394
Anal. Chem. 1988, 58,394-397
(20) Bose, M. In “Progress in NMR Spectroscopy, Volume 4”; Emsley, J. W., Feeney, F., Sutcliffe, L. H., Eds.; Pergamon Press: New York, 1969; pp 335-440. (21) Wind, R. A.; Trommel, J.; Smidt, J. Fuel 1979, 58, 900-901. (22) Wind, R. A.; Trommel, J.; Smldt, J. Fuel 1982, 61, 398-400. (23) Wind, R. A,; Duijvestijn, M. J.: Lugt, C. V. D.; $mi&, J.; Vriend, J. “Magnetic Resonance. Introduction, Advanced Topics and Applications to Fossil Energy”; Petrakls, L., Fraissard, J. P., Eds.; D. Reidel Publishing Co.: Boston, MA, 1984; NATO AS1 Series C, No. 124, pp 461-484. (24) Vanderhart, D. L.; Retcofsky, H. L. Preprints of the 1976 Coal Chemistry Workshop; Stanford Research Institute, Menlo Park, CA, August 26-27, 1976; pp 202-218. (25) Hagaman, E. W.; Woody, M. C. Proceedings of the International Conference on Coal Science, Dusseidorf, West Germany, Sept 7-9, 1981; pp 807-81 1. (26) Wemmer, D. E.; Pines, A.; Whitehurst, D. D. fhilos. Trans. R . SOC. London, A 1981, 300, 15. (27) Sullivan, M. J.; Maciei, G. E. Anal. Chem. 1982, 5 4 , 1615-1623. (28) Dudley, R. L.; Fyfe, C. A. Fuel 1982, 8 1 , 651-657. (29) Kalman, J. R. “Magnetic Resonance. Introductlon, Advanced Topics and Applications to Fossll Energy”; Petrakis, L., Fraissard, J. P., Eds.; D. Reidel Publlshing Co.: Boston, MA, 1984; NATO AS1 Serles C, No. 557-567. 124.. rDD r - (30) Hagaman, E. W.; Chambers, R. R. Prepr. Pap.-Am. Chem. S O ~ . , Div. FuelChem. 1985, 30, 188-191.
(31) Hartmann, S. R.; Hahn, E. L. Phys. Rev. 1982, 128, 2042-2053. (32) Schaefer, J.; Stejskal, E. 0.; Buchdahi, R. Macromolecules 1977, IO, 384-405. (33) Garroway, A. N.; Moniz, W. 6.; Resing, H. A. I n “Carbon-13 NMR in Polymer Science”; Pasika, W. M., Ed.; American Chemical Society: Washington, DC, 1979; ACS Symposium Series No. 103, pp 67-87. (34) Liona, R.; Rose, K.; Hlppo, E. J. J . Org. Chem. 1981, 46, 277-283. (35) Liona, R.; Brons, G. J. Am. Chem. SOC. 1981, 103, 1735-1742. (36) Chambers, R. R., Jr.; Hagaman, E. W.; Woody, M. C.; Smlth, K. E.; McKamey, D. R. Fuel 1985, 64, 1349-1354. (37) Mehring, M. “High Resolution NMR Spectroscopy in Solids”; SpringerVerlag, Berlin, 1976; Chapter 4. (38) Murphy, P. D.; Cassady, T. J.; Gerstein, B. C. Fuel 1982, 8 1 , 1233-1 240. (39) Murphy, P. D.; Gerstein, B. C. Anal. Chem. 1982, 5 4 , 522-525. (40) Alemany, L. 6.; Grant, D. M.; Alger, T. D.; Pugmire, R. J. J . Am. Chem. SOC. 1983, 105, 6697-6704.
RECEIVED for review June 14,1985. Accepted October 1,1985. This research was sponsored by the Division of Chemical Sciences, Office of Basic Energy Sciences, US.Department of Energy under Contract DE-AC05-840R21400 with Martin Marietta Energy Systems, Inc.
Synchrotron Radiation Excited X-ray Fluorescence Analysis Using Total Reflection of X-rays Atsuo Iida,* Atsushi Yoshinaga, Kenji Sakurai, and Yohichi Gohshi Department of Industrial Chemistry, Faculty of Engineering, University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113, Japan
Trace-element analysls by synchrotron radlation excited energy dispersive X-ray fluorescence was carried out uslng an X-ray reflector as a sample support. The samples used were aqueous soiutlons dried on the reflector. A very hlgh signal-to-background ratlo was achieved owing to the remarkable reduction of scattered radiation. The minimum detectlon llmit oblalned by monchromatic excttatlon was less than 1 ppb or 1 pg. The angular dependences of fluorescence signals and scattered radiation are analyzed In detali to clarify the process by whlch the signal-to-background ratio was improved. The optlmai experimental condltions for attalning a high signal-lo-background ratio and the advantages of using synchrotron radiatlon are discussed.
Energy dispersive X-ray fluorescence (EDXRF) spectrometry has been used as a routine analytical tool because it is nondestructive, rapid, and accurate. Despite its advantages, there are several drawbacks, such as relatively low sensitivity and rather tedious calibration and correction procedures for quantitative analysis. In order to improve the minimum detection limit (MDL), the key factor is background reduction in addition to signal enhancement. Recently, high sensitivity in XRF analysis has been attained by using synchrotron radiation (SR) (l-g), in which the high brightness, energy tunability, and polarization of SR are effectively utilized. In the SR experiment, scattered radiation was greatly reduced owing to the effective utilization of the polarization of SR and to the use of a thin sample. This reduction in scattered radiation means that there is an increase in the fraction of the X-ray fluorescence signal in the total photon flux from the sample and also a decrease in the background beneath the
fluorescence signal (9). The scattered radiation still remaining in the spectra obtained by SR (6) was caused mainly by the sample itself and the sample support. In order to reduce the background still further, the use of total reflection of X-rays, first reported by Yoneda and Horiuchi (lo), is promising. Total X-ray reflection occurs when a collimated beam impinges on an optically flat surface (reflector) a t an angle below the critical angle, which is typically in the region of a few milliradians. The penetration of X-rays is very shallow under total reflection conditions, typically less than a few tens of angstroms. When the reflector is used as a sample support for EDXRF analysis, scattered radiation is reduced substantially, leading to high sensitivity. Furthermore, when the sample is used in solution, the procedure for the interference correction is greatly simplified due to the thin sample. Even with conventional X-ray tubes, the MDLs reported were in the concentration range of parts per billion or in nanograms (10-18). There have been efforts to improve this technique in terms of practicability and higher sensitivity; apparatus has been developed for quasi-monochromatic excitation using an X-ray mirror in front of the reflector (15,16) and also for monochromatic excitation using a crystal (19). Its recent application to seawater analysis (20) demonstrates the high sensitivity of this technique. SR is the best excitation source for the total reflection scheme, since its high brightness and natural collimation give a high flux density in the monochromatic beam, and by using its polarization, residual scattered radiation can be further reduced. In this paper, the effects of monochromatized SR excitation on the signal-to-background ratio of the total reflection method are studied in detail. The decrease in scattered radiation with this technique is generally attributed to the very shallow penetration of the incident X-rays, but a
0003-2700/86/0358-0394$01.50/00 1986 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 58, NO. 2, FEBRUARY 1986
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Figure 2. Diagram of the sample chamber (plan): (SI) slit; (IC) ionlzatlon chamber. detailed analysis has not been attempted before. The angular dependences of fluorescence signals and scattered radiation are measured in detail to clarify the process by which the signal-to-background ratio is improved. On the basis of this analysis the optimal experimental conditions for attaining a high signal-to-background ratio are discussed. Some practical problems encountered in quantitative analysis are also discussed.
EXPERIMENTAL SECTION Most of the previous experiments used an optical flat made of fused quartz as a reflector. The reflector material must have a smooth, flat surface, high purity, chemical resistance, and be easily available. In the present study, we used two types of reflectors; X-ray reflectors made of fused quartz with a flatness of X/20, where X = 589 nm, and commercially available mirror polished silicon wafers. Though the surface smoothness of the fused quartz reflectors is superior (21,22) the silicon wafers are much more easily available. The samples used were aqueous solutions of metal ions at various concentrations. An aliquot of 2 p L was pipetted onto the reflector and was then dried. The spot size of the sample was about 1 mm in diameter. To check the effects of the size and the uniformity of the sample on the reflector, the followingsamples were also prepared: 0.05% poly(viny1alcohol) (PVA) was added to the aqueous solution when the fused quartz reflector was used (16).Since a bare silicon surface is hydrophobic, the silicon wafer was dipped into nitric acid before pipetting to form a hydrophilic thin oxide layer. The final spot size on the silicon wafer was up to about 3 mm in diameter in this case. The experiment was carried out using SR from the storage ring at the Photon Factory (PF) using beam line (BL) 4A. As detailed descriptions of the beam line and the EDXRF system used have been given elsewhere (6, 7), Figure 1 shows only a schematic drawing of the experimental setup. Three types of primary monochromators were used; a silicon (111)crystal (Figure la), a synthetic microstructure, and a total reflection mirror in conjunction with an A1 absorber (Figure lb). The latter two types are usually known as wide band monochromators (7,23, 24). Figure 2 shows the sample chamber. The 'monochromatized X-ray beam was collimated by Ta slits to less than 40 pm in horizontal width, then propagated through the ionization chamber for intensity monitoring, and then impinged onto the surface of the reflector at a glancing angle of a few milliradians. For the crystal monochromator, the vertical dimension of the incident beam was about 3 mm, which covered most of the vertical di-
Flgure 3. Angular dependence of the Zn ( 0 )and SI (0)fluorescence signals, which are from the sample and the reflector,respectively, the scattered radiation (X), and the reflected beam (A). The dotted line shows sample inhomogeneity. vergence of SR at BL 4A. The reflector was mounted on a rotational and translational stage, driven by step motors. The reflected beam was monitored by another ionization chamber followed by a half slit to remove the direct beam. The Si(Li) detector, with an active area of 12 mm2,was set at right angles to the reflector surface. The distance between the sample and the Si(Li) detector was less than 5 mm for the crystal monochromator. This arrangement makes it possible to reduce the scattered radiation by making use of the polarization of SR and also to enhance the signal intensity by making the solid angle of the detector very large. The sample chamber can be evacuated or filled with He, but most of the measurements were made in air, because both scattering and attenuation by air were practically negligible due to the short distance between the sample and the detector. The electronic signal from the Si(Li) detector was processed through an amplifier, stored in a multichannel analyzer, and then recorded on floppy disks. A counting time of 100 s was adopted. The intensities obtained were normalized by the output of the primary intensity monitor when necessary.
ANGULAR DEPENDENCE OF SIGNALS The angular dependences of fluorescence signals and scattered radiation were measured in detail using monochromatic beams. A perfect silicon (111)crystal was used as a monochromator. The excitation energy was 11.8 keV. A series of aqueous solutions of Zn, Mn, and Ca a t various concentrations were used as samples. The distance between the sample and the Si(Li) detector was 3 mm. Figure 3 shows the intensities of the Zn and Si fluorescence signals, which are from the sample and the reflector, respectively, the scattered radiation, and the reflected beam as a function of the glancing angle (0). The absolute angle was determined by adjusting the point at which the Si fluorescence curve rose sharply to the calculated critical angle (OJ, which in this case is 2.6 mrad. The spectra corresponding to angle positions A and B in Figure 3 are shown in parts a and b of Figure 4, respectively. A high signal-to-scattered radiation ratio was obtained under the total reflection conditions in Figure 4a. Figures 3 and 4 clearly show how the signal-to-background ratio is improved by using an X-ray mirror as a sample support. The intensity of the fluorescence signal from the sample depends on the intensity of the X-rays just above the reflector, where a standing wave is created by the interference between an incident and a coherently related reflected beam. In Figure 5a, the solid curve is the calculated X-ray intensity a t the reflector surface for which the interference effect is taken into account (25-27). The X-ray intensity in the actual sample is the one integrated over the,sample thickness and is shown in Figure 5b as the solid curve, which is in good agreement with the experimental result shown in Figure 3. From this analysis, one can say that the fluorescence signal from the sample is doubled by the incident and the reflected beams under total reflection conditions. But when the sample is thin
ANALYTICAL CHEMISTRY, VOL. 58, NO. 2, FEBRUARY 1986
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enough, or nonuniform in thickness, the fluorescence signal from the sample shows a more complicated angular dependence as shown in Figure 3 at lower angle region, because of the interference phenomenon. The slow decrease in the reflection intensity below 8, in Figure 3 is due to the reduction of the effective cross section
of the sample for the primary beam width of 40 hm. The Si fluorescence signal from the reflector and scattered radiation are weak below e, increase steeply around Bo and increase monotonically above 8,. The intensities of the Si fluorescence signal and the scattered radiation depend essentially on both the thickness of the layer involved in the reflection process and the X-ray intensity at the surface. The dashed curve in Figure 5a shows the penetration depth of the incident X-rays as a function of the glancing angle (25). The product of the penetration depth and the X-ray intensity at the surface (solid line in Figure 5a) is shown by a dashed line in Figure 5b and is in good agreement with the Si fluorescence and scattered radiation intensities shown in Figure 3. Scattered radiation is also affected by imperfections in the reflector surface such as flow and dust and the amount of the sample. These factors explain the excess scattered radiation observed below 8,.
RESULTS AND DISCUSSION From Figure 3 it can be seen that the highest signal-tobackground ratio is attained at an angle lower than 9,. However, the intensity observed below 8, sometimes differed from the calculated one due to the sample inhomogeneity mentioned above and to misalignment of the X-ray optics. Due to the natural collimation of SR in the vertical direction, and also due to the Gaussian distribution of the incident beam, the intensity of the signal is very sensitive to the position of the sample relative to the beam. These factors reduce the reproducibility of the X-ray intensity measured. In order to make analysis reliable, it is advisable to measure the intensities of both fluorescence and scattered radiation at different angles; for instance, at angles slightly lower, much lower, and much higher than 6., In regard to sample preparation, a sample about 1mm in diameter is preferable, because the dried sample often shows concentric inhomogeneity even after the pretreatment described in the Experimental Section, and the beam size of SR is relatively small. With these points in mind, the reliability of the present method was examined. We used the internal standard method rather than the measurement of the absolute intensity, because the absolute intensity of the signal depends on the glancing angle and also on the primary beam intensity, which in the case of SR decreases as a function of time. In the present experiment, the intensity ratio of Zn to the internal standard of Co was confirmed to be proportional to the ratio of concentrations of the two elements. On the basis of analysis in the previous section, the normalization of the signal from the sample by the intensity of the fluorescence signal or the scattered radiation from the reflector was also found to be effective, providing that the angular dependences of signal intensities were similar to those in Figure 3 or Figure 5b. For the determination of the MDL, the criterion of a signal at least 3 times the square root of the background wm adopted. The MDL obtained by using the perfect Si monochromator was about 3 ppb in terms of the concentration of the original aqueous solution and 6 pg in absolute amount. By use of the lapped silicon monochromator, which has higher reflecting power than the perfect crystal, the MDL was improved to 0.5 PPb or 1 Pg. Though monochromatic excitation has the advantage of selective excitation of a particular element, the photon flux density is relatively low. To examine the effects of excitation modes, wide band-pass monochromators using a combination of a reflection mirror and an absorber or layered synthetic microstructure were used as the excitation source. Figure 6 shows the spectrum obtained from a sample containing 5 ppm Ca, Mn, and Zn on a fused quartz reflector with a reflection mirror/absorber combination. Since the flux density was very high, the distance between the sample and the Si(Li) detector
ANALYTICAL CHEMISTRY, VOL. 58, NO. 2, FEBRUARY 1986
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SR excited EDXRF using a X-ray mirror as a sample support attained a high signal-to-background ratio. The advantages of using SR in total reflection EDXRF are as follows: (1) The high signal-to-background ratio attained by the use of monochromatic excitation is difficult to achieve with a conventional X-ray tube. (2) The naturally collimated beam from the storage ring is appropriate for excitation at a small glancing angle. (3) Residual scattered radiation is further reduced because of the polarization of SR. (4) By use of a crystal monochromator, the selective excitation of a particular element is possible. For instance, elemental sensitivity can be enhanced by adjusting the excitation energy to just above its absorption edge, and overlapping peaks can be removed by setting the excitation energy between the relevant absorption edges ( 6 ) . Since the amount of sample needed for these measurements is very small, this method is promising for micro- and traceelement analysis.
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ACKNOWLEDGMENT
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The authors thank K. Kohra and T. Sasaki of the Photon Factory (PF) for their interest in this work. The authors also thank T. Matsushita and the other PF staff whose help made the experiment possible. Registry No. Si, 7440-21-3;Ca, 7440-70-2;Mn, 7439-96-5;Zn, 7440-66-6;Clz,7782-50-5;K, 7440-09-7;Fe, 7439-89-6;Cu, 744050-8; water, 7732-18-5.
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was more than 30 mm in this particular experiment in order to avoid detector saturation. When the detector was close to the sample, the vertical beam size had to be reduced to less than a few hundred micrometers to keep the total counting rate constant. The MDL in relative concentration using this arrangement was about the same as by monochromatic ex- ’ citation using the crystal. The MDL in absolute amount was estimated as less than 0.1 pg, but it was difficult to make an accurate estimate because of sample inhomogeneity. Within the concentration range of the samples we used, the total counting rate was determined mainly by the fluorescence signals themselves. If a sample of lower concentration had been used, the MDL would probably have been improved still further. The application of this technique to the analysis of environmental water is now in progress. Figure 7 shows an example of a spectrum obtained from wastewater. We would finally like to discuss some practical problems concerning the reflector material. The surface of the reflector should be optically flat, but the difference between the optical flat made of fused quartz and the mirror-polished silicon wafer was small. Since residual scattered radiation below Bc was negligible in our concentration range, the silicon wafer is in practice usable as an X-ray reflector. In order to increase the X-ray flux density for more diluted samples, a reflector made of a high-2 element is preferable because of its larger critical angle. But although K and L fluorescence signals from reflectors, such as Ge and Pt, sometimes interfere with the sample signal, the Si fluorescence signal from a Si or SiOz reflector seems in practice to have no serious effect on the sample signal.
i22j (23) (24) (25) (26) (27)
Sparks, C. J., Jr. “Synchrotron Radiation Research”; Wlnick, H., Donlach, S., Eds.; Plenum: New York, 1980; Chapter 14. Knoechei, A.; Petersen, W.; Tolklehn, G. Nucl. Instrum. Methods 1983, 208, 659-685. Jones, K. W.; Gordon, B. M.; Hanson, A. L.; Hastlngs, J. B.; Howell, M. R.; Kraner, H. W.; Chen, J. R. Nucl. Instrum. Methods 1984, 63, 225-231. Bos, A. J. J.; Vis, R. D.; Verfuel, H.; Prlns, M.; Davies, S. T.; Bowen, D. K.; Makianic, J.; Vaikomic, V. Nucl. Instrum. Methods 1984, 63, 232-240. Giifrich, J. V.; Skeiton, E. F.; Odari, S. B.; Kirkland, J. P.; Nagei, D. J. Anal. Chem. 1983, 55, 232-240. Iida, A.; Sakurai, K.; Matsushlta, T.; Gohshi, Y. Nucl. Instrum. Methods 1985, 228, 556-563. Iida, A,; Matsushita, T.; Gohshi, Y. Nucl. Instrum. Methods 1985, A235, 597-602. Jaklevlc, J. M.; Giauque, R. D.; Thompson, A. C. Nucl. Instrum. Metho d ~1985, B l O I 1 1 , 303-308. Goulding, F. S.; Jaklevic, J. M. Nucl. Instrum. Methods 1977, 142, 323-332. Yoneda, Y.; Horiuchi, T. Rev. Scl. Instrum. 1971, 42, 1069-1070. Aiglnger, H.; Wobrauchek, P. Nucl. Instrum. Methods 1974, f 14, 157-158. Wobrauchek, P.; Aiginger, H. Anal. Chem. 1975, 47, 852-855. Wobrauchek, P.; Aiginger, H. X-Ray Spectrom. 1979, 8 , 57-62. Knoth, J.; Schwenke, H. Fresenlus’ 2. Anal. Chem. 1978, 291, 200-204. Knoth, J.; Schwenke, H.; Fresenlus’ Z . Anal. Chem. 1980, 301, 7-9. Schwenke, H.; Knoth, J. Nucl. Instrum. Methods 1982, 193, 239-243. Wobrauchek, P.; Aiginger, H. Spectrochim. Acta, fat? 6 1980, 358, 607-614. Knoth, J.; Schwenke, H. Fresenius’ Z . Anal. Chem. 1979, 294, 273-274. Ilda, A.; Gohshi, Y. Jpn. J . Appl. Phys. 1984, 2 3 , 1543-1544. Knoechei, A.; Prange, A. Fresenlus’ Z . Anal. Chem. 1981, 306, 252-258. Bilderback, D. H. Nucl. Instrum. Methods 1982, 195, 85-89. IBiiderback, D. H. Nucl. Instrum. Methods 1982, 195. 91-95. Giifrich, J. V.; Nagel, D. J.; Barbee, T. W., Appl. Spectrosc. 1982, 36, 58-6 1. Biiderback, D. H. Nucl. Instrum. Methods 1982, 195, 67-72. Parrat, L. 0 . fhys. Rev. 1954, 9 5 , 359-369. Vineyard, 0 . H. fhys. Rev. 6 : Condens. Matter 1982, 2 6 . 4146-4 159. Born, M.; Wolf, E. “Prlnciples of Optics”; Pergamon: New York, 1970.
RECEIVED for review June 17,1985. Accepted August 27,1985.
