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Comparison of conventional and total reflection excitation geometry for fluorescence X-ray absorption spectroscopy on droplet samples
Spectrochimica Acta Part B 58 (2003) 2239–2244
Comparison of conventional and total reflection excitation geometry for fluorescence X-ray absorption spectroscopy on droplet samples夞 G. Falkenberga,*, G. Pepponib, C. Strelib, P. Wobrauschekb a
Hamburger Synchrotronstrahlungslabor HASYLAB at Deutsches Elektronen-Synchrotron DESY, Notkestr. 85, 22607 Hamburg, Germany b ¨ ¨ Atominstitut der Osterreichischen Universitaten, TU Wien, Stadionallee 2, A-1020 Vienna, Austria Received 15 January 2003; accepted 10 June 2003
Abstract X-ray absorption fine structure (XAFS) experiments in fluorescence mode have been performed in total reflection excitation geometry and conventional 458y458 excitationydetection geometry for comparison. The experimental results have shown that XAFS measurements are feasible under normal total reflection X-ray fluorescence (TXRF) conditions, i.e. on droplet samples, with excitation in grazing incidence and using a TXRF experimental chamber. The application of the total reflection excitation geometry for XAFS measurements increases the sensitivity compared to the conventional geometry leading to lower accessible concentration ranges. However, XAFS under total reflection excitation condition fails for highly concentrated samples because of the self-absorption effect. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: Total reflection; X-ray absorption fine structure spectroscopy; Pb; Self-absorption; Fluorescence mode
1. Introduction X-ray absorption fine structure (XAFS) is widely used to study the local physical and electronic environment of specific atomic species in materials. The fluorescence yield detection mode of 夞 This paper was presented at the 9th Symposium on Total Reflection X-Ray Fluorescence Analysis and Related Methods, held in Madeira, Portugal, September 2002, and is published in the Special Issue of Spectrochimica Acta Part B, dedicated to that conference. *Corresponding author. Tel.: q49-40-8998-2933; fax: q4940-8998-2787. E-mail address: [email protected] (G. Falkenberg).
XAFS is known to be very effective in the study of dilute samples and thin layers, and measurements on micrometer-sized samples are feasible using micro-beam techniques. Low concentrations and minute amounts are also the subject of total reflection X-ray analysis (TXRF), but for sizes several orders of magnitude smaller than that for fluorescence XAFS. Using synchrotron radiation (SR), TXRF is capable of detecting and quantifying traces down to the fg absolute mass range and concentrations in the ppt range and only a small amount of sample in the range of ml or mg is needed for analysis w1–3x. In this study, it was investigated if the applica-
0584-8547/03/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.sab.2003.06.006
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G. Falkenberg et al. / Spectrochimica Acta Part B 58 (2003) 2239–2244
Table 1 Comparison of experimental conditions for fluorescence XAFS measurements in TXRF geometry and conventional XRF geometry
Incident beam cross-section Detector solid angle Double excitation Use of polarisation Incidence angle Penetration path length Homogeneity
tion of the TXRF geometry makes sense also for XAFS experiments, in case only minute sample amounts in the milligram range are available. The experiments were performed at the fluorescence beamline L at HASYLAByDESY, which recently has been equipped with a combination of Si(1 1 1) double-monochromators for spectroscopy and multilayer double-monochromators for micro-synchrotron radiation X-ray fluorescence analysis (m-SRXRF) and SR-TXRF w4,5x. SR-TXRF already has been applied successfully to samples with difficult matrices, such as metal alloys and detection limits for Nb of 0.8 pg corresponding to 40 ngyg for the specific samples w6x have been extrapolated, whereas for an ideal sample detection limits for Ni in the range of 7 fg were obtained w7x. Micro-SRXRF of minute amounts can also be performed making use of thin sample supports in the standard 458y458 XRF geometry. A comparison between the two techniques for the analysis of metal alloys has shown a higher sensitivity and lower detection limits for TXRF w3x. In this study, a similar comparison of geometries is performed for XAFS measurements in the near edge region. 2. Experimental Experiments on standard samples were performed at the fluorescence beamline L at HASYLAByDESY, which provides a combination of Si(1 1 1) double-monochromators for spectroscopy and multilayer double-monochromators for mSRXRF and TXRF. The TXRF chamber of the Atominstitut, Vienna, with vertical reflector and side-looking detector geometry was attached to the beamline for the XAFS measurements in total reflection excitation geometry. The same samples
TXRF geometry
XRF geometry
2=0.05 mm2 0.11 sr Yes Yes 0.18 3 mm Low
1=1 mm2 0.005 sr No Yes 458 -0.1 mm Low
were measured in conventional geometry with incidence and detection angles of 458 to the sample surface using the permanently installed microXAFS experiment for comparison. The absorption spectra were recorded in fluorescent mode, tuning the excitation energy near the LIII absorption edge of Pb (13 044 eV) by stepping the Si(1 1 1) monochromator with an increment of 0.5 eV. The fluorescence yield was detected at an angle of 908 to the incoming beam using energy-dispersive Si(Li) detectors in both set-ups and fluorescence spectra were acquired at every energy step. More details of the experimental conditions are given in Table 1. PbO3 was used as standard compound for the experiments. A minute amount of pure PbCO3 powder was added to 3 ml of water. Two-hundred microliters were taken from the saturated suspension and 200 ml of 1 ppm Ni solution was added for calibration. Fifty microliters of the Ni-doped pure PbCO3 suspension were taken and 200 ml water and 20 mg Al2O3 powder were added for the preparation of the diluted sample. A quantity of 3 ml of the pure and diluted sample suspension, respectively, was pipetted on a quartz reflector for the measurements in TXRF geometry and on a Mylar foil of 0.8-mm thickness for the measurements in conventional geometry. After vacuum dry, the droplets had a size of 1 mm2 on the Mylar foil. The droplet size on the reflector was much larger (;⭋ 3–4 mm). The amount of Pb in the droplets was calculated from the intensity ratio of Ni and Pb characteristic X-ray fluorescence lines in the spectra. Droplets of the pure PbCO3 suspension contained 200 ng Pb and droplets of the diluted PbCO3 suspension contained 40 ng Pb,
G. Falkenberg et al. / Spectrochimica Acta Part B 58 (2003) 2239–2244
Fig. 1. Fluorescence spectra of (a) pure and (b) diluted PbCO3 droplets taken in TXRF geometry (black) and conventional XRF geometry (gray). The excitation energy is 13 100 eV, the sample time is 6 s in (a) and 10 s in (b).
which corresponds to a relative concentration of 200 ppm Pb in Al2O3. The TXRF chamber was evacuated before measurement, while the experiments using the XRF set-up were performed in air. 3. Results and discussion Fig. 1 shows fluorescence spectra of (a) pure and (b) diluted PbCO3 samples acquired with TXRF and XRF set-ups at an excitation energy of 13 100 eV, which is well above the LIII absorption edge of Pb. In all spectra the lines of the Pb-LIII
2241
series are prominent, but the scatter peak surmounts the fluorescence peaks in the diluted samples. Note for a comparison of line intensities that a conversion gain of 40 eVychannel was used in the TXRF set-up and of 20 eVychannel was used in XRF set-up. The fluorescence spectra shown in Fig. 1 are single measurements taken from XAFS scans around the Pb-LIII near edge region. Complete XAFS scans of both PbCO3 sample systems acquired in both geometries are shown in Fig. 2. The XAFS signal is the Pb La line intensity, which is approximated by the sum of counts in a region of interest covering the line. The shapes of the spectra, which carries information about the bonding configuration of the Pb atoms, are similar in all spectra, and using the TXRF geometry more than two times higher count rates of Pb lines were achieved. In Table 1 experimental conditions for both geometries are given. In TXRF geometry, the detector can be placed much closer to the sample, which results in a larger detector solid angle and higher sensitivity, and, due to the reflection to the incident beam, the effective exciting beam intensity is doubled. However, the largest possible detector solid angle was not employed in our measurements, since the detector was equipped with a detector collimator in order to reduce blank values of Pb. Measurements on samples with concentration levels one order of magnitude smaller with similar count rates seem reasonable in TXRF geometry by modification of the detector collimator. A section of the XAFS spectra close to the Pb-LIII edge is shown in Fig. 3a and b. The spectra were normalized to the height of the edge jump in each spectrum for a comparison of the oscillations of the XAFS signal. A XAFS spectrum of a pure PbCO3 sample acquired under ideal conditions (homogeneous film, absorption coefficient, md;1) in transmission mode is given for comparison. The XAFS measured on the droplet sample does not follow exactly the shape of the XAFS of the ideal sample. This observation is related to the inhomogeneous nature of the droplets, since the inhomogeneity of samples is known to deteriorate the XAFS oscillations. The data measured using the TXRF set-up show less variance around the main course of the oscillations
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G. Falkenberg et al. / Spectrochimica Acta Part B 58 (2003) 2239–2244
and for the dilute samples the amplitudes are similar in both set-ups. This observation can be explained by the self-absorption effect. When the penetration depth of the incident X-ray beam is
Fig. 2. Fluorescence XAFS spectra of (a) pure and (b) diluted PbCO3 taken in TXRF geometry (triangles) and conventional XRF geometry (squares). The step size 0.5 eV, the sample time per step is 6 s in (a) and 10 s in (b).
because of better counting statistics for both pure and diluted samples. However, the TXRF set-up provides reduced amplitudes of the oscillations compared to the XRF set-up for the pure sample,
Fig. 3. Section of the XAFS spectra shown in Fig. 2 of (a) pure and (b) diluted PbCO3 taken in TXRF geometry (triangles) and conventional XRF geometry (squares). A PbCO3 absorption spectrum recorded in transmission mode is added for comparison.
G. Falkenberg et al. / Spectrochimica Acta Part B 58 (2003) 2239–2244
2243
Fig. 4. Grazing incidence–normal detection (A) and normal incidence–GE (B) experimental configurations and schematic representation of the path length of the exciting and fluorescence radiation in the sample.
much less than the sample size and the escape depth of the fluorescence radiation, the dependence between absorption and fluorescence is not linear, except for dilute samples. A decreased absorption coefficient at the bottom position of a XAFS oscillation, for example, will increase the penetration depth of the incident beam. If the fluorescence photons excited in the additional volume can reach the detector, the XAFS oscillations are attenuated or disappear in the extreme case. The self-absorption effect is much more distinct in TXRF geometry, since the path length of the incident beam is 3 mm compared to -0.1 mm in XRF geometry and, in our case, the absorption along the path of the incident beam cannot be neglected for the pure PbCO3 sample (md(0.5). ¨ Suzuki w8x, Troger et al. w9x and Pfalzer et al. w10x have studied the problem of self-absorption and suggested a normal incidence–grazing-exit (GE) geometry instead of the grazing incidence– normal detection used in this work. In normal incidence the self-absorption effect vanishes, because the sample seen by the primary beam is
‘thin’. The two configurations have been represented graphically in Fig. 4. The GE technique has also been applied to chemical analysis in the past w11–14x. The GE excitation–detection geometry presents the advantages of the TXRF geometry for the background reduction and sensitivity (except the double excitation by both the incident and reflected beam). Problems related to GE-XRF technique, namely the hampered quantification of measurements due to high absorption in the sample, do not appear for GE-XAFS measurements. The application of this geometrical configuration and a comparison of GE-XAFS with XAFS in TXRF and XRF geometries are planned for the future. 4. Conclusion The experimental results show that the total reflection excitation geometry can be employed for fluorescence XAFS measurements on droplet samples. The chemical (trace-) elements analysis performed by SR-TXRF can be complemented by
2244
G. Falkenberg et al. / Spectrochimica Acta Part B 58 (2003) 2239–2244
chemical speciation analysis by XAFS spectroscopy on the same sample and in the same experimental set-up at HASYLAB beamline L. The TXRF geometry facilitates increased sensitivity leading to lower accessible concentration ranges compared to the conventional XRF geometry. However, it should be noted that the long penetration path length of the beam in the droplet in TXRF geometry leads to serious problems due to the self-absorption effect for higher amounts of concentrated droplet samples, restricting the application of the technique to very small amounts or highly diluted samples. References w1x P. Wobrauschek, P. Kregsamer, W. Ladisich, C. Streli, ¨ S. Pahlke, L. Fabry, S. Garbe, M. Haller, A. Knochel, M. Radtke, Nucl. Instrum. Meth. A 363 (1995) 619. w2x P. Wobrauschek, R. Gorgl, ¨ P. Kregsamer, Ch. Streli, S. ¨ Pahlke, L. Fabry, M. Haller, A. Knochel, M. Radtke, Spectrochim. Acta B 52 (1997) 901–906. w3x G. Pepponi, P. Wobrauschek, C. Streli, N. Zoger, ¨ F. ¨ X-ray Spectrom. 30 (2001) 267–272. Hegedus,
w4x G. Falkenberg, O. Clauss, A. Swiderski, Th. Tschentscher, Nucl. Instrum. Meth. A 467–468 (2001) 737–740. w5x G. Falkenberg, Th. Tschentscher, O. Clauss, HASYLAB Annual Report 2001, Available from: http:yywww-hasylab.desy.deyscienceyannual_reportsy2001_reporty part1yinterny5720.pdf. w6x G. Pepponi, P. Wobrauschek, F. Hegedus, ¨ C. Streli, N. ¨ Zoger, C. Jokubonis, G. Falkenberg, H. Grimmer, Spectrochim. Acta Part B 56 (2001) 2063–2071. w7x G. Pepponi, C. Streli, P. Wobrauschek, S. Zamini, N. ¨ Zoger, G. Falkenberg, Comparison of SR-TXRF excitation–detection geometries for samples with differing matrices, Spectrochim. Acta Part B 58 (2003) 2139– 2144. w8x Y. Suzuki, Phys. Rev. B 39 (5) (1989) 3393–3395. w9x L. Troger, ¨ D. Arvanitis, K. Baberschke, H. Michaelis, U. Grimm, E. Zschech, Phys. Rev. B 46 (6) (1992) 3283–3289. w10x P. Pfalzer, J.-P. Urbach, M. Klemm, S. Horn, M.L. deBoer, A.I. Frenkel, J.P. Kirkland, Phys. Rev. B 60 (13) (1999) 9335–9339. w11x P. DeBokx, Kok, Bailleul, G. Wiener, H. Urbach, Spectrochim. Acta Part B 52 (1997) 829–840. w12x K. Tsuji, K. Wagatsuma, Oku, X-ray Spectrom. 29 (2000) 3155–3160. w13x M. Claes, R. VanGrieken, P. DeBokx, X-ray Spectrom. 26 (1997) 153. w14x P. DeBokx, H. Urbach, Rev. Sci. Instrum. 66 (1) (1995) 15.
Comparison of conventional and total reflection excitation geometry for fluorescence X-ray absorption spectroscopy on droplet samples夞 G. Falkenberga,*, G. Pepponib, C. Strelib, P. Wobrauschekb a
Hamburger Synchrotronstrahlungslabor HASYLAB at Deutsches Elektronen-Synchrotron DESY, Notkestr. 85, 22607 Hamburg, Germany b ¨ ¨ Atominstitut der Osterreichischen Universitaten, TU Wien, Stadionallee 2, A-1020 Vienna, Austria Received 15 January 2003; accepted 10 June 2003
Abstract X-ray absorption fine structure (XAFS) experiments in fluorescence mode have been performed in total reflection excitation geometry and conventional 458y458 excitationydetection geometry for comparison. The experimental results have shown that XAFS measurements are feasible under normal total reflection X-ray fluorescence (TXRF) conditions, i.e. on droplet samples, with excitation in grazing incidence and using a TXRF experimental chamber. The application of the total reflection excitation geometry for XAFS measurements increases the sensitivity compared to the conventional geometry leading to lower accessible concentration ranges. However, XAFS under total reflection excitation condition fails for highly concentrated samples because of the self-absorption effect. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: Total reflection; X-ray absorption fine structure spectroscopy; Pb; Self-absorption; Fluorescence mode
1. Introduction X-ray absorption fine structure (XAFS) is widely used to study the local physical and electronic environment of specific atomic species in materials. The fluorescence yield detection mode of 夞 This paper was presented at the 9th Symposium on Total Reflection X-Ray Fluorescence Analysis and Related Methods, held in Madeira, Portugal, September 2002, and is published in the Special Issue of Spectrochimica Acta Part B, dedicated to that conference. *Corresponding author. Tel.: q49-40-8998-2933; fax: q4940-8998-2787. E-mail address: [email protected] (G. Falkenberg).
XAFS is known to be very effective in the study of dilute samples and thin layers, and measurements on micrometer-sized samples are feasible using micro-beam techniques. Low concentrations and minute amounts are also the subject of total reflection X-ray analysis (TXRF), but for sizes several orders of magnitude smaller than that for fluorescence XAFS. Using synchrotron radiation (SR), TXRF is capable of detecting and quantifying traces down to the fg absolute mass range and concentrations in the ppt range and only a small amount of sample in the range of ml or mg is needed for analysis w1–3x. In this study, it was investigated if the applica-
0584-8547/03/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.sab.2003.06.006
2240
G. Falkenberg et al. / Spectrochimica Acta Part B 58 (2003) 2239–2244
Table 1 Comparison of experimental conditions for fluorescence XAFS measurements in TXRF geometry and conventional XRF geometry
Incident beam cross-section Detector solid angle Double excitation Use of polarisation Incidence angle Penetration path length Homogeneity
tion of the TXRF geometry makes sense also for XAFS experiments, in case only minute sample amounts in the milligram range are available. The experiments were performed at the fluorescence beamline L at HASYLAByDESY, which recently has been equipped with a combination of Si(1 1 1) double-monochromators for spectroscopy and multilayer double-monochromators for micro-synchrotron radiation X-ray fluorescence analysis (m-SRXRF) and SR-TXRF w4,5x. SR-TXRF already has been applied successfully to samples with difficult matrices, such as metal alloys and detection limits for Nb of 0.8 pg corresponding to 40 ngyg for the specific samples w6x have been extrapolated, whereas for an ideal sample detection limits for Ni in the range of 7 fg were obtained w7x. Micro-SRXRF of minute amounts can also be performed making use of thin sample supports in the standard 458y458 XRF geometry. A comparison between the two techniques for the analysis of metal alloys has shown a higher sensitivity and lower detection limits for TXRF w3x. In this study, a similar comparison of geometries is performed for XAFS measurements in the near edge region. 2. Experimental Experiments on standard samples were performed at the fluorescence beamline L at HASYLAByDESY, which provides a combination of Si(1 1 1) double-monochromators for spectroscopy and multilayer double-monochromators for mSRXRF and TXRF. The TXRF chamber of the Atominstitut, Vienna, with vertical reflector and side-looking detector geometry was attached to the beamline for the XAFS measurements in total reflection excitation geometry. The same samples
TXRF geometry
XRF geometry
2=0.05 mm2 0.11 sr Yes Yes 0.18 3 mm Low
1=1 mm2 0.005 sr No Yes 458 -0.1 mm Low
were measured in conventional geometry with incidence and detection angles of 458 to the sample surface using the permanently installed microXAFS experiment for comparison. The absorption spectra were recorded in fluorescent mode, tuning the excitation energy near the LIII absorption edge of Pb (13 044 eV) by stepping the Si(1 1 1) monochromator with an increment of 0.5 eV. The fluorescence yield was detected at an angle of 908 to the incoming beam using energy-dispersive Si(Li) detectors in both set-ups and fluorescence spectra were acquired at every energy step. More details of the experimental conditions are given in Table 1. PbO3 was used as standard compound for the experiments. A minute amount of pure PbCO3 powder was added to 3 ml of water. Two-hundred microliters were taken from the saturated suspension and 200 ml of 1 ppm Ni solution was added for calibration. Fifty microliters of the Ni-doped pure PbCO3 suspension were taken and 200 ml water and 20 mg Al2O3 powder were added for the preparation of the diluted sample. A quantity of 3 ml of the pure and diluted sample suspension, respectively, was pipetted on a quartz reflector for the measurements in TXRF geometry and on a Mylar foil of 0.8-mm thickness for the measurements in conventional geometry. After vacuum dry, the droplets had a size of 1 mm2 on the Mylar foil. The droplet size on the reflector was much larger (;⭋ 3–4 mm). The amount of Pb in the droplets was calculated from the intensity ratio of Ni and Pb characteristic X-ray fluorescence lines in the spectra. Droplets of the pure PbCO3 suspension contained 200 ng Pb and droplets of the diluted PbCO3 suspension contained 40 ng Pb,
G. Falkenberg et al. / Spectrochimica Acta Part B 58 (2003) 2239–2244
Fig. 1. Fluorescence spectra of (a) pure and (b) diluted PbCO3 droplets taken in TXRF geometry (black) and conventional XRF geometry (gray). The excitation energy is 13 100 eV, the sample time is 6 s in (a) and 10 s in (b).
which corresponds to a relative concentration of 200 ppm Pb in Al2O3. The TXRF chamber was evacuated before measurement, while the experiments using the XRF set-up were performed in air. 3. Results and discussion Fig. 1 shows fluorescence spectra of (a) pure and (b) diluted PbCO3 samples acquired with TXRF and XRF set-ups at an excitation energy of 13 100 eV, which is well above the LIII absorption edge of Pb. In all spectra the lines of the Pb-LIII
2241
series are prominent, but the scatter peak surmounts the fluorescence peaks in the diluted samples. Note for a comparison of line intensities that a conversion gain of 40 eVychannel was used in the TXRF set-up and of 20 eVychannel was used in XRF set-up. The fluorescence spectra shown in Fig. 1 are single measurements taken from XAFS scans around the Pb-LIII near edge region. Complete XAFS scans of both PbCO3 sample systems acquired in both geometries are shown in Fig. 2. The XAFS signal is the Pb La line intensity, which is approximated by the sum of counts in a region of interest covering the line. The shapes of the spectra, which carries information about the bonding configuration of the Pb atoms, are similar in all spectra, and using the TXRF geometry more than two times higher count rates of Pb lines were achieved. In Table 1 experimental conditions for both geometries are given. In TXRF geometry, the detector can be placed much closer to the sample, which results in a larger detector solid angle and higher sensitivity, and, due to the reflection to the incident beam, the effective exciting beam intensity is doubled. However, the largest possible detector solid angle was not employed in our measurements, since the detector was equipped with a detector collimator in order to reduce blank values of Pb. Measurements on samples with concentration levels one order of magnitude smaller with similar count rates seem reasonable in TXRF geometry by modification of the detector collimator. A section of the XAFS spectra close to the Pb-LIII edge is shown in Fig. 3a and b. The spectra were normalized to the height of the edge jump in each spectrum for a comparison of the oscillations of the XAFS signal. A XAFS spectrum of a pure PbCO3 sample acquired under ideal conditions (homogeneous film, absorption coefficient, md;1) in transmission mode is given for comparison. The XAFS measured on the droplet sample does not follow exactly the shape of the XAFS of the ideal sample. This observation is related to the inhomogeneous nature of the droplets, since the inhomogeneity of samples is known to deteriorate the XAFS oscillations. The data measured using the TXRF set-up show less variance around the main course of the oscillations
2242
G. Falkenberg et al. / Spectrochimica Acta Part B 58 (2003) 2239–2244
and for the dilute samples the amplitudes are similar in both set-ups. This observation can be explained by the self-absorption effect. When the penetration depth of the incident X-ray beam is
Fig. 2. Fluorescence XAFS spectra of (a) pure and (b) diluted PbCO3 taken in TXRF geometry (triangles) and conventional XRF geometry (squares). The step size 0.5 eV, the sample time per step is 6 s in (a) and 10 s in (b).
because of better counting statistics for both pure and diluted samples. However, the TXRF set-up provides reduced amplitudes of the oscillations compared to the XRF set-up for the pure sample,
Fig. 3. Section of the XAFS spectra shown in Fig. 2 of (a) pure and (b) diluted PbCO3 taken in TXRF geometry (triangles) and conventional XRF geometry (squares). A PbCO3 absorption spectrum recorded in transmission mode is added for comparison.
G. Falkenberg et al. / Spectrochimica Acta Part B 58 (2003) 2239–2244
2243
Fig. 4. Grazing incidence–normal detection (A) and normal incidence–GE (B) experimental configurations and schematic representation of the path length of the exciting and fluorescence radiation in the sample.
much less than the sample size and the escape depth of the fluorescence radiation, the dependence between absorption and fluorescence is not linear, except for dilute samples. A decreased absorption coefficient at the bottom position of a XAFS oscillation, for example, will increase the penetration depth of the incident beam. If the fluorescence photons excited in the additional volume can reach the detector, the XAFS oscillations are attenuated or disappear in the extreme case. The self-absorption effect is much more distinct in TXRF geometry, since the path length of the incident beam is 3 mm compared to -0.1 mm in XRF geometry and, in our case, the absorption along the path of the incident beam cannot be neglected for the pure PbCO3 sample (md(0.5). ¨ Suzuki w8x, Troger et al. w9x and Pfalzer et al. w10x have studied the problem of self-absorption and suggested a normal incidence–grazing-exit (GE) geometry instead of the grazing incidence– normal detection used in this work. In normal incidence the self-absorption effect vanishes, because the sample seen by the primary beam is
‘thin’. The two configurations have been represented graphically in Fig. 4. The GE technique has also been applied to chemical analysis in the past w11–14x. The GE excitation–detection geometry presents the advantages of the TXRF geometry for the background reduction and sensitivity (except the double excitation by both the incident and reflected beam). Problems related to GE-XRF technique, namely the hampered quantification of measurements due to high absorption in the sample, do not appear for GE-XAFS measurements. The application of this geometrical configuration and a comparison of GE-XAFS with XAFS in TXRF and XRF geometries are planned for the future. 4. Conclusion The experimental results show that the total reflection excitation geometry can be employed for fluorescence XAFS measurements on droplet samples. The chemical (trace-) elements analysis performed by SR-TXRF can be complemented by
2244
G. Falkenberg et al. / Spectrochimica Acta Part B 58 (2003) 2239–2244
chemical speciation analysis by XAFS spectroscopy on the same sample and in the same experimental set-up at HASYLAB beamline L. The TXRF geometry facilitates increased sensitivity leading to lower accessible concentration ranges compared to the conventional XRF geometry. However, it should be noted that the long penetration path length of the beam in the droplet in TXRF geometry leads to serious problems due to the self-absorption effect for higher amounts of concentrated droplet samples, restricting the application of the technique to very small amounts or highly diluted samples. References w1x P. Wobrauschek, P. Kregsamer, W. Ladisich, C. Streli, ¨ S. Pahlke, L. Fabry, S. Garbe, M. Haller, A. Knochel, M. Radtke, Nucl. Instrum. Meth. A 363 (1995) 619. w2x P. Wobrauschek, R. Gorgl, ¨ P. Kregsamer, Ch. Streli, S. ¨ Pahlke, L. Fabry, M. Haller, A. Knochel, M. Radtke, Spectrochim. Acta B 52 (1997) 901–906. w3x G. Pepponi, P. Wobrauschek, C. Streli, N. Zoger, ¨ F. ¨ X-ray Spectrom. 30 (2001) 267–272. Hegedus,
w4x G. Falkenberg, O. Clauss, A. Swiderski, Th. Tschentscher, Nucl. Instrum. Meth. A 467–468 (2001) 737–740. w5x G. Falkenberg, Th. Tschentscher, O. Clauss, HASYLAB Annual Report 2001, Available from: http:yywww-hasylab.desy.deyscienceyannual_reportsy2001_reporty part1yinterny5720.pdf. w6x G. Pepponi, P. Wobrauschek, F. Hegedus, ¨ C. Streli, N. ¨ Zoger, C. Jokubonis, G. Falkenberg, H. Grimmer, Spectrochim. Acta Part B 56 (2001) 2063–2071. w7x G. Pepponi, C. Streli, P. Wobrauschek, S. Zamini, N. ¨ Zoger, G. Falkenberg, Comparison of SR-TXRF excitation–detection geometries for samples with differing matrices, Spectrochim. Acta Part B 58 (2003) 2139– 2144. w8x Y. Suzuki, Phys. Rev. B 39 (5) (1989) 3393–3395. w9x L. Troger, ¨ D. Arvanitis, K. Baberschke, H. Michaelis, U. Grimm, E. Zschech, Phys. Rev. B 46 (6) (1992) 3283–3289. w10x P. Pfalzer, J.-P. Urbach, M. Klemm, S. Horn, M.L. deBoer, A.I. Frenkel, J.P. Kirkland, Phys. Rev. B 60 (13) (1999) 9335–9339. w11x P. DeBokx, Kok, Bailleul, G. Wiener, H. Urbach, Spectrochim. Acta Part B 52 (1997) 829–840. w12x K. Tsuji, K. Wagatsuma, Oku, X-ray Spectrom. 29 (2000) 3155–3160. w13x M. Claes, R. VanGrieken, P. DeBokx, X-ray Spectrom. 26 (1997) 153. w14x P. DeBokx, H. Urbach, Rev. Sci. Instrum. 66 (1) (1995) 15.