Analytical Applications of X-Ray Excited Optical Fluorescence Spectra: The Internal Standard Principle Edward L. DeKalb, Velmer A. Fassel, Takeshi Taniguehi,‘ and T . R. Saranathan2 Institute for Atomic Research and Department of Chemistry, Iowa State University, Ames, Iowa 50010
The intensities of X-ray-induced optical fluorescence emitted by trace concentrations of various rare earth elements in a suitable matrix are shown to be enhanced by low concentrations of residual impurities, and to be suppressed by somewhat higher concentrations of the same impurities. A variety of other operational variables, such as the time and temperature course of the sample ignition, the degree of absorption of HzO and COS, and drifts in power output of the X-ray tube and in the detector response sensitivity, can also influence the fluorescence intensities. It is shown that the use of an internal standard will satisfactorily compensate for many of these effects.
THESHARP-LINE optical fluorescence spectra emitted by ions of the rare earth elements when certain matrices are irradiated by X-rays (1-3) have recently been applied to the quantitative determination of fractional ppm amounts of PryNd, Sm, Eu, Gd, Tb, Dy, Er, Tm, and Yb in YZOS and GdzOa matrices (4, 5 ) . In the analytical papers by Linares, Schroeder, and Hurlburt ( 4 ) and by Cosgrove, Oblas, Walters, and Bracco (5) no mention is made of the sensitive dependence of fluorescence intensities on the kind and amount of residual impurities present in the host. Under ultraviolet excitation conditions, such dependence is commonly observed (6-8). Our studies of fluorescence emission under X-ray excitation Present address, Asahi Chemical Industry Co., Ltd., Tokyo, Japan. Present address, B.A.R.C., Trombay, Bombay-74, India. (1) J. Makovsky, W. Low, and S . Yatsiv, Physics Letters, 2, 186
(1962).
(2) W. Low, J. Makovsky, and S. Yatsivin, “Quantum Elec-
tronics-Paris 1963 Conference,” Columbia University Press, New York, N.Y., p 655, 1964. (3) V. E. Derr and J. J. Gallagher, ibid., p 817. (4) R. C. Linares, J. B. Schroeder, and L. A. Hurlburt, Spectrochim. New York, N.Y., Acta, 21, 1915 1965. (5) J. F. Cosgrove, D. W. Oblas, R. M. Walters, and D. J. Bracco, Electrochem. Technol., 6, 137 (1968). (6) P. Pringsheim, “Fluorescence and Phosphorescence,” Interscience, New York, 1949, pp 322-8. (7) C. G. Peattie and L. B. Rogers, Spectrochim. Acta, 7 , 321 (1956). (8) L. 6.Van Uitert and S . Iida, J. Chem. Phys., 37, 986 (1962).
conditions have clearly demonstrated that the emitted intensities are markedly influenced by other impurities present at the ppm level in the host matrix. In this paper, a description of these and other interference effects is given, and a technique for internally compensating for these effects on the fluorescent emission is outlined. EXPERIMENTAL FACILITIES AND PROCEDURES Apparatus. The experimental equipment employed is similar to that described by Linares et al. In our facility the X-ray source consists of a Machlett OEG-50 tungsten-target X-ray tube driven by a Norelco power supply. A sample chamber, an exploded view of which is shown in Figure 1, provides for the completely safe exchange of samples even though the X-ray tube ( a ) continues in operation at full power. During activation of the sample in this chamber, a small access door (b) is blocked in the closed position by a large lever (e). To exchange samples, the lever and an attached shutter ( d ) are rotated so that the X-ray beam is completely interrupted before the door can be raised. The sample is accurately and reproducibly positioned at an angle of 45” to the incident X-ray beam through the use of a vee-way mounting block (e) and mating sample holder ( f ) . The optical path is inclined at an angle of 45” to the sample surface, and at 90” to the X-ray beam. A quartz lens focuses an image of the fluorescing sample onto the entrance slit of a 25-cm focal length grating spectrometer. The entrance and exit slits of the spectrometer are set ?t 100 nm, and the spectrum is scanned at the rate of 200 A per minute. The fluorescent radiation is detected with an S-20 response, fused silica-window photomultiplier (RCA 7268). The photocurrent is amplified (Keithley Model 417 Picoammeter) and recorded with a two-pen strip-chart recorder. One of the pens is one tenth as sensitive as the other, which permits a greater range of spectral inteflsities to be recorded during a single scan of wavelength. For the observation of some of the data reported in this paper, such as those shown in Figures 2-6, the photomultiplier voltage was adjusted to bring the photocurrents of the various spectral lines into an appropriate range for optimal plotting of the data. Sample Preparation, All samples were prepared by combining dilute H N 0 3solutions of the various elements, followed by evaporation, dehydration, and ignition at appropriate
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Figure 2. Effect of iron addition of the fluorescent emission of 200 ppm of Sm, Eu, Tb, and Dy in YZO3
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temperatures. An oxalic acid precipitation was avoided because incomplete precipitation, which could significantly alter the rare earth concentrations at trace levels, has been reported (9). The ignited oxide was ground to a fine powder and then pressed into an aluminum planchet with the aid of a hydraulic press. EXPERIMENTAL RESULTS AND DISCUSSION
Figures 2 and 3 show the intensity behavior of the optical fluorescent emission of 200 ppm of Sm, ELI,Tb, and Dy in a highly purified Y Z O Shost, with the addition of increasing amounts of iron, chromium, or calcium. The enhancements observed at the lower concentration additions, which may be greater than a factor of two, are indeed surprising. We can offer no explanation for this enhancement effect. The (9) M. M. Woyski and R. E. Harris, in "Treatise on'Analytical Chemistry," Part 11, Vol. 8, Interscience,New York, 1963, p 29. I
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Figure 4. Effect of iron addition on the fluorescent emission of 2 ppm Pr, 3 ppm Sm, and 5 ppm Dy in ThOz, and of calcium addition on the fluorescent emission of 100 ppm of Sm, Eu, Tb, and Dy in GdzOa
validity of these observations was substantiated by repeated experiments which yielded essentially similar results. For several other YzOamatrices, a degree of enhancement lower than that shown in Figures 2 and 3 was observed. Whether these hosts contained a higher total non-rare earth impurity content has not been unequivocally established. Figure 4 shows essentially similar depressions of the fluorescence emission for Pr, Sm, and Dy in a T h o z matrix as the iron impurity level is increased, and for Sm, Eu, Tb, and Dy in a GdzOa host as the calcium impurity level is varied. Since these experiments may have been performed at total non-rare earth impurity levels greater than those for which enhancement may be observed, we have chosen to use dotted lines to extend the curves below 50-ppm impurity additions. Linares et al. ( 4 ) have indicated that spurious results arising from energy transfer mechanisms between rare earth ions should not be expected at concentrations below 0.1 atomic per cent. Also, Cosgrove et al. ( 5 ) have provided experimental evidence that interactions of this type were not operative in an YzOa host when rare earth ion dopant levels of 100 ppm were investigated. However, our experimental results show that dopant concentrations of ytterbium or cerium at levels less than 100 ppm can markedly reduce the fluorescent emission of Pr in Laz03or of Pr, Sm, Eu, Gd, and Dy in T h o z hosts. These observations are shown in Figure 5. APPLICATION OF THE INTERNAL STANDARD PRINCIPLE
The typical enhancement-suppression effects illustrated in Figures 2 to 5 imply that precise knowledge of the nature and concentrations of all impurities is a prerequisite for quan-
Pr4957A I
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Figure 7. Typical analytical curve using 50 ppm of Sm as the internal standard titative analytical applications of these spectra. Fortunately, this is not so. A characteristic exhibited by both the enhancement and suppression effects so far observed is the nearly parallel behavior of the intensities among the rare earths. Thus, though the individual relative intensities undergo wide excursions, the intensity ratios remain essentially invariant. These observations immediately suggest the application of the internal standard principle (IO) so widely used in optical emission spectroscopy to these radiations as well. The merit of employing intensity ratio measurements to compensate internally for impurity enhancement or depression effects is excellently illustrated in Figure 6, where the same data in Figure 3 are replotted as the relative intensity of Eu, Tb, and Dy with reference to the Sm emission. A typical application of the internal standard principle is shown in Figure 7. For this application, the Y ~ 0 host 3 was doped with 50 ppm Sm before the evaporation-dehydration-ignition preparation steps were undertaken. The amount of Sm added was far above the concentration normally found in ion-exchange column-purified Y203 when either EDTA or citric acid is used as the chelating eluant (11). Although two (10) W. Gerlach, “Foundation and Methods of Chemical Analysis by the Emission Spectrum,” Chap. V, Adam Hilger, London,
1929. (11) D. B. James, J. E. Powell, and F. H. Spedding, J. Znorg. Nucl. Chem., 19, 133 (1961).
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Figure 9. Effectiveness of internal standardization in compensating for both day-to-day fluctuations and the effect of ignition time on net intensities of separately prepared Y 2 0 3 samples containing 10 ppm Tb and 50 ppm Sm 2084
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Figure 8. Effectiveness of internal standardization in compensating for day-to-day fluctuations in net intensities of separately prepared Y z O ~samples containing 10 ppm Tb and 50 ppm Sm of the standard samples were deliberately doped with iron, and one with calcium, the working curve is essentially straight over the concentration range studied. Similar calibrating curves have been prepared and successfully utilized for the routine quantitative determination of Eu, Gd, Tb, and Dy in highly purified Yzo3. The emission intensities of X-ray induced optical fluorescence spectra are also dependent on the time and temperature course of the ignition, on the degree of absorption of HzO and C 0 2 ,and on drifts in power output of the X-ray tube and in the detector response sensitivity. Linares et al. ( 4 ) have already observed that measurement of intensity ratios compensated for day-to-day variations in the emission intensities caused by differences in details of preparation. (After this manuscript was submitted for publication, the paper by Burke and Woods (12) came to our attention. Burke and Woods also observed that the emission intensities were strongly dependent on the details of preparation of the crystalline host and that measurement of internal standard intensity ratios compensated for this effect.) The effectiveness of the internal standard principle in compensating for these run-to-run and day-to-day variations is illustrated in Figure 8. For these experiments eight identical samples were prepared and analyzed on different days, with all operating conditions maintained as nearly constant as possible. Although the standard deviations in intensity of both the terbium and samarium fluorescent lines are greater than 16 %, the intensity ratio of the pair is much less variable, with only a 4 % standard deviation. A similar study on the effect of ignition time is shown in Figure 9. The study was run twice on different days, to provide two data points for each ignition period. Again, it is clear that internal standardization has compensated for the effects of both time of ignition and day-to-day variations in net fluorescent emission intensities. The internal standardization principle described in this paper has also been applied in a quantitative procedure for the determination of fractional ppm amounts of several rare earths in reactor grade thorium metal (13). This analytical method will be described in a subsequent communication. RECEIVED for review July 9, 1968. Accepted September 16, 1968. (12) W. E. Burke and D. L. Woods, Conference on Applications of X-Ray Analysis, Denver, Colo., Aug 1967. (13) T. R. Saranathan, V. A. Fassel, and E. L. DeKalb, Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Cleveland, Ohio, March 1968, Paper #267.
