Education, and Welfare, Public Health Service, Center for Disease Control, National Institute for Occupational Safety and Health, Rockvilk, Md. 20852 (June 1975). J. Nagata, Trans. Jpn. Pathol. Soc., 27, 426-434 (1937). J. Nagata, G a m , 38, 174-201 (1944). T. Yoshida et ai., Gann, 35, 272-274 (1941) as reported in PHS 149, Original. R . Kinosita (1936) as reported in PHS 149, Original. H. Maruya, Osaka Igakkai Zassi, 36, 527-528 (1937). N. Ito et al., Cancer Res., 29, 1137-1145 (1969). M. Umeda. Gann, 46, 367-371, 597-602 (1955): Gann, 47, 153-158. 595-599 (1946), as reported in PHS 149, Supplement 11. J. McCann. E. Choi, E. Yamasaki, and B. N. Ames, Proc. Natl. Acad. Scl. USA, 72, 5135 (1975). B. N. Ames, H. 0. Kammen, and E. Yamasaki. Proc. Natl. Acad. Sci., USA, 72, 2423-2427 (1975). C. Burnett, B. Lanman, R. Giovaccini, G. Wolcott, and R. Scala, Cosmet. roxicol., 13, 353-357 (1975). G. A. Campbell. T. J. Dearlove, and W. C. Meluch, J . Cell. Plast., 12, 222-226 (1976). A . S. Tompa, Anal. Chem., 44, 1056-1058 (1972). C. L. Wilson, U.S. Patent 2,921,866, Jan. 19, 1960. D. J. David and H. B. Staley, Analytical Chemistry of the Polyurethanes, High Polymers XVI, Part 111”. Wiley Interscience, New York, N.Y., 1969. M. Genchev and A. B. Atanasova, Much. Tr. Visshiya Med. Inst. Sofiya, 6, 25 (1959); (Chem. Abstr.. 51, 16210 g). E. A . Emelin and T. V. Lipina, U.S.S.RPatent 474, 734, June 25, 1975 (Chem. Abstr., 83, 115455h). E. A. Emelin, Zh. Anal. Khim., 30, 335-339 (Chem. Abstr., 82, 171644). F. Willebourdse, Q. Quick, and E. T. Bishop, Anal. Chem., 40, 1455-1458 (1968). C. R. Boufford, J . Gas Chromatogr., 6 , 438-440 (1968). S. Goldstein, A. A. Kopf, and R. Feinland, Proc. Joint Conf. Cosmet. Sci., 19-38 (1968). 1. Pinter, M. Kramer, and J. Kleeberg, J . Parfum Kosmet., 46, 61-64 (1965) (Chem. Abstr., 63, 1652d). G. F. Macke, J . Chromatogr., 36, 537-539 (1968). C. M. Kottemann, J . Assoc. Off. Anal. Chem., 49, 954-959 (1966). A. Mathias, Anal. Chem., 38, 1931-1932 (1966). E. Rinde and W. Troll. Anal. Chem., 48, 542-544 (1976). S. Udenfriend, S. Stein, P. Bohlen. W. Dairman. and W. Leimgruber, Science, 178, 171 (1972).
Table 111. Duplicate Analyses of Extracts of Hydrophilic Polyether Urethane Foams Foam No.
A
B C D E
F
TDA in Foam, ppm Plate 1 Plate 2 11.3 15.8 15.4 11.2 15.5 10.4
9.4 15.8 15.4 12.2 16.6 11.3
applicable to bis(4-aminopheny1)methane (MDA) and the aliphatic amines bis(4-aminocyclohexyl)methane (reduced MDA) and isophorone diamine. In the case of the reduced MDA, the intensity of the fluorescence is too low to detect less than 100 ppm by the present method. Obviously, the use of larger foam samples can overcome this problem. We have not yet looked for any other amines. Occasionally there is interference from polymeric amines, diethylenetriamine, 4,4’-diaminodiphenylurea, and other amino contaminants. An experienced operator can soon learn to recognize these by the shape, intensity, and position of the spots on the plate. Polymeric amines tend to stay a t their initial position on the plate. Poor separations can usually be overcome by adjustments in the composition of the developing solution.
ACKNOWLEDGMENT We thank S. E. Arquette and M. F. Schroader for their excellent technical assistance and for several valuable contributions to method development.
LITERATURE CITED (1) “Suspected Carcinogens-A subfile of the NIOSH TOXICSubstances List”, H. E. Christensen and T. T. Luginbyhl, Ed., U.S.Department of Health,
RECEIVED for review May 11, 1977. Accepted July 14, 1977.
Ultra-Trace Method for Lanthanide Ion Determination by Selective Laser Excitation Frederick J. Gustafson and John. C. Wright* Department of Chemistry, University of Wisconsin, Madison, Wisconsin 53706
Conventional coprecipitation techniques have been combined with laser spectroscopy to provide a new technique for trace analysis that exhibits excellent selectivity and detectivity. The technlque is here applied to lanthanide Ion determinations by coprecipitatingthe lanthanide in calcium fluoride. A series of different crystallographlc sltes result for the lanthanide Ions because of the charge compensation required for the Ln3+ substituting for Ca2+. Proper ignltion condltions can produce an analytically useful site distrlbution where a single oxygen compensated site predominates. When 0.06 mol CaF, was precipitatedfrom 40-L solutions contalnlng 50 pg/mL to 1.25 pg/mL of erbium, the fluorescence intensity from the single site was directly proportional to erbium concentration with an RSD of 8%. The estimated detection limit is 25 fg/mL of erbium. The technique is shown to be highly Selective and free from interference from other lanthanide ions.
A new technique has been developed for trace analysis by selective excitation of probe ion luminescence (SEPIL). The technique requires the formation of a crystalline host lattice 1680
ANALYTICAL CHEMISTRY, VOL. 49, NO. 12, OCTOBER 1977
in the presence of a spectroscopically active ion whose transitions are sharp and dependent upon the local crystalline environment. The sharp lines permit a tunable laser to excite a specific ion in a specific site with a high selectivity. T h e technique can be used to determine the spectroscopically active ion directly as will be described in this paper or it can be used to determine other ions that enter the lattice and perturb the crystal field levels of the spectroscopically active probe ion as will be described in subsequent papers. In this paper, we demonstrate the use of the technique for lanthanide ion analysis by coprecipitation in a calcium fluoride host precipitate. Although most of the lanthanide ions can be determined by SEPIL, the CaF2:Er3+system was chosen as a representative example because of the detailed studies that have been performed on single crystal samples in this laboratory and elsewhere. This information has provided the basis for a fundamental understanding of the coprecipitation-laser spectroscopy system which promoted the rational development of the analytical technique. When calcium fluoride is precipitated in a solution containing lanthanide ions, the lanthanide ions coprecipitate and enter the lattice substitutionally a t calcium sites. After the
precipitate is aged, recovered, and dried, a narrow bandwidth, tunable dye laser is used to selectively excite fluorescence from specific lanthanide ions at specific crystallographic sites. The resulting fluorescence intensity is linearly related to the concentration of the lanthanide ion in the original solution. T h e observed lower limit of detection of 25 pg/mL erbium was set by laboratory contamination but it is estimated that elimination of this contamination would give a lower limit of 25 fg/mL. T h e method is highly selective t o the lanthanide ion of interest and is capable of multi-ion determinations. There are several inherent advantages in the SEPIL method. The coprecipitation serves as a preconcentration and a separation step as well as providing the ordered environment required for efficient fluorescence. The selectivity of the laser excitation eliminates the complex fluorescence spectra with overlapping peaks and sloping baselines that are observed with conventional excitation sources. Finally, the high spectral intensity of the dye laser and the ability to efficiently deliver this energy to the small area on the sample that is being observed provides the extraordinary sensitivities that have been demonstrated for laser excitation. The detection limit reported in this paper for erbium by the SEPIL technique is lower than that obtainable by neutron activation ( I ) , inductively coupled plasma emission (2),ultraviolet continuum source excitation of solid state luminescence (3),mercury line source excitation of solid state luminescence ( 4 ) , and x-ray excited optical luminescence of erbium in Y P 0 4 ( 5 ) . Shand quoted a detection limit of 0.1 wg/g erbium in CaF2 in the case where erbium is positively identified, by employing x-ray excited optical luminescence (6). Without positive identification (selectivity), the detection limit extrapolated by Shand from a CaF2 sample containing 1 pg/g erbium was 0.0001 pg/g erbium. A number of methods for lanthanide ion determination by luminescence are related to the work described in this paper in that they incorporate the ions in a lattice. None of these methods, however, permits the same selectivity that is demonstrated by the SEPIL technique. Peattie and Rogers (7, 8 ) determined samarium by high temperature sintering with calcium tungstate, and europium and samarium by coprecipitation in calcium sulfate followed by mercury line source excitation. Ozawa and Toryu (3) have determined many lanthanides in yttrium oxide utilizing a dual monochromator spectrofluorometer. Standards were prepared by coprecipitating lanthanide ions in yttrium oxalate which was then ignited to the oxide. Cathodoluminescence has been used by Wickersheim et al. (9) and by Larach and Schrader (10-12) to determine lanthanides in yttrium oxide and zinc sulfide host lattices. Polu6ktov and co-workers have published a series of papers on the determination of lanthanide ions using phosphors by LaOCl (13),YOF (14), CeOs (15), ScV04 ( 1 6 ) , and YV04 ( 4 ) excited by a mercury line source. Fassel and co-workers have determined lanthanides in several types of real samples by x-ray excitation of optical luminescence of sinters of the sample and various oxides (17, 18).
THEORY The spectra of charge compensated systems such as CaF2:Er3+have only recently begun to be understood. Since an understanding of the physical mechanisms responsible for the observed spectra is essential to an understanding of the material in this paper, a brief summary of the directly relevant theory will be published. When an aliovalent ion such as Er3+ is substituted for a cation in a host lattice such as CaF,, a charge compensation is required. Since the defects in the alkaline earth fluorides are of the anti-Frenkel type, the charge compensation in CaF2:Er3+ is provided by an interstitial symmetry relative to a fluoride ion in positions of CdVor CBV given erbium ion. These two sites have been labeled A and
24 22 20 18
'E
16
0
14
0
I2
x
IO
m
8 6 4
2
0
Flgure 1. 4f electron energy levels of trivalent erbium in LaCI,
B, respectively (19). If the fluoride compensation is distant from an erbium ion, the erbium possesses cubic symmetry. Clustering of several erbium-fluoride interstitial pairs can occur leading to a large number of different sites labeled C, C', D ( l a through lk), and D (2a through 2d) according to the pattern of the crystal field splittings (19, 20). T h e relative populations of the sites is determined by a solid state equilibrium that is frozen at a temperature where the defects lose their mobility, typically 550 OC (21). AU of the site distribution work has been performed on large single crystal samples grown in the absence of oxygen and water vapor. These crystals have been labeled Type I1 (22). If the crystals are not grown in an oxygen-free environment, a new series of sites is observed that have been ascribed to oxygen compensation of the erbium Such crystals have been labeled Type I (23, 24). Each of the different sites described above has an unique crystal field which lifts the degeneracy of the res+lrLJ electronic manifolds of the 4fn5s25p' electron configuration. A series of J + crystal field levels is formed for erbium since it is a Kramers' ion (25). T h e ordering and mzgnitude of the splitting is completely dependent upon the immediate ionic environment about the erbium ion and the distinctive splitting pattern serves to identify that particular crystallographic site. T h e shielding effects of the outer 5s25p2closed shells result in the sharply defined crystal field levels of the 4F shell as well as providing ithe lanthanides with remarkably similar chemical properties (26). The total crystal field splitting is seldom more than 200 cm-l because of the shielding. T h e lower electronic manifolds are typically separated by several thousand cm-l as is evident from the Er3+ levels shown in Figure 1 (27). T h e position of these manifolds is therefore approximately independent of type of crystallographic site or type of matrix. Since the positions of the electronic manifolds change completely between different lanthanides (27),it is easy to qualitatively identify a lanthanide by looking a t transitions between manifolds. Electric dipole transitions will occur between manifolds within the 4F configuration if the crystal field mixes even parity configurations into the 4f" configuration (28). For the cubic site in CaF2,such mixing is not permitted and no optical transitions have been observed for this site in CaF2:Er3+.The mixing is greatest for cases where the lattice ions interact covalently with the lanthanide ion as occurs in the case of oxygen compensated sites. T h e enhanced mixing increases the radiative transition rates. ANALYTICAL CHEMISTRY, VOL. 49, NO. 12, OCTOBER 1977
1681
The lanthanide ions amenable to SEPIL will include those having electronic manifolds accessible for laser excitation and which fluoresce. The fluorescence quantum efficiency depends upon the nonradiative relaxation rates that can quench the fluorescence. If the separation between the manifold of interest and the next lower energy manifold is smaller than about 2400 cm-', the energy difference can be efficiently lost t o the lattice through multiphonon relaxation (29). A large gap between levels is therefore required for a level to emit efficiently. A second quenching process can occur if two ions can exchange energy in such a way that the total energy of the pair is decreased, with the difference in energy being dissipated by lattice phonons. T h e level populated initially becomes quenched. This quenching process occurs very efficiently in charge-compensated systems where two or more ions can cluster with each other. The presence of this process has been used as a test of whether a given site contains a single erbium ion or a cluster of ions (30). At low concentrations of erbium, quenching of this type is not significant. With the criteria listed above it is apparent that Pr, Nd, P m , Sm, Eu, Gd, T b , Dy, Ho, Er, T m , and Yb can all be determined by the SEPIL technique.
EXPERIMENTAL Apparatus. A complete description of the experimental apparatus used to study precipitates is given by Miller et al. (31). A brief description of the essential instrumental features is given here for completeness. A pulsed nitrogen laser with a ca. 10-ns pulsewidth pumps a tunable dye laser that has a measured bandwidth of 0.012-0.02 nm with coumarin 1 dye. The spatially narrow dye laser beam is focused onto the sample holder housed in a cryogenic refrigerator at 13 K. The sample holder is a copper block with a series of shallow holes drilled in the face, each holding a different sample. These holes are arranged vertically so that a new sample can be introduced into the fixed optic axis by simply lifting the cryogenic refrigerator and inserting an aluminum spacer to keep it at the proper predetermined height (31). Fluorescence from the samples is first focused onto a mechanical chopper adjusted to block sub-microsecond broadband scatter from the sample that could damage the photomultiplier. The fluorescence is then directed to a 1-m monochromator with a bandpass between 0.02 and 0.3 nm that is capable of monitoring specific fluorescence lines from individual lanthanide ion sites. A 0.25-m monochromator with a 6.6-nm bandpass is used to monitor many fluorescence lines from a series of sites. The 1-m monochromator is always employed unless the 0.25-m monochromator is specified. A conventional gated integrator and strip chart recorder provide the intensity readout. Besides being able to selectively monitor fluorescence from individual sites, it is possible to selectively excite ( 1 9 ) individual sites by simply tuning the dye laser to an absorption transition of that site. This procedure results in single site spectra which are free from background signals or interference from other site or ion spectra (19). Two methods are used to obtain quantitative relative intensities of the same sites in a series of samples. In the first method, either the 0.25-m or the 1-m monochromator is set for a specific fluorescence wavelength while the dye laser is scanned over a range of wavelengths to obtain an excitation spectrum of one sample. Excitation spectra are obtained under the same conditions for the entire sample series as the dye laser power is being constantly monitored for gross changes. Relative peak areas or peak heights are then used as the relative intensities between samples. With the second intensity measurement procedure, both the dye laser and the 1-m monochromator are tuned to specific absorption and fluorescence transitions of a single lanthanide ion site and the resulting steady-state intensity is recorded for the series of samples. Again, the dye laser power is constantly monitored for gross changes. The relative heights of the steady-state intensites are used directly as relative intensities. Reagents. Coumarin 1 dye (7-diethylamino-4-methyl coumarin) at a concentration of 1 mg/mL in absolute ethanol was used in the dye laser to excite the H(4F5,z)manifold of erbium and the I(5F1)manifold of holmium. The erbium G(4F712)and the terbium A('D4) manifolds were excited with the combination 1682
ANALYTICAL CHEMISTRY, VOL. 49, NO. 12, OCTOBER 1977
of coumarin 30 ( 2 mg/mL in methanol) and coumarin 1 in a 1:l mixture. Approximately 0.03 M solutions of erbium, holmium, and terbium chlorides in M nitric acid (AR) were prepared from the hydrated chlorides (99.9% with respect to other rare earths, Research Organic/Inorganic Chemicals, Belleville, N.J.) and standardized by determining chloride according to the Fajans method. Lanthanide solutions of lower concentration were prepared by dilution into M "OB. To minimize introduction of lanthanides into reagents or samples by contaminated glassware, all glassware was rinsed twice with 3 M H N 0 3 and then rinsed with distilled, deionized water having a specific resistance greater than 10 MR cm. All other chemicals used were analytical reagent grade and used as supplied. All solutions were stored in plastic bottles except the 3 M "OB. Solutions of 1.32 M calcium nitrate in M HN03 were prepared by weighing and diluting Ca(N03)z.4Hz0(J. T. Baker Chemical Company) while 1.2 M ammonium, potassium, and sodium fluoride solutions were prepared by weighing and diluting NH4F (J. T. Baker), KF.2H20 (Fisher Scientific Company), and NaF (J. T. Baker). The calcium and fluoride solutions were standardized against each other by a precipitation pH titration of the calcium solution by the fluoride solution. The end point was measured by determining the point of maximum slope in the pH vs. milliliters of fluoride added curve. The calcium and fluoride solutions were accurate to within 0.2% of each other. The fluoride solution was then standardized by titration against a standard calcium solution prepared from CaC03and HN03. The value agreed within 0.3% with the nominal concentrations determined for the calcium and fluoride solutions. Procedure. Any deviation from the following sample preparation procedure will be noted in the Results section. Five mL of 1.32 M Ca(NO& are pipetted into a 250-mL beaker, followed by the addition of a solution of the lanthanides to be studied. The M "OB. A 50-mL volume is increased to 40 mL by adding plastic syringe is filled with 0.3 M NH,F and inserted into an automatic syringe calibrated to deliver 40 mL in 4 min 15 s. The fluoride flows through rubber tubing and down the side of the beaker to the calcium solution which is being vigorously stirred. After adding 40 mL of the fluoride solution t o precipitate 91 % of the calcium, the calcium fluoride precipitate is recovered by one of two methods, each giving identical results. One method involves centrifuging the precipitate for 1 h at 2700 rpm in polycarbonate tubes. Lower speeds do not cause settling of the small calcium fluoride particles. The precipitate is transferred to a glass vial, dried at 80 O C for at least 1 h or overnight in a desiccator, and ground in an alumina mortar that was rinsed with 3 M HN03 and dried. With the second recovery technique, the precipitate is filtered through a 0.45-pm pore size membrane filter (No. GA-6 Metricel, Gelmen Instrument Co., Ann Arbor, Mich.). After drying on the filter paper in air for a few hours, the powder is placed between pieces of weighing paper and broken into fine particles with a spatula. Precipitates recovered by both techniques are ignited in small platinum crucibles in a Lindberg Lab Box Furnace (Model 51844, rise time to temperature is 10 min, or Model 51748, rise time to temperature is about 1 h, Lindberg Co., Watertown, Wis.). Ignition temperature is an important experimental parameter and must be specified whenever experimental results are reported. After ignition, the powder is finely ground in an alumina mortar. The powder is then placed in a hole on the sample holder and pounded into a pellet with the flat end of a rod that is supported so that the face of the pellet will be smooth and parallel to all the other samples. Tape can be placed over the holes already containing a sample to prevent contamination during the formation of other pellets. Each sample requires about 10 mg of precipitate. The temperature of a typical series of samples was measured by determining the Boltzman distribution of crystal field level populations to determine if good thermal contact was being made between the precipitate and the sample holder. The method has been described by Tallant and Wright (19) for single crystals of CaF2:Er3*and was used on the CaF:Er3+ precipitates. All the samples during one run were found to be at the same temperature within the standard deviation of the temperature measurement technique. With a Pyrex window directly in front of the samples t o absorb some of the infrared room radiation, the sample
A
rrm
L"
445
1
446
-
1
1
Relative
'
447 448 X (nm)
'
"
449
450
-
B Site
Figure 2. Z(41,5,2) H(4F,,,) excitation spectrum of unignited CaF,: 0.1 mol % Er3+obtained by monitoring E(4S312) Z(4115/2)fluorescence at 550 nm with the 0.25-m monochromator
temperature was measured t o be 13 f 1 K from four samples, which compares favorably with the 12.2 0.4 K obtained with a single crystal. A method for measuring temperature differences of less than 1 K between samples is given in the concentration dependence section. X-ray Powder Diffraction. X-ray diffraction patterns of precipitates were obtained on a Norelco X-ray Powder Diffraction instrument (North American Phillips Co.). The x-ray source was copper K a radiation from a tube operated at 35 kV and 18 mA. Single crystals of anhydrous calcium fluoride serving as a standard were obtained (Optovac Inc., North Brookfield, Mass.) and ground in an alumina mortar. Exposure time was 4 h.
*
RESULTS A N D D I S C U S S I O N C h a r a c t e r i s t i c s of Unignited Precipitates. Figure 2 is a n excitation spectrum with broadband monitoring of fluorescence of unignited CaF2:0.1 mol % Er3+showing lines from all the sites present. This spectrum was broken into the individual site spectra by the selective excitation technique and the A, B, C, C', and LL sites of single crystal samples (19, 20) were observed while the D 1 and D2 cluster sites were not observed. In addition the E l , E2, E3, and E4 sites, labeled E sites in Figure 4,were found in the precipitate spectra. They are believed to be single erbium sites because no energy transfer could be observed (30). The F site is also unique to t h e precipitate. I t is evidenced by a broadband excitation continuum about 1.2 nm wide, indicating that F site erbium ions are in a large number of sites that differ slightly in their local site symmetries. Because the dye laser has a narrow bandwidth of ca. 0.02 nm, it is possible to excite in a narrow region of the F site continuum and observe fluorescence line narrowing in the resulting fluorescence spectrum. A continuum of different fluorescence lines was produced by incremental excitation wavelength changes within the 1.2 nm line width indicating a continuum of different sites. Thus, the F site is unique in that it cannot be characterized by a unique series of crystal field levels. This behavior is the result of a disturbance in the short range lattice order caused by such processes as the entrapment of large molecules or ions such as H 2 0 , NH4+, or NO3- a t random positions in the lattice, strain broadening of the crystal field around the erbium ion, or the presence of erbium fluoride unit cells in the CaF2 lattice. T h e F site does not involve clusters of erbium ions, however, as determined by a search for energy transfer (30). The presence of so many different sites in unignited calcium fluoride precipitates makes their use for determining lanthanide concentrations possible only if the site distribution stays the same as the lanthanide concentration is varied. This has been found not t o be the case. As Figure 3 shows, when the total erbium concentration decreases, the B site intensity drops very steeply. This steep decline is partly due to three phenomena. T h e first is a change in site distribution. T h e LL site actually becomes more intense than the B site at lower erbium concentrations, while the F site was shown to decrease in intensity less steeply than the B site. In addition, the solid state equilibrium between nonfluorescing cubic sites, in-
o LLSite
I-
p
--
5-
'0
01 02 03 04 05 Erbium Concentration ( m o l 70)
Figure 3. Dependence of the B and LL site intensities on total erbium concentration in mol % Er with respect to Ca in unignited CaF,. H, 445.8 nm for LL and 446.44 nm for B while Excitation was 2 monitoring E 2 fluorescence at 539 nm with the 0.25-m monochromator
--
terstitial fluorides, and sites such as A and B consisting of an erbium ion charge compensated by a nearby interstitial fluoride (ErF) will favor dissociation of the E r F sites into the cubic site and a free interstitial ion a t low erbium concentrations. For example, if the ratio of cubic t o E r F sites is assumed t o be unity at a total erbium concentration of 0.05 mol ?&,a 10-fold decrease in dopant concentration will result in only a 6-fold decrease in the cubic site population, but a factor of 34 decrease in the E r F concentration. The detection limit for the erbium B site in unignited calcium fluoride exciting Z H and monitoring E Z (Figure 1) is 0.005 mol % or 1.25 pg/mL Er3+. This high detection limit, coupled with the nonlinearity of the calibration curve and the large number of different sites present renders the unignited precipitates unsuitable for analytical work. C h a r a c t e r i s t i c s of Ignited Precipitates. When erbium doped precipitates of calcium fluoride are ignited a t different temperatures and cooled slowly in the furnace, the D1 and D2 cluster sites (19) known from single crystal work appear, while the other sites observed become less important. T h e D sites appear a t 350 "C and 400 "C ignition temperatures for CaF2:0.1 mol % Er3+as shown in Figure 4c and d, while before 350 "C, the site distribution remains relatively unchanged (Figure 4a and b). At 500 "C and 700 "C, all the lines from these sites disappear and are replaced by the lines of an erbium site which is associated with oxygen compensation and labeled the G1 site. In addition to these large scale changes in t h e spectrum, there is also a sharpening of all of the line widths as the precipitates are heated to higher temperatures. This sharpening is consistent with the explanation that the calcium fluoride lattice is becoming more perfect through thermal aging (see Figure 4e and f). Several processes may contribute to the perfecting of the lattice. All calcium fluoride precipitates contain ca. 3.5% water (32)and a slow loss of moisture occurs with ignition up to 400 "C (33). A 0.5% weight loss after heating to 105 "C can occur through partial conversion to Ca(OH)F, while at 400 "C, a conversion of about 2.5% to the oxide may occur (34). In addition, microscopic strains may be relieved as the microcrystals are annealed. T o confirm that even strong ignition left the precipitate substantially in the form of CaF,, x-ray powder diffraction
-
+
ANALYTICAL CHEMISTRY, VOL. 49, NO. 12, OCTOBER 1977
1683
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Figure 5. Effect of post-ignition cooling rate on the G site distribution in CaF,: mol % E?'. Z H excitation spectra, monitoring E Z fluorescence at 548 nm with the 0.25-m monochromator. Ignited 3 h at 770 O C IO 000
I1
r Relative lnlenslly
looot Site8 GI c-
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Figure 4. Site distribution in CaF,:O. 1 mol % Er3+ at several ignition temperatures. Z H excitation spectra, monitoring E Z fluorescence at 550 nm with t h e 0.25-m monochromator. Ignition times were 3 h. Precipitates were cooled slowly after ignition. (a) 155 O C ignition, muttiply intensity by 33 to correct for recorder sensitivity. (b) 245 OC ignition, multiply intensity by 10. (c) 350 O C ignition, multiply intensity by 76. (d) 400 O C ignition, muttiply intensity by 76. (e)500 O C ignition, multiply intensity by 45.6. ( f ) 700 O C ignition, multiply intensity by 190 was performed on a ground single crystal of CaF2 and on precipitates of CaF2:10-4mol 70Er3+ ignited a t 1100 "C for 3 h and CaF2:0.5 mol 70Er3+ ignited for 3 h at 700 "C. All three samples had exactly the same powder diffraction pattern with no extra lines appearing in any pattern. The G1 site that becomes prominent in precipitates heated above 500 "C is thus not the result of any bulk change in the stoichiometry of the CaF2 lattice. Although the G1 site is not observed in oxygen-free (Type 11) single crystals of CaF2:Er3+, i t is observed in Type I single crystals which were grown in an oxygen containing atmosphere. The site has been identified as an oxygen compensated erbium where an oxygen replaces a normal lattice fluoride ion with trigonal symmetry relative to the erbium (35). The wavelengths of the G1 site spectrum shown in Figure 4f agree to within 0.03 nm of the wavelengths given by Leung (23) for single crystals of CaF2:Er3+. At still higher ignition temperatures, three additional sites become important (see Figure 5b). These sites are called G2, G3, and G4 and are also thought t o involve oxygen compensation of the erbium ion. When the ignition temperature is varied from 430 OC to 1100 OC for a series of precipitates (CaF2:10-4mol Er3+),and they are cooled slowly for 2 h to room temperature, the order of appearance of sites is GI, G2, G3, and then G4 (Figure 6). T h e G1 site reaches maximum intensity first followed by the G2, G3, and finally the G4 site. .Thus a t higher ignition temperatures, the G4 site becomes more intense a t the expense of the G1 site. In mol 70 Er3+ precipitates, the four G sites are the only ones observed b u t at lo-' mol 70 Er3+,the H site is observed as described below. If an ignited precipitate is cooled quickly to room temperature by removing i t from the furnace after ignition, the G4 site intensity increases dramatically, while the G1 site becomes less important as shown in Figure 5 . Rapid cooling 1684
ANALYTICAL CHEMISTRY, VOL. 49, NO. 12, OCTOBER 1977
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lgn~tion Temporalwe
la C)
Figure 6. Dependence of G site distribution on ignition temperature in CaF,: 10-4 mol % Er3+. Fixed excitation and fluorescence wavelength technique. Ignited for 3 h and cooled slowly. The size of t h e symbol indicates the error in measuring a single intensity, not errors associated with precipitation and ignition reproducibility. G1 site, excite 449.22 nm, monitor 546.06 nm. G2 site, excite 450.48 nm, monitor 547.84 nm. G3 site, excite 452.61 nm, monitor 552.64 nm. G4 site, excite 454.57 nm, monitor 549.88 nm also increases the G2 and G3 site intensities. When a precipitate is a t an elevated temperature, a site distribution equilibrium is established that is characteristic of the high temperature. Rapid cooling would cause the high temperature site distribution to be frozen in the lattice, while slower cooling allows the ions time to re-equilibrate to the distribution characteristic of a lower ignition temperature and rapid cooling. T o test these conclusions, a precipitate which had been rapidly cooled was reignited to 770 OC and cooled slowly. The G1 site increased lo3 times with respect to the G4 site, but the result is still more G4 site than in a n equivalent but fresh precipitate heated to 770 "C and cooled slowly. This indicates that the site distribution is not completely reversible. If the ignition time is increased beyond the usual 2 or 3 h for a more concentrated precipitate, CaF2:10-2 mol % Er3+, the H site appears (Figure 7 ) . When the ignition time increased to 24 h, the H site was shown to grow a t the expense of both the G1 and G4 sites. The H site does not follow the pattern of the four G sites because i t does not appear in precipitates less concentrated than about W 3mol % Er3+even after a 24-h ignition period. This concentration dependence suggested that the H site was an oxygen compensated cluster
A
a) 5 7 h r Ignition a'
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pa, 0 :
y, '
G2 d
i
L
448 449
1 1
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'
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450 451 b)
i
'
u
452 453
I i
L
454
455 456
446
I hr Ignition
447
448 449
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G4
m
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1
I 448 449
I
450 451
452
453
I
X(nm)
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Flgure 7. Appearance of t h e H site in CaF2:10-' mol YO Er3+ with prolonged ignition at 1000 "C. Z H excitation spectra, monitoring E 2 fluorescence at 549 nm with the 0.25 m monochromator. (a)
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446
L
454 455 456
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448 4 4 9
45C
451
452
453
454
455
446
h(nm1
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Figure 8. Sites present in CaF2:10-4mol % Er3+ single crystal after ignition. Z H excitation spectra, monitoring E Z fluorescence at (a) 540 nm and (b) 560 nm with the 0.25-m monochromator. Cooled slowly
5.7-h ignition, slow cooling. (b) 1.0-h ignition, slow cooling
site, but the site exhibited no energy transfer. Phillips and Hanlon (36) and Bontinck (37) have partially converted CaF, to CaO by igniting a t around 1000 "C for extended time periods. They verified the presence of a separate CaO phase by x-ray powder diffraction. We coprecipitated erbium in CaC204and ignited a t 1000 "C for 3 h assuring complete conversion to CaO. T h e only site present in CaO:Er3+ was the H site and a perfect match was noted between the spectra for the H site in CaO and CaF,. Two models have been proposed for oxygen compensated sites. In one case the erbium ion is surrounded by seven fluorides and one oxygen, all on the corners of a cube with erbium in the center (38). T h e second model involves an erbium ion a t the center of a cube with four oxygens, one fluoride, and three vacancies a t the corners. Both sites are fully charge compensated and both have CSvsymmetry. Yang e t al. (38)proposed these two configurations on the basis of the site symmetry and the hyperfine structure in the E P R spectrum of oxygen compensated CaF,:Gd3+ single crystals. Reddy et al. (39, 40) proved the existence of both sites using ENDOR of "F, 'H, and " 0 on crystals of CaF2:Yb3+. In both studies a more prolonged heating time resulted in more of the four-oxygen site. T o verify that all four G sites are intrinsic sites and not characteristic only of the precipitate, a single crystal of CaF2:10-4mol % Er3+(Optovac, Inc.) was ignited a t 1000 "C under vacuum for more than 5 h. Ignition under vacuum prevented total conversion of the erbium sites to four-oxygen compensation. T h e Z(411j/2) H(*FSl2)excitation spectrum of the crystal after ignition in Figure 8 clearly shows all four G sites as well as the fluoride-compensated A and B sites. T h e G1 and G4 sites correlate with the E P R and optical work by Zverev and Smirnov who found two sites with Csv symmetry (35, 41). They report seven wavelengths for one site that agree with those we observe for the G1 site and twelve for the other site that agree with those of the G4 site, meaning t h a t the G l and G4 sites have CSvsymmetry. They also list three wavelengths that agree with the G2 site and four wavelengths t h a t agree with the G3 site for fluorescence Z which they were not able to assign transitions from E to any specific site. T h e conversion of fluoride compensation to oxygen compensation probably involves the reaction of trapped water within the precipitates with fluoride interstitials forming free H F and leaving oxide ions in anionic lattice positions. In the analytical work we report, ignition conditions were chosen that
447
-+
a1
447
448
449
450
451
452
453
4 5 4 455
456
C)
Figure 9. Excitation spectra of ignited CaF,: mol % Er3+:10-3mol % Ho3+. (a) Monitoring the erbium G1 site E --* Z fluorescence at 546.06 nm (b) Monitoring holmium E Z fluorescence and erbium E Z fluorescence from all sites present at 552 nm with the 0.25-m monochromator. (c)Monitoring a holmium oxygen compensated site fluorescence at 555.64 nm
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produce only the G1 site and we have used the intensity of transitions from this site to monitor erbium concentration. Ignition of any other lanthanide in calcium fluoride under these same conditions should produce single, oxygencompensated sites which have sharp line transitions that can be selectively excited. The energy levels of europium, holmium, and erbium shown in Reference 27 clearly indicate that spectroscopic separation of europium from holmium and erbium should be simple because there are few overlapping transitions. The very similar energy level diagrams of holmium and H(4F5,z) and erbium, particularly the Z(4115/2), E(4S3/2), manifolds of erbium and the Z(518),E(5S2),and I(5F1)manifolds of holmium, suggested that a precipitate doped with these two lanthanides would be a satisfactory test of the selective excitation system. Three excitation spectra (Z H, Er3+, and Z I, Ho3+) were obtained of a precipitate containing both holmium and erbium ignited at 1000 "C. In Figure 9b, the 0.25-m monochromator is used to monitor all the sites present in the precipitate. The excitation transitions
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ANALYTICAL CHEMISTRY, VOL. 49, NO. 12, OCTOBER 1977
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for both ions are sufficiently sharp that no overlap of excitation lines occurs, and selective excitation of either erbium or holmium alone is possible by simply tuning the dye laser to an absorption transition of the ion of interest. By monitoring a narrow holmium fluorescence line with the 1-m monochromator, the excitation spectrum will show only holmium lines from one specific site (Figure 9c). Similarly, the erbium G1 site can be selectively monitored, and Figure 9a shows its excitation spectrum free of any holmium interference. Thus for any case where one excitation line per lanthanide ion is clear of interferences from other lanthanides, monitoring the fluorescence with a large fluorescence bandpass would be sufficient to ensure selective excitation and would take advantage of the smaller f / z of the 0.25-m monochromator to increase signal to noise. The selectivity of excitation is limited by the fact that part of the dye laser pulse striking the randomly oriented calcium fluoride particles is converted into a broadband continuum at wavelengths which can in turn excite fluorescence in other ions or other sites of the same ion in the crystal. For example, in a precipitate of CaF2:0.1mol 70 Er3+,the laser was tuned to 447.1 nm (see Figure 9a), which is not an excitation wavelength of the erbium G1 site, and a G1 site fluorescence spectrum was recorded with an intensity of about 0.1% of that excited by the Z1 HI excitation line at 449.21 nm. This would cause a problem if a sample contains several lanthanide ions at much different concentrations and with closely matched energy levels. I t would therefore not be possible to use a low resolution monochromator under these conditions. T h e use of a higher resolution monochromator to monitor specific fluorescence lines of particular ions would eliminate this problem. Reproducibility a n d Accuracy. T h e average relative standard deviation (RSD) of eight separate measurements of a total of 34 samples from the same precipitation was 8 % , measured by the fixed excitation, fixed fluorescence wavelength technique on the CaF2:Er3+ G1 site. When four separate precipitates were prepared and ignited a t the same time, the RSD of the G1 site intensity of the four was again 870, which is consistent with the 10% precision reported by Ozawa and Toryu (3) for the solid-state luminescence determination of lanthanides in Y203. With an internal reference, Tb3+,compensating for the imperfect precipitate pellet surface, the RSD was lowered to 5.5% based on one series of five measurements. The above measurements were all made on CaF2:10-4 mol 70,a moderate erbium concentration. For good reproducibility it is essential that the surfaces of each precipitate in the sample holder be as parallel and as flat as possible since most of the fluorescence from a sample originates in erbium ions near the precipitate surface because scattering by sample particles attenuates the laser. Thus, if the precipitate sample in the sample holder is thick enough so that the laser is effectively 100% attenuated before reaching t h e bottom of the sample, the fluorescence intensity out of the sample is independent of its exact thickness. The dependence of the total relative fluorescence from a sample as a function of the distance the laser penetrates into the sample was experimentally determined in the following manner. A series of precipitates was run in which a weighed CaF2:10-' mol % Er3+ sample layer on the bottom of the sample port was covered by a weighed layer of CaF2 containing no erbium. T h e intensities of these precipitates were compared with a sample consisting entirely of CaF2:10-2mol 70 Er3+. Thus their relative intensities contain information on the amount of laser light scattered from the sample before it reaches the part of the sample containing erbium and fluorescence scattered before it leaves the sample. The experimental values are given in Figure 10 along with an expression for the depth dependence of the scatter. We assumed scatter to be geometrical
loo!
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A N A L Y T I C A L CHEMISTRY, VOL. 49, NO. 12, OCTOBER 1977
01 0 2 0 3 0 4 0 5 0 6 Loser Path Depth ( m r n )
Figure 10. Relative fluorescence intensity from a sample as a function of the thickness of a layer of calcium fluoride on top of the sample
loot
501
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'0
t
Fluorescence Relative 2o Intensity
a) b)
2t/
' 0805
0
Excite Excite
02 05
002 Ob5
Z It o G I Z It o G 2
5
.2
Erbium Concentration (mol %)
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Figure 11. Concentration dependence of erbium fluorescence intensity G1 excitation at 488.39 nm, at high erbium concentrations. (a) Z, monitoring E Z fluorescence at 546.06 nm. (b) Z1 G2excitation Z fluorescence at 546.06 nm at 483.98 nm, monitoring E
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(particles bigger than the wavelength of light), having an exponential depth dependence e*' where x is the distance into the sample in the direction of the laser and a is the scattering parameter. The sample depth was always chosen to completely attenuate the laser beam within the sample. Concentration Dependence. In a series of precipitations the erbium concentration was varied from 50 pg/mL (2 X mol 70Er3+)to 125 kg/mL (0.5 mol % Er3+)in 14 steps, while each precipitate contained mol 7'0 Tb3+ which served as an internal reference. The precipitates were all ignited at 700 "C for 2 h. Fixed excitation, fixed fluorescence wavelength intensity comparisons of both erbium and terbium were made between four samples at one time. Erbium was excited a t 488.39 nm (411512 4Fj/2)and monitored a t 546.06 nm (4S3,2 4115,2).Terbium was excited at 487.34 nm (7Fs 5D4)and monitored at 540.44 nm (5D4 'F5). Then one sample was kept the same and three new samples were added in order to relate the relative intensities of all the samples together, until the entire series had been run. T h e log-log plot of erbium fluorescence intensity is linear in erbium concentration between 50 pg/mL and 1.25 pg/mL with and without the terbium internal reference. The erbium intensity curve is still mol % Er3+)although i t usable above 1.25 pg/mL (5 X is nonlinear as shown in Figure 11. The linear portion of the two calibration curves was fit to a line by taking the logarithm of the concentration and relative intensity values and performing a least-squares analysis. The antilog of the equation determined by least squares was taken after assuming a y intercept of zero to obtain Equation 1,
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I(Er") 0: [Er3+]0.90 (1) for the dependence of the erbium relative intensity, I(Er3+). on erbium concentration. When the erbium intensity was ratioed to the terbium intensity from the same sample, the equation Z(Er3+) a
[Er3+]0.99
I(Tb3+)
was obtained. The linear correlation coefficient, r , for both logarithmic data sets used to determine the lines of Equations 1 and 2 was calculated to be 0.99, indicating that both of the data sets are truly linear. T o determine the deviation of the actual data from the least-squares curves in Equations 1 and 2, the standard deviation of the relative deviation of the data from the curves (RSD) was calculated to be 13% for erbium intensity alone and $70for erbium intensity with terbium as an internal reference. The value for the RSD of the erbium intensity alone is larger than the RSD obtained for the reproducibility because all the samples could not be measured in a single group a t the same time. When CaF2is precipitated by the normal procedure without adding any erbium, a background erbium concentration of 25 pg/mL Er3+ (CaF2:10-' mol % Er3+)is observed arising from contamination in the laboratory. The origin of this residual contamination has not been investigated. A minimum detectable concentration of 25 fg/mL can be estimated from the S / N ratio present a t the contamination limit if the calibration curve is linear over the entire range of concentrations, and the 0.25-m monochromator is used to monitor fluorescence. This corresponds to CaF2:10-10mol % Er3+. Since only 7 mg of precipitate in the sample holder would be necessary to obtain a spectrum, and only ca. 20% of the total area of the precipitate is sampled by the dye laser beam, the absolute amount of erbium fluorescing a t the estimated minimum detectable concentration of 25 fg/mL is 3 fg. The detection limit could be further reduced by precipitating less calcium fluoride while keeping the erbium concentration in solution the same. The bending of the calibration curve above 1.25 pg/mL Er3+ (CaF2:5X mol 70E$+) shown in Figure 11is nearly always observed when lanthanides are determined by solid-state luminescence. T h e explanations proposed for this behavior include the formation of dimer or cluster sites of rare earths a t higher concentrations (30), the inner filter effect caused by absorption of the exciting light by a lanthanide in high concentrations ( 3 ) , quenching of fluorescence from a lanthanide ion by a lanthanide or other ion (42),and quenching of fluorescence by energy transfer to another lanthanide ion (30). T o simply eliminate the high concentration problem, the original sample can be diluted or more calcium fluoride can be precipitated. I n a n effort to determine the exact cause of the bending of the experimental curve, the following mechanisms of erbium intensity loss were explored: self-absorption of fluorescence, incomplete coprecipitation of erbium a t high concentrations, incomplete conversion from fluoride to oxygen compensated sites, formation of Er-Er or Er-Tb cluster sites, appearance of other sites such as the other G sites or the H site, fluorescence quenching, and the inner filter effect. Selfabsorption of fluorescence cannot be a problem in the case we have studied because the erbium fluorescence transition Z3 (4S3,2 4115J2) and the Z3 population monitored is E l of the Z1 population. A study of the GI site a t 13 K is 4 X of the completeness of coprecipitation in the coprecipitation section showed that at an erbium concentration as high as 25 Kg/mL corresponding to CaF2:0.1mol % Er3+,less than 0.5% of the erbium originally present remained in solution after precipitation. This means that erbium distributes very fa-
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vorably in CaF2 and that incomplete coprecipitation is not the mechanism of calibration curve bending. The conversion of fluoride compensated sites to oxygen compensation is very efficient. No fluoride compensated sites were observed at maximum instrumental sensitivity after CaF2:0.5 mol % Er3+ was ignited to 700 OC for 2 h and cooled slowly. Thus all the erbium must be oxygen-compensated or in the nonfluorescing cubic site. The cubic site has been shown to decrease in concentration when lanthanide doped single crystals are ignited in the presence of water vapor. This experiment was performed by Yang et al. (38)on CaF:!:Gd3+using EPR. When CaF, precipitates, about 3.5% water is coprecipitated (329, which would be enough to convert even 0.5 mol % Er3+to oxygen compensation. In addition, the precipitates are always ignited in air, another oxygen source. The conclusion is that the conversion to oxygen-compensated sites is complete for the concentrations studied here. T h e other oxygen-compensated erbium sites, G2, G3, G4, and H, have been observed in the CaF2:0.05 mol % Er3+precipitate used in this study, but each site is a t most a factor of 160 000 less intense than the G1 site. The H site appears at 0.05 mol % Er3+under the ignition conditions used in this experiment, but then decreases with increasing erbium concentration. Thus the appearance of the H site at high erbium concentrations cannot explain the increasingly great deviations from linearity in the erbium calibration curve. By monitoring all possible tranZ for erbium with the 0.25-m monochromator sitions E while Z H excitation spectra were taken, no other new sites were observed, which rules out the possibility of fluorescing Er-Er or Er-Tb cluster sites forming. Further confirmation of this can be seen in Figure 4d and e. As the D cluster sites that predominate at the 400 "C ignition temperature change to the oxygen-compensated G1 site a t 500 "C, there is no evidence of a n intermediate oxygen-compensated erbium cluster site. If fluorescence quenching were the curve bending mechanism, the excited state lifetime of the level being monitored would decrease (42). T h e lifetimes of the 5D4manifold of manifold of erbium were measured terbium and the E(4S312) for a series of highly concentrated CaF2:Er3+precipitates to 0.5 mol % Er3+. The system used containing from 2 X to measure the lifetimes has a reproducibility of about f 2 % . T h e results showed that there is no appreciable change in either the erbium or terbium lifetime. Since the 2 X mol % Er3+ precipitate is in the linear region of the calibration curve and its lifetimes are essentially the same as those of 0.2 mol % Er3+,we conclude that fluorescence quenching does not cause the calibration curve nonlinearity. T o study the inner filter effect, two excitation transitions were pumped to give the erbium concentration dependence shown in Figure 11. In this figure, the fluorescence intensities of CaF55 X mol 70Er3+for the two transitions were made equal for purposes of illustration. The only difference between the two curves is that Z1 GI is strongly absorbing and Z1 G2 is not. If the bending is caused by the attenuation of GI the laser by a strongly absorbing transition, the Z1 transition should show more pronounced bending than the Zl G2 transition. This effect is shown in Figure 13. However, the oscillator strength of the Z1 GI transition of the G1 site in CaF2:Er3+is ca. 50 times that of the Z1 G2 transition. The Z1 G2 transition is thus almost nonabsorbing compared with the Z1 GI transition but the calibration curve in Figure 11 still shows bending. This indicates that an additional unidentified mechanism is operative in causing the nonlinearity a t high concentrations. T h e transitions excited in Figure 11 originate from the ground Z1 crystal field level which is highly populated a t 13 K. Transitions originating in Z2 would have a lower absorption
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Table I. Percent Erbium Coprecipitated Estimated by Reprecipitating the Supernate Separate supernate 5 days Separate supernate immediately after precipitation after precipitation Concentration of Er3+ Ca2' Er3+ Ca *+ erbium original precipitated, coprecipitated, precipitated, coprecipitated, precipitation, % % % % 4 m L 97.2 0.25 25 99.2 97.6 50 0.25 50 99.5 75 96.9 75 0.25 99.9 100 99.3 100 0.25 92.9 25 25 50 99.6 95.5 25 50 75 99.1 93.5 25 75 99.6 100 95.6 100 25 due to the small Z2 level population and could be used to determine erbium at high concentrations because the absorption of the laser would be much smaller from Z L than from Z1. But because the Z2 population is only about 0.4% of the Z1 population for the G1 site at 13 K, the Z2 population will be a very sensitive function of sample temperature. The Z2 population can increase twofold with a one-degree increase in sample temperature at 13 K. Since the temperature difference between different samples in the same holder can be as much as 1 K, the Z L transitions are less satisfactory for analytical work, although they do provide a sensitive means of measuring the temperature difference between samples. Coprecipitation of Erbium. A set of experiments were performed to determine whether coprecipitation of erbium in calcium fluoride is favorable and whether the fraction of erbium coprecipitated depends on the total amount of erbium present. The degree of erbium coprecipitation when different fractions of calcium are precipitated was also investigated. Precipitations were conducted by the normal procedure, where enough ammonium fluoride was added to precipitate 2570, 50%, 7570, or 100% of the total calcium present. The precipitates were centrifuged immediately, and the supernate was decanted and centrifuged again to remove most of the calcium fluoride particles. The clean supernates were precipitated in the normal manner and compared spectroscopically with a standard CaF2:10-' mol % E r i + precipitate which was prepared and ignited at the same time as the rest of the precipitates. The results are given in Table I. At 25% precipitated, 97% of the erbium had already been incorporated into the calcium fluoride lattice and removed from the solution, which is a very favorable distribution. This distribution is not affected if the precipitate is aged for five days, although the calcium fluoride particle size does increase. I t appears that the distribution is more favorable after the five-day aging period. The cause of this is the greater ease in separating the larger particles from the supernate after aging which means that the separations performed immediately after the precipitation were probably incomplete. Even at the high erbium concentration of 0.1 mol % , the distribution of erbium into the CaF, is very favorable. It should be added that no spectral transitions are observed in ignited precipitates that would come from an ErF3 precipitate, as predicted from solubility calculations that show ErF3 precipitates should not form under the conditions used in this study. Figure 12 shows how the G1 site intensity varies with the initial unbuffered solution pH. In the pH 3 to 9.5 range, there is no significant change in intensity due to pH. A change in intensity above p H of 9.5 may be caused by the precipitation of Er(OH)3which would be expected to precipitate before the addition of fluoride, and in fact calculations show that Er(0H) should form a t any p H greater than 7. Effects of Other Ions. The precipitates discussed in the previous sections were prepared by adding a NH4F solution to a Ca(N03)2solution to precipitate calcium fluoride. The
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Relative loo/ Fluorescence Intensity
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ANALYTICAL CHEMISTRY, VOL. 49, NO. 12, OCTOBER 1977
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20
'0
2
I
3
4
5
7
6
8
9
IO
Initial Solution pH
Flgure 12. Dependence of G1 site intensity on unbuffered solution pH before precipitation. Precipitates are CaF,: mol YO Er3'. Excite Z, H, at 449.21 nm while monitoring E, Z, fluorescence at 546.06 nm. Ignited 3 h at 700 OC and cooled slowly
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0 )
Zbg?-ilNa
1 I
1 '
_ _ ~ I _
447
448
449
450
45
452
453
-
454
455
- ~ -
-_i,s-/_-----4 4 1 4 4 8 4 4 9 4 5 0 451 4 5 2 453 4 5 4
I ,
456
I.J__
_--
'j
_ic_~ ,'-,-Add
h -i
448
455
I
c i 50ug'mlNa
447
456
449
-
450
451
452
453
454
~~
455
456
X(nrn:
Figure 13. Effect o i sodium on the site distribution in ignited CaF,:10-4 mol % E?', Excite 2 H while monitoring E Z fluorescence at 550 nm with the 0.25-m monochromator. (a) 2 pg/mL Na' present before precipitation, muttiply intensity by 1.6. (b) 10 pg/mL Na' present, multiply intensity by 1.6. (c) 50 pg/mL Na' present, multiply intensity by 1 +
large ionic radii of NH4+and NO3- make their coprecipitation in the calcium fluoride lattice unlikely and thus incapable of causing any changes in the spectrum. In general, it is known that an ion whose ionic radius differs by not more than 15% of a host lattice ion can distribute favorably in the host lattice (43),as exemplified by the favorable coprecipitation of erbium in calcium fluoride, where the ionic radii of calcium and erbium differ by 10%. CaC12 and NaF have also been used as sources of calcium and fluoride and they cause observable changes in the erbium spectrum. A chloride concentration of 0.165 M in the initial solution caused a broadband excitation
background to appear whose intensity near 449.2 nm in a CaF,:10-6 mol % Er3+ignited a t 600 OC for 3.5 h was equal to the intensity of the G1 site Z1 HI excitation line in the same precipitate. Because the effect of chloride is to produce one broad peak rather than a series of new sites, the chloride ion probably enters the lattice randomly relative to the erbium ion positions resulting in a disruption of the short range order that inhomogeneously broadens transitions. Sodium has many effects on the erbium spectrum. As the amount of sodium added to the calcium-erbium solution before ammonium fluoride addition increases, the G2, G3, G4, and H sites become more intense a t the expense of the G1 site (Figure 13). Six satellite sites appear around the intrinsic G1 and G4 sites that are thought to be caused by a sodium ion near enough to a G1 or G4 site to cause a shift in the crystal field levels of that site. These satellite sites look very much like G1 or G4 sites that have been simply shifted to higher or lower wavelengths. In addition, incorporation of sodium into the lattice is accompanied by inhomogeneous broadening of the excitation line widths of the G1 site lines as shown in Figure 13, because of a disturbance of the lattice order. T h e charge compensation expected when sodium coprecipitates in calcium fluoride is the introduction of a fluoride ion vacancy into the lattice. The presence of fluoride vacancies increases the mobilities of ions in the lattice and therefore enhances the kinetics of the conversion from fluoride compensation to oxygen compensation. If the site distribution is controlled by the kinetics, the introduction of additional fluoride vacancies would have the same effect as increasing the ignition temperature of a precipitate containing no sodium. This is the mechanism we propose for the dependence of the G4 site population on sodium concentration even a t low ignition temperatures. Fluoride vacancies would also disrupt the lattice order causing broad lines. The precipitates formed with Na’ present exhibit additional satellite lines around both the G1 and G4 sites that indicate additional sites are formed (see Figure 13). There are indications that these lines are associated with sites where a sodium ion is near an erbium ion. The intensity of lines from this site depends upon the concentration of sodium in the original solution. In addition if 750 pg/mL of lithium is present during coprecipitation instead of sodium, the ignited precipitate contains the G1 through G4 sites that are found when the kinetics of conversion to oxygen compensation are enhanced. None of the satellite lines seen with sodium are present in these precipitates. Instead, two new sites appear which depend upon the lithium concentration. This observation could form the basis of a method for sodium or lithium determination by SEPIL. Other ions of similar size might also substitute for calcium, creating the possibility of determining these ions.
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LITERATURE CITED (1) D I Ryabchikov and V A Ryabukhin, “Analytical Chemistry of the
(2) (3) (4) (5) (6) (7) (8) (9) (10) (1 1) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21) (22) (23) (24)
(25) (26) (27)
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RECEILXD for review January 21, 197‘7. Accepted June 7, 1977. This research is supported by the National Science Foundation under Grant No. MPS74-24394. I t was presented in part at the 27th Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Cleveland, Ohio, March 1976.
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