Anal. Chem. 1987, 59, 427-432
for some of the observed quenching since the lifetime change is much larger in the presence of Ag if Ab*R is also present (Table I). However, as shown in Figure 8, the lifetime of Ab*F is also significantly reduced by Ag alone, without Ab*R. The first decrease in T~ occurs in the presence of small amounts of Ag and may be due to self-quenchingby fluorescein labels on different Ab molecules brought together on a single multivalent Ag molecule (4), as well as to quenching of Ab*F upon binding to Ag. As Ag concentration increases and the binding ratio approaches 1 Ab*F per Ag, TF increases. A second region of decreasing lifetime is then observed at higher Ag concentrations, perhaps due to the formation of clusters of Ab*F on the antigen surfaces (4), again bringing fluorescein labels on different antibodies in closer proximity to facilitate quenching.
CONCLUSIONS The selectivity of the noncompetitive immunofluorometric technique described for homogeneous determinations of lactoferrin is greatly enhanced first by the use of phase-resolution to exploit fluorescence lifetime differences between the free and antigen-bound labeled antibody and, second, by the use of energy transfer from the monitored donor label to a receptor label on a separate antibody also bound to the multivalent antigen. Of the various competitive and noncompetitive techniques studied, the excitation transfer noncompetitive approach gave the best selectivity and sensitivity. Excellent precision was observed for calibration curves, with a relatively large dynamic range. Good accuracy and reproducibility were obtained for the determination of synthetic unknowns. Lactoferrin was chosen as the model system for these studies because of the availability of both lactoferrin and antilactoferrin antibody (in a highly purified form) labeled with several different fluorophores capable of excitation transfer. Because rhodamine-labeled lactoferrin was not
427
available, we could not evaluate the competitive technique with excitation transfer from Ab*F to Ag*R. This system would be an interesting subject for future studies. It would also be worthwhile to study the competitive schemes with antigens that have a similar degree of labeling. However, because labeled antibodies are often easier to obtain or prepare than labeled antigens, the noncompetitive excitation transfer technique with two labeled antibody species may prove more convenient and widely applicable than the competitive approach.
LITERATURE CITED (1) Nakamura, R. M. Ciin. Lab. Assays: [Pap. Annu. Clin. Lab. Assays COnf.1 4th 1983, 33-60. (2) Smith, D. S.; Hassan, M.; Nargessl, R. D. I n Modern Fiuorescence Spectroscopy; Wehry, E. L., Ed.; Plenum: New York, 1981; Chapter 4. (3) Hemmiia, I. Ciin. Chem. (Winston-Salem, N.C.)1985, 31, 359-370. (4) Ullman, E. F.; Schwartzberg, M.; Rubenstein, K. E. J. Biol. Chem. 1976, 251, 4172-4178. ( 5 ) Tltus, J. A.; Haugland, R.; Sharrow, S. 0.; Segal, D. M. J . Immunol. Methods 1982, 50. 193-204. (6) Mattheis, J. R.; Mitchell, G. W.; Spencer, R. D. I n New Directions in Molecular Luminescence; ASTM STP 822; Eastwood, D., Ed.; American Society for Testing and Materials: Philadelphia, PA, 1963; pp 50-64. (7) Lakowicz, J. R. Principles of Fiuorescence Spectroscopy; Plenum: New York, 1983; Chapter 4. (8) Bright, F. V.; McGown, L. B. Taianta 1985, 32, 15-18. (9) Tahboub, Y.; McGown, L. B. Anal. Chim. Acta 1988. 182, 185-191. (10) Blackberg. L.; Hernell. 0. FEBS Left. 1980, 109, 180-184. (11) Querinjean, P.;Masson, P. L.; Heremans, J. F. Euf. J . Biochem. 1071. 20. 420-425. -. _. (12) Nagasawa, T.; Kiyosawa, I.; Kuwahara. K. J . Dairy Sci. 1972, 55, 1651-1659. (13) Lonnerdal. B.; Forsum, E.: Hambraeus, L. Am. J. Clln. Nutr. 1976, 2 9 , 1127-1133. (14) Saarinen, U. M.; Slimes, M. A. Pediatr. Res. 1979, 13, 143-147.
..~..
RECEIVED for review August 25,1986. Accepted October 16, 1986. This work was supported by the National Science Foundation (Grant CHE-8403759).
Determination of Heavy Siderophile Elements in Geological Samples via Selective Excitation of Probe Ion Luminescence R. J. Haskell' and J. C. Wright* Department of Chemistry, University of Wisconsin-Madison, Madison, Wisconsin 53706
Previously developed techniques for selective excitation of probe ion luminescence (SEPIL) for Re and Os ultratrace measurements are appiled to a variety of geobgicai samples. The ions arqobserved via their incorporatlon into anttfluorlte A,SnX, (A = K, Rb, Cs; X = CI, Br) host lattices. The effect of the host on the optical characteristlcs of the dopant is discussed in the framework of possible trade-offs that need to be made in real sample analysis. Sample preparation procedures that have been optimized for the determination of these elements in varlous sampks are described and Justified. Good agreement wRh several NBS, USBM, and NIM standards is reported.
Heavy siderophilic elements, such as rhenium and osmium, are of interest to geochemists to identify differentiation 'Present address: Control Division, The Upjohn Co., Kalamazoo, MI 49001. 0003-2700/87/0359-0427$01.50/0
processes in primordal earth (1). Low abundances of these elements on planetary crusts allow them to become indicators for the extraterrestrial origin of spherules in some deep-sea sediments (2), stratospheric dust (2), and the lunar surface (3). For this reason, the high noble-metal concentration in the cretateous-tertiary boundary layer, deposited 65 million years ago, was taken as evidence for a meteor that resulted in mass extinctions (4). In addition, the intrinsic value of these elements makes their recovery economically feasible from materials even where they are present at parts per billion levels (5).
While all of the traditional spectrometric techniques have been applied to these elements (6, 7),results are generally disappointing. Instrumental detection limits are poor, and interferences from base (8)and precious (9) metals are a severe problem, particularly for flame methods. Electrothermal techniques (IO) are limited by the refractory nature of the elements (Re bp 5600 "C).Fire assay, a frequently employed decomposition/preconcentration technique, is highly de@ 1987 American Chemlcal Society
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ANALYTICAL CHEMISTRY, VOL. 59, NO. 3, FEBRUARY 1, 1987
pendent upon the sample matrix and proficiency of the analyst (11). Also the large sample quantities, 1-lo00 g, required by this method may not be available for rare materials. Instrumental neutron activation (INAA) is generally not sensitive enough because of the interferences from the complex matrices involved, while radiochemical neutron activation (RNAA) is expensive, time consuming, and inconvenient (3). Selective excitation of probe ion luminescence (SEPIL) has been shown to be a sensitive and selective technique for the analysis of a variety of elements with detection limits that often exceed those of RNAA (12). The advantages of sensitivity and selectivity are maximized when a tunable dye laser is employed as the fluorescence excitation source. Emissive and nonemissive actinides (13-15) and transition metals (16, 17) have been observed via SEPIL. The analyte metal ions are coprecipitated with a suitable host precipitate, and the laser excited fluorewnce is used to measure the ions selectively. The nonemissive ions are observed indirectly via the formation of dimer sites in the precipitate where the second member of the dimer is an emissive lanthanide. Rhenium and osmium in solution at low parts per trillion levels have been quantified via SEPIL methods by coprecipitating these metals with AzSnX6(A = K, Rb, Cs; X = C1, Br) precipitates that have the antifluorite structure and using laser excited fluorescence (18, 19). There is little mention in the literature regarding the use of SEPIL in practical analytical problems. The goal of this paper is to discuss the considerations that must be taken into account and to demonstrate the application of this technique to the determination of rhenium and osmium in geological samples. While rhenium and osmium can exist in a variety of oxidation states (Re, +2, +4, +6, +7; Os, +2, +3, +4, +6, +8), only the +4 state has been observed to demonstrate characteristic sharp line optical transitions. Therefore, steps must be taken during the analytical procedure to ensure that the analyte is quantitatively removed from the geological matrix and converted to the tetravalent state prior to coprecipitation. Sample preparation procedures that have been optimized for the determination of these ions in various samples are described and justified.
EXPERIMENTAL SECTION Apparatus. The nitrogen-pumped dye laser system used in this study has been described previously (20). 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 cooled photomultiplier (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 days, and then rinsed with DDI water. All reagents were ACS reagent grade unless otherwise specified. Solutions (2.5 M) of HzSnBrs and H,SnCl,j were prepared from HBr, Br2,and Sn (99.999%, Aldrich) or HCl, Clz, and Sn via the methods described in ref 18. Solutions (0.21 M) of KzSnBrs or K2SnC&were prepared from the corresponding hexahalostannic acid and either KBr or KC1. Stock solutions (lo-, M) of K2SnCI,(Spex, 99.99%) in 8 M HCl and KzSnBr6in 8 M HBr were prepared as in ref 19. Analytical standards were prepared by serial dilution of lo9 M stock solutions of KzReC16in 1 M HCl while K2ReBr6in 3 M HBr were made from KReO, (Spex, 99.99%) (18). Procedure. Geological samples were analyzed by dissolving the sample by fusion or high-temperature acid digestion methods, reducing the Re or Os to the tetravalent state, and coprecipitating the analyte metals in AzMX6precipitates. The details of the preparation method differed for Re and Os. The fluoresence measurements were performed with the laser system (201,and the fluoresence intensities were compared against those of
Table I. Rhenium Content of Samples Determined via SEPIL
sample Canadian concentrate Arizona concentrate SRM 332 Canadian ore Arizona ore Utah ore SRM 331 KRe0, standard
Re content expected found 1090 f 30 ppm' 134 ppmb 10.2 f 0.2 pprn' 220-560 ppbd 30-135 ppbd
1111 f 176 ppmf 111 f 3 ppmf 10.2 f 1.8 ppmf 441 f 15 ppbs 75.5 f 4.9 ppbg 45 f 8 ppbe
43 f 16 ppb' 530 pptrd
27.3 f 5.7 ppbs 520 pptr
a Determined via spectrophotometry. Determined via atomic absorption by supplier. Reference 18. See text. e Certified NBS value. fusing fusion/open breaker reduction. "sing bomb/ closed reflux apparatus.
standards prepared by coprecipitation of known amounts of Re or Os. A summary of the samples analyzed in this work is given in Table I. The rhenium content of the ore concentrates was unknown. An extraction/spectrophotometricprocedure developed by Budesinsky (21) was used to analyze those ore concentrates that had a rhenium content greater than 500 ppm. These highly enriched materials were useful for optimization procedures. The sample preparation for geological materials containing greater than 1ppm of Re is described in ref 18. A sample (250-350 mg) is fused with NaOH and Naz02whereupon the cake is decomposed with water and acidified with HCl. After filtration, an aliquot of this solution is taken and reduced with H3PO2 in boiling HCl for 1h. After cooling,this solution is used as a dopant solution in the procedures outlined above for the production of doped KzSnC&precipitates. Decomposition of geological materials containing less than 1 ppm of Re is as follows: 250-300 mg of sample is weighed into a Teflon-lined acid digestion bomb (Parr4749) containing a 0.5-in. Teflon-coated stir bar. Then 1.0 mL of 49% HF (Ultrapure, Aldrich),2.0 mL of 38% HCl, 0.5 mL of HzO,and 1.0 mL of 71% HNOZare added, and the bomb is sealed, and heated in a glycerine bath with stirring at 210 "C for 1 h. The bomb is allowed to cool, and the contents are transferred and diluted to 25.0 mL with a saturated aqueous solution of boric acid. A sample (5.0 mL) is added to a 100-mLround-bottom flask along with 2.0 mL of KI solution (0.16 g/mL) and 30 mL of 8 M HBr. The flask is connected to a cold water condenser. On top of the condenser is an adapter to which Tygon tubing is attached. The tubing ran into a few milliliters of water. The contents of the flask were heated to boiling for 1 h, and the condenser walls are rinsed down with water after 30 min. The flask is allowed to cool to room temperature while still attached to the condenser. The mixture is transferred and diluted to 100 mL with water. The resulting sample solution is treated as a dopant in the precipitate-forming procedure described above in which KzSnBreis used as the host. Sample preparation for osmium-containing materials is as follows: 95-100 mg of sample is weighed into a zirconium crucible (B. J. Scientific), mixed with 800 mg of 3:l Naz02:NaOH,and covered with another 200 mg of fusion reagent. The crucible is covered and heated at 610 "C for 10 min, swirling after 5 min. After being cooled with water, the crucible is dropped into a 100-mL round-bottom flask containing 30 mL of 8 M HBr at 0 "C and 0.5 mL of an HBr solution that is saturated with SnC12. The flask is sealed immediately with parafilm, left in an ice-water bath for 30 min, and warmed to room temperature. The flask is opened and attached to the reflux apparatus described above, and the contents are boiled for 1 h. The flask is cooled to room temperature. If there is a brown coloring (from Fe3+)in the solution at this stage, the Sn2+solution is added dropwise until the coloring disappears. The mixture is then transferred and diluted to 50 mL with 8 M HBr. The resulting solution is treated as a dopant in the precipitate-forming procedure described above in which RbzSnBrs is used as the host. Doped precipitates used as standards are prepared as above by using the stock Os&z- and Rex$ solutions as dopants. These solutions do not undergo the sample preparation procedures but
ANALYTICAL CHEMISTRY, VOL. 59, NO. 3, FEBRUARY 1, 1987
are used after dissolution of the appropriate salt in acid. Doped K2SnX6precipitates were prepared according to the methods outlined in ref 18. A mixture of a 3.0-mL portion of K2SnX, and 1.0-3.0-mL aliquot of dopant solution (either dissolved samples or standards) was heated to 90 O C until precipprecipitates were itation occurred. Doped Rb&h& and Cs&& prepared according to the methods outlined in ref 19. A 50% excess of RbCl or CsCl, dissolved in HX, was added to a mixture of 0.5 mL of HzSnX6and 1.0-3.0 mL of dopant solution. Precipitation occurred immediately upon addition of the alkali halide. In all m e s samples and standards were prepared simultaneously. Potassium precipitates are placed in one of ten 3-mmdiameter depressions in a copper block (two columns of five) and packed by using a stainless steel rod and gentle tapping with a hammer. Each column contained one sample and four standards having analyte doping levels bracketing that of the sample. For rubidium and cesium precipitates, a pellet of undoped K2SnBr6is packed into the depression first and the doped material is then pressed into place with the rod and finger pressure. The copper block is screwed onto a cryogenic refrigerator and cooled to 11 K. A calibration curve is generated from the standards by subtracting the signal measured at a background wavelength from that obtained at the analytical wavelength.
429
b) Re
RESULTS AND DISCUSSION Formation of individual sites via charge compensation mechanisms is not expected in the dopant-host systems under study because the dopant ion substitutes uniquely for the tetravalent tin. It is possible, however, that the simultaneous incorporationof different species into the host will occur. The narrow band nature of the dye laser and the optical transitions of the dopant allow for the selective observation of individual species within the host. Two degrees of freedom in selectivity are potentidy available, one in excitation and one in emission. To selectively observe emission from one species, the dye laser wavelength is set to a value at which only that species is absorbing. Even if several different species have overlapping emission spectra, only one will be emitting as only that one has been excited. The same logic applies to the reverse case in which there is overlap of excitation wavelengths; an emission wavelength can be chosen so only one species is observed even though several may have been excited. Since the ligand-to-metal charge-transfer (LMCT) bands for these ions overlap, excitation at 443 nm will result in the emission of both species in a host lattice that has been simultaneously doped with each. Figure l a shows an example in the case of RbzSnBr6doped with 0.15 mol 70each ReBrtand Significant overlap of emission between the two species is evident. Parts b and c of Figure 1show the emission from the ReBrt- and OsBr2- ions, respectively, brought about by the selective excitation of the individual species. Rhenium was excited via the rs(4Aa) Fa (T%) u4 transition at 663.4 nm and osmium via the rl (3T1g) rl ?Alg) v4 transition at 614.5 nm. A detailed description of the changes that occur in the dopant optical spectra as a function of the host employed can be found in ref 18 for rhenium and ref 19 for osmium. Control over spectral parameters via choice of a host permits the analyst to optimize the spectroscopy of the analyte for a given sample matrix. For example, if a species were to be incorporated into the host lattice that contributed a short-lived fluorescence to the overall analytical signal, detection of analyte emission could be temporally delayed until the interfering fluorescence has decayed away. A host should be chosen to maximize the difference in the lifetimes of the analyte and interference so that the delay and gate used in the gated detection can provide maximum discrimination. In this example, CszSnC1, would be a good choice and KzSnBr6would be a poor one. Unfortunately, the latter host generally affords lower detection limits and a trade-off will become necessary in some cases.
--
743
750
7 57
764
771
A (nm)
Figure 1. Demonstration of SEPIL in Rb,SnBr, doped with 0.15 mol % each Re&:and OsBrt-: (a) simultaneous emission of Re4+ (2T,,) (4A2g)) and Os4+ ('Alg) (3T1g))exciting at 443 nm (LMCT); (b) emission of Re4+ brought about selective excitation of (4A28) (2T2,) u4 at 663.4 nm; (c) emission of Os" brought about by selective excitation of (ql,) ('Alg) v4 at 614.5 nm.
- r8
r8
-+
(rl
re
rl
- r4 - rl
(r,
A brief summary follows of the characteristics that can be varied depending upon the particular AzM& host employed. Choice of the host A ion will control selectivity as it is this ion that influences the line widths. The cesium hosts have the maximum resolution, but the potassium hosts have the lower detection limits. The A ion has only a small effect on emission decay rates. No one A ion is best in all cases, but potassium, rubidium, and cesium were commonly used. Quaternary amines were never employed as no emission was observed from salts containing these ions. Presumably fluorescence quenching occurs through nonradiative relaxation via the energetic N-H vibrations. Waters of hydration present in the sodium salts were observed to have a similar effect. Choice of the M ion controls the physical characteristics of the host as well as its chemical stability. The M ion needs to be photophysically inert to prevent its interfering with the observation of the dopant ion. While lead, zirconium, and hafnium were all potential M ions, tetravalent tin was found to be most satisfactory in both regards. The most significant choice is the one involving the X ion. Use of the bromide hosts simultaneously results in orders of magnitude lower detection limits and a much faster emission decay rate than when a chloride host is employed (18,19). The emission lifetime can change by a factor of 10 in the case of some dopants. No one host yields a greater tolerance to interferences over any other: however, the ions that will interfere in each host may be different so that the choice of a halide can be influenced by the type of interferences that are anticipated. Fe3+is an example of an ion that interferes strongly in bromide hosts, but not in chloride hosts. Iodide
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ANALYTICAL CHEMISTRY, VOL. 59, NO.
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5.c
Table 11. Relative Level of ReBrt-, Observed in K2SnBr6, as a Function of the Amount of KI Employed in Reduction Step . *
P u)
re1 Re
..
7
..
'
2.5
73 z
8
0
-
621.3
622 7
6220
Mnm)
Figure 2. Photon counts vs. monochromator setting (stepping over the ('A, ) ,'l (qIg) v4 transttion of 5.5 X 10-lo mole fraction K,SnBr,:Osbe2-; excRlng charge-transfer band at 443 nm).
rl
salts decomposed rapidly under laboratory conditions; fluoride salts were not explored. Qualitative identification of the dopant a t low levels is carried out by stepping the monochromator or dye laser over an appropriate optical transition. The emission intensity is integrated by gated photon counting over 4000 laser pulses for each of 10-15 sampling points along the chosen wavelength region. The entire process of acquiring data takes about 1 h. A correction for the decrease in dye laser intensity over time is required when dyes subject to photodegradation are employed. Figure 2 shows an example for stepping the monochromator over the rl (lAl,) rl (3T1,)v4 transition of 5.5 X lo-'' mol fraction KzSnBr6:OsBr62-. Rhenium. Two general types of decomposition procedures were investigated. The first was a fusion method, similar to the one described above. The acidification step was carried out in a centrifuge tube so the sample could be centrifuged to remove a white suspension of silicates that resulted from acidification. After centrifugation, the white solid was washed several times with HC1. The second method entailed heating the sample in a Teflon-lined acid decomposition bomb with a mixture of HF, HC1, and "0% Advantages of the first method include speed, safety, and the capability of achieving high temperature while the disadvantages include high solid content of resulting solutions, possible loss of volatiles during the heating step, contamination of sample due to impurities inherent in fusion reagents, and the inability to completely solublize aluminosilicates. The bomb method has the advantages of the use of purer reagents, more complete decomposition of silicates, and retention of volatiles during heating. The disadvantages include potential danger, time, and inability to obtain temperatures above 250 "C. In both cases, the Re is present in the +7 oxidation state and a quantitative reduction to the +4 state must be performed before it can be measured. The initial sample preparation procedure employed fusion decomposition. However, detectable quantities of Re4+were obtained only when a closed reflux apparatus was employed and the reduction was accomplished with HBr so that ReBrs2was produced. Rhenium is reported to become volatile, presumably as Rez07or HRe04, in acidic solution at elevated temperatures (22). Therefore, it is possible that some loss of analyte occurs from the open beaker before reduction of rhenium to the nonvolatile ReBrBZ-anion is complete. The recovery of rhenium from the ore to the doped precipitate was still not quantitative. In order to identify where in the sample preparation procedure rhenium was being lost, we added a portion of rhenium standard to solutions at various stages in the procedure. The
-
re1 Re signal
signal mmol of KI
(15%)
0.96 1.4 1.9 2.9
1.00 f 0.15 0.98 & 0.15 1.01 f 0.15 0.91 f 0.14
mmol of KI
(15%)
3.9
0.50 f 0.08 0.63 f 0.09 0.67 f 0.10
4.8 6.7
final solutions were used as dopants in the preparation of KZSnBr6:ReBr6'-precipitates. Examination of signal level in this series of samples indicated that losses of rhenium were occurring in both the centrifugation and reduction steps, probably as the result of volatilization. As a fusion procedure required the centrifugation step to remove undecomposed silicates, it was abandoned in lieu of a bomb decomposition method. Complete dissolution of ore samples could be brought about by heating the sample with a mixture of HF, HC1, and HN03 at 210 "C. Boric acid was added upon completion to complex the excess fluorides. A variety of reagents were examined for use in the reduction step after the acid digestion in the bomb. H3P02/HBr, Na2C2O4/HBr,or Sn2+/HBryielded no observable signal even from an ore spiked with a standard before decomposition. Some species left over from the decomposition process, probably NO3-,must be preventing the formation of ReBrs2as an H3P02/HBr reduction after a fusion decomposition results in an unambiguous rhenium signal. The sample resulting from the acid digestion could be reduced with potassium iodide and HBr to produce large quantities of ReBr:-. With the assumption that 20% of the NO3- originally present in the 1.0 mL of HN03 employed to decompose the sample is converted to NO during the reduction step, a 10-fold excess of electrons would be supplied by 9.6 mmol of KI. Table I1 shows that less than 1.9 mmol of I- should be used in the reduction step. This decrease may be the result of a competing reaction in which formation of the stable Re162-anion occurs. It should also be mentioned that appreciable reduction of the nitric acid occurs in the decomposition step, so that there is certainly less than 9.6 mmol of NO3- present at the start of the reduction. SEPIL analysis was carried out by excitation of the LMCT bands at 443 nm and observation of the F7 (zTzg) rs (4A 1 v3 at 767.4 nm when K$"Br6 was used as the host. Excitation of the rS (4A2,) rs (2Tz,) v4 transition at 640.7 nm and observation of the r7(zTzp) Fa (4A2g)v4 transition at 722.8 nm was used for the K2SnC16host. Ores and ore products were the first geological samples to be analyzed for rhenium by using the SEPIL technique. They contain appreciable quantities of the element and were used for optimization of the sample preparation methodology. The primary geological form of rhenium is as Resz, which is exclusively associated with molybdenite, MoS2 In an industrial process, the ore is ground to a fine powder before being introduced into a floatation step. In this process, the bulk of the aluminosilicates are removed (as mill tails), leaving a primary concentrate consisting of mostly MoSz, CuSz, and traces of Res2 Depending upon the grade of the ore, a second floatation step may be carried out to separate the copper, producing a secondary concentrate containing MoSz and Res2 First ore concentrates, then the ores themselves, and finally the mill tails were subjected to rhenium determination via SEPIL methods. Several samples were procured for the optimization procedure. The first was a secondary concentrate from Canada, containing approximately 1000-1500 ppm of rhenium (ob-
-
-+
-
ANALYTICAL CHEMISTRY, VOL. 59, NO. 3, FEBRUARY 1, 1987
tained from Island Copper Mine, Utah Mines Ltd., Port Hardy, British Columbia). The extraction/spectrophotometric method described in the Experimental Section showed the rhenium content was 1090 ppm. The second was a concentration from Arizona, containing 134 ppm of rhenium as assayed by an extractionfatomic absorption method (obtained from Sierrita Mine, Duval Corp., Sahaurita, AZ). An NBS (SRM 332)copper concentrate, containing a certified rhenium content of 10.2 f 0.2 ppm, was also analyzed. Ores, from which the first two concentrates were produced, and a third ore from Utah (obtained from Bingham Mine, Kennecott Corporation, Utah Copper Division, Magna, UT), all containing unknown quantities of rhenium, were also obtained. An NBS (SRM 331)copper ore mill tail containing a certified rhenium content of 43 f 16 ppb was the material with the lowest rhenium content analyzed. Table I shows the results obtained for the variety of samples analyzed and the procedure employed for each. INAA was unsuccessful at determining the rhenium content in all three ores and SRM 331 due to interferences from Mo, Eu, Se, and a generally high background radiation level. In most cases, the mineralization of an ore with molybdenite is about 0.02-0.1 %. In particular, for the Arizona ore it is about 0.1% (23).If the rhenium is carried along quantitatively with the molybdenum during the floatation processes, then a value of approximately 130 ppb would be expected in the ore given the measured 134 ppm value of the concentrate (see Table I). However, this sample is a secondary concentrate, and some rhenium would be expected to be lost during the first floatation step. Therefore, 130 ppb represents a high ceiling, and 76 ppb becomes a reasonable result. The mineralization in the Canadian ore is about 0.02-0.05%, predicting a value of 500 ppb of rhenium in the ore given the 1090 ppm value of the concentrate (see Table I). However, this ore is known for very high rhenium content (24) so the experimental value of 455 ppb is reasonable as well. The value obtained for the NBS standard, 27 f 6 ppb, is in reasonable agreement with the certified rhenium content of 43 f 16 ppb. It is thus demonstrated that the SEPIL methods developed for rhenium determination are applicable to samples containing wide ranges of analyte. Osmium. Two standard ores were obtained for use in optimizing sample preparation as well as estimating the accuracy and precision of the SEPIL methods for the determination of osmium in naturally occurring materials. The first, SARM-7 (NIM, Johannesburg, South Africa), has a certified Os content of 63 f 7 ppb. The second, USBM-Pt-A (USBM, Denver, CO), contains approximately 110 ppb of osmium. Both ores originate from the Bushveld complex of South Africa and contain large quantities of sulfide minerals. Complete decompositionof platinum group element (PGE) ores is difficult. PGE's, either as sulfide minerals or in the native state, tend to be resistant to all but the most vigorous conditions. The mineral osmiridium (OsIr), in particular, is very difficult to solubilize due to its high density and chemical inertness. Since it is likely that the osmium will be in the highest possible oxidation step (+8) after the decomposition process, loss of analyte via the formation of volatile OsOl is an important concern. Acid bomb digestion methods were found to be unsuitable for Os analysis in ores because of volatilization losses. A variety of reagent mixtures were examined for use in the acid methods. A combination of either HF, HC1, HN03, and HzO or HF, HC1, and HzOzeffected complete decomposition. A temperature of 220 "C must be maintained for at least 5 h. The high pressures and temperatures in the bomb resulted in the diffusion of OsO, through the Teflon liner in the bomb. The osmium signal intensity detected in the final precipitate
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was inversely proportional to both the decomposition time and temperature. Higher temperature (>150"C) and short times (1h) reduced signal more effectively than lower temperature (110 "C) and long times (65 h). Treatment of standard solutions with conditions sufficiently vigorous to decompose an ore yielded dopant solutions containing hardly any detectable osmium. No osmium is detected when peroxide is employed, and higher pressure builds up in this case than when nitric acid is used. In addition, the solution produced by the procedure is very unstable, losing appreciable quantities of osmium after 24 h, again probably because of Os04 volatilization. Fusion techniques however proved quite satisfactory for sample decomposition although it is not clear why the fusion retained osmium better than the bomb methods considering the strong oxidant and high temperature employed. The osmium may be converted into various nonvolatile forms (25). It is interesting to note, however, that when a standard OsBr2solution is treated with a NazOzfusion decomposition, complete loss of osmium occurs whereas osmium originating in a geological matrix is retained. Possibly, the slow introduction of osmium into the flux as the ore decomposes allows time for the osmium to be rendered nonvolatile. Such a process may have no time to occur if the osmium is originally present in the crucible as a readily available species. Complete decomposition was noted for 100-mg samples treated with a 10-fold excess of either 3:l or 4:l Naz02:NaOHin a covered zirconium crucible for 10 min at 595-640 "C. Finally, it should be pointed out that the solution resulting from the dissolution of the cake will be less oxidizing than the strongly acidic nitrate solution produced by the bomb method. Therefore, loss of Os04 is less likely to occur. A variety of reagents were examined for use as reductants of the fusion cake. When the fusion procedure was followed by a 1-h reflux in 8 M HBr, the efficiency with which the osmium converted to OsBr2- increases for the following order of reagents: KI < HBr (alone) C Sn2+(as a saturated solution of SnC12in 8 M HBr). It is somewhat surprising that KI in HBr is less effective than HBr alone. A possible explanation is that OsIs2-is being produced in a competing side reaction when I- is present. Sn2+is a well-known reducer of PGEs, so its position in the order is reasonable. The presence of H3PO2either alone or in conjunction with any of the other reagents after either decomposition step ultimately results in precipitates containing no observable OsBrs2-. Given the highly reducing nature of H3P02,it seems likely that excessive reduction is occurring. The dissolution of the fusion cake in HBr at the start of the reduction introduces a significant chance of losing osmium through spattering or volatilization. Appreciable loss occurred via one or both of these mechanisms when the crucible was immersed in a beaker containing 8 M HBr at 0 "C and the contents were transferred to the reflux aparatus. Custommade crucibles small enough to fit through the neck of the 100-mL flask were substituted for the larger crucibles. Once cooled, the crucible could be dropped into the flask containing 0 "C HBr, thus eliminating loss through spattering. This modification decreased loss of analyte, but the method still produced dopant solutions containing 20-30 % less OsB$ (as assayed via SEPIL) than would be predicted on the basis of the amount of osmium known to be present in the original sample. Sealing the flask immediately after dropping in the crucible and keeping the flask immersed in an ice-water bath for 30 min before refluxing reduced the loss of analyte to less than 5% . Osmium concentrations of 53 f 7 and 103 f 7 ppb were found in the SARM-7 and USBM-Pt-A ores, respectively, when the procedure described in the Experimental Section
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Anal. Chem. 1987, 59, 432-436
was used. These results agree well with the values of 63 f 7 ppb certified for the former ore and approximately 110 ppb for the latter. Excitation of the rl (Tlg) rl ('Al 1 v4 transition at 614.5 nm and observation of the rl ('Alg) ) v4 transition at 753.9 nm prevented interference from Ptf+ when the RbzSnBrahost was used.
-
(%rl
+
CONCLUSIONS The methods described in this paper require ca. 12 h of sample preparation for four samples by an experienced analyst. The methods are appropriate for the determination of Re and Os present a t low parts per billion concentrations in geological materials. Although the SEPIL technique itself has demonstrated ppt detection limits in solution (12,18,19),the need to decompose a solid sample introduces a dilution factor of at least 100. These characteristics can be improved by those concerned with routine analysis. It should be noted that a small sample size (
