766
Anal. Chem. 1986, 58,766-771
Ultratrace Determination of Osmium by Laser Excitation of Precipitates R. J. Haskell and J. C. Wright* Department of Chemistry, University of Wisconsin, Madison, Wisconsin 53706
The techniques of seiectlve excitation of probe ion luminescence (SEPIL) are applied to the analysts of geological samples for osmium at uitratrace ieveis. os4+is coprecipttated into A,SnX, (A = K, Rb, Cs; X = CI, Br) host lattices and then observed at Cryogenic temperatures. Laser excitation of the resulting narrow optical transitions of osmium provldes for excellent selectlvlty and sensltlvlty. Detectlon limits of 3 pg/mL are shown along wlth suggestions for lowerlng the limit still further. Remarkable freedom from Interferences Is demonstrated, and analysts of geochemlcal standards is reported.
Highly siderophile elements, such as osmium, are of interest when studying the formation of the Earth's mantle (I), the cosmic abundance of the elements (2),and the influx of extraterrestrial material onto the Earth's surface (3). The economic value of osmium also spurs interest in the petrogenesis of ores containing recoverable quantities of the platinum group elements (PGEs) ( 4 ) . An extremely rare element, osmium is usually determined by one of several methods (5). Fire assay followed by a spectrometric method, such as atomic absorption (6) or spectrophotometry (3, is commonly employed for the PGEs. However, the tendency to lose osmium as 050,during the cupellation step requires the use of radiotracers or an alternative method of isolation from the collector, e.g., distillation. Once isolated, problems arising from the high volatility of OsO, limit detection with flame methods, while electrothermal techniques are limited by the highly refractory nature of Os metal (bp 5000 "C). Interelement interferences are also a problem (8). Fire assay methods usually require large amounts, 1-10 g, of sample, which may not be available if rare substances, such as extraterrestrial materials, are to be analyzed. Radiochemical neutron activation analysis (RNAA) is the most frequently employed measurement method, sometimes in conjunction with fire assay collection (9). The requirements of a nuclear reactor, a lab equipped for radiochemistry, and the preparation time per sample can render RNAA expensive and inconvenient, Frequently, distillations and anion exchange separations are required in the separation procedures (10). There are also methods specifically designed to decompose and analyze osmium-containing materials. An example is a hybrid atomic fluorescence technique in which the sample is decomposed by treatment with an oxidizing flux a t 1250 "C, whereupon the volatilized Os04is then reduced by gaseous sodium metal (11).
Selective excitation of probe ion luminescence (SEPIL) has been shown to be a sensitive and selective method for the determination of many elements. When doped into various host materials, such as CaF2, and observed at cryogenic temperatures, the lanthanides have been determined a t levels below that of RNAA (12). The specificity afforded by the sharp line optical transitions under such conditions is fully exploited by the use of a narrow band dye laser to excite emission and a high-resolution monochromator to observe it. Low detection limits are achieved due to the power of the 0003-2700/86/0358-0766$01.50/0
excitation source, and the favorable incorporation of analyte into the host lattice. Other elements observed via SEPIL include fluorescent and nonfluorescent transition metals (13, 14)and actinides (15-17). The nonfluorescing ions are observed when they are substituted into dimer sites in which the second member of the resulting pair is a fluorescing rare earth. Excellent results have been obtained with the SEPIL methods for the determination of rhenium when doped into antifluorite, A2SnX, (A = K, Rb, Cs; X = C1, Br), lattices (18). This study will report similar results for osmium. Dorain et al. (19) were the first to observe that osmium in the tetravalent state would give rise to sharp line absorption transitions a t cryogenic temperatures when doped into single crystals of Cs2ZrCl6. Osmium emission, in a Cs2HfC16host, was first reported by Reinberg (20). Since then, there have been numerous examples of intra- and interconfigurational d-d absorption (21, 22) and magnetic circular dichroism spectra (23, 22). There are reports of absorption studies assigning charge transfer bands (24,25)in the literature as well. Luminescence arising from d-d transitions of osmium doped into various cubic and tetragonal hosts has also been investigated (26,27). Only one crystallographic site has been observed as the dopant ion substitutes uniquely for the tetravalent host cation.
EXPERIMENTAL SECTION Apparatus. The nitrogen-pumped dye laser system used in this study has been described previously (28). The sample was cooled to approximately 11K with a closed cycle helium refrigerator. Gated photon counting equipment was employed for the detection of low light levels, while gated integration was used for higher levels. f / 2 collection optics coupled the emitted light into a f/6.8 1-m monochromator fitted with a dry-ice-cooledphotomultiplier (EM1 9658R, 0.5 nA dark current at -60 "C). Reagents. Distilled deionized (DDI) water was used at all times. Glass and plasticware were washed in microsoap, soaked in 10% HNO, for several hours, and then rinsed with DDI water. All reagents were ACS reagent grade unless otherwise specified. HzSnBrs and HzSnClswere prepared from HBr, Br,, and Sn (99.999%,Aldrich) or HC1, C12, and Sn via the methods described in ref 18. The 0.21 M solutions of K2SnBr6and K2SnC16were made from the corresponding hexahalostannic acid and either KBr or KCI. Stock solutions of K2OsCl6were prepared by mixing weighed samples of the solid (99.99%,Spex) and 1M HC1. Stock solutions of Kz0sBr6were made by treating weighed portions of K2OsC16 with concentrated HBr and evaporating the mixture almost to dryness on a hotplata This process was repeated a total of 3 times. Solutionswere then diluted to volume with 3 M HBr. Analytical standards were obtained via serial dilution of the respective stock solutions with 1M or 8 M HC1 being used for the chloride salt and 3 M or 8 M HBr for the bromide. Procedure. Osmium doped K2SnXswas formed by adding 1.0-3.0 mL of the dopant solution (in 1 M HC1 or 3 M HBr) to 3.0 mL of the desired host solution and heating the mixture at 80 "C in a laminar flow hood until numerous crystals had formed (about 4 h). The sample was then allowed to sit overnight. The solid was isolated by suction filtration (Whatman No. 50), dried on a watchglass in a 90 "C oven for 15 min, ground gently with a porcelain mortar and pestle, soaked for 10 min in 2 mL of diethyl ether, dried @gainat 90 OC for 10 min, ground to a fine powder, and stored in a glass vial. 0 1986 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 58, NO. 4, APRIL 1986
767
v4
6
ai
il d+L v3
572
568
A
574
576
570
2
v4
b)
A
v3 F
E
3 n
0 s W
4
n
d) \
v3
\
\ \
r,
L
0 602
Flgure I. Energy level diagram for the :,t configuration of Cs,SnBr,:OsBr,*showing the effects of both electrostatic repulsion and spin-orbit coupling. The energies of the ('E,) and ('TQ) states are approximate.
r3
rs
Doped Rb2Sn& and CszSnX6salts were prepared by adding 5 mL of 8 M HX, containing a 50% excess of either RbCl(99+%, Aldrich) or CsCl(99.9%,Alfa), in a dropwise fashion to a stirred mixture of 0.5 mL of 1.25 M HzSnXsand 3.0 mL of dopant (in 8 M HX). The solid was suction filtered, and rinsed with 2-3 mL of 0 "C 8 M HX. Air was pulled through the precipitate for 10 min followed by drying on a watchglass in a 90 "C oven for 15 min and grinding in an agate mortar and pestle. After isolation, the precipitates were a fine, slightly sticky powder. The chloride salts were white; the bromide salts were light yellow. In all cases samples and standards were prepared simultaneously. Ore samples (90-100 mg) were decomposed at 610 "C for 10 min in a zirconium crucible with a Na202/NaOHflux. The crucible was dropped into a round-bottom flask containing 40 mL of 8 M HBr at 0 "C. The mixture was then refluxed for 1h, cooled to room temperature, and transferred to a 50-mL volumetric flask with HBr. The above precipitation method was used to produce 480 mg (0.62mmol) of RbzSnBr6.The resulting doped precipitates were examined with the dye laser system on the same day that they were formed. Precipitates were placed in one of ten 3-mm diameter depressions in a copper block (two columns of five) and then packed using a stainless steel rod and gentle tapping with a hammer. Each column of samples had either one sample and four standards or two samples and three standards. The block was screwed onto the bottom of the refrigerator cold tip and placed under vacuum. The difference in emission intensity between that measured at an analytical wavelength and that measured at a background wavelength was used to generate a calibration curve from the standards. An integration time of 250 s (4000 laser shots) was used with photon counting. Signal levels were monitored for about 3 min when gated integration was employed.
RESULTS AND DISCUSSION Osmium(1V) is a d4 transition metal that exhibits a large amount of spin-orbit coupling due to its high 2 number. Electrostatic repulsion splits the tz; configuration into four terms: (lowest energy), lT@,lEg,and 'A, (highest energy). Spin-orbit coupling splits the low energy, IT,,, state into rl
606
- r4
610
t614
618
A(nm)
Flgure 2. Excitation spectra of rl ('A,,) for various Os-doped hosts, (3T1,)v,: (a) K,SnCI,; (b) Rb,SnCI,; (c) monitoring ('Alg) K,SnBr,; (d) Rb,SnBr,. The analytical line is indicated with a down arrow, and the background wavelength is indicated with an up arrow.
(ground state), r4(ca. 2800 cm-l), (ca. 4800 cm-l), and r3 (ca. 4900 cm-l) states. The three remaining terms are unsplit but are relabeled r5(ca. 10500 cm-l), r3(ca. 11000 cm-l), and rl (ca. 17 000 cm-l), respectively, according to the Bethe notation. The exact energy of these states depends on the particular host material and the halide ligand. The energy levels for C ~ ~ s n B r ~ : o sare B r given ~ ~ - in Figure 1. Pertinent luminescence and excitation spectra are shown in Figures 2-5. Both emission and excitation are characteristic of the ungerade vibrations, v6 (tz,),v4 (tlu),v3 (t,,,), of the octahedral 0~x2complex anion. The v6 band is quite weak in the rl (lAlg) rl (3T,g)and rl (3T1g) r, (lAlg) electronic transitions as it is forbidden in an octahedral symmetry. Due to the small energy difference between the r3and r5 (3T1,) states, the vibronic bands of the respective electronic origins overlap to a large degree. The overlap makes assignment of some spectral lines to a specific electronic transition difficult. The excitation spectra of levels above 17000 cm-l for the OSBq2- ion have been found to arise from a ligand-to-metal charge transfer (LMCT) process (24). These bands are relatively broad and much less structured in comparison to the d-d transitions. The LMCT bands of the bromide complex are about 8000 cm-l lower in energy than the same bands of chloride complexes (25). with the exception of the magnetic dipole (MD) allowed Fl (,Al,) r4(3T1g) transition, no 0-0 transitions are visible in the cubic rubidium and cesium hosts. The potassium salts undergo a phase transition a t reduced temperature so that the dopant ion is in a Dllh environment. This change in symmetry causes the v6 band of the rl (lAl,) rl (3T1g)and I'l (3T1,) rl (lAlg) electronic transitions to become electric dipole allowed and is thus barely visible in Figures 2 and 3. The phase transition also causes the electronic origin of the rl (,Alg) r5 (3T,g)transition to become MD allowed, but
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768
ANALYTICAL CHEMISTRY, VOL. 58, NO. 4, APRIL 1986
Table I. Detection Limitsa for
0~x2-Using AzSnXBHosts
-
excitation transition
rl ('T1,)
X = Br
KzSnX6 CTb
(7.6 X 10-B)c 43 (3.6 x lo-*)
r4(3T1,)
Rb2SnX,
(1.0 x 10-8) 84 (7.2 X
327 (2.8 X lo-')
CszSnX6 CT
rl (
(1.4 x 10-7) 1500 (1.3 x lo4)
100 (8.1 x
rl ( i ~ l g )
CT
('AIg)
65 (5.5 X
1.270 (1.1 x 10-6)
1 ~ ~ ~ )
8650 (7.3 x 10-6)
665 (5.7 x 10-7) "Limits are in picograms of Os per 0.63 mmol of host. bCT indicates excitation to charge transfer at 441 nm. eMole percent of Os relative to Sn given in parentheses. dLimit >50 ng (4.2 X mol %). vA
a)
A
v3
sa7!
692
b)
697
702
707
v4
v6
J+,L 582
5S6t
590
594
59s
f 693
"4
699
705
7i1
7i7
v4
C)
C)
A
'6
v3
768
%T J
1
744
d)
d)
623
6i5
627
762
v4
v3
0-0
A+621 t
V6
AA 2
v3
6i9
756
750
-+
&
t
747
752
757 X(nm 1
Xhm)
762
- r4
767
Figure 3. Emission spectra of the Pi ('Al,) ri (3Ti,) transition for various Os-doped hosts, exciting I?, ('Alg)v4: (a) K,SnCI,; (b) Rb,SnCI,; (c) K,SnBr,; (d) Rb,SnBr,. The analytical line Is indicated with a down arrow, and the background wavelength is indicated with an up arrow.
Flgure 4. Emission spectra of the ri ('Aig) (3Tig)transition for various Osdoped hosts, exciting rl ('Alg)v4: (a) K,SnCI,; (b) Rb,SnCl,; (c) K,SnBr,; (d) Rb,SnBr,. The analytical line is indicated with a down arrow, and the background wavelength is indicated with an up arrow.
the line is buried in the phonon side bands. According to group theory, all but the totally symmetric rl states should split upon lowering the osmium site symmetry from Oh to D4h since it is a non-Kramers ion. The splitting is readily observable in the rl (IA1J r3,r5 (3T1g)transitions of K2SnBr6:0sBr62-(Figure sa). As the 0-0 lines for the two electronic transitions are not easily seen, the extent of the splitting is undetermined. For some of the vibronic bands (e.g., rl ('AIg) r5(3T1$v6and v4),the splitting is obvious, but for others (e.g., rl ( Alg) r3(3T1g)vsand rl PAlg) rS (3Tlg)v3),there is only a slight broadening. There is also observable splitting in the rl (IAlg) r4 (3T1,)emission transition, but not all of the vibronic bands can be assigned. Calibration curves constructed from the plots of emission intensity vs. dopant concentration for K2SnC16:0sC1~were linear from 195 ppm (0.16 mol %) to at least 1.92 ppb (1.6 X lo4 mol %) with an extrapolated detection limit (twice the signal-to-noiseratio) of 1.3 ppb (1.1X lo4 mol %). This result
is achieved when 1.0 mL of dopant is used and the (3T1,) rl ('Alg)v4excitation and rl PAl,) r4(3T1g)v4emission transitions are employed. A linear concentration dependence was observed from 195 ppm (0.16 mol %) to 45 ppt (3.7 x mol %) for K2SnBr6:OsBra-when the charge transfer bands are excited (441 nm) and emission arising from the rl (lAlg) rl (3T1,)v4transition is observed. Again, 1.0 mL of dopant solution is used. These detection limits are equivalentto those found for lanthanide and actinide analysis using SEPIL. Table I shows the absolute detection limits for osmium in a variety of hosts for different combinations of excitation and emission transitions. The general trend is that of lower detection limits being achieved using potassium hosts, higher limits with cesium hosts, and intermediate limits with rubidium hosts. Because of the lower energy of the bromide LMCT bands there is increased charge transfer character in the excited states of these salts as compared to those of the chloride salts. Such an increase causes the d-d optical tran-
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 4, APRIL 1986
Table 11. Observed Lifetimes, in Microseconds, of State for Os-Doped AzSnXBHosts"
I'l
769
('Aig)
A2SnX8 A
x = c1
X = Br
K Rb cs
23.5 24.3 31.6
7.4 8.2 11.1
- rl - r4
'Exciting charge transfer bands at 441 nm, monitoring rl (.'Aglg) (3T1,)v4for X = Br. Exciting rl (lAlg)v4and monitoring rl (lAlg) (3Tl,)v4for X = C1.
SI4
821
828
835
si2
907
914
6'
SS6
893
900
$ "4
886
-rs r,
893
900
X (nrn 1
r,
907
-
914
Flgure 5. Emission spectra of the ('Alg) r 3 (3T1g)(primed), and I', (.'AlQ) ("r,,) (unprimed) transitions for various Osdoped hosts, exciting ('AIg)v4: (a) K,SnCI,; (b) Rb,StlCI& (C) K,SnBr& (d) RbzSnBr,. The analytical line is Indicated with a down arrow.
sitions of the bromide complex to become more allowed than is the case for the chloride complex. Therefore, the bromide hosta would be expected to afford lower detection limits than the chloride hosts. This expectation is confirmed in Table I. In addition, it is possible to directly excite the very allowed bromide LMCT bands with the dye laser due to their lower energy, decreasing detection limits still further. The corresponding bands of the chloride are inaccessible with our laser system. There are some anomalies in the above generalizations, but they are due to differences in the efficiencies of the laser dyes employed. At very low photon count rates, the contribution to the total signal from a broad background fluorescence of unknown origin becomes apparent. The background scales with the laser power so that the signal-to-noise ratio will improve as the square root of the laser power, but only if the fluctuations in the background level are determined by shot noise. This case is usually encountered because the intensity of the background emission is so low. Some combination of excitation and emission transitions could not be used for trace analysis as the large amount of laser scatter from the precipitate would have damaged the photomultiplier. Thus detection limits resulting from excitation of the rl (lAl,) state and emission to the rl (3T1,)state are not reported. The use of a method to temporally block the laser scatter would obviate the problem. Choice of an analytical wavelength depends on the detection limit required and the potential interferents that may be present. It should be noted that modifications to the procedures outlined above will reduce detection limits. The mole percent of dopant present in the host is the fundamental quantity determining the minimum amount of osmium that is detectable. Changes in the method so that the analyte can be extracted from a larger sample volume will therefore lower detection limits. Thus, the low solubility of some hosts will
provide for remarkably low concentration detection limits in that large sample aliquots are possible. Employing KzSnBr6 as a host, a detection limit of 3 pg/mL is achievable if 3.0 mL of the analyte solution is used. Using greater volumes will decrease detection limits still further. The sparingly soluble rubidium and cesium salts will allow for the preconcentration of osmium from even larger sample solutions, although this advantage will be somewhat offset by the lower absolute detection limit afforded by these hosts. Another change that will reduce detection limits is to produce doped precipitate on a smaller scale. Only 6 mg of material is packed into the depressions on the sample holder while the above procedure produces 0.63 mmol (426 mg, in the case of K2SnBr6). Therefore, of all the osmium present in the original aliquot of sample solution, only 6/430 is actually observed. If a micropreparation procedure was employed producing only that amount of precipitate needed to fill a sample depression, the absolute detection limits should be lowered by a factor of 70. In the case of K2SnBr6with charge transfer excitation, the limit would then be 130 fg. In general, when charge transfer excitation is used, the correction for background emission is made by changing the monochromator wavelength to an emission-free region. An increase in selectivity could be brought about, if necessary, by changing both the excitation and observation wavelengths. Observed lifetimes of the rl (lAlg)excited state in various AzSnX6 materials doped with 0.15 mol % Os are given in Table 11. In all cases, the lifetime increases for the sequence Cs > Rb > K, and C1> Br. The luminescence decay for the chloride hosts is a single exponential, while the decay for the bromide hosts is slightly nonexponential at the given dopant concentration, perhaps indicating an energy transfer process (18). The shorter lifetimes encountered when the bromide lattices are used may be the result of increased coupling of the OsBr2- anion to the host as compared to that of OSC&~thus allowing for more efficient nonradiative relaxation. The larger coupling of the bromide could be due to the larger size of the metal-ligand bonds and the increased flexibility of the bromide complex. The temperature dependence of the rl (3T1g) Fl (lA1g) excitation spectrum of 0.16 mol % CszSnC16:OsC&2is shown in Figure 6. Relative intensities and luminescence lifetimes are given in the figure caption. At 40 K there is little difference in the spectrum from that at 11K. The phonon bands at 585 nm are slightly more structured and the triplet at 580.5 nm is less resolved. However, the intensity has dropped to 23% of its initial value. By 77 K anti-Stokes phonon bands have appeared at 588 nm, and the triplet is not resolved. Increased broadening of the vibronic bands and significant contribution to the spectrum by Stokes and anti-Stokes phonon bands has occurred by 120 K. There was no observable emission in the sample at room temperature. The lifetime of the emitting level remained unchanged until the temperature began to exceed 77 K. No emission could be detected for 0.16 mol % KzSnBr6:0sBr2-until the temperature was reduced to at least 50 K. The importance of tem-
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ANALYTlCAL CHEMISTRY, VOL. 58, NO. 4,APRIL 1986
Table 111. Interference Ions and Levels a)
AzSnBr,:OsBr2- a A = Kb
interference level, ppm
"3
5000
AI3+ (29), CaZf (20), Mg2+ (33)
50-500
A = Rbc
Fe3+(0.07-0.7) Cu2+ (0.7-7) Cr3+ (0.8-8) Ni2+(7)
Fez+(7)
r'l
1600-16000 9500-95000
SO-:
1000-100000
SO?- (200)
co2+(7)
I- (2-20) (16-160)
Interference levels in more percent relative to the Sn are given in parentheses. b1.6 X lo4 mol % OsBr2- relative to AzSnBre. 7.7 X lom7mol % OsBrR2relative to A,SnBr,.
I\
57 3
577
58 I
585
589
X (nm 1
r,
Flgure 6. Temperature dependence of excitation to ('A,J in 0.16 mol % Cs2SnCI,:OsCI,*- (luminescence lifetimes and relative emission intensities are given in parentheses): (a) 11 K (30 ps, 1.00); (b) 40 K (28 ps, 0.23); (c) 77 K (26 ps, 0.071); (d) 120 K (7 ps, 0.019).
perature stability is thus demonstrated, especially for the bromide hosts. The signal levels observed when using KzSnBr6as the host showed a high degree of irreproducibility (>30% relative standard deviation) when dopant concentrations of less than 500 ppt were employed. It was noted that when host crystals were formed from more concentrated (> 10 ppm), colored OsBrt- solutions, most of the osmium coloring was contained in the first 50% of the precipitate as it formed. While implying that the dopant ion is favorably incorporated into the host lattice, this observation also emphasizes the importance of sufficiently grinding the precipitate after drying so as to obtain a homogeneous distribution of osmium throughout the host material. At extremely low dopant concentrations (5 X lo-' mol TO),it may be impossible to achieve the necessary degree of homogeneity by grinding alone. The consequence would be irreproducible results. Attempts to dilute and mix KzSnBr,:OsBrt- with undoped RbzSnBrsby grinding with a mortar and pestle yielded Samples that showed very high sample-to-sample emission intensity variations (>50% relative standard deviation). This result suggests that grinding is an inadequate means of homogenizing the dopant distribution throughout the sample. In response to the above problem, RbzSnBr6was employed as the host material. Due to its reduced solubility compared to that of the potassium salts, it was possible to rapidly produce a fine suspension of doped Rb2SnBr6precipitate in a stirred solution. The osmium is again coprecipitated with the first 30-50% of host material formed, but the final suspension after precipitation is visibly more homogeneous than is the case for the potassium host crystals. Thus less emphasis is placed on the grinding step as the small precipitate particles are already well mixed even before filtration. It was noted that RbzSnBrs tended to become dislodged from the sample holder at cryogenic temperatures, falling out of the depressions as disks. Since good thermal contact be-
tween the holder and the sample is a prerequisite for reproducible emission intensity measurements, a layer of KzSnBr6 was packed into the sample holes before the RbzSnBr6to act as a binder between the observed sample and the sample holder. Gentle tapping with a hammer was used to pack the K2SnBrs,but finger pressure was used for the RbzSnBr6layer as use of the hammer produced gross irregularities on the surface of the packed sample disk. Good reproducibilities (7% relative standard deviation) were obtained with RbzSnBr6as a host. Interference studies were carried out by adding a measured amount of potential interferent to the host/dopant solution before the formation of a solid precipitate. Two standards and three test samples, each with a different interferent concentration,were prepared for each ion studied. A reduction in signal for those samples containing a given concentration of the test ion was considered to be sufficient evidence of an interference. The rl (3T,,) rl (lAlg)v4and rl (lAlg) r4 (3Tl,)v4.transitions were used for excitation and emission, respectively. Results are shown in Table 111. The results of similar studies for rhenium (18)indicate that the dominant interference is from those ions that are strongly colored. These ions would be expected to interfere via the absorption of exciting and/or emitted light. Other mechanisms by which an interference can be brought about are deexcitation of the emitting state by energy transfer or a perturbation of the solid-state equilibria. No new lines are generated in the spectra of those samples with a test ion, nor is there any change in the observed lifetimes of the emitting state. Therefore deexcitation is probably not occurring. As the incorporation of osmium(1V) into the host requires no charge compensation, it is not expected that perturbed solid-state equilibria will play a significant role in an interference mechanism either. Bromide complexes of most potential cationic interferences are more strongly colored in the visible than chloride complexes. Since the AzSnBrshosts provide for much lower detection limits than the chloride hosts, interference studies were only carried out on bromide salts. The interferences are expected to be less severe for the AzSnCls materials due to the decreased molar absorptivities of the chloride complexes. The interference caused by iron strongly depends on the valence state of the cation. Fe3+strongly interfered at the 50-500 ppm level in both the potassium and rubidium salts whereas Fez+interferes at some level greater than 5000 ppm. The difference in concentration can be explained based on the observation that Fe3+in HBr is a dark brown while Fez+ is essentially colorless, Since the Sn(I1) procedure used to reduce the Os04to OsBr2- will also ensure that all iron present is in the divalent state, large concentrations of iron can be present in the sample to be analyzed without its having an
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 4, APRIL 1986
adverse effect on the osmium determination. This is fortunate as osmium often occurs in samples containing significant amounts of iron. The lower solubility of the rubidium salt allowed more extensive rinsing with acid after filtration than did the highly soluble potassium host. It was hoped that the additional rinsing would reduce the coloration imparted by the presence of some of the more strongly interfering ions such as Fe3+. Unfortunately, repeated rinsing did not remove any of the interfering ions, implying that there is some incorporationinto the host lattice. The large difference in precipitation time of the rubidium and the potassium hosts (1min vs. 4 h) may also provide for a different degree of interferent coprecipitation. The ether washings of those potassium salts containing colored interferents were observed to be colored themselves. It can be therefore be assumed that the levels at which interferences occur due to colored ions would be lower were it not for the ether washing step. Interferences in the SEPIL determination of rhenium using KzSnC16 as a host (18) were reduced by as much as an order of magnitude by redissolving the doped solid in fresh 1M HC1 and then reprecipitating. The same procedure should be applicable to osmium using the potassium host for the first precipitation and the rubidium or cesium salt for the second. The low solubility of the rubidium and cesium salts would preclude their use in both precipitation steps. One ion, Pt4+,was observed to interfere by contributing additional luminescence of its own when the dye laser wavelength was set to excite the osmium charge transfer bands and the monochromator was set to observe the rl (lAlg) rl (3Tlg)v4transition. The platinum emission, arising from the lTIg(tzieg) lAlg (tz,") interconfigurational transition, is very broad, extending from 650 to 820 nm. The lifetime of the PtBrs2- is about 160 ps when excited with 441-nm light and when K2SnBr6is employed as the host lattice. The osmium luminescence transitions are narrow, so there is little change in any background signal due to the platinum emission when the wavelength setting of the monochromator is moved on and off of the osmium analytical line. Thus large osmium signals are not influenced when platinum is present. However, the large number of photon counts contributed by the platinum luminescence contains enough statistical noise to overwhelm a small osmium signal. The platinum interference can be completely eliminated by using the r1(3Tlg) rI(lAlg) transition for excitation and the r1(lAlg) I'4(3T1,) transition for emission as is done for Table 111. Other platinum group elements and rhenium are not expected to emit under these observationconditions either and were, therefore, not studied. The accuracy and precision of the technique for the analysis of geological samples was demonstrated by the analysis of the two standard ore samples. The first, SARM-7 (NIM, Johannesburg, South Africa), has a certified osmium content of 63 f 7 ppb, and the second, USBM-Pt-A PGE ore (USBM, Denver, CO), has an uncertified concentration of 110 ppb. Both ores originate from the Bushveld complex in South Africa. The above SEPIL methods yielded values of 53 f 7 ppb and 103 f 17 ppb for SARM-7 and USBM-Pt-A, respectively. Three milliliters of dopant solution, containing approximately 150-200 ppt osmium, was used. The concentration of platinum in the solution produced from SARM-7 was 5.8 ppb; from the USBM-Pt-A ore it was 8.9 ppb. The transitions appropriate to eliminate the interference from platinum were successfully employed.
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osmium. There are a greater variety of optical transitions to choose from than there are for a similar rhenium determination. Excitation of the broad LMCT bands provides for the minimum detection limit, but only at the expense of decreased selectivity and the possibility of an interference from platinum. Choice of the emission transition depends on the selectivity and sensitivity desired as well as the absorption profiles of any interferents that may be present. Trade-offs are also present for the selection of a suitable host, the more sensitive potassium hosts giving poorer reproducibilities. A reasonable balance .between, these two considerations is achieved with the use of the RbzSnBr6host. In addition, the cesium and rubidium hosts give somewhat improved selectivity since the transition line widths are narrower. While detection of osmium is possible in chloride hosts above 77 K, the detection limits are high and significantly better limits are possible at 11K. If the bromide hosts are used to obtain lower detection limits, the temperature should be lower than 30 K. The use of SEPIL as a viable tool for the analysis of materials containing both osmium and rhenium has now been demonstrated. Similar methods may prove to be useful for the analysis of other fluorescent, octahedrally complexed ions such as PtX?-, PdX:-, and IrX:-. Work on such techniques is currently under way. Registry No. Osc&", 16871-52-6;OsBr6", 16920-04-0;K2SnC&, 16923-42-5;K2SnBr6,17362-95-7;Rb2SnC&,17362-92-4;RbzSnBr8, 17362-96-8; CszSnC1,, 17362-93-5; Cs2SnBr6,17362-97-9; os, 7440-04-2.
LITERATURE CITED
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CONCLUSIONS In a practical situation, there are a number of options that need to be considered if SEPIL is to be used to analyze for
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RECEIVED for review September 3,1985. Accepted November 4,1985. This research was supported by the National Science Foundation under Grant CHE8306084. R.J.H. gratefully acknowledges the financial support of the Procter and Gamble co.
