Anal. Chem. 1983, 55,
substrate for chemical1,y modified electrode surfaces (11). Inexpensive base-metal F’MF materials could certainly provide a highly conductive substrate for a large number of electrode surfaces. Bulk P M F is available in a number of metals with various porosities and can be easily formed or cut into any required shape. As is the case with RVC, the choice of several porosities permits user control over the optical and electrolytic characteristics of the electrode. PMF has at least two advantages over RVC of being less fragile and exhibiting lower electrical resistance. The PMF-CITE combines the desirable characteristics of a metal or metal oxide thin-layer electrode with the favorable structure of the RVC-OTE; such that electrochemical equilibrium can be attained in a short time while a substantial optical path length can be maintained by using only a small volume of solution. In conclusion, the PMF-OTE has been shown to be a new, simple, and highly versatile OTE for spectroelectrochemical studies of redox systems. In particular, the PMF-OTE is quite useful for studies of weakly absorbing splecies and/or dilute solutions, where optimization of the cell path length is required. Many applications will be found1 for using P M F as optically transparent electrodes as well as for use as conventional working electrodes where versatility, porosity, and large electroactive surface area are advantageous, such as for bulk electrolysis in flow-through process chemistry and for electrochemical detectors in liquid chromatographic systems. ACKNOWLEDGMENT The authors wish to thank G. Mamantov of The University of Tennessee (Knoxville), L. N. Klatt of Oak Ridge National
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Laboratory, and F. Gorman and J. M. Stamper of Astro Met Associates, Inc., for their helpful discussions. The form-fitting cell described herein was fabricated expertly by R. W. Poole, B. R. Vermillion, and E. R. Mellon of Oak Ridge National Laboratory. Registry No. Eu, 7440-53-1; Ce, 7440-45-1; K,C03, 584-08-7; nickel, 7440-02-0;gold, 7440-57-5;mercury, 7439-97-6;ferricyanide, 13408-62-3;ferrocyanide, 13408-63-4. LITERATURE C I T E D (1) Helneman, W. R. Anal. Chem. 1980, 5 0 , 390A-402A. (2) Kuwana, T.; Helneman, W. R. Acc. Chem. Res. 1976. 9 , 241-248. (3) Brewster, J. D.: Anderson, J. L. Anal. Chem. 1982, 5 4 , 2560-2566, and references therein. , E.; Mamantov, G. Anal. Chem. 1977, 49, 1470-1472. (4) N o ~ e l l V. (5) Bulletin No. 66-1-A Rev. 2, Astro Met Assoclates, Inc., Cinclnnatl, OH 45215. (6) DeAngelis, T. P.; Heineman, W. R. J . Chem. Educ. 1978, 5 3 , 594-597. (7) Heineman, W. R.; DeAngells, T. P.; Goelz, J. F. Anal. Chem. 1975, 4 7 , 1364-1369. ( 8 ) Hellwege, H. E.; Hobart, D. E.; Peterson, J. R., 1982, data to be submitted for publication. (9) Peterson, J. R. et al. U S . Department of Energy Document No. DOE/ ER/04447-152; pp 18-26. (10) Hobart, D. E.; Samhoun, K.; Young, J. P.; Norveil, V. E.; Mamantov, G.; Peterson, J. R. Inorg. Nucl. Chem. Leff. 1980, 16, 321-328. (11) Murray, R. W. Acc. Chem. Res. 1980, 13, 135-141.
RECEIVED for review February 28,1983. Accepted April 15, 1983. This research sponsored by the Division of Chemical Sciences, Office of Basic Energy Sciences, U S . Department of Energy, under Contracts DE-AS05-76ER04447 with The University of Tennessee (Knoxville) and W-7405-eng-26with the Union Carbide Corporation.
Determination of Thorium in Geological Materials by X-ray Fluorescence Spectrometry after Anion Exchange Extraction I wan Roelandts Department of Geology, P~~trology and Geoche?mistry,University of Licige, B 4000 Sari Tilman, LlSge 1, Belgium
Precise and accurate determinations of thorium in rocks and minerals of diverse nature are of importance not only for understanding its geochemical behavior during magmatic differentiation but also for the development of nuclear technology. A survey of the literature (1)shows, that radiochemical or nondestructive neutron activation analysis (using thermal, epithermal, or delayed neutrons) and maw spectrometry are the most widespread and particularly suited methods for the estimation of thorium a t the microgram and submicrogram levek of concentration in geological specimens. Unfortunately, these elaborate techniques require specialized instrumentation not available in most geochemical and industrial laboratories. Over the last few years, X-ray fluorescence spectrometry (XRF) has proved to be a useful analytical technique in geochemistry, mainly for bulk analysis of common silicate rocks. For certain trace elements present a t low levels of concentration, conventional separation and preconcentration procedures have been proposed in order to enhance both sensitivity and accuracy. Ion exchange i13a convenient and extremely efficient method for this purpose. Van Niekerk et al. ( 2 )have previously reported on a cation exchange X-ray fluorescence method for thorium, but an EDTA titration seemed to offer more accurate and flexible results (3). Anion exchange preconcentratiion is probably a more selective and favorable technique ( 4 ) . The aim of the present investigation was to verify the ap-
plicability of a combined batch anion-exchange-XRF method for the determination of thorium in geological samples. The so-called “matrix effects” which are severe drawbacks in these complex geological matrices will thus be overcome to a large extent after the removal of the main constituents. The calibration will be achieved by using pure chemical standards processed in the same way as the samples and therefore be independent upon internationally available geochemical reference materials. Two Canadian syenite reference rocks (SY-2, SY-3) as well as some South African thorium ores and concentrates were analyzed to control if the outlined procedure is suitable in practice. EXPERIMENTAL SECTION Apparatus. Batch equilibrations were performed by means of a Turbula system Schatz WAB shaking machine. A stainless steel die and a hydraulic press (Weber, Stuttgart, Uhlbach, Germany) were used for pelletizing the cellulose supports (5). All X-ray measurements were performed by using a CGR-alpha 2020 semiautomatic spectrometer (Compagnie GBnBrale de Radiologie, France) p equipped with a six-position sample changer. The spectrometer was interfaced to a Hewlett-Packard 9815A calculator. Reagents. The resin Dowex 1x8 (lOC-200 mesh, chloride form) was supplied by Fluka, Buchs, Switzerland. The nitrate form of the anion-exchange resin was prepared from the chloride form
0003-2701~/83/0355-1637$01.50/0 0 1983 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 55, NO. 9, AUGUST 1983
Table I. Instrumental Parameters for Thorium X-ray tube molybdenum voltage, k V 50 current, m A 50 analyzing crystal LiF220 (2d = 2.848) collimator, Fm 150 detector scintillation peak (28, deg) Th La 39.23 path vacuum counting time (s) 50 sample spinner on by washing with nitric acid, as has earlier been described in detail (5). Cellulose powder, Schleicher and Schull, no. 123 (Dassel, West Germany) was used to prepare the support material for the resin beads. Thorium stock standard solution (5 mg of Th/mL) was prepared from analytical reagent grade Th(N03)4.4H20(UCB, Belgium). Procedure. Cellulose Support for the Resin. The support for the resin beads used in the XRF analysis consisted of a cellulose disk. About 2 g of cellulose powder was poured into a 27 mm diameter stainless steel die and pelletized under 10 tons of pressure to give a firm plate 2 mm thick, which proved quite satisfactory. A small round disk of double-sided self-adhesive Duoplex foil (X-film, Type DX) was fixed on each cellulose support. Calibration Standards. Known amounts of thorium were pipetted into round-bottom Pyrex tubes containing 5 mL of 5% boric acid solution and the mixture was carefully evaporated t o dryness. The residue was taken up in 25 mL of 8 M nitric acid. Portions (250 mg) of dry anionic resin in the nitrate form were weighed and added to each solution. The stoppered tubes were shaken mechanically with a Turbula mixer for 4 h to ensure equilibrium. A t the end of this period the resin was allowed to settle. The resin was filtered off through a sintered glass crucible and dried overnight in an oven at 120 "C. The resin beads (about 40 mg) were then uniformly spread in a thin layer over the adhesive surface of the cellulose rigid support. Samples. Appropriate amounts (0.25-1.0 g) of finely powdered samples were accurately weighed into a platinum dish and dissolved with a mixture of 10 mL of concentrated nitric acid and 20 mL of 40% hydrofluoric acid. After complete evaporation to dryness on a water bath, the attack residue was treated with 10 mL of concentrated nitric acid and again evaporated just to dryness. This step was repeated several times. The residue was dissolved in 10 mL of concentrated nitric acid + 5 mL of 5% boric acid solution and evaporated gently to dryness on a hot plate. The final evaporation residue was taken up in 10 mL of 8 M nitric acid and transferred to the Pyrex stoppered tube containing about 250 mg (accurately weighed) of dry anionic resin in the nitrate form. Three further 5-mL portions of 8 M nitric acid washing solution were added and the ion exchange procedure described above was carried out. After equilibration, the resin was recovered and dried, and pellets were prepared following the procedure used for the standard samples. X-ray Fluorescence Analysis. The experimental XRF parameters used in this study are shown in Table I. Background intensity was measured separately at the same 20 Bragg angle of the analyte and under the identical instrument conditions on a "blank" pellet, prepared with thorium-free resin. All intensities were corrected for the background so obtained. In order to compensate for inhomogeneity, two pellets were prepared from each sample and the two readings averaged. In the concentration range studied, the reproducibility of the readings was better than 2%.
RESULTS AND DISCUSSION Ion Exchange Procedure. While it has been well established that thorium, in contrast to uranium, was not adsorbed practically on strongly basic anion exchange resins from hydrochloric acid solutions of any acid concentration, because of its low tendency to form negatively charged chloride com-
-lo7 -106 8
2
c
2
5 -105 e
a
"
-*m
E
.. z
-104
3 102
106
103
105
p g Th/ZlOmg r e s i n
Figure 1. Exchange capacity of the resin.
plexes (6), several investigators have reported that it was strongly retained on the same resins from nitric acid at high molarities. The existence of a stable anionic nitrate complex of the formula [Th(N03)6]2-has thus been suggested to explain the adsorption mechanism (4). Distribution coefficients, Kd, for thorium have been determined a t different concentrations of nitric acid under static and dynamic conditions (7-9). A systematic survey of the anion-exchange behavior of various elements in nitric acid solutions have been carried out by Ichikawa et al. (IO). From these relevant data, it can be seen that the adsorption of thorium on the resin was highest in the At that acid concentration, the region from 7 to 8 M "OB. Kd values for thorium were found to be about 200-300. On the other hand, the Kd values for major elements present in geological materials including alkali, alkaline earths, iron, etc. were far below that of thorium (Kd C 1). I t must be noted that the thorium separations hitherto carried out in order to permit its subsequent determination by suitable analytical methods utilized the column technique. Previous experience with the batch equilibration method showed that this process presents several distinct advantages with respect to a follow up by a determination using X-ray fluorescence directly on the resin particles, e.g., a smaller quantity of resin is required and a homogeneous distribution of the analyte on the resin is achieved, which simplifies sample handling prior to XRF analysis. In a preliminary stage of this investigation, a study was undertaken to determine the rate a t which exchange equilibrium was attained. All data in this work were obtained after an agitation time of 4 h. Exchange Capacity. The exchange capacity of the resin under our working conditions was determined by shaking 250-mg portions of the dry resin (nitrate form) with increasing As can be amounts of T h in 25 mL volumes of 8 M "OB. seen in Figure 1, the correlation between T h concentration and X-ray response of the resin is linear up to about 1 2 mg of Th/250 mg of resin. Beyond this value, the curve starts to deviate from the straight line. The capacity of the resin was thus found to be 1 mequiv of Th/g dry resin. Analytical Curve. It is common for quantitative XRF routine analysis of geological samples to make use of well established international geochemical reference materials to calibrate the instrument. Ideally, the reference materials must have as nearly as possible the same chemically composition as the samples submitted to analysis to obtain the required accuracy. Since the geological matrix i s variable, a wide variety
ANALYTICAL CHEMISTRY, VOL. 55, NO. 9, AUGUST 1983
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Table 11. Results for the Determination of Thorium (Expressed as ThO,) in CCRMP Syenite Rocks and MINTEK ThoriumOres and Concentrates no. of Tho, samples Tho, sample type sample source analyzed this work “accepted” values ref 432 ppm 20 syenite rock SY -2 CCRMP 3 413 * 67 ppm 20 3 1102 i 7 3 ppm 1127 ppm syenite rock CCRMP SY -3 303 i: 18 ppm 22 2 306 i: 6 ppm thorium orea 51/70 MINTEK 22 4 2953 i 109 ppm 2906 i 79 ppm thorium orea 50170 MINTEK 2.59 * 0.24% 21,22 6 2.64 * 0.11% monazite 50171 MINTEK 21,22 9 5.77 * 0.11% 5.81 * 0.16% 56/68 MINTEK monazite a Prepared mixtures .with 56/68 ( 2 1 ). k 1 standard deviation of analytical results. of reference materials is therefore needed. Although there has been a proliferation of ”standard rocks” in recent years, the lack of mineral and ore samples whose T h content is accurately known is a serious deficiency. It was thus considered desirable to use an independent method of calibration. Accordingly, synthetic calibration standards of thorium were prepared over a large concentration range. XRF Analysis. Direct XRF trace dement analysis on pressed powder pellets present several drawbacks. In addition to the need for well chemically analyzed reference materials of similar composition t o the samples, a correction for “matrix effeds” is imperative (11). More or less effective methods have been put forth to compensate for these effects (12-15). The problem concerning the choice of background and the computation of the real background under the peak measured can also be very complex (16,17). In the low concentration range, measurement in the vicinity of the analyte line wavelength can introduce important,errors, mainly in the short wavelength region where the background is sloping and has high intensity. Particle size effects and eventual spectral interferences are also to be corrected for. On the other hand, mder our operating conditions, the resin target assumes, to a certain extent, the nature of a “dilute solution”, that is attractive for XRF analysis. Basic advantages of the outlined technique may be summarized as follows: (1) homogeneous and reproducible matrix of very low atomic number element,s; (2) similarity of the resin grain size distribution in both samples and in the calibration standards; (3) absence of absorption and/or enhancement effects (e.g., the enhancement of T h La due to large quantities of rare earths occurring in monazite, dlanite, etc. as pointed out by Hart et al. (18) may be ignored because these elements are not adsorbed on the resin); (4) obviously the measurement of background is simple, it may be measured from a “blank” at the 20 angle for the analyte line; (5) elimination of possible interfering elements by preliminary separation; (6) ease in preparation of calibration standards and good linearity range of the calibration curvle. Limit of Detection, Precision, a n d Accuracy. Under the above operating conditions, the lower limit of detection (with 95% confidence) has been calculated according to Currie’s convention (19) and found to be equal to 13 pg of Th/250 mg of resin. This is quite sufficient for the range of concentrations found in thorium bearing minerals and ores. To obtain some idea about the precision and the accuracy of the present procedure, we analyzed two Canadian syenite reference rocks from CCRMP, SY-2 and SY-3, and a suite of South African (MINTEK) reference minerals, designated 51/70, 50170, 50171, and 56/68. Independent portions of each sample were submitted to the entire procedure and analyzed. The mean values obtained f one standard deviatiion of analytical results (based on n samples) are reported in Table 11.
For comparison, the values proposed by the originators (20-22) for these samples are also listed. It can readily be seen that the XRF data obtained by the method described above compare well with the averages from other geoanalysts using a variety of analytical procedures, with no systematic deviations being observed. These results show that the proposed method appears to be relatively precise and accurate for exploration geochemistry. ACKNOWLEDGMENT The interest of J. Belliere is appreciated. J. C. Duchesne is acknowledged for X-ray fluorescence facilities. The author is grateful to S. Abbey (Canada) and E. J. Ring (South Africa) for supplying the samples analyzed in this study and to V. Miocque, G. Bologne, G. Delhaze, and L. Lejeune for their assistance. Registry No. Thorium, 7440-29-1. LITERATURE CITED (1) Gladney, E. S.;Burns, C. E.; Roelandts, I.Geostand. Newsl. 1983, 7, 3-226. (2) Van Niekerk, J. N.; Streiow, F. W. E.; Wybenga, F. T. J . Appl. SpectrOSC. 1961, 15, 121-124. (3) Strelow, F. W. E. Anal. Chem. 1961, 33, 1648-1650. (4) Korkisch, J. “Modern Methods for the Separation of Rarer Metal Ions”; Pergamon Press: Oxford, 1969. (5) Roelandts, I . Anal. Chem. 1981, 53, 676-680. (6) Kraus, K. A.; Moore, G. E.; Nelson, F. J. Am. Chem. SOC. 1956, 78, 2692-2695. (7) Danon, J. J. Am. Chem. SOC. 1956, 78, 5953-5955. (8) Carswell, D. J. J. Inorg. Nucl. Chem. 1857, 3 , 384-387. (9) Bunney, L. R.; Ballou, N. E.; Pascual, J. P.; Foti, S. Anal. Chem. 195g, 31, 324-326. (10) Ichikawa, F.; Uruno, S.;Imai, H. Bull. Chem. SOC.Jpn. 1961, 3 4 , 952-955. (11) Norrish, K.; Chappei, 8. W. “Physical Methods in Determlnative Mlneralogy”; Zussmann, J., Ed.; Academic Press: London, 1967; pp 161-214. (12) Reynolds, R. C. Am. Mineral. 1867, 52, 1493-1502. (13) Leake. B. E.; Hendry, G. L.; Aucott, J. W.; Lunel, T.; Howarth, R. J. Chem. Geol. 1969, 5, 7-86. (14) Murad, E. Anal. Chim. Acta 1973, 6 7 , 37-53. (15) Nesbitt, R. W.; Mastins, H.; Stolz, G. W.; Bruce, D. R. Chem. Geol. 1976, 18, 203-213. (16) Wilband, J. T. Am. Mineral. 1975, 6 0 , 320-323. (17) Feather, C. E.; Wiilis, J. P. X-Ray Spectrom. 1976, 5 , 41-48. (18) Hart, R. J.; Reid, D. L.; Stuckless, J. S.;Welke, H. J. Chem. Geol. 1960, 29, 345-350. (19) Currie, L. A. Anal. Chem. 1968, 4 0 , 586-593. (20) Abbey, S. Geostand. Newsl. 1980, 4 , 163-190. (21) Stoch, M.; Ring, E. NIM Report No. 2042, Randburg Feb. 22, 1980; 25 PP (22) Ring, E. J. Mintek, South Africa, personal communication July 15, 1982.
RECEIVED for review December 1, 1982. Accepted April 20, 1983. The X-ray equipment used for this study was purchased with funds from the Belgian “Fonds de la Recherche Fondamentale Collective”, Collectif Interuniversitaire de GBochimie Instrumentale, under Contract No. 2.4521.76. This work was presented at the 23rd Colloquium Spectroscopicum Internationale, Amsterdam, The Netherlands, June 26 to July 1, 1983.
