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Anal. Chem. 1982, 54, 2636-2637
Selective Laser Excitation of Terbium( I I I ) In Lanthanum( I I I ) Fluoride Precipitates D. D. Ensor and C. G. Pippin Chemistry Department, Tennessee Technological Unlversity, Cookeville, Tennessee 3850 1
J. P. Young* and J. M. Ramsey Analytical Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830
The analysis of lanthanides by selective laser excitation in coprecipitated CaFz has been reported by Gustafson and Wright ( I ) . This technique was shown to be extremely sensitive, exhibiting detection limits in the range of 1pg/mL for several lanthanides. The fluorescence spectra of the lanthanides were characterized by four different excitation sites. These sites result from charge compensation when a +3 ion is substituted in a +2 ion matrix. Various ignition temperatures were used to produce a specific site. The technique is sensitive to various interfering ions including Na+ and has a reproducibility of 4 4 % relative standard deviation (RSD) (2). This work will report the use of LaF, as the host precipitate for the study of Tb(II1) and Er(II1). The use of LaF, as a coprecipitating agent for rare earths is a common technique. The Tb(II1) or Er(III), due to size and charge similarities, should occupy a single site in the lattice and thus greatly simplify their excitation and emission response. As a result, the procedure would be simplified and perhaps less sensitive to environmentally significant interfering ions.
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EXPERIMENTAL SECTION Reagents. All reagents used in the present study were analytical reagent grade and used without further purification. Stock solutions of 0.1 M La(III), Er(III), and Tb(II1) were prepared by dissolving the hydrated chlorides (99.9%) in 0.001 M HC1 to prevent hydrolysis of the rare earth metal ions. The lanthanide stock solutions were standardized by EDTA titration using xylenol orange indicator. The NH4F, NaC1, CaCl,, and FeC1, were prepared by dissolving a weighed sample of undried salt in the appropriate volume of distilled, deionized water and analyzed by using standard analytical techniques. Procedure. All glassware was prerinsed with dilute nitric acid followed by repeated rinsing with distilled, deionized water. A 25-mL aliquot of 0.1 M LaC1, was added to a 250-mL beaker and the pH adjusted to 2 using concentrated HC1. A specified concentration of Tb(II1) or Er(II1) was obtained by analytical dilutions of the standardized stock solution. A small volume (0.5 mL or less) of this diluted stock solution was added to the Lac4 solution. If interferences were to be studied, the required concentrations were added at this point. The total volume of each solution was adjusted to 50.0 mL before precipitation. Thirty milliliters of 0.3 M NHIF was added dropwise to the solution over a period of 1 min with constant stirring. The resulting precipitate was digested for 15 min at 60 O C and dowed to settle overnight. The supernatant was decanted and the precipitate transferred to a centrifuge cone and washed twice with deionized water. The precipitate was dried at 110 "C for several hours. After removal from the centrifuge tube, the sample was ground into a powder with a mortar and pestle. The precipitates were transferred to porcelain crucibles and ignited for 2 h in a muffle furnace at temperatures between 200 and 800 "C for various samples. After cooling, the powders were then loaded into 2 mm square Pyrex cuvettes and sealed. Apparatus. The laser excitation system consisted of a Chromatix CMX-4 pulsed dye laser operating at an average power of 8 mW with coumarin 102 dye (Eastman Kodak Chemical Co.). The laser was focused on the sample in a liquid helium cooled cryostat (8 K) with a 25-cm lens. The samples were positioned at approximately 60" to the incident beam and the fluorescence
EMISSION S P E C T R U M
560 W A V E L E N G T H (nmi
580
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Figure 1. The emission and excitation spectra of Tb(II1) in a LaF, precipitate.
emission was collected with a lens and directed into a 0.5-m monochromator (f/lO). The output from the monochromator was monitored with a 1P28 photomultiplier tube. The PMT signal was sent to a gated boxcar integrator with its output plotted on an x-y recorder. RESULTS AND DISCUSSION The Tb(II1) ion was found to coprecipitate in LaF3 and occupy a single lattice site (which produced an excitation spectrum with a single line at 486 nm). Two emission bands were observed when the sample was excited at 486 nm (7FG 6D4);these relaxations corresponded to 6D4 'F5 and SD4 7F4. The excitation and emission spectra of Tb(II1) are reproduced in Figure 1. The 6D4 7F6transition was the most intense, and a peak in this manifold, at 543 nm, was chosen as the analytical peak. A number of different precipitating and heating procedures were carried out to determine the optimum sample preparation techniques. It was determined that the precipitations carried out in the pH range of 2-3 yielded the best results. At a pH below 2, a precipitate formed that was slow to settle out, and a t a pH above 3 the fluorescence of the Tb(II1) was quenched, possibly due to oxyfluoride or hydroxide formation. A study of various ignition temperatures revealed no change in the excitation site of Tb(II1) up to 800 OC. The major effect of heating the samples was a sharpening of the emission peaks and removal of background fluorescence if the sample was heated in the range of 400-600 "C. As a result, the standard procedure was to heat all Tb(II1) samples a t 450 "C for 2 h. The detection limit for Tb(II1) using the present experimental apparatus was 1 X lo4 mol % Tb(II1) in the LaF,
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0003-2700/82/0354-2836$0 1.2510 0 1982 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 54, NO. 14, DECEMBER 1982 54'3.3 538.9
538.0
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541.0
544.0
WAVELENGTH (nm)
Figure 2. The emission spectrum of Er(II1) in a LaF, precipitate and the emission spectrum of Er(1II) in a LaF, precipitate containlng Tb(111). The excitation wavelength was 484.0 nm. (Base line is offset for graphical presentation.)
precipitate. This value was obtained by the extrapolation of a log-log calibration plot of the relative emission intensity vs. the mol % Tb(II1) to a signal-to-noiseratio of one. The lowest experimentally measured concentration was 6.6 X lo4 mol % Tb(II1). Such a mole ratio corresponds to a Tb(II1) concentration of 3.4 X lo4 IM in the solution before precipitation. The precision of the technique was found to be 7.0-8.0% for identical samples. 'The primary factors in precision are positioning of sample lholder and fluctuation of laser power. The incorporation of an internal standard, Er(III), provided a method for improving the precision. The Er(II1) emission spectra alone and with Tb(II1) in the LaF, precipitate are shown in Figure 2. The peak at 538.9 nm remans unaffected by the presence of Tb(II1). The concentration of Er(II1) was always below 2 X lo-, mol % since cluster formation has been observed for these two ions at higher concentrations (3). A calibration curve over the Tb(II1) concentration range of 1 X to 6.6 X l@mol % using Er(II1) as an internal standard
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(constant concentration of 1 x 10" mol %) improved the precision to better than 4%. The presence of interfering ions in the precipitating media has been investigated. The preparation of precipitates in either chloride or nitrate media with these anion concentrations up to 0.5 M showed no changes in fluoresence intensities nor were any energy shifta observed. The interference of Na', Ca2+,and Fe3+ions was analyzed with concentrations in the M, 1 X lo4 M, and 1 X precipitating media of 1.0 X M, respectively. The results indicated that the excitation spectrum of Tb(II1) remained unchanged in the presence of sodium and iron ions; however, the intensity of the analytical peak for Tb(II1) is reduced 25% in a precipitating media which had a Na+ concentration of 1 X lo-, M but was unaffected at a Na+ concentration of 1 X loa M. The presence of calcium ions did change the emission spectrum but the intensity of the analytical peak at 543 nm was not affected. The presence of large amounts of Ca2+could, of course, provide additional sites for the dopant ion and therefore interfere with the technique. The use of LaF3 as a precipitating agent for the selective laser excitation of lanthanide ions shows promise of being better than CaF2 The less complicated emission spectra and m p l e preparation are a major advantage. The sensitivity is within 1 order of magnitude of the CaFz and can be significantly improved with the use of increased laser power, increased monochromator throughput, and an improved detection system. Although the LaF, is susceptible to cation interferences (Na(I), Ca(II), Fe(II1)) it is less so than the CaFz system. It is significant that it is not necessary to use ionic buffers to remove or reduce interferences from common ions as is the case with the CaF2 host (1). The use of LaF, as a carrier should make this method easily applied to the detection of amenable trivalent 4f and 5f metal ions on an ultratrace level.
LITERATURE CITED (1) Gustafson, F. J.; Wrlght, J. C. Anal. Chem. 1070, 5 7 , 1762-1774. (2) Johnston, M. V.; Wright, J. C. Anal. Chem. 1081, 53, 1054-1060. (3) Tallant, D. R.; Miller, M. P.; Wright, J. C. J . Chem. Phys. 1076, 65, 510-521.
RECE~VED for review June 14,1982. Accepted August 9,1982. Research sponsored by the Division of Chemical Sciences, Office of Basic Energy Sciences, U.S. Department of Energy, under Contracts DE-AS05-79ER10489 with TTU and W7405-eng-26 with Union Carbide Corp.
ADDENDA Use of Chelex-100 To Maintain Constant Metal Activity and Its Application to Characterization of Metal Complexation
L. L. Hendrickson, M. A. Turner, and R. B. Corey (Anal. Chem. 1982,54, 1633-1637). The work was supported by the College of Agricultural and Life Sciences, University of Wisconsin-Madison, and by the U.S. Environmental Protection Agency (Grant R80461401).
