Energy Dispersive X-Ray Fluorescence Analysis in Ion-Exchange Studies
Joseph F. Mucci and Robert L. Stearns Vassar College Poughkeepsie. New York 12601
Energy dispersive X-ray fluorescence analysis is one of the simplest techniques. for obtaining elemental analysis and it provides information over a coniinuous range of &merits and concentrations.' The combination of rapid, semi-quantitative energy dispersive fluorescence X-ray analysis techniques with standard ion-exchange experiments provides a simple, interesting, and educational introduction to this relatively new method of analysis. Snecificallv. " . the ex~erimentis concerned with nerforming the following ion-exchange processes using the styrene tvoe. sulfonic cation-exchanee resin Dowex 50W-X4 (50id0 mesh) CS+ Dowex SO-& + H+ Dowex 50-CS+ (1) (1) W+ Dowex E M & + + 2 ~ s ' Dowex 50-Co2+(2) (11) 2Y3+ Dowex SDCq2+ Xo* Dowex 50-Y,3f (3) (111) ' The exchange products (i.e. (I), (II), and (111) above) are subjected to X-ray fluorescence analysis, as described below, to demonstrate the complete ion exchange as indicated in eqns. (11, (2). and (3).
+
+ +
-
+ + +
lon-Exchange Procedure A small column (10 cm long and 1.2 cm i.d.) containing Dowex 50W-X4 is put in H+ form by use of 1M HCI solution. The column is then washed clean of excess H+ and C1- with distilled water. Then 500 ml of 0.050 M CsCl is allowed to flow through the column by syphon action, the rate of flow (1 m1/3 min) being regulated with a Hoffman clamp. A diagram of the ion-exchange &-up can he found in the article by Mucei, e t aL2 The column is neat washed clean of excess Cs+ and CI- by use of distilled water. The exchange product (i.e. Dowex 50-Cs+) is then removed from the column and while still damp placed in a simple aluminum sample holder as described below and subjected to X-ray fluorescence analysis. Following this, the Dowex 50-Cs+is put hack in the column and 500 ml of CoClz is allowed to flow through the column displacing the Cs+ with Co2+.Again the column is washed clean of excess Co2+and C1- and a portion of the resin is analyzed by fluorescence analysis to show that indeed this displacement has taken place. An identical procedure is followed to displace the Co2+with Y3+ and again the displacement is confirmed hy analysis of the Dowex 50-Y3+. X-ray Analysis Procedure Figure 1 shows the general arrangement for the X-ray measurements. The basic idea is to use X-rays from a radioactive source which are energetic enough to remove K or L electrons from atoms of the element of interest. These atoms then emit characteristic X-rays which are detected by a lithium drifted silicon detector, the output from which, after appropriate amplification, is analyzed by a multichannel pulse height analyzer and displayed on a standard oscilloscope screen. The oscilloscope screen provides a linear horizontal energy display of the X-rays coming from the sample. For further details concerning nondispersive X-ray energy analysis see Porter and Woldseth.' Porter, D. E.,and Woldseth, R.,Anol Chem, 45,604A, (1973). 2Mucci, J. F., Spiegel, D. E., and Steams, R. L., J. CHEM. EDUC., 38,348, (1961). 750 / Journal of Chemical Education
I t would of course he possible to use a wavelength dispersive spectrometer, i.e. a Bragg spectrometer, for which the excitation would almost necessarily have to be done with a high intensity X-ray machine. The fluorescence X-rays leaving the sample would be collimated to form a beam incident on a single crystal such as LiF, calcite, or ADP, from which they would be scattered constructively a t the Bragg angle given by the familiar relationship nh = 2d sins. Here 8 is the Bragg angle, h is the wavelength, d is the crystal spacing, and n is an integer. The disadvantages of this technique include the high intensity required for excitation, the need to scan through a wide range of angles, the desirability of using different crystals for different wavelengths ranges and the lack of a linear relationship between angle and wavelength. Although the energy resolution is usually better for such wavelength dispersive systems, this increased resolution is rarely necessary for either qualitative or quantitative analysis. Although the electronic equipment required for an energy dispersive system is complex, the use of such a system with radioactive excitation sources is considerably simpler than the use of a wavelength dispersive set-up and it is this simplicity that has encouraged us t o use X-ray fluorescence techniques in the undergraduate laboratory. The ion-exchange experiment described here is but one simple example of the use of energy dispersive X-ray fluorescence for simple qualitative analysis. If care is taken with the use of standard samples and with sample preparation a certain amount of quantitative analysis can be done. The analysis of water samples from polluted lakes or streams is one example of a simple quantitative experiment which can be done by concentratine samnles of known comoosition. The anolication in a wide variety of analytical chemistry experiments involving both anions and cations is possible in a relatively quick, non-destructive, and pedagogically interesting manner. In this experiment the detector used was a lithium drifted silicon detector 6 mm in diameter and 3 mm thick with a system resolution of approximately 200 eV a t 6.4 keV ohGined from princetoncamma ~ e i Inc., h Princeton, New Jersey. The signal from the Ortec Model TC 202 BLR amplifier is fed to a Nuclear Data ND 180 512 channel analyzer and the resulting pulse height distrihution is displayed on a Tektronix Model 503 oscilloscope. The information from the oscilloscope can be read very easily and quickly by eye, if necessary, using an expanded horizontal display, permitting the student to identify the fluorescence X-rays from a particular sample after exposures which typically lasted only a few minutes. The equipment must be calibrated and this provides an additional worthwhile exercise for the student who uses samples of known composition to make a graph or table from which unknown energies can be read. For these experiments the sample holders used were simply small aluminum blocks 1 in. square by about 0.5 in. thick. A layer of Dowex resin about ?$ in. thick was spread over a recessed area on the top of such an aluminum block so that the "sample" seen by the detector was a sample of resin approximately 1 X 1 X # in. thick. When slightly damp the resin would stick nicely in such a holder so that it could be set on its side and adjusted to about 45' to the axis of the counter so as to be exposed to the bombarding &.
largements for these illustrations. Energy in units of keV is plotted linearly on the horizontal axis while the number of counts measured a t a particular energy is plotted vertically. Figure 2a is a 20-min exposure using the 57Fe source to excite a Dowex sample saturated with Cs+. The Cs K, and Kg peaks a t 30.8 keV and 35 keV show u p clearly a t the right (KB2is barely visible). The peaks t o the left of the cesium peaks are background due apparently to indium somewhere in the detector assembly and to the 14.4-keV gamma ray from the source. Figure 2b is also a 20-min exposure showing the displacement of the Cs+ by Co2+. The cesium is clearly gone and the presence of the cobalt is indicated by the small peak a t the left of the display. In Figure 2c, the same sample is excited using the lWCd source and the cobalt K, and Kg peaks are clearly visible a t the left. The energy scale is the same as in 2a and 2b but the exposure time is only 7 min. The large peaks a t the center of the display are produced by the silver X-rays from the source (about 22 keV) scattered from the sample at about 90'. It is interesting to note that each of the peaks (K, and Kg) is split into two parts by Compton scattering. The smaller peak in each case is the "original" or unmodified peak while the larger is the Compton scattered peak. Calculation of the displacement of these peaks (about 1 keV) provides an interesting exercise for the student. Figure 3a shows the results of exciting the same sample used in Figure 2c except that the energy scale has heen expanded by a factor of 2.5. The cobalt peaks have therefore moved nearer the middle of the display and are essentially all that can be seen in this 10-min exposure. Figure 3b shows the spectrum from the resin after the Y3+ has displaced the cobalt. The excitation source and energy scale are the same as for Figure 3a. The system is extremely sen-
Figure 1. General anangemsnt f w the X-ray measurements.
X-rays from the source and also permit fluorescence Xrays to reach the counter as indicated in Figure 1. The three elements used in this experiment were cesium, cobalt, and yttrium which have K binding energies of 35.96, 7.71, and 17.04 keV, respectively. We observed K X-rays in all three cases although in the case of cesium we could have used, in principle, the L X-rays which have energies between 4.3 and 4.6 keV. The signal to noise ratio in this region was not good and we found the K, X-rays to be much easier to detect. For measurements on cobalt and yttrium, a 2-mCi '09Cd source was used providing silver X-rays at ahout 22 keV. For cesium, we used a 1.5-mCi cobalt 57 source which provides iron X-rays a t 6.4 keV and, for our purposes, a gamma ray a t 122 keV. The sources were mounted in lead holders for safety purposes but because the experiment was so simple, i.e. the source was merely set a t the appropriate height beside the detector, students were not permitted to use the sources unsupervised. Discussion
Figures 2 and 3 show the experimental results as photographed on the oscilloscope screen and traced from en-
~ ~ c n e r
a
SNE~OY
b
~ W S ~ O I
C
Figure 2. Experimental resuns, energy versus number of counts f w (a) I'Fe source to excite a Dowex sample saturated with Cs+ (20-min exposure): (b) dispiacement of Cs+ by Co2+ (20-min exposure): and (c)the same sample as (b) with a 7-min exposure.
Figure 3. Experimental reeuns for (a) the same sample as Figure 2(c) with the energy scale expanded by a factor of 2.5 (10-min exposure): (b) the resin aner Y3+ has displaced the cobait (Z-min run) (source and energy scale same as (a): and (c)same as (b) except for 10-min run).
Volume 52, Number 11. November 1975 / 751
sitive to the Y since the K binding energy is fairly close t o the silver K, X-ray energy. This is a 2-min run. Note that the cobalt has been completely displaced. T o make this more convincing, Figure 3c shows a much longer 10-min rim in which the Y K, peak has overflowed the memory capacity for the display setting used and in which the Co peaks are still not detectable. The two small peaks which seem to bracket the region of the cobalt K, peak are from iron K,, in the detector system on the left and from Cu K,, (from the Al sample holder) on the right. General References ~
~~
Woldseth, R.. "X-Ray EnergySpedrometry..'Kevex Corp., 1913. Kolthoff, I. M.,Sandell, E. B., M A " , E. J.,andBmckenstein,S..'~Quantitativ.Chemiesl Andysia? The Macmillan Co., N.Y.. 1969. Kunin, Robert. "lon~ExchenpeResins: 2nd Ed., John Wiley and Sans, be..1958. Marinsky, J. A,, (Editor), "Ion Exchange: Marcel Dckker, N.Y.,Vol. I, 1966: Vol. 11, 1969. Benin. E. P.. "Principles and Prectiee af X-Ray Speetmmetrie Analysis? Plenum Pmrr. ,970.
752 / Journal of Chemical Education
Birks. L. S.. ef al., "Excitation ofCbars&htie X-Rays by Protom. Eleetmm, and P.imari X-Rays." J. Appl. Phys., 35,2578, (1964). Frankel, R. S.. Aitkin, D. W.. "Large Area Silicon Detsdars," in "Applications of Lo. Enorgy X- and Gamma Rays," Gordon & Breach, 1971. Giaugue, R. D.. et sl., '"Trace Element Determination with Semimndudo~Deteetor X-Ray Spetmmeter:'Anol. Chsm., Vol. 45.671, (1973). Hanaen. J. S., et *I.. "Aeeurste Efficiency Cdibration and Proputies 01 Scmicondueto. Deteetoteeto for Low Enerm Photons? Nucl Inatrum. Methods. 106,365 (1973). SpeJaklevic, J. M.. and Godding, F.S.. "Semiconductor Deteetar X - b y Fluo-na trometry Applied to Environmental and Biological Analysis: lEEE Transanions in Nuei Sei, Vol. NS-19. (1972). Johnson. G. G., White. E. W., "X-Ray Emission Wavelewlhs and keV Tables for Nondiffrsetive Anelyda? ASTM Data SeriesDS46,1970. Kneip, T. J.. and Laurer. G. R,'"Isotope Excited X-Ray Fluoremnee.'. A n d Chem., Vol. 44, No. 14. (Dee. 1972). Lucas-Tooth, H. J.. and pyne. C.. "The Accurate Determinsfion of Major ConsfituenU by X-Ray Fluorescent Andysis in the Presence of Large Interclement Effecfs: id". X-Ray Anal., 7,52341, (1964). nhodes. J. R., et al.,"~nergyoispemive X - ~ a ymission Spenrametry for ~ultieloment Andysir aiAir Particulates? ISA Trsnssetlons. Val. 11. 1972. p. 337. Stephenson, A , " T h e o r e t d Analysis of Quantitative X-Ray Emission Dsfs: GI-. Rocksand Metals"Ano1. Chem., 43.1761-4, (1971). , s.. "caleuiation ~ a t h o d nfor ~iuorescentx ~ n a ySpenromeCriri, J. w.. and ~ i r k rL. try,"Anal. Cham.. 40, 1080 11963).
