2.1 CHEMICAL SEPARATION OF OSMIUM
Prior to mass-spectrometric determination of Os-isotope ratios, osmium has to be separated from the matrix and subsequently cleaned in a series of steps involving either a NiS fire-assay or leaching, distillation of OsO4 from a 4N sulfuric acid Cr(VI)-solution into HBr (modified after Luck, 1982) and finally a single resin bead microchemical cleaning procedure (Reisberg et al., 1991; Falkner, 1992) or microdistillation. For acidified hydrogen peroxide leaching of sediments, the Ni-S fire-assay step is omitted. Acid dissolution of samples in a reduced environment (Reisberg et al., 1993), or in oxidizing environments as employed in Carius tube digestions (Shirey and Walker, 1995 and refs. therein) are also widely used.
2.1.1 FIRE-ASSAY
The high partition coefficients of platinum group elements (PGE) between NiS liquid and silicate melts attests to the efficiency of immiscible NiS liquids as a concentrator of PGE (Crocket et al., 1992). The total recovery for Os is usually better than 90%, for Re ~60%, depending on sample composition.
Coors ceramic crucibles and lids are washed in Milli-Q water (or soaked in dilute HNO3) and dried. The flux-mixture (Borax, Ni-powder, and S-powder) is prepared using a flux/sample ratio of ~0.8 - 2 (for felsic material) or 2 - 4 (for mafic material, basalts successfully have been fused with a ratio of 1), and a Borax/Ni-powder/S-powder ratio of 13/2/1. By changing this ratio systematically, Esser (1991) showed that about half of the total blank is associated with the NiS. The blank for this flux is usually ~0.5 - 1.5 pg Os/g flux with an 187Os/186Os of ~2 - 6. Blank data show a strong negative correlation between Os concentration and isotopic composition, indicating the presence of two Os components, one with high Os concentration and low 187Os/186Os and one with low Os concentration and high 187Os/186Os. The Carius tube technique, in comparison, yields a total blank of ~5 pg Os and 10 pg Re, and it is limited to sample sizes <4g due to the size of the Carius tube (Shirey and Walker, 1995). High-purity silica powder must be added to samples not containing silica (e.g., Mn-nodules). The reagents are weighted into a clean plastic vial, ground carefully with a glass rod and homogenized thoroughly. A mask must be worn while handling the Ni-powder (carcinogenic) and gloves should be used while handling the Ni-powder and the S-powder (skin irritant). In order to ŇcleanÓ the flux mixture and achieve lower blank levels, the flux reagents can be fused in the presence of NiS, the resulting glass crushed and powdered and then used as a flux for samples. Preliminary analyses of fused Na-borax suggest that Os concentration and isotope ratio is lowered by ~50%.
Samples are weighed into a ceramic crucible (use an anti-static pistol) on a microbalance. A crater is formed in the middle of the sample using a clean test tube, and spike solution (either pure Os or mixed PGE) is dripped into this crater (calculate the weight of spike by taking the difference in the weight of the spike bottle). Carefully cover the spike with sample powder by shaking the crucible slightly. Cover the crucible with a lid and allow the spike solution to evaporate (overnight).
After the spike has dried, sample and spike are thoroughly mixed using a glass test tube. Flux is added step by step and mixed with the sample. To insure that none of the spike has adhered to the glass test tube, a crater is formed in the center of the sample/flux mix with the test tube, the test tube is cleaned with a small strip of Kimwipe, and the Kimwipe is placed in the crater. The crucible is shaken slightly to cover the Kimwipe with the mixture, then covered with a ceramic lid. Finally, the crucible with the lid is placed into a simple fire-clay crucible or another Coors ceramic crucible as a precaution against loss of melt in case the sample crucible cracks or dissolves during the fusion.
The NiS-bead is dissolved in an Erlenmeyer flask filled with ~125 ml 6.2N HCl (quartz distilled) on a hotplate at ~200° - 250°C for ~6 - 12 h (or overnight using a timer), depending on the size of the bead. Dissolution is characterized by vigorous bubbling (H2S), the development of an intense green color (aqueous Ni2+), and the formation of fluffy black residues floating in solution or creeping up the sides of the flask. These residues are probably (Cu, Hg, Ag, Sb, Sb,...) sulfides (Skoog and West, 1982), and radiotracer experiments by Esser (1991) show that Os (and Re) is almost quantitatively partitioned into these aggregates. PGE sulfides are resistant to attack by acids other than nitric or perchloric (Cotton and Wilkinson, 1988). During the dissolution the Erlenmeyer flask is covered with a Teflon lid. The hotplate should be turned off immediately after the bead is dissolved to prevent the sample solution from splattering out of the flask by sporadic, vigorous boiling. The green solution must be filtered through a 0.45 µm cellulose filter paper using a vacuum hand pump as soon as it is cooled down to avoid formation of elemental sulfur, which clogs the filter paper. The insoluble residue accumulates on the filter paper. Sulfur-rich samples sometimes leave a fluffy, yellowish-white material floating on the surface of the green solution.
This native sulfur accumulates on the filter paper during filtration and can seal the filter paper immediately and thus terminate filtration.
Even if filtration is possible, the sulfur accumulation on the filter paper may cause problems during the distillation. The filter paper can be split in half if the sample was spiked for Os and other PGE. One half is transferred directly into the distillation unit (Os, Ru) while the other half is transferred into a 7 ml Teflon beaker (Ir, Pt, Pd, Ru). The filter paper will dissolve within ~2 h in cold conc. HNO3 and the solution can be processed for subsequent PGE analysis.
2.1.2 PEROXIDE LEACH
The acidic hydrogen peroxide leaching technique is used to selectively leach the hydrogenous (seawater, river water) Os-component that is presumed to be bound on Fe-Mn-oxyhydroxides. The technique used to reduce these oxyhydroxides is modified from Pegram et al. (1992). An initial 10% acetic acid leaching step is required only if the sample contains carbonates. Carbonate-free pelagic sediments are leached with a solution consisting of 2 ml Ultrex H2O2 (30%), 2 ml concentrated H2SO4 (Seastar), and 90 ml Milli-Q-UV H2O. This leaching solution is much weaker than that used by Pegram et al. (1992). Furthermore the samples are not sonicated before they are left overnight at room temperature.
The samples can also be leached in a stepwise procedure to check the influence of different leaching solutions and leaching times on the analytical results. These samples are leached in a solution consisting of 1 ml Ultrex H2O2 (30%), 1 ml concentrated H2SO4 (Seastar), and 90 ml Milli-Q-UV H2O. In the presence of H2O2, ferric iron is reduced to ferrous iron and the reaction can be observed by the production of small oxygen bubbles resulting from oxidation of H2O2. After being placed in an ultrasonic bath for 20 min the reaction usually ceases and the samples are centrifuged. Iron-rich samples (Mn-nodules, bog and lake ores, large samples of basalt and peridotite) will consume much more H2O2 (up to 100 ml per gram sample) to complete the reaction. Such large amounts of H2O2 should be added slowly and successively to avoid vigorous reaction.
The total blank of this leaching procedure is of the order of 0.1 - 0.2 pg Os with an 187Os/186Os of ~1.3 (Fisher H2O2). The leachate is processed in the usual way. The separation and purification of Osmium is described in detail in the following chapters. The leachates must be heated in the fully assembled still for ~30 min to oxidize excess H2O2, before the Cr(VI)-solution can be added. All chemicals used in the distillation can be predistilled to drive off Os. This can be done in the same round-bottom flask (covered with a Teflon lid) used later
2.1.3 DISTILLATION
The distillation unit consists of a round-bottom Pyrex flask placed in a heater, two condensers, and a collector. All parts except for the O-rings (Teflon) are made of Pyrex and are cleaned/stored in a hot concentrated HNO3 bath (5000 ml glass) to oxidize and vaporize any Os. The stills are rinsed thoroughly before each use, first with deionized water followed by Milli-Q-UV water. After the distillation unit is cleaned, the filter paper is added directly into the round-bottom flask and dissolved in 8 ml concentrated H2SO4 (i.e., 16N Seastar H2SO4, boiling point = 327°C). If the sample was spiked for complementary PGE, the filter paper is split and only half of the filter paper is added to the still. Then 64 ml of Milli-Q-UV water is added to the round-bottom flask to dilute the conc. H2SO4 solution to a 4N H2SO4 solution (boiling point of 110° - 120°C). OsO4 is volatile at ~130°C (Re2O7 at 270°C). To ensure complete oxidation of Os, 0.5 to 1 ml of the Cr(VI)-solution is added to the 4N H2SO4 solution. An oxidizing solution will be orange or brown and change as more Cr is reduced during the distillation. More Cr(VI)-solution must be added if the color is still reducing (to dark green/blue because of Cr3+).
2.1.4 SINGLE RESIN BEAD CHEMISTRY
The dilute HBr solution containing the Os is dried down on a hotplate at 120° - 150°C. The final drop (100 µl) is transferred into a small (3 ml) round-bottom Teflon beaker and dried down at lower temperature (<100°C) to a final volume of 1 - 2 µl. It is important to watch the drop carefully during this step in order to avoid complete dryness.
In the meantime several large Chelex 20 resin beads are picked from the resin stock. This can be done on a strip of Parafilm® using yellow pipette tips as tweezers. All handling should be done in a clean air flow box. The beads should be large enough not to be sucked into the opening of the yellow pipette tip. Each bead is stored in a drop of clean water on Parafilm®. Falkner (1992) showed that the size of the bead does not affect the partitioning of Os onto the bead because the number of available exchange sites (0.4 meq/ml) far outweighs the amount of Os atoms present.
Once the HBr-volume is 1 - 2 µl, ten times more Milli-Q-UV H2O is added to the small beaker so that the HBr solution does not exceed 1N. To this dilute HBr solution, one large resin bead is added. Radiotracer experiments demonstrate that uptake of Os decrease linearly with increasing solution volume at 1% per 2 µl. These experiments also show that Br- starts to compete with Os complexes for exchange sites when the concentration exceeds 0.4N HBr or the total amount of Br exceeds 50 µmol. The beaker is then placed in a carrousel tilted to ~45° and rotated slowly (~30 rpm) for at least 2 h. After 2 - 3 h of rotation, the small Teflon beakers are opened, 10 µl Milli-Q-UV H2O is added into each lid, and the bead is transferred from the weak HBr solution with a yellow pipette tip to the water in the cap to be carefully rinsed. No Os is lost during this cleaning procedure (Falkner, 1992). In the meantime 20 µl ultrapure HBr is added to each new conical Teflon beaker. The rinsed bead is transferred to the conical Teflon beaker to elute the osmium in the concentrated HBr. The Os is eluted from the bead in two steps by placing the bead in conc. HBr and sonicating for 30 min.
After ~30 min, the conical Teflon beakers are opened and the concentrated HBr containing most (70% - 80%) of the osmium is transferred to the lid of the conical Teflon beaker while the resin bead remains in the beaker. Again, 20 µl of concentrated HBr is added to the beaker, the beaker is sealed with Parafilm® and placed in the ultrasonic bath for additional 30 min. Care must be taken with the HBr in the lid, because it contains most of the osmium. Therefore it is necessary to mark the conical Teflon beakers and the respective lids to make sure that lids and beakers are not mixed up during the chemical separation. After the second eluting step, the bead is taken out of the beaker and added to the second 20 µl HBr that contains the remaining 20% - 30% of the osmium. The total yield of the chemical separation is usually on the order of 95%.
The 40 µl concentrated HBr have to be dried down in the conical beakers prior to loading the samples. This should be done carefully to avoid drying down the solution completely. Watch the samples while they are drying down at low heat (< 100°C) to a final volume of ~1 µl. The total chemical separation procedure takes about 4 h.
2.1.5 MICRODISTILLATION
Instead of using an ion excange resin for the cleanup-step, final purification can be achieved using microdistillation in an inverted conical Teflon vial (see Birck et al., 1997 for details). The HBr containing the Os is dried down at low temperature in the lid of an inverted, closed conical Teflon vial. The evaporated HBr recondenses on the walls of the inverted conical Teflon vial. About 40 µl of an H2SO4-Cr6+ solution is added to the dried Os in the lid, the beaker is carefully sealed and placed on a hotplate at 70°C for ~3 h. The acidic, oxidizing, Os-containing solution in the lid forms volatile OsO4, which is trapped on the film of HBr in the conical Teflon beaker. Once the solution in the lid is completely dry, the conical Teflon beaker is carefully inverted to ensure that no HBr drops run down the walls into the lid. Once inverted, the lid can be removed and the HBr collected at the conical bottom of the Teflon beaker. After drying to ~1 - 2 µl this solution can be directly loaded onto the filament.
2.1.6 LOADING THE SAMPLES
The platinum filaments and filament holders are washed in Ethanol and Milli-Q H2O and dried in an oven. The filaments are mounted on the filament holders. Outgassing is not required.
The final drop of concentrated HBr (~1 µl) with all the Os is taken up with a clean (washed in 6.2N HCl, rinsed several times in Milli-Q H2O and dried) spaghetti tip using a micro-pipette syringe and dried down over a small hotplate to a final loading volume of 0.02 - 0.1 µl. This sample is loaded on a platinum filament be repeated loading and drying steps at ~0.5 A. The loading procedure can be controlled by observing the sample and filament with binocular microscope magnification. The loading spot should be as tiny as possible with as little residue as possible. The residue on the filament is mainly organic leftover from the chemical separation.
The filaments are then placed in an outgassing machine for 4 - 6 h to reduce the Os(Br6)2- to the metal at low filament temperatures (dim orange). Prior to running Os on NIMA-B, the high ionization potential of Os requires that an ionization modifier be added to act as an electron donor. Barium chloride, barium nitrate or lanthanum chloride solutions are most commonly used. A tiny drop (~0.1 µl) of a saturated Ba(NO3)2 solution is added on top of the reduced osmium and dried down at ~0.5 A. The Os can now be measured as OsO3- by negative thermal ionization mass spectrometry following the techniques described in detail by Všlkening et al. (1991), Creaser et al (1991), Hauri (1992), and Hauri and Hart (1993).
2.2 COMPLEMENTARY PGE
A detailed description of this method was published by Ravizza and Pyle (1997) and only a brief summary is given here. The other half of the filter paper not used for distilling Os is transfered to a 7 ml screw-cap Teflon vial. About 3 - 4 ml concentrated, Teflon-distilled HNO3 is added to the vial. The cellulose filter paper dissolves within 2 h at ambient temperature. The solution is then dried down at ~60°C to 10 µl and diluted with Milli-Q-UV water to 0.5 ml (2% HNO3). This solution can directly be analyzed by ICP-MS. Analytical blanks are 1 - 2 pg/g for Os and Ir, and 15 - 30 pg/g for Pt and Pd. Platinum and Pd blanks are controlled by the purity of the Ni powder, whereas Ir and Os blanks seem to be controlled mainly by the purity of the Na-borate.
2.3 CHEMICAL SEPARATION OF RHENIUM
Detailed descriptions for the chemical separation of rhenium are given in Luck (1982), Reisberg et al. (1991, 1993), Hauri (1992) and Hauri and Hart (1993).
2.3.1 DISSOLUTION
100 - 200 mg of sample powder is weighed into a 10 ml Teflon screw cap beaker together with ~50 mg 185Re-spike solution (1.8 ng/g Re). About 5 ml of concentrated HNO3 is added to dissolve carbonates and oxidize organic matter. The beaker is sealed and placed on a hotplate (200°C) overnight. The solution is then dried down carefully and a HF-HCl-HNO3 mixture is added to destroy silicates. The beaker is placed on a hotplate at ~200°C overnight. Rather than using acid digestion in srew-cap Teflon beakers, pressurized digestion using a microwave digestion system can also be used. The resulting solution should be clear (yellow) without any residue. If any residue is present, the clear solution is transferred to another Teflon beaker and the residue subjected to another acid dissolution. Finally, the clear solution is dried down carefully to avoid complete dryness (formation of insoluble fluorides) and a few ml of concentrated HNO3 are added to form soluble nitrates. This solution is dried down completely nd dissolved again in 10 ml 0.5N HNO3. Prior to loading the samples, the solution should be centrifuged to separate any solid residue.
2.3.2 CHEMICAL SEPARATION
ml AG1-X8 (200 - 400 mesh) anion exchange resin is slurried into HDPE columns. The columns are washed three times with 5 ml 4N HNO3 (reagent grade) and a fourth time with 5 ml Teflon-distilled 4N HNO3. Thereafter, the resin is conditioned two times with 1 ml Teflon-distilled 0.2N HNO3 and once with 5 ml Teflon-distilled 0.2N HNO3. After conditioning, the sample is loaded (10 ml 0.5N HNO3) and subsequently be rinsed three times with each 5 ml 0.2N HNO3. Rhenium is eluted by adding three fractions of 3 ml 6N HNO3 each and the solution is collected in a clean Teflon screw cap beaker (15 ml).
Rhenium can now be measure either by ICP-MS, or by thermal ionization mass spectrometry following the techniques described by Všlkening et al. (1991), Creaser et al. (1991), Hauri (1992), and Hauri and Hart (1993). For ICP-MS measurement, the solution is dried down carefully at low temperature (~75°C) and dissolved again in 20 µl 4N HNO3. This solution is then transferred to a clean centrifuge vial and diluted to 0.5 ml with Milli-Q-UV H2O.
For cleaning the columns, they are carefully rinsed with Milli-Q-UV H2O to remove adhering resin and placed in 8N HNO3 for a few days. Finally the columns are stored in Milli-Q-UV H2O.
The mixture is fused for ~75 min in a muffle furnace at ~1000°C. Carbonate-rich samples should not be fused for more than 1 h because the carbonate melt is less viscous than a silicate melt and burns through the ceramic crucible more easily. Samples rich in Corg. have to be fused longer and hotter, and in extreme cases uncovered to allow oxidation of the carbon (Esser, 1991). The crucible and fused glass is cooled, crushed, and the NiS bead(s) collected. As the viscosity of the melt increases, as in MgO-rich peridotites, the NiS beads are less likely to coalesce into one large bead. The weight of the NiS-bead(s) is/are compared with the weight of the Ni- and S-powder originally taken. The yield is usually on the order of 80% - 95%, and yields in excess of 100% are common in heavy-metal polluted sediments. The NiS-beads are susceptible to alteration, especially in the lab-air. If one does not plan to continue with the bead dissolution, it is better not to crush the crucible and the glass. The bead is perfectly safe in the glass.
The still should be assembled quickly and the joints should be sealed with Teflon tape. The collector tube is filled with ~5 ml concentrated ultrapure HBr and is cooled in an ice water bath to condense OsO4. If H2O2 is still present in the round-bottom flask, small amounts of H2O2 oxidize HBr to Br2, turning the HBr in the trap orange-brown. After connecting the cooling water and carrier gas supply tubes, the cooling water and the carrier gas (filtered clean-lab air) are opened. The carrier gas is needed to ensure transfer of the volatile OsO4 from the round-bottom flask into the collector tube. A simple fish tank pump with an in-line air filter can be used for this purpose. The distillation should begin at medium temperature below the boiling point of the solution. After ~30 min, the cooling water supply to the first condenser is closed, and distillation continues another 30 min. The temperature of the H2SO4-Cr solution is increased to, or slightly above, boiling and the distillation is allowed to proceed to completion. A measure of the distillation progress is the increasing volume in the collector tube, caused by water vapor carried over with the Os. Yield experiments show that most of the Os is distilled into the HBr fairly rapidly (90% within 2 h) before the water dilutes the HBr. After the collector tube is full (~15 ml) and the distillation can be stopped. Most of the dilute HBr is transferred into a clean 15 ml Teflon beaker, sealed and placed in an oven at 80° - 100°C overnight to ensure complexation of Os as Os(Br6)2-; Os8+ is reduced to Os4+. This solution is used either for the single resin bead chemistry or the microdistillation. A few milliliters of the dilute HBr can be saved in a small 3 ml Teflon beaker for Ru isotope measurements using ICP-MS. The distillation unit can be weighed prior to and after the distillation to make sure that no leak has occurred during the distillation. The loss in weight should not exceed a few grams. After disassembling the still, 1 ml H2O2 (30%) is added to the remaining solution in the round-
bottom flask to reduce the oxidizing solution (color change from brown/orange to dark green). The excess H2O2 is added to reduce Cr to a non-toxic form. The still must be rinsed carefully with Milli-Q water and placed in hot concentrated HNO3 for at least 2 h prior to its next use to ensure that all Os has been removed from the Pyrex surfaces.
