1774
ANALYTICAL CHEMISTRY, VOL. 51, NO. 11, SEPTEMBER 1979
at the expense of a poorer reproducibility. Several approaches were attempted to improve the reproducibility while not changing the influence of interferences but these were unsuccessful. There are numerous modifications which are being pursued to solve these problems.
LITERATURE CITED (1) F. J. Gustafson and J. C. Wright, Anal. Chem., 49,1680-1689 (1977). (2) M. Servigne, Ann. Chim. Anal., 45,273-280 (1940). (3) C.G. Peattie and L. B. Rogers, Spectrochim. Acta, 9,307-322 (1957). (4) C. G. Peattie and L. B. Rogers, Spectrochim. Acta, 7,321-348 (1956). (5) R. Reisfeld and E. Biron, Talanta, 17, 104-108 (1970). (6) L. Ozawa and T. Toryu, Anal. Chem., 40, 187-190 (1968). (7) N. S. Poluiktov, N. I.Smirdova, and N. P. Efryushina, J . Anal. Chem. USSR, 25, 616-619 (1970). (8) N. S.PoluBktov, N. I.Smirdova, and N. P. Efryushina, J . Anal. Chem. USSR, 27, 1466-1469 (1972). (9) N. S.PoluBktov, N. I. Smirdova, and N. P. Efryushina. J . Ana/. Chem. USSR, 25, 1633-1636 (1970). (10)N. S.PoluBktov and S. A. Gava, J. Anal. Chem. USSR, 25, 1489-1492 (1970). (11) N. S.PoluBktov. R. A. Vitkun. and S.A. Gava, J . Anal. Chem. USSR, 24, 540-543 (1969). (12) D. I.Ryabchikov and V. A. Ryabukhin, "Analytical Chemistry of the Elements: Yttrium and the Lanthanide Elements", Ann Arbor-Humphrey Science Publishers, Ann Arbor, Mich., 1970,p 193. (13) M. P. Miller, D. R. Tallant, F. J. Gustafson, and J. C. Wright, Anal. Chem., 49. 1474-1482 (1977). (14) L. A. Haskin, T. R. Wideman, and M. A. Haskin, J . Radioanal. Chem., I, 337-348 (1968). (15) R. H. Hamers, University of Wisconsin-Madison, unpublished work, 1977. (16) H. A. Doerner and M. Hoskins, J . Am. Chem. SOC., 47,662 (1925). (17) T. Rs. Reddy, E. R. Davies, J. M. Baker, D. N. Chambers, R. C. Newman, and B. Ozbay, Phys. Lett. A , 36, 231-232 (1971).
(18) D. N. Chambers, Phys. Letf. A , 37,77-78 (1971). (19) G. H. Dieke, H. M. Crosswhite, and 6.Dunn, J. Opt. SOC Am., 51,820 (1961). (20)G. D. Dieke, "Spectra and Energy Levels of Rare Earth Ions in Crystals", Interscience Publishers, New York, 1968. (21) F. J. Gustafson, Ph.D. Thesis, University of Wisconsin-Madison, 1978; available at University Microfilms, Ann Arbor, Mich.
(22) B. di Bartolo, "Optical Interactions in Solids", John Wiley, New Yo&, 1968, n 32R
(23)D. R. Tallant, M. P. Miller, and J. C. Wright, J. Chem. Phys., 65,510-521 (1976). (24) D. R. Taliant and J. C. Wright, J . Chem. Phys., 63,2074-2085 (1975). (25) R. C. Elser, in "Trace Analysis: Spectroscopic Methods for Elements", J. D. Winefordner, Ed., John Wiley, New York, 1976,p 383. (26) V. A . Fassel and R. N. Kniseley. Anal. Chem., 46,lllOA-1120A (1974). (27)A. P. D'Silva and V. A. Fassel. Anal. Chem., 45, 542-547 (1973). (28)A. P. D'Silva and V. A. Fassel, Anal. Chem., 46, 996-999 (1974). (29) W. Shand, J . Mater. Sci., 3, 344-348 (1968). (30) M. Miller, Ph.D. Thesis, University of Wisconsin-Madison, 1978;avaihble at University Microfilms, Ann Arbor, Mich.
(31)J. Short and R. Roy, J . Phys. Chem., 67, 1860 (1963). (32)R. W. Ure, J . Chem. Phys., 26, 1363-1373 (1957). (33)W. L. Phillips, Jr., and J. E. Hanlon, J . Am. Ceram. SOC.,46,447-449 (1963). (34) H. A. Laitinen, "Chemical Analysis", McGraw-Hill Book Co., New York, 1960,pp 165-181. (35) F. A . Cotton and G. Wilkinsen, "Advanced Inorganic Chemistry", 3rd ed.. Interscience Publishers, New York. 1972,p 811,p 930. (36)J. C. Wright, Anal. Chem., 49, 1690-1702 (1977). (37)L. G. Van Uitert and R. R. Soden, J . Chem. Phys., 36,517-519 (1982).
R E C E ~for ~D review March 26, 1979. Accepted June 22, 1979. This research was supported by funds from the Science Foundation under Grant No. CHE 74-24394.
Trace Analysis of Nonfluorescent Ions by Associative Clustering with a Fluorescent Probe Murray V. Johnston and John C. Wright Deparfment of Chemistry, University of Wisconsin, Madison, Wisconsin 53706
A new method for ultra-trace determination of the nonfluorescent rare earths, La, Ce, Gd, Lu, as well as yttrium and thorium has been developed using the technique of selectively excited probe ion luminescence (SEPIL). An Er3+ ion acts as a fluorescent probe of a CaF, precipitate to detect the presence of other ions which have coprecipitated into the lattice. Association of the Er3+ ion with a nonfiuorescent analyte Ion forms a new crystallographic site in the lattice which can be selectively excited with a tunable dye laser. An aliquot of an unknown solution is added to a solution containing calcium and erbium. When fluoride is added to precipitate CaF,, the erbium and analyte ions coprecipitate. The solid is ignited at a high temperature and then cooled to 13 K to observe fluorescence. Calibration curves are linear over four orders of magnitude with estimated detection limits as low as 4 pg and an RSD of 5 %.
Selectively excited probe ion luminescence (SEPIL) has been shown to be a potentially useful technique in the determination of trace inorganic ions ( I ) . Gustafson (2) and Gustafson and Wright ( 3 ) have developed a method for the determination of fluorescent rare earths by coprecipitation into CaF2. Fluorescence from a specific crystallographic site of a specific rare earth ion in the lattice can be selectively excited and monitored with a nitrogen laser pumped dye laser and a high resolution monochromator. Wright (4) has show 0003-2700/79/0351-1774$01 .OO/O
that this technique can be applied to the analysis of nonfluorescent ions as well if a new crystallographic site is formed by the presence of a nonfluorescent impurity ion residing near a fluorescent probe ion in the lattice. Associative clustering of rare earth ions to form new crystallographic sites in CaF2 is found to occur a t high dopant concentrations in numbers much greater than that predicted by statistical arguments. In the CaF,:Er"+ fluoride compensated system, where the extra positive charge accompanying an Er3+ substitution into the lattice is charge compensated by an interstitial fluoride ion, several sites have been observed on the basis of their unique crystal field splittings of the Er3+electronic levels (5). Sites containing two or more rare earth ions can be identified on the basis of their strong concentration dependence, complex fluorescence transients, and energy upconversion. In chemical analysis, oxygen compensation of the valence mismatch between a trivalent rare earth and the CaF, lattice is found to be superior to fluoride compensation in obtaining intense transitions ( 2 ) . The intrinsic fluoride compensation of CaF2 is converted into oxygen compensation by igniting a CaF2 precipitate in air. The reaction for this process is shown in Equation 1 using the notation of Kroger and Vink.
Here, the subscript "Ca" denotes an ion in a normal calcium site in the lattice, the subscript "F" an ion in a normal fluoride site, and the subscript "i" an ion in an interstitial site. The 1979 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 51, NO. 11 SEPTEMBER 1979
superscript "x" denotes a 0 net charge, relative to the normal lattice. Entities enclosed in parentheses indicate associated defects. Trace amounts of the fluorescent rare earths Pr, Nd, Sm, Eu, T b , Dy, Ho, E r , and T m which have been coprecipitated into CaF, and converted to oxygen compensation have been determined by selective laser excitation (2). The upper concentration limit of this technique is determined by the point a t which clustering becomes important and an appreciable amount of the rare earth is no longer in the single ion site. T h e overall cluster equilibrium is governed by the total amount of all rare earths in the lattice. Therefore, a large concentration of a nonfluorescent rare earth will cause an interference in the determination of trace amounts of a fluorescent rare earth. The nonfluorescent ion produces cluster sites of its own which the fluorescent analyte can randomly substitute into. This process results in a decrease of the fluorescent ion single ion site concentration and. hence, a n interference t o the analysis. The method presented in this paper is a simple extension of the interference problem. The roles of the fluorescent and nonfluorescent ions are reversed. A fluorescent probe will be present in the lattice at a high concentration and independently determine the number and type of clusters formed. T h e nonfluorescent analyte, present at a much lower concentration, can then substitute into these clusters to give "mixed" sites containing both probe and analyte. The solid state equilibrium describing this situation for a generalized case is illustrated below. (P = fluorescent probe, A = nonfluorescent analyte) P + P * P , t P + P , + P = e t c .
+
+
A
A
1
1,
PA P
t
+
PA
A
z \
a P,A P
+
P , A P,A
Equilibria involving more than one analyte ion will be negligible at low analyte concentrations. Species of the type PA, P2A, etc. will both fluoresce and exhibit a n analyte concentration dependence.
EXPERIMENTAL Apparatus. A complete discussion of our nitrogen laser pumped dye laser and detection system has already been published (6) and will not be presented here. Procedure. All glassware is rinsed with 3 M HN03 and deionized water prior to use to remove any traces of rare earth ions. Five milliliters of a solution 0.4 M in KNO, and LiN03and 2.5 mL of 1.20 M Ca(N03)*are pipeted into a 250-mL beaker. To it are added 0.2 mL of 3 X M Er(N0J3 and the proper amount of analyte solution to make up the correct mole percent. Mole percent is calculated on the basis of material present in solution before precipitation. For interference studies with Sa+, a volume of lo-' M NaN0, stock solution was added at this point. The sodium concentration, in ppm, was calculated with respect to the final solution prior to precipitation. The solution is diluted to 20 mL with deionized water and precipitated with 18 mL 0.3 M NH4F which is added via a calibrated automatic syringe at a rate of 10 mL/min. This process precipitates 90% of the calcium in solution, yielding about 210 mg of sample. A previous study (2) showed that essentially all of a rare earth present in solution will coprecipitate into the lattice under these conditions. The precipitate is allowed to settle for 1 day, and is then filtered through 0.45-fim pore size membrane filter paper (No. GA-6 Metricel, Gelman Instrument Co., Ann Arbor, Mich.),washed with deionized water, and air dried. The white solid is ground with a mortar and pestle, transferred to a Vycor crucible and ignited in a Lindberg Lab Box Furnace (Model 51844) for 3 h at 510 "C. The furnace is controlled by an external analog temperature controller (Model 917, Eurotherm Corporation, Isaac Newton Center, Reston, Va.) which is set to provide a rise-time to the set temperature of 10 min with an overshoot of less than 5 "C and a temperature stability off 0.5 "C at the equilibrium temperature. When the ignition is finished, the controller is unplugged and the
1775
furnace is allowed to cool by itself to less than 150 "C. (The cooling time constant of our heating element measured with an external thermocouple is approximately 50 min above 400 "C and 60 min below it.) The sample is reground and then pressed into a copper sample holder. About 10 mg of solid is required. The steady-state fluorescence intensity at 13 K of each sample is obtained by tuning the dye laser and monochromator to the appropriate wavelength transitions. Reagents. Reagent grade Ca(N0,)2.4H20(Mallinckrodt) and NH,F (J. T. Baker Chemical Company) were used to prepare Ca(N03)2and NH,F solutions. Rare earth stock solutions approximately 3 X M were prepared and standardized by an EDTA-Cu2+back titration procedure (7). Nitrate solutions were prepared from the oxides of La, Eu, Gd, Er (99.9% pure, ROC/RIC Corporation) and Lu (99.99% pure, Spex Industries) by dissolving them in concentrated HNO, (High Pure Chemical Corp.). The resulting solution was evaporated on a hot plate and the remaining solid dissolved in deionized water. 'rh(N03)4-4H20 (99.9% pure, ROC/RIC Corporation) and CeCl3#7H20 (99.99% pure, Spex Industries) were dissolved directly in deionized water. Dilutions were made to obtain 3 X M and 3 X M solutions. The lithium and potassium solution was prepared from reagent grade LiN0, (J. T. Baker Chemical Company) and KNOB (Matheson, Coleman and Bell). For interference studies, reagent grade NaNO, (Mallinckrodt) was used. Coumarin 1 dye (7-diethylamine-4-methylcoumarin, Eastman Kodak Co.) at a concentration of 1 mg/mL in 95% ethanol was used to excite Z H in E?+. A mixture of Coumarin 30 (Eastman Kodak Co.) at 2 mg/mL in methanol and Coumarin 1 in a 1:4 volume ratio was used to excite Z G. Rhodamine 6G (Eastman Kodak Co.) was used to excite 7F0 'Do in Eu".
-
--
RESULTS AND DISCUSSION In this discussion, we will first consider the specific example of lanthanum doped CaF2:RE3+,where Re3+ is a fluorescent rare earth, to demonstrate the utility of our technique. Next, we will generalize this to include all possible analytes in both single and multicomponent analysis. Finally, we will discuss some of the more important aspects of sample preparation and briefly show how they can be manipulated t o reduce interference effects. I n t r i n s i c Site Fluorescence. Figure l a is an excitation spectrum of the 'Fo 5Dotransition in CaF2:0.1 mol % Eu3+ while monitoring fluorescence from all sites simultaneously with a low resolution monochromator. Since the transition is between two levels having J = 0, there can be only one excitation line per site. The line at 573.55 nm is the G1 oxygen compensated single ion site of Eu3+ (2). The lines in the region of 576.4 nm are strongly dependent upon concentration and are attributed to cluster sites of two or more europium ions together. If 0.1 mol % lanthanum is added to the lattice, two new sites appear in the 576.4-nm region which are shifted slightly to higher energy from the intrinsic cluster sites (Figure Ib). These lines must belong to sites containing both La3+ and Eu3+. (The relative amounts of the cluster sites vs. the Eu3+G1 site in Figures l a and l b are not the same since, in the second case, La3+exerts an additional influence upon the overall cluster equilibrium.) If the dye laser is tuned t o excitation line number 1 (an Eu3+-La3+ cluster site) of Figure lb, the fluorescence spectrum in Figure 2a, is obtained when a 1-m monochromator is scanned through the :'DO 'F,, 'F, electronic transitions. A similar spectrum for excitation line number 2 (an Eu3+-Eu3+ cluster site) of Figure l b is found in Figure 2b. In each case the fluorescence lines are broadened because the terminal 7F1and 7F2levels relax quickly to the 'Fo ground state. Inspection of Figures lb, 2a, and 2b reveals that the crystal field splittings are too small t o permit a completely selective excitation of t h e Eu3+-La3+ site fluorescence with respect t o the intrinsic Eu3+-Eu3+ fluorescence. This problem is illustrated by the excitation spectrum in Figure IC. Here, a 1-m monochromator was used to reject as much intrinsic site fluorescence as possible by
-
-
1776
ANALYTICAL CHEMISTRY, VOL. 51, NO. 11, SEPTEMBER 1979 Enerpy [ x ~ 0c 3 rn~l) 0.1 Eu
01
J y 573
574
576
575
t l
577
t I Er
a) 573
574
575
576
--
577
x Flgure 1. (a) Excitation spectrum 7F0 5D0 of CaF,:Eu (0.1%) monitoring fluorescence from all sites, 5D0 7F,. (b) Excitation spectrum 7F0 %, of CaF,:Eu (0.1%), La (0.1%) monitoring fluorescence from all sites, 5D0 7F2. (C) Excitation spectrum F' , 5D0of CaF,:Eu (0.1%), La (0.1'YO) with 1-m monochromator set at 607.1 nm and a bandDass of 0.5 A ("It.)
-
-
-
F l u o r e s c e n c e from an E u - L a aite
I 605
610
615
620
625
630
635
F l u o r e s c e n c e from an E u - E u site
2 Er's b)
Figure 3. (a)Er3+ electronic levels. (b) EPf-Er3+ dimer electronic levels. (c) Fluorescence quenching from E by energy transfer
laxation rate has an exponential dependence upon the energy separation between the two levels involved in the process. The E level of Er3+in a single ion site fluoresces strongly since the energy gap to D is large. In a dimer, however, the state where one Er3+is excited to the E level and the other is in the ground state can efficiently relax to a state where one of the ions is excited to A and the other to Y. This state in turn can relax to a state where one Er3+is excited to D and the other is in the ground state. This process is summarized in Figure 3c. The net result is that fluorescence from the E level is quenched when two or more erbium ions are tightly coupled. In CaF2:Er3' (0.02%),only two sites of importance are present: The G1 single ion site and the I site, which is most probably a dimer. The fluorescent lifetimes of these two sites out of the E level are 120 ps for the G1 site and 52 ps for the I site. The I site lifetime is so short compared to the G1 site lifetime that we can use a conventional gated integrator to discard virtually all of its fluorescent intensity. This effect is illustrated in Figure 4. Figure 4a is an H transition in CaF2:Er3+ excitation spectrum of the Z (0.02%) while monitoring all fluorescence from D Z. Since the H manifold is a 4F5i2level and erbium is a Kramer's ion, we expect to see three excitation lines per site, corresponding to transitions from the ground state to the three nondegenerate MJ levels in H. As shown in Figure 3a and 3b, the radiative lifetimes of the two sites out of D should be close since the energy gaps to the next lower level for the two are similar. Thus, we observe excitation lines for both sites. The I site has more than three lines due to exchange interactions between the two erbium ions which lifts the Kramer's degeneracy of the =kMJlevels in the ground state and in the excited state manifold. If we now perform the same experiment only monitoring the fluorescence from E, the excitation lines of the I site are removed (Figure 4b). In this experiment, a 50-ps delay between the laser pulse and the gated integrator was employed to reject virtually all fluorescence from the I site while retaining most of it from G1. This procedure will discriminate against all sites in the lattice which contain two or more erbium ions. The analytes we have chosen to study all have closed or ' / 2 filled subshells and therefore no accessible spectroscopic levels. Cooperative energy transfer from erbium to these other ions in the mixed clusters is not possible, so the fluorescent lifetimes for the E level for these sites should be similar to that of the G1 site.
-
605
610
615
620
X inm.1
625
-
630
635
Figure 2. (a) Fluorescence spectrum 'Do 7F,, 7F, with dye laser tuned to excitation line # 1 in Figure 1. Linewidths are not limited by the monochromator resolution. (b) Fluorescence spectrum 5D0 7F,, 7F, with dye laser tuned to excitation line #2 in Figure 1 +
monitoring fluorescence from the 607.1-nm line in the Eu3+-La3+ fluorescence spectrum. If the lanthanum concentration is reduced, our ability to observe the mixed site fluorescence is quickly eliminated by overlap with the intrinsic site fluorescence. Exactly analogous results were obtained with europium and lanthanum concentrations of 0.01%, except that the total number of cluster sites present relative to the single ion is reduced. Therefore, this technique can be useful in trace analysis only if the interferences due to intrinsic cluster site fluorescence are removed. Fluorescence from intrinsic clusters is many times quenched by cooperative energy transfer between ions in the site (8,9). Figures 3a and 3b show energy level diagrams for a single Er3+ ion and an Er3+-Er3+dimer. The dimer levels are obtained by taking all possible combinations of the single ion levels two at a time. Nonradiative relaxation from one level to the next lower level occurs by emission of lattice phonons. The re-
Er-Er C)
-
ANALYTICAL CHEMISTRY, VOL. 51, NO. 11, SEPTEMBER 1979
1777
a) 0.02 E r
cl 0 . 0 2 Er, 0.01 La
La-I \I
J p - 2
447
44 8
449
447
b) 0.02 E r
7
I
440
-
448
d) 0 . 0 2 Er,
447
1
-
449
La
1
I
449
448
447
449
Lf3
-
--
H of CaF,:Er (0.02%) monitoring fluorescence from all sites, D Z. Multiply by 0.01. (b) Excitation Flgure 4. (a) Excitation spectrum Z H H of CaF,:Er (0.02%) monitoring all fluorescence, E Z, with a 5 0 - p ~delay. Multiply by 20. (c) Excitation spectrum Z spectrum Z of CaF,:Er (0.02%), La (0.01% ) monitoring fluorescence from all sites, E Z. Multiply by 10. (d) Excitation spectrum ;Z H of C.aF,:Er (0.02%), monitoring fluorescence from all sites, E Z. Multiply by 1 La
-
-
C l u s t e r Equilibria. Figure 4c is an excitation spectrum of Z H in CaF2:Er (0.02%), La (0.01%) monitoring fluorescence from all sites. At this concentration of lanthanum, three sites in addition to the erbium G1 site, designated La-1, La-2, and La-3, are observed. If the lanthanum concentration is dropped by a factor of ten, a single lanthanum site, La-1, becomes predominant (Figure 4d). The extra line which appears in Figure 4d is a “hotband” transition arising from a thermally populated level in the ground state manifold of the G1 site. Fluorescence spectra of the G1, I, and La-1 sites appear in Figure 5. As in excitation, the I site fluorescence transitions are split by exchange interactions which serve to broaden the lines. Inspection of Figures 4 and 5 shows that if we were not able to discriminate against the I site fluorescence, spectral overlap between it and the La-1 site in both excitation and fluorescence would again limit us as it did in CaF2:Eu,La. Selective excitation of the La-1 site with respect to the G I site is easily done. The concentration dependences of the La-1, La-2, and La-3 sites were studied by preparing two sets of precipitates. The first set had a constant amount of Er3+ (0.02%) with lanthanum concentrations ranging from 0.02 to mol %. The second set contained a constant amount of La3+ (0.02%) and erbium concentrations ranging from 0.02 to 5 X lo4%. Slopes from log-log plots of fluorescent intensity vs. concentration are given in Table I. Since, a t the concentrations studied, both dopant ions affect the overall cluster equilibrium, changing one concentration with respect to the other will affect the cluster populations in a complex way determined by the
-
+
Table I. Concentration Dependence of La-1, La-2, and La-3 Sites
site La- 1 La- 2
La-3
slope of log-log plot vs. [Er“]
slope of lOg-lOg
plot vs. [La”]
0.88
0.8t
0.89 0.90
1.82 2.65
details of the site equilibria. Thus, we would not necessarily expect the slope of a log-log plot to bel a simple whole number corresponding to the number of ions present within the cluster. An approximately linear dependence upon the erbium concentration is observed for each site. ‘The same is true for the lanthanum concentration dependence of the La-1 site. However, the strong lanthanum dependence of the La-2 and La-3 sites shows that these correspond t o clusters containing a t least two lanthanum ions. In an analytically useful situation where the analyte concentration is much less than the probe concentration (Le., if [Er3+]= 0.02% then [La3+]4 the only important erbium-lanthanum site is La-1, an erbium-lanthanum dimer. A series of precipitates with lanthanum concentrations ranging from to 5 x were prepared and the fluorescent intensity of the La-1 site was monitored. In this experiment, an erbium concentration of 0.02% was used since it gave optimum fluorescent intensity from the La-1 site when [La3+] = At higher erbium concentrations, line broadening of excitation and fluorescence lines occurs since
1778
ANALYTICAL CHEMISTRY, VOL. 51, NO. 11, SEPTEMBER 1979
Table 11. Important Parameters in the SEPIL Technique ionic radius
E lifetime,
site
of analyte, ~f
PS
258 253 24 6
La- 1
1.16
Gd-1 Lu-1 Y- 1 Ce-1
1.05
0.98 1.02
Th- 1
1.19
1.14e
231 3 88 372
S I N limitn
mol % 4x 2x 6x 3x 2x
___
pg/mLb
Pgc
8 46 16
47
lo-' lo-*
4 4 8
2 x
contamination limit,d mol %
8 15 4 4 7
< 5 x 10''
a Extrapolation of intensity vs. concentration curve to S I N = 1. Based upon the 20-mL solution volume prior to precipitation. Based upon the 1 0 mg of sample needed for laser excitation. Due t o impurities in our reagent erbium. e Trivalent state; 0.97 A for tetravalent ion. f Reference 12. Other radii are 1 . 1 2 A for Ca2+and 1.00 A for Er3+.
X (nm 1
Figure 5. Fluorescence spectra of (a) erbium G1 site, (b) La-1 site,
and (c) erbium I site neighboring sites are close enough to provide long range perturbations of the probe ions levels. Below 0.02%, the formation of single ion sites is preferred relative to cluster sites. The Z G transition was excited since its absorption coefficient is larger than Z H and, thus, greater fluorescent intensities are produced. Fluorescence was still monitored from E Z. To account for changes in optical alignment and positioning of individual samples in the laser beam, the G1 site fluorescent intensity was measured as an internal reference. In the regime we are working, the lanthanum present has virtually no effect on the cluster equilibrium so the G1 intensity is constant. A log-log plot of Intensity La-l/Intensity G1 vs. concentration is linear from to lo4% with a slope (of 1.03) calculated by least squares. At 5 x an anomalously high intensity is obtained which is probably due to impurities in the reagents. The erbium used in this experiment is pure to 1 part in l o 4 with respect to other trivalent rare earths and, thus, establishes the lower limit of detectability of this technique. Similar results are obtained H but the overall fluorescent intensities when exciting Z
-
-
-
are reduced by a factor of eight. The average relative standard deviation (RSD) of five separate samples having mol % lanthanum is 5%. The relative amount of analyte residing in cluster sites with erbium with respect to single ion sites in the lattice was estimated by preparing two precipitates, CaF2:Er(10-3%)and CaF2:Er(10-3%),Y(10-2%). Yttrium, like lanthanum, yields a single mixed dimer site at low concentrations. When yttrium is added, the erbium G1 intensity decreases to 45% of its value when no yttrium is present. This means that 55% of the erbium goes into cluster sites with yttrium. Therefore, in an analytical situation where the roles of erbium and yttrium are reversed, about half of the nonfluorescent ions present in the lattice are bound to fluorescent probe ions. Multiple Ion Analysis. Application of this technique to other nonfluorescent ions is limited by two factors. First, the ion must quantitatively coprecipitate into CaF2. This condition requires that the ion have both a similar ionic radius and a similar coordination number to the host cation. Second, its solid state chemistry must be similar to that of the fluorescent probe, erbium, so that there is a driving force for "mixed" cluster formation. The ions which fulfill these conditions and can be determined in chemical analysis are La3+,Ce3+, Gd3+,Lu3+, Y3+, and Th". All exhibit cluster equilibria similar to that of La3+ described above. At concentrations below only one "mixed" cluster site is observed, an erbium-analyte dimer. Data pertinent to the analysis of each of the above ions, including detection limits, excitation and fluorescence wavelengths, and fluorescent lifetimes of the E manifold, appear in Tables I1 and 111. All of the analytes except for thorium have abnormally high fluorescent intensities a t 5 x due to impurities in the reagent erbium. A multiple ion analysis of these ions is also possible. Figures 6a and 6b are excitation spectra of Z G and Z H, respectively, monitoring fluorescence from all sites in the precipitate CaF2:Er (0.02%), La, Ce, Gd, Lu, Y, T h (2 x The crystal field splittings in the G manifold are rather large, so spectral overlap between levels of different analyte cluster sites is small. Experimentally it is found that the lowest energy level of the manifold has a much larger absorption than any other and is therefore the transition H lines in Figure 6b are chosen for the analysis. The Z much closer, so accidental overlap of the various sites is a bigger problem. The magnitudes of the crystal field splittings of the various Er3+-analyte dimer transitions depend upon the ionic radius and effective charge of the analyte, and upon how differently the ground and excited manifolds interact with the crystal field. Lanthanum, gadolinium, lutetium, and yttrium are all stable only in the '3 state and have very similar excitation and fluorescence spectra. The Gd-1 site has a much broader linewidth than the rest due to exchange interactions between the 8Sground state of Gd3+and the Er3+electronic manifolds which removes the degeneracy of the 8S.The other ions all have S = 0 in their ground states and do not exhibit this effect.
-
-
-
ANALYTICAL CHEMISTRY. VOL 51, NO. 11, SEPTEMBER 1979
-
Table 111. Selected Excitation and Fluorescence Wavelengths, nmavb Z- H
Z- G
E- Z
La-1
449.08 448.68 447.30
486.84
547.30 545.08
Gd-1'
449.1 448.8 441.4
487.1
547.2 54 5.2
Lu-1
449.19 448.88 447.47
487.14
547.15 545.30
Y- 1
449.21 448.89 447.48
487.12
547.28 545.40
Ce-1
448.32 447.99 446.88
486.24
545.32 544.29 543.87
Th- 1
448.49 448.18 446.89
486.30
545.83 544.39 544.09
Er3+,Gl
449.22 448.36 448.16
488.39
546.05 544.63
site
The most a Wavelengths were measured i 0.02 nm. intense transitions in each manifold are listed first. Except where noted, linewidths are limited by the laser or Linewidths are 1 A . monochromator bandwidth.
486
487
468
1779
is especially evident in E Z fluorescence where the relative spacing and intensities of the lines do not resemble those of the other four ions and in the magnitude of the E level lifetimes (Table 11). In aqueous solutions, Ce3+and Th" are the stable species which enter the lattice during coprecipitation. However, the similarity of the cerium and thorium site spectra suggests that during the ignition to form oxygen compensated sites, cerium is oxidized to the tetravalent state. This conclusion is consistent with several features in the Ce-1 spectrum. Cerium(III), an f' ion. has a low lying electronic level a t about 2200 cm Fluorescence from erbium in the E manifold should be quenched in Er"+-Ce'+ dimers by cooperative energy transfer from Er3+ to Ce3+,leaving the erbium in the D manifold. On this basis, we would not expect to see Ce-1 site fluorescence, especially with a longer lifetime than the La-1 site, where energy transfer cannot occur. Also, the Ce-1 lines are no broader than those of Th-1, La-1, Lu-I, and Y-1, suggesting that exchange interactions are not present and that the cerium species in the dimer has an S = 0 ground state. For all sites except Gd-1, the excitation linewidths are limited by the laser linewidth (about 0.025 nim), so instrumental improvements may further increase the degree of selectivity. The clustering of ions to form "mixed" sites with erbium is not limited to the five discussed here. All of the fluorescent rare earths will participate in a similar process if introduced into the lattice. However, their cluster sites with erbium will not fluoresce owing to energy transfer aniong the various levels on both ions. This technique, therefore, is specific toward nonfluorescent, spectroscopically "inert" ions. Sample P r e p a r a t i o n a n d Interferences. Since the details of our sample preparation method have already been presented in the Experimental section, this discussion will be limited to some of the more important aspects of it. Ignition is by far the most important step in the procedure since it controls the development of oxygen compensation. Earlier work on single ion sites of fluorescent rare earths in CaFz revealed that four classes of oxygen comperisation can be produced ( 2 ) . The sites resulting from these, C:l, G2, G3, and G4, differ in the number of oxide ions and fluoride vacancies which surround the rare earth ion. When ignited in air, pure calcium fluoride will slowly react to form calcium oxide, the basic step being:
~ F +F HJO(,,
489
Ah.)
-
2HF
+ V F + O'F
Here, VF refers to a fluoride lattice vacancy. The superscript denotes a +1 charge relative to the normd lattice while a ""' denotes a -1 charge. The introduction of a rare earth to the lattice produces a point defect around which oxide ions and fluoride vacancies can cluster. Since there are eight fluoride sites surrounding a single cation position, a maximum of four oxide ions can be accommodated. The overall process can be described by a set of sequential steps: "a''
bl
,
446
447
448
X(nm.1
449
-
450
Figure 6. (a) Excitation spectrum Z G of CaF,:Er (0.02%), La, Ce, Gd. Lu, Y, Th (2 X monitoring fluorescence from all sites. (b) Excitation spectrum Z H monitoring fluorescence from all sites
- -
--
Lutetium and yttrium excitation lines coincide for all possible Z H, Z G, and Z F transitions, but they are separated by about 0.1 nm in E Z fluorescence which may be exploited in selective excitation. Crystal field splittings of the thorium and cerium sites differ markedly from the rest. This difference
For the dimer sites, only one higher order oxygen compensated site has been observed and the energies of its transitions suggest that it is analogous to the G2 single ion site: (Erca.Laca.2Fi)X+ (Erca-Laca-20F)X e La-1 (Erc,.Laca.30F.VF) Since this is a nonequilibrium situation, ignition time and temperature must be controlled if reproducible site distri-
1780
ANALYTICAL CHEMISTRY, VOL. 51, NO. 11, SEPTEMBER 1979 5-
Intensity
(orbiirory, units)
3.
2-
I-
I
447
L.
400
,/
>
600
, 800
,
.
1000
T e m p e r a t u r e ('Ci
Figure 7. Temperature dependence of La.1 (0)and sodium ( 0 )sites in CaF,:Er (0.02%), La Na (250 ppm)
butions are to be obtained. Additionally, ions which do not coprecipitate quantitatively may still enter the lattice and affect the quasi-equilibrium by increasing the number and mobility of defects. In single ion studies, it was found that the addition of a lithium and potassium buffer to the coprecipitation solution stabilized the relative distribution of oxygen compensated sites in the presence of interfering ions (2). The buffer also promoted conversion to oxygen compensation a t a lower ignition temperature than without. We have found these general results to apply to our cluster sites, too. The ignition temperature dependence of cluster site formation is based upon two competing processes. As the temperature is increased, the dimer site fluorescence is reduced by the formation of higher order oxygen compensated sites and by the dissociation of dimers into single ion sites. This latter effect has been observed in fluoride compensated CaF2:Er3+single crystals also (10, 11). If the ignition temperature is kept too low, incomplete conversion to oxygen compensation is obtained. A plot of ignition temperature vs. La-1 fluorescent intensity relative to G I intensity is given in Figure 7. We have found the La-1 site intensity to be constant within experimental error over a range of temperature from 500 to 540 "C. Ignition time has a similar effect although the dependence is not as pronounced. Ignition for very long periods results in the formation of higher order oxygen compensates sites, while too short an ignition time leaves incomplete conversion to oxygen compensation. In our preparation procedure, a range of 2.5 to 5 h of ignition a t 510 "C gave optimum intensities for all erbium-analyte dimers. T h e presence of sodium in the lattice has two interfering effects. I t catalyzes the formation of higher order oxygen compensates sites and produces sites of its own. These sites can overlap with the principal analyte sites causing an interference a t low analyte concentrations (Figure 8). The ignition temperature dependence of Er-Na clusters is shown in Figure 7 . Above 550 "C, fluorescence from these sites increases sharply. This is outside the optimum ignition range for erbium-analyte dimer formation, so the interference due t o spectral overlap with sodium sites is discriminated against as a direct consequence of the ignition procedure. Sodium has an unfavorable distribution coefficient for coprecipitation into CaF2. The amount of this ion, or any other one with the same property, which enters the lattice can be reduced by aging the precipitate in solution, washing it after filtration, or precipitating less than 100% of the calcium. AI1 of these steps are included in the sample preparation procedure.
440
-
449
h(nrn.~
Figure 8. Excitation spectrum Z H of CaF,:Er (0.02%), La Na (250 ppm) monitoring fluorescence from all sites. The additional lines not found in Figure 4d are due to Er3+-Na+ clusters
Fluorescent rare earths are not inhibited from entering the lattice by this procedure, but their effect is somewhat limited. Because of the inherently sharp linewidths that rare earth ions possess, our selective excitation-fluorescence technique removes any interference due to fluorescence from other single ion sites. Fluorescent rare earths can induce changes in the overall cluster equilibrium. If the concentration of all ions which can associate with erbium in the lattice exceeds then the cluster equilibrium is no longer controlled bythe erbium alone and changes can result in the G1 and analyte site concentrations. This effect can be eliminated by diluting the original solution.
CONCLUSIONS We have shown the the SEPIL technique can be successfully applied to trace analysis of the nonfluorescent rare earths, yttrium and thorium, yielding detection limits that are competitive with comparable techniques. Associative clustering of the analytes with a fluorescent rare earth probe in calcium fluoride produces "mixed" cluster sites which both fluoresce and display a concentration dependence upon the analyte. The fluorescent probe has been shown to act as a buffer in establishing and controlling the overall cluster equilibrium as the analyte concentration is varied. Interference due to fluorescence from intrinsic probe ion sites was eliminated by a judicious choice of the probe ion and its fluorescence transition. Other major sources of interference were identified, and a sample preparation procedure was designed to minimize their effects.
LITERATURE CITED (1) Wright, J. C.; Gustafson, F. J. Anal. Chem. 1978, 50, 1147A. (2) Gustafson, F. J. Ph.D. Thesis, University of Wisconsin, Madison, Wis., 1978. (3) Gustafson, F. J.; Wright, J. C. Anal. Chem. 1977, 49, 1680. (4) Wright, J. C. Anal. Chem. 1977, 49, 1690. (5) Tallant, D. R.; Wright, J. C. J . Chem. Phys. 1975, 63, 2074. (6) Miller, M. P.; Taiiant, D. R.; Gustafson, F. J.; Wright, J. C. Anal. Chem. 1977, 49, 1474. (7) Haskin, L. A,; Wildeman, T. R.; Haskin. M. A. J . Radioanal. Chem. 1968, 7 , 337. (8) Tallant, D. R.; Miller, M. P.; Wright, J. C. J . Chem. Phys. 1976, 65, 510. (9) Wright, J. C. "Up-Conversion and Excited State Energy Transfer", from "ToDics in ADDlied Physics", Vol. 15, Fong, F. K., Ed.; SDrinaer-Verlaa: . Beriin, 1976; Chapte; 4. (10) Tallant, D. R.; Moore, D. S.;Wright, J. C. J . Chem. Phys. 1977, 67, 2897. (11) Moore, D. S.;Wright, J. C. Phys. Rev. Lett., submitted. (12) Shannon, R. D. Acta Crystallogr. Sect. A 1978, 32, 751
RECEIVED for review March 28,1979. Accepted June 22, 1979. This research was supported by the National Science Foundation under Grant no. CHE-74-24394 A-1.
