to the Byzantine gold coins, providing future reliable dating criteria. 4. Conclusions

Fig. 8. Golden tesserae from Hierapolis (about 1 cm x 1 cm in size) with the apparent gold leave sandwished between two glass layers, after removal on the corner of the top glass layer.

The newly developed D2XRF setup, which consists of an energy dispersive pnCCD chip as detector and a crystal for wavelength dispersion, allows with a simple geometry the detection of highly resolved wavelength dispersive X-ray fluorescence spectra within 264 channels. The simultaneously detected energy range is typically in the range of 1 keV, which means 4 eV/channel. The best demonstrated FWHM energy resolution in first order reflection is 13 eV for the Cu-Kα line. Energy resolution and detected range can be easily modified by changing the distances from sample to crystal, respectively from crystal to pnCCD. The ability to use energy resolved single photon counting with the pnCCD makes the use of additional collimators unnecessary and results in spectra with highly reduced background.. The ability of D2XRF was demonstrated by the excellent results obtained for the analysis of Pt in thin gold foils and leaves, a difficult analytical configuration, and the achieved LOD below 1 mg/kg at thick

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Table 2 Compilation of analysed samples. Reference

RM 8058 Rus Abydos 2.2 Abydos 2.3 Hierapolis 1 Hierapolis 2

Origin

Standard Control sample ‘North Cemetery’ Abydos, Egypt St. Philips basilica Hierapolis, Turkey

Thickness

Main elements %

mm

Au

Ag

Cu

1 0.5 b0.05 b0.05 b0.001 b0.001

99.99 90.3 85.2 77.5 96.3 98.4

0.3 13.8 21.2 3.6 1.0

9.4 1.0 1.3 0.1 0.6

samples that can only currently be reached by D2XRF. The concentrations of Pt present in the Egyptian gold foils from Abydos and in the gold leaves in the Byzantine tesserae from Hierapolis could be quantified, evidencing the use of alluvial gold. In the case of Ancient Egypt, these first data seems to show that the different workshops had common gold suppliers. In the case of the golden tesserae data revealed the possibilities offered by D2XRF to attest whether they were produced by using monetary alloys. In the future, this technique will be applied to analytical challenges like the nondestructive detection of lead and bismuth in gold. The D2XRF setup is routinely available at the beamlines BAMline and mySpot at BESSY II. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Acknowledgment We thank HZB for the allocation of synchrotron radiation beamtime, Dr. Ian Shaw from the University of Liverpool and Dr. Elisabetta Neri from UMR8167 CNRS for providing access to the archaeological samples. References [1] M.F. Guerra, T. Calligaro, Gold traces to trace gold, J. Archaeol. Sci. 31 (2004) 1199–1208. [2] Q. Lemasson, B. Moignard, C. Pacheco, L. Pichon, M.F. Guerra, Fast mapping of gold jewellery from ancient Egypt with PIXE: Searching for hard-solders and PGE inclusions, Talanta 143 (2015) 279–286. [3] M.W. Hinds, G. Bevan, R.W. Burgess, The non-destructive determination of Pt in ancient Roman gold coins by XRF spectrometry, J. Anal. At. Spectrom. 29 (2014) 1799–1805. [4] M.F. Guerra, T. Calligaro, M. Radtke, I. Reiche, H. Riesemeier, Fingerprinting ancient gold by measuring Pt with spatially resolved high energy Sy-XRF, Nucl. Instrum. Methods Phys. Res., Sect. B: Beam Interactions with Materials and Atoms 240 (2005) 505–511. [5] M.F. Guerra, M. Radtke, I. Reiche, H. Riesemeier, E. Strub, Analysis of trace elements in gold alloys by SR-XRF at high energy at the BAMline, Nucl. Instrum. Methods Phys. Res., Sect. B: Beam Interactions with Materials and Atoms 266 (2008) 2334–2338. [6] M. Radtke, I. Reiche, U. Reinholz, H. Riesemeier, M.F. Guerra, Beyond the Great Wall: gold of the silk roads and the first empire of the steppes, Anal. Chem. 85 (2013) 1650–1656. [7] A. von Bohlen, R. Hergenroder, C. Sternemann, M. Paulus, M. Radtke, H. Riesemeier, Wavelength dispersive synchrotron microprobe used for material analysis, Instrum. Sci. Technol. 33 (2005) 137–150.

Known Pt concentration

Measured Pt concentration

LOD Pt

μg/g

μg/g

μg/g

40 192-314

Standard 242 μg/g 227 207 103 143

3 5 5 5 15 15

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