Anal. Chem. 1988, 6 0 , 1231-1234
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Low-Blank Chemical Separation of Rhenium and Osmium from Gram Quantities of Silicate Rock for Measurement by Resonance Ionization Mass Spectrometry Richard J. W a l k e r National Measurement Laboratory, Center for Analytical Chemistry, A 21 Physics Building, National Bureau of Standards, Gaithersburg, Maryland 20899
Procedures have been developed so that nanogram quantttles of Re and Os can be cleanly and efflclently separated from gram quantltles of sllicate rock. Concentrations of these elements can subsequently be determined by M o p e dliutlon and the isotopic composltlon of Os obtalned by uslng resonance ionization mass spectrometry. Typically, 1-10 g of rock powder Is spiked wlth lWReand 'O0Osand then dissolved In sealed dlgestlon vessefs made of Teflon fluorocarbon resln by uslng a comblnatlon of HF, HCI, and ethanol. Once the rock Is In solution and equilibrated wtth the spike, the solution Is evaporated to dryness and the r d u e redholved In H2SOI. Ceric sulfate Is added to the solution and Os Is distilled from the rock matrix as 080, by using a reflux condenser-dlstillation apparatus. The OsO, is trapped and reduced in a 3:l mixture of HCI and ethanol. Re Is extracted from the H,SOJrock solution into a solution of trlbenzylamine In chloroform. Re Is subsequently back-extracted Into concentrated ammonium hydroxlde. Chemlcal blanks are 80 mesh) are added to the digestion vessels. Appropriate quantities of accurately weighed spike solutions are added to separate Teflon beakers. Approximately 6 g of ethanol, 10 g of 10 mol/L HCl, and 10 g of concentrated HF are subsequently added to the beakers. These solutions are quickly and quantitatively added to the digestion vessels containing the sample powders. The vessels are then immediately sealed and heated to 383 K for 8 h. Rapid sealing is required because exothermic heating (occurring within 30 s) resulting from rock/acid reactions could release volatile Os04 from the rock matrix and result in Os loss before spike/sample Os equilibration can occur. Furthermore, ethanol and HC1 must be present in excess during the digestion to reduce all Osw that is released from the rock matrix to the stable hexachlorcwmate. It should be noted that the ethanol must be present during the digestion/heating and not added following the heating stage because Teflon is permiable to OsO, at elevated temperatures. Another reductant, such as HBr, can be used in place of the HCl/ethanol mixture. Upon cooling, the digestion vessel is opened and placed under an infrared heat lamp until the solution has dried. Heating should never exceed approximately 473 K because of the possible loss of the somewhat volatile Re207,which sublimes at 540 K. Ethanol/HCl/HF digestions and subsequent evaporations should be repeated 3-5 times to ensure the complete digestion of the rock
and remove most silicon from the system as a volatile silicon fluoride. The amount of ethanol that is used can be reduced with each digestion as the amount of unreacted silicate decreases. Final digestion is accomplished by adding 10 g of concentrated sulfuric acid to the residue, sealing the digestion vessel, and heating to 413 K for 8 h. Because of the large volume of material digested, the resulting solution is rarely clear and typically contains some precipitate. The solution is ready to add to the distillation vessel. Assembly of Distillation Apparatus. The distillation apparatus is assembled as shown in Figure 1. Ground glass joints are sealed with purified water or concentrated HzS04and tightly clamped. Organic greases should not be used as they may trap OsO, and facilitate its reduction. If water is used as the sealant, the joints must be kept moist during the distillation. Approximately 20 mL of a 3:l mixture of 10 mol/L HCl and ethanol is added to the trap and chilled with ice. Nzis bled into the apparatus and adjusted to a flow rate at which several bubbles pass into the trapping solution per second. Plumbing for the chilled water condensers should also be attached just prior to distillation. Addition of Sample to Apparatus and Oxidation. The separation of Os from the rock/sulfuric solution is achieved by oxidizing the Os in solution from the stable hexachlorosmate (bp >673 K) to the volatile Os04 (bp -383 K) and distilling it from the more refractory Re207 and other rock components. To oxidize Os, the distillation flask is removed from the apparatus and the sample solution is added. An appropriate quantity of ceric sulfate (2 g/g of rock) is also added, along with enough water to dilute the acid to approximately 6 mol/L H2S04. Upon addition of the water, the solution begins to heat because of the exothermal reaction between water and sulfuric acid. Such heating could potentially result in the loss of OsO,, so the flask should be quickly reattached to the apparatus. Heat is applied with a variable resistance heating mantle. The solution is boiled a t approximately 378-383 K. Chilled water is circulated through the condensers during the entire distillation. Distillation is continued approximately 1.5 h to ensure maximum recovery of Os from the solution. Some SiF., may accumulate on the condensers but should not clog the apparatus if the rock was sufficiently desilicified. Upon completion of the distillation, the trapping solution is poured into a 20-mL Teflon container, capped, and sealed with Teflon tape, then heated to 363 K f x 1.5 h. This step reduces the Os back to the stable chloride form. Once this solution is taken to dryness with a heat lamp, the sample is suitable for loading or storage. Solvent Extraction of Re. Following the distillation of Os, Re resides as perrhenic acid in the 3-6 mol/L rock/HpS04solution. Separation can be achieved via either anion exchange chromatography (J.W. Morgan, personal communication, 1987) or solvent extraction. The solvent extraction technique was chosen for this study because of its speed and simplicity. The technique reported is slightly modified from previous reports (26, 27). All Ce4+that remains in solution following distillation must first be reduced to Ce3+,otherwise it will partially coextract into the solvent phase, severely reducing the extraction efficiency of Re. To facilitate the reduction, approximately 5 mL of H202is slowly added dropwise to the solution. The solution is then returned to the original digestion vessel and an equal volume of a solution of tribenzylamine in chloroform (approximately 1wt W ) is added to the vessel. The vessel is sealed, shaken for about 5 min, and then poured into a separatory funnel. Re is strongly partitioned into the organic phase. When the separation of the two immiscible phases is complete, the denser chloroform is removed and returned to the digestion vessel. The acid/rock solution can be discarded. An equal volume of concentrated ammonium hydroxide is added to the vessel and again the vessel is capped and shaken for 5 min. Re is strongly partitioned into the more basic aqueous phase. The denser chloroform is removed and discarded when separation of the immiscible phases is complete and the ammonium hydroxide is slowly dried with a heat lamp in a Teflon beaker. Several milliliters of aqua regia is added to the resulting residue to eliminate any remaining ammonium phases and to assure that Re is in its highest oxidation state. At this stage, Re is separated from most of the rock matrix; however, small quantities of sulfuric acid and rock components inevitably coextract. As a further purification step, the solvent
ANALYTICAL CHEMISTRY, VOL. 60, NO. 11, JUNE 1, 1988
extraction process is repeated using only 2-mL quantities of reagents. To do this, the residue that remains once the aqua regia solution is taken to dryness is picked up with 2 mL of 4 mol/L HzS04and placed in a 6-mL Teflon vessel with cap. About 2 mL of the chloroform/tribenzylaminesolution is added and the vessel is capped and shaken for 5 min. The acid is completely removed with a pipet and discarded. Two milliliters of concentrated ammonium hydroxide is added and the process repeated. The chloroform is removed with a pipet and the aqueous phase is again slowly dried with a heat lamp. The residue that remains, following a subsequent aqua regia dry-down, consists typically of 1WLof sulfuric acid from which the Re can be extracted during the filament loading procedure. Filament Loading. The filament loading procedure further purifies the sample by the selective absorption of Re and Os from low-normality HC1 or H2SO4solutions into anion exchange beads. Both Re and Os have a strong affinity for these beads (28). The filament design has been previously discussed (13). Sample loading procedures are conducted on a Teflon tape substrate that is attached to a glass microscope slide. Approximately 10-20 anion exchange beads (chloride form, 1X8,200-400 mesh) are placed in 2-4 WLof 3 mol/L HC1 containing the separated Os. Beads are soaked for several hours in this solution, the solution is then dried with a heat lamp and the beads are extracted from the residue with a tungsten needle. These beads are picked up with 1 pL of 3 mol/L HCl, using a l-wL pipet with Teflon tips, and deposited on the filament. The purification/loading step for Re is similar except that the residual sulfuric solution is diluted to 1-5 mol/L HZSO4and 10-20 beads are added for several hours. Upon equilibration, the solution is pipetted away from the beads. The beads are then picked up with 1 pL of 3 mol/L HCl and loaded onto the filament. To facilitate the reduction of the rhenium and osmium chlorides in the vacuum of the mass spectrometer by heating, the beads are covered with a layer of flexible collodion, followed by a layer of graphite (applied as an ethanol slurry). Mass Spectrometry. The resonance ionization mass spectrometer has been previously described (13). In brief, a Nd-YAG pulsed laser system tuned to wavelengths of 297.16 and 297.69 nm for Os and Re, respectively, is used to selectively photoionize the atomic species of these elements from a gas-phase reservoir produced from a hot Ta filament (2273-3273 K). A 6-in. mass spectrometer, designed at NBS, is used to magnetically select isotopes for measurement. Background signals are measured on-mass but approximately 0.1 nm off the resonance ionization wavelength. Quantification of the pulsed signal is achieved by using a 17-stage ion multiplier interfaced to a transient digitizer. The mass spectrometer is currently capable of making isotopic ratio measurements to better than 1% precision and accuracy when >lo0 pg of each of two isotopes is present and is limited by counting statistics to 1-5% when 0.05 ng), presumably
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derived from either the crucible or the flux (J. W. Morgan, personal communication, 1987). Fire assays have also been used with success to digest large quantities of silicate (16,29,30). Sample powders are mixed with nickel powder, sulfur, and a flux and fused at 1273 K (16). During the fusion, Os, and to a lesser extent Re, extract into a nickel-sulfide button that is physically separated from the rock matrix and flux upon cooling. Further purification and concentration are generally required and must be achieved by using other techniques. As with the simple fusion technique, sample digestion is complete, even with refractory phases, and large quantities of sample (up to 25 g) can be processed. The main drawback to this technique is again the significant Re blank that the process imparts (R. Teng, personal communication, 1987). Acid/ bomb silicate digestions have been commonly applied by geologists when examining most other isotope systems. This technique has also been used with success for most SIMS isotope dilution and isotope composition measurements of Re and Os (6,26). The technique is generally very clean because most reagents can be specifically purified of the elements of interest. For this reason the technique was chosen for development by this study. Disadvantages of this technique include slowness, possibility of incomplete digestion, and possible loss of any volatile Os that may form during the digestion. The described digestion process is designed to slowly break down and dissolve minerals, retain all Os released from the minerals, equilibrate it with enriched isotopes, and remove most of the silicon from the solution. Choice of Oxidant. Numerous oxidants have previously been used with success to oxidize Os for distillation, including HzOz(18),nitric acid (31),and dichromate (27). For this study each of these oxidants was assessed for both efficiency a t oxidizing Os in rock solutions (determined by using the radioactive tracer lglOs) and purity. It was found that HzOz works well with small samples but is unwieldy when used to oxidize solutions containing large quantities of rock (>0.5 g) because of its extreme reactivity. Oxidation using nitric acid proved to be slow and incomplete, especially when attempting to oxidize the hexachlorosmate. Dichromate is very efficient; however, all commercially available forms that were examined during this study imparted a significant Re blank to the solutions (>500 pg/g added). I t is difficult to remove this Re. Aqua regia was also examined as a suitable oxidant. It is both clean and efficient but is unsuitable for samples with significant Fe because it will lead to the production of volatile iron chlorides that will codistill with Os. Ceric sulfate was chosen as the oxidant for this work because it serves as an effective oxidant and can be purified of Re and Os by boiling in concentrated sulfuric acid, with no reduction of the ceric sulfate (as occurs with dichromate). Compositional Determinations. The application of the Re-Os isotope system requires that the concentrations of Re and Os, and the isotopic composition of Os, be precisely determined for a given sample. Re and Os concentrations are determined by isotope dilution. As noted previously, accurately measured quantities of highly enriched lE5Re(>99%) and 1900s (>96%) are equilibrated with the Re and Os in a sample during digestion. The 185Re/187Re and 1wOs/1920sof the Re and Os subsequently separated from the sample are precisely measured with the mass spectrometer, and their concentrations are calculated from these ratios (32). For geological purposes, the concentration of lE70smust also be determined. This isotope (produced from the decay of IE7Re)is usually reported as the 1E70s/1860s. However, because all of the Os isotopic ratios are invariant in nature, except for those with 1870s, IE7Oscan be measured versus any other Os
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ANALYTICAL CHEMISTRY, VOL. 60, NO. 11, JUNE 1, 1988
Table I. Duplicate Analyses' of Re and Os Concentrations and *870s/1"Os in Silicates (Komatiites) sample
Re, ppb
Os, ppb
218309
1.08 f 0.015 1.00 f 0.020 0.945 f 0.020 0.924 f 0.025 1.27 f 0.013 1.40 f 0.020 0.962 f 0.015 1.17 f 0.013
1.23 f 0.016 1.19 f 0.018
218308 218326 218360
1.45 f 0.015 1.35 f 0.019 1.45 f 0.015 1.51 f 0.015 3.31 f 0.040 2.20 f 0.043
1s70s/'ffiOs 2.40 f 0.066 2.35 f 0.070 1.96 f 0.040 1.89 f 0.038 2.51 f 0.088 2.60 f 0.070 1.40 f 0.018 1.80 f 0.044
Uncertainties cited are based o n t h e r a t i o measurement counti n g statistics.
isotope and the 1s70s/1s60s ratio can be calculated. It is noted that whichever other isotope is used in the lS7Oscompositional determination, both it and the 1870smust be corrected for contributions from the spike (e.g. the lWOsspike has 1.8% 1920~).
Blanks. Total analytical blanks for the chemical procedure were determined by adding 100 pg of '%Re and 20 pg of '%'Os to Teflon digestion vessels and treating this spiked sample the same as an analytical sample. Blanks were run several times during the development of the chemistry and were always
