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Optimization of a glancing angle for simultaneous trace elemental analysis by using a portable total reflection X-ray fluorescence spectrometer
Spectrochimica Acta Part B 64 (2009) 288–290
Contents lists available at ScienceDirect
Spectrochimica Acta Part B j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s a b
Technical note
Optimization of a glancing angle for simultaneous trace elemental analysis by using a portable total reflection X-ray fluorescence spectrometer Shinsuke Kunimura ⁎, Daisuke Watanabe, Jun Kawai Department of Materials Science and Engineering, Kyoto University, Sakyo-ku, Kyoto, 606-8501, Japan
a r t i c l e
i n f o
Article history: Received 29 June 2007 Accepted 18 February 2009 Available online 3 March 2009 Keywords: Portable spectrometer Total reflection X-ray fluorescence Detection limit Nanogram Glancing angle
a b s t r a c t By using a portable total reflection X-ray fluorescence spectrometer with a 1 W X-ray tube, a specimen containing nanograms of Ca, Sc, Ti, V, Cr, Mn, Fe, and Ni is measured at several glancing angles of incident X-rays. Continuum X-rays are used as the excitation source. The intensities of the spectral background which degrades sensitivity to trace elements are decreased with a decrease of the glancing angle, and all these elements are detected at the glancing angle of 0.13° smaller than the critical angle for total reflection of the incident X-rays (0.20°). An optimum glancing angle for simultaneously detecting these trace elements is around 0.13°, and detection limits at 0.13° are sub-nanograms to ten nanograms. © 2009 Elsevier B.V. All rights reserved.
1. Introduction The total reflection X-ray fluorescence (TXRF) analysis [1–3] is a spectrometric method for trace elemental analysis. By using total reflection of incident X-rays on a sample holder with a specular surface, the intensities of the spectral background due to scattered Xrays are decreased. Therefore, highly sensitive analysis is performed by the TXRF analysis. Iida et al. [4,5] reported that using monochromatic X-rays is more effective for improving sensitivity to trace elements than using non-monochromatic X-rays. Since then, monochromatic X-rays have been often used for trace elemental analysis in the TXRF analysis. Detection limits down to femtograms (10− 15 g) are obtained by using a monochromatic synchrotron radiation [6,7]. On the other hand, we have developed a portable TXRF spectrometer using continuum X-rays from a 1 W X-ray tube, and a detection limit of 1 ng for Cr was achieved [8]. Trace elements in leaching test solutions of soils [9] and drinking water [10] were detected by using the portable spectrometer. The X-rays with continuous energy distribution are used as the excitation source, and the reflectivity of the incident X-rays on a sample holder at a glancing angle varies according to the X-ray energies. Therefore, the signal to background ratios of each fluorescent X-ray peak are different at each glancing angle. In the present paper, a specimen containing nanograms of Ca and 3d transition metal elements is measured at several glancing
⁎ Corresponding author. Tel.: +81 75 753 5483; fax: +81 75 753 5436. E-mail address: [email protected] (S. Kunimura). 0584-8547/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.sab.2009.02.004
angles, and an optimum glancing angle for simultaneous trace elemental analysis is experimentally determined. 2. Experimental section The details of the spectrometer were reported in the previous paper [8], and they are briefly summarized in this section. An X-ray tube with a W target “N6601” (Hamamatsu Photonics Co., Hamamatsu, Japan) was used. The X-ray tube was operated at 9.5 kV and 150 µA, and it emitted continuum X-rays. Almost all the W M-lines as the characteristic X-rays were absorbed by a 300 µm thick Be window of the X-ray tube. A waveguide was used as a collimator, and it restricted the incident X-rays to 50 µm in height and 10 mm in width. A quartz optical flat was used as a sample holder. A Si PIN detector “X123” (Amptek Inc., Bedford, MA) was used. A mixed standard solution containing 0.1 ppm each of Ca, Sc, Ti, V, Cr, Mn, and Fe, and 0.3 ppm of Ni was prepared from each standard solution. A 40 µL portion of the mixed standard solution was pipetted and dried on the optical flat twice; the total volume of 80 µL of the mixed standard solution was mounted on the optical flat. The dry residue contained 8 ng each of Ca, Sc, Ti, V, Cr, Mn, and Fe and 24 ng of Ni. The specimen was measured at several glancing angles (ϕ = 0.06°, 0.13°, 0.20°, 0.27°, and 0.34°). The critical angle of total reflection for 9.5 keV X-rays on the optical flat is 0.20°, and it is increased with a decrease of X-ray energies. Measurements were performed in air for 600 s. Detection limits were determined by the equation as follows:
Detection limit =
3m INet
rffiffiffiffiffiffiffi IBG t
ð1Þ;
S. Kunimura et al. / Spectrochimica Acta Part B 64 (2009) 288–290
289
where m is the amount of an element (ng), INet is the net intensity of the fluorescent X-ray peak (counts/s), IBG is the background intensity of the fluorescent X-ray peak (counts/s), and t is the counting time (s). 3. Results and discussion Fig. 1 shows the X-ray fluorescence spectra of the specimen measured at several glancing angles. Nanograms of Ca, Sc, Ti, V, Cr, Mn, Fe, and Ni were detected at the glancing angle of 0.13°. The maximum energy of the X-rays from the X-ray tube was 9.5 keV, and the intensities of the X-rays higher than the Ni K absorption edge energy (8.3 keV) were low. Therefore, the net intensity of the Ni Kα peak was lower than those of the Kα peaks of the other elements. The critical Xray energy of total reflection is decreased with an increase of a glancing angle, and increasing glancing angle resulted in higher background. Therefore, it was difficult to detect the Mn Kα peak at 0.34°, the Fe Kα peak at 0.27° and 0.34°, and the Ni Kα peak at 0.20°, 0.27°, and 0.34°. The net intensities of the Ca, Sc, Ti, and V Kα peaks (counts/s) at 0.06° were one order of magnitude lower than those at 0.13°. It was difficult to detect the Cr, Mn, Fe, and Ni Kα peaks at 0.06°. These results show that the excitation efficiency for the elements at
Fig. 2. Detection limits for Ca, Sc, Ti, V, Cr, Mn, Fe, and Ni at glancing angles of 0.06°, 0.13°, 0.20°, 0.27°, and 0.34°.
0.13° became higher than that at 0.06° because of higher intensities of the standing waves formed by the incident and reflected X-rays in the dry residue. The Ar and Si Kα peaks were due to air containing 0.93% Ar and the quartz optical flat. Because the Ti and V standard solutions contained H2SO4, the S Kα peak was detected. The net intensity of the Si Kα peak was increased with the increase of the glancing angle because of an increase of the X-ray penetration depth into the optical flat. Detection limits at each glancing angle are shown in Fig. 2. The Kβ peaks overlapped with the Kα peaks of the neighbor elements. In the present paper, the influence of the Kβ peaks was not considered for calculating the net intensities of the Kα peaks because the net intensities of the Kβ peaks would be negligibly low. For example, the Fe Kβ peak was not detected in Fig. 1. The detection limits for Ca, Sc, Ti, V, Cr, Mn, and Ni were minimum at 0.13°, and they were subnanograms to ten nanograms. The detection limit for Fe at 0.13° was similar to that at 0.20°, and they were a few nanograms. Consequently, an optimum glancing angle for detecting these trace elements simultaneously was around 0.13°. 4. Conclusions
Fig. 1. Representative measured X-ray fluorescence spectra of a specimen containing 8 ng each of Ca, Sc, Ti, V, Cr, Mn, and Fe and 24 ng of Ni at glancing angles (ϕ) of 0.06°, 0.13°, 0.20°, 0.27°, and 0.34°.
A specimen containing nanograms of elements was measured at several glancing angles by using a portable TXRF spectrometer with a 1 W X-ray tube, and an optimum glancing angle for detecting the elements in the specimen was experimentally determined. Although continuum X-rays were used, the intensities of the spectral background were low enough to detect these trace elements at a glancing
290
S. Kunimura et al. / Spectrochimica Acta Part B 64 (2009) 288–290
angle that was smaller than the critical angle of 0.20°. An optimum glancing angle was around 0.13°, and detection limits were subnanograms to ten nanograms. Although an optimum glancing angle varies according to character of specimen (e.g. thickness), the continuum X-rays from the low power X-ray source are useful for simultaneous trace elemental analysis when measurements are performed at each optimum glancing angle. Acknowledgment The present research was financially supported by SENTAN, JST and Asahi Glass Foundation. One of the present authors (S.K.) thanks JSPS for research fellowships for young scientists. References [1] Y. Yoneda, T. Horiuchi, Optical flats for use in X-ray spectrochemical microanalysis, Rev. Sci. Instrum. 42 (1971) 1069–1070.
[2] R. Klockenkämper, Total Reflection X-ray Fluorescence Analysis, Wiley, New York, 1997. [3] P. Wobrauschek, Total reflection X-ray fluorescence analysis—a review, X-ray Spectrom. 36 (2007) 289–300. [4] A. Iida, Y. Gohshi, Total-reflection X-ray fluorescence analysis using monochromatic beam, Jpn. J. Appl. Phys. 23 (1984) 1543–1544. [5] A. Iida, A. Yoshinaga, K. Sakurai, Y. Gohshi, Synchrotron radiation excited X-ray fluorescence analysis using total reflection of X-rays, Anal. Chem. 58 (1986) 394–397. [6] P. Wobrauschek, R. Gorgl, P. Kregsamer, C. Streli, S. Pahlke, L. Fabry, M. Haller, A. Knochel, M. Radtke, Analysis of Ni on Si-wafer surfaces using synchrotron radiation excited total reflection X-ray fluorescence analysis, Spectrochim. Acta Part B 52 (1997) 901–906. [7] P. Pianetta, K. Baur, A. Singh, S. Brennan, J. Kerner, D. Werho, J. Wang, Application of synchrotron radiation to TXRF analysis of metal contamination on silicon wafer surfaces, Thin Solid Films 373 (2000) 222–226. [8] S. Kunimura, J. Kawai, Portable total reflection X-ray fluorescence spectrometer for nanogram Cr detection limit, Anal. Chem. 79 (2007) 2593–2595. [9] S. Kunimura, J. Kawai, K. Marumo, Measurements of leaching test solutions of soils by a portable total reflection X-ray fluorescence spectrometer, Adv. X-ray Chem. Anal. Japan 38 (2007) 367–370. [10] S. Kunimura, J. Kawai, Trace elemental analysis of commercial bottled drinking water by a portable total reflection X-ray fluorescence spectrometer, Anal. Sci. 23 (2007) 1185–1188.
Contents lists available at ScienceDirect
Spectrochimica Acta Part B j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s a b
Technical note
Optimization of a glancing angle for simultaneous trace elemental analysis by using a portable total reflection X-ray fluorescence spectrometer Shinsuke Kunimura ⁎, Daisuke Watanabe, Jun Kawai Department of Materials Science and Engineering, Kyoto University, Sakyo-ku, Kyoto, 606-8501, Japan
a r t i c l e
i n f o
Article history: Received 29 June 2007 Accepted 18 February 2009 Available online 3 March 2009 Keywords: Portable spectrometer Total reflection X-ray fluorescence Detection limit Nanogram Glancing angle
a b s t r a c t By using a portable total reflection X-ray fluorescence spectrometer with a 1 W X-ray tube, a specimen containing nanograms of Ca, Sc, Ti, V, Cr, Mn, Fe, and Ni is measured at several glancing angles of incident X-rays. Continuum X-rays are used as the excitation source. The intensities of the spectral background which degrades sensitivity to trace elements are decreased with a decrease of the glancing angle, and all these elements are detected at the glancing angle of 0.13° smaller than the critical angle for total reflection of the incident X-rays (0.20°). An optimum glancing angle for simultaneously detecting these trace elements is around 0.13°, and detection limits at 0.13° are sub-nanograms to ten nanograms. © 2009 Elsevier B.V. All rights reserved.
1. Introduction The total reflection X-ray fluorescence (TXRF) analysis [1–3] is a spectrometric method for trace elemental analysis. By using total reflection of incident X-rays on a sample holder with a specular surface, the intensities of the spectral background due to scattered Xrays are decreased. Therefore, highly sensitive analysis is performed by the TXRF analysis. Iida et al. [4,5] reported that using monochromatic X-rays is more effective for improving sensitivity to trace elements than using non-monochromatic X-rays. Since then, monochromatic X-rays have been often used for trace elemental analysis in the TXRF analysis. Detection limits down to femtograms (10− 15 g) are obtained by using a monochromatic synchrotron radiation [6,7]. On the other hand, we have developed a portable TXRF spectrometer using continuum X-rays from a 1 W X-ray tube, and a detection limit of 1 ng for Cr was achieved [8]. Trace elements in leaching test solutions of soils [9] and drinking water [10] were detected by using the portable spectrometer. The X-rays with continuous energy distribution are used as the excitation source, and the reflectivity of the incident X-rays on a sample holder at a glancing angle varies according to the X-ray energies. Therefore, the signal to background ratios of each fluorescent X-ray peak are different at each glancing angle. In the present paper, a specimen containing nanograms of Ca and 3d transition metal elements is measured at several glancing
⁎ Corresponding author. Tel.: +81 75 753 5483; fax: +81 75 753 5436. E-mail address: [email protected] (S. Kunimura). 0584-8547/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.sab.2009.02.004
angles, and an optimum glancing angle for simultaneous trace elemental analysis is experimentally determined. 2. Experimental section The details of the spectrometer were reported in the previous paper [8], and they are briefly summarized in this section. An X-ray tube with a W target “N6601” (Hamamatsu Photonics Co., Hamamatsu, Japan) was used. The X-ray tube was operated at 9.5 kV and 150 µA, and it emitted continuum X-rays. Almost all the W M-lines as the characteristic X-rays were absorbed by a 300 µm thick Be window of the X-ray tube. A waveguide was used as a collimator, and it restricted the incident X-rays to 50 µm in height and 10 mm in width. A quartz optical flat was used as a sample holder. A Si PIN detector “X123” (Amptek Inc., Bedford, MA) was used. A mixed standard solution containing 0.1 ppm each of Ca, Sc, Ti, V, Cr, Mn, and Fe, and 0.3 ppm of Ni was prepared from each standard solution. A 40 µL portion of the mixed standard solution was pipetted and dried on the optical flat twice; the total volume of 80 µL of the mixed standard solution was mounted on the optical flat. The dry residue contained 8 ng each of Ca, Sc, Ti, V, Cr, Mn, and Fe and 24 ng of Ni. The specimen was measured at several glancing angles (ϕ = 0.06°, 0.13°, 0.20°, 0.27°, and 0.34°). The critical angle of total reflection for 9.5 keV X-rays on the optical flat is 0.20°, and it is increased with a decrease of X-ray energies. Measurements were performed in air for 600 s. Detection limits were determined by the equation as follows:
Detection limit =
3m INet
rffiffiffiffiffiffiffi IBG t
ð1Þ;
S. Kunimura et al. / Spectrochimica Acta Part B 64 (2009) 288–290
289
where m is the amount of an element (ng), INet is the net intensity of the fluorescent X-ray peak (counts/s), IBG is the background intensity of the fluorescent X-ray peak (counts/s), and t is the counting time (s). 3. Results and discussion Fig. 1 shows the X-ray fluorescence spectra of the specimen measured at several glancing angles. Nanograms of Ca, Sc, Ti, V, Cr, Mn, Fe, and Ni were detected at the glancing angle of 0.13°. The maximum energy of the X-rays from the X-ray tube was 9.5 keV, and the intensities of the X-rays higher than the Ni K absorption edge energy (8.3 keV) were low. Therefore, the net intensity of the Ni Kα peak was lower than those of the Kα peaks of the other elements. The critical Xray energy of total reflection is decreased with an increase of a glancing angle, and increasing glancing angle resulted in higher background. Therefore, it was difficult to detect the Mn Kα peak at 0.34°, the Fe Kα peak at 0.27° and 0.34°, and the Ni Kα peak at 0.20°, 0.27°, and 0.34°. The net intensities of the Ca, Sc, Ti, and V Kα peaks (counts/s) at 0.06° were one order of magnitude lower than those at 0.13°. It was difficult to detect the Cr, Mn, Fe, and Ni Kα peaks at 0.06°. These results show that the excitation efficiency for the elements at
Fig. 2. Detection limits for Ca, Sc, Ti, V, Cr, Mn, Fe, and Ni at glancing angles of 0.06°, 0.13°, 0.20°, 0.27°, and 0.34°.
0.13° became higher than that at 0.06° because of higher intensities of the standing waves formed by the incident and reflected X-rays in the dry residue. The Ar and Si Kα peaks were due to air containing 0.93% Ar and the quartz optical flat. Because the Ti and V standard solutions contained H2SO4, the S Kα peak was detected. The net intensity of the Si Kα peak was increased with the increase of the glancing angle because of an increase of the X-ray penetration depth into the optical flat. Detection limits at each glancing angle are shown in Fig. 2. The Kβ peaks overlapped with the Kα peaks of the neighbor elements. In the present paper, the influence of the Kβ peaks was not considered for calculating the net intensities of the Kα peaks because the net intensities of the Kβ peaks would be negligibly low. For example, the Fe Kβ peak was not detected in Fig. 1. The detection limits for Ca, Sc, Ti, V, Cr, Mn, and Ni were minimum at 0.13°, and they were subnanograms to ten nanograms. The detection limit for Fe at 0.13° was similar to that at 0.20°, and they were a few nanograms. Consequently, an optimum glancing angle for detecting these trace elements simultaneously was around 0.13°. 4. Conclusions
Fig. 1. Representative measured X-ray fluorescence spectra of a specimen containing 8 ng each of Ca, Sc, Ti, V, Cr, Mn, and Fe and 24 ng of Ni at glancing angles (ϕ) of 0.06°, 0.13°, 0.20°, 0.27°, and 0.34°.
A specimen containing nanograms of elements was measured at several glancing angles by using a portable TXRF spectrometer with a 1 W X-ray tube, and an optimum glancing angle for detecting the elements in the specimen was experimentally determined. Although continuum X-rays were used, the intensities of the spectral background were low enough to detect these trace elements at a glancing
290
S. Kunimura et al. / Spectrochimica Acta Part B 64 (2009) 288–290
angle that was smaller than the critical angle of 0.20°. An optimum glancing angle was around 0.13°, and detection limits were subnanograms to ten nanograms. Although an optimum glancing angle varies according to character of specimen (e.g. thickness), the continuum X-rays from the low power X-ray source are useful for simultaneous trace elemental analysis when measurements are performed at each optimum glancing angle. Acknowledgment The present research was financially supported by SENTAN, JST and Asahi Glass Foundation. One of the present authors (S.K.) thanks JSPS for research fellowships for young scientists. References [1] Y. Yoneda, T. Horiuchi, Optical flats for use in X-ray spectrochemical microanalysis, Rev. Sci. Instrum. 42 (1971) 1069–1070.
[2] R. Klockenkämper, Total Reflection X-ray Fluorescence Analysis, Wiley, New York, 1997. [3] P. Wobrauschek, Total reflection X-ray fluorescence analysis—a review, X-ray Spectrom. 36 (2007) 289–300. [4] A. Iida, Y. Gohshi, Total-reflection X-ray fluorescence analysis using monochromatic beam, Jpn. J. Appl. Phys. 23 (1984) 1543–1544. [5] A. Iida, A. Yoshinaga, K. Sakurai, Y. Gohshi, Synchrotron radiation excited X-ray fluorescence analysis using total reflection of X-rays, Anal. Chem. 58 (1986) 394–397. [6] P. Wobrauschek, R. Gorgl, P. Kregsamer, C. Streli, S. Pahlke, L. Fabry, M. Haller, A. Knochel, M. Radtke, Analysis of Ni on Si-wafer surfaces using synchrotron radiation excited total reflection X-ray fluorescence analysis, Spectrochim. Acta Part B 52 (1997) 901–906. [7] P. Pianetta, K. Baur, A. Singh, S. Brennan, J. Kerner, D. Werho, J. Wang, Application of synchrotron radiation to TXRF analysis of metal contamination on silicon wafer surfaces, Thin Solid Films 373 (2000) 222–226. [8] S. Kunimura, J. Kawai, Portable total reflection X-ray fluorescence spectrometer for nanogram Cr detection limit, Anal. Chem. 79 (2007) 2593–2595. [9] S. Kunimura, J. Kawai, K. Marumo, Measurements of leaching test solutions of soils by a portable total reflection X-ray fluorescence spectrometer, Adv. X-ray Chem. Anal. Japan 38 (2007) 367–370. [10] S. Kunimura, J. Kawai, Trace elemental analysis of commercial bottled drinking water by a portable total reflection X-ray fluorescence spectrometer, Anal. Sci. 23 (2007) 1185–1188.