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Anal. Chem. 1986. 58,616-620
values. One would except very little or no error with the R T P measurements because the R T P emission spectrum is normally far removed from the excitation spectrum. This was the case for the compounds investigated. In the case of RTF, there was a small amount of overlap between the excitation spectrum and RTF emission spectrum for the compounds on the solid substrates. Similar results were found for the solution luminescence spectra. Table I compares the low-temperature and room-temperature solution luminescence quantum yields for compounds in ethanol solution. It can be seen that most of the compounds have relatively high quantum yields at low temperature except for the phosphorescence of 5,6-BQ in 0.4 M HCl. The increase in quantum yield a t low temperature could be partly due to the decreased rate of oxygen diffusion in the solid phase. In addition the RTF quantum yields of 5,6-BQ in 0.1 M HC1 and 0.1 M HBr are low. Also the reproducibility of the quantum yield determinations is very good. Table I1 gives the R T F and R T P quantum yield values of the compounds adsorbed on solid supports with and without nitrogen passing over the surface. In all cases, higher quantum yields were obtained with nitrogen present for both RTF and R T P except for the R T F of 5,6-BQ on filter paper. Higher quantum yield values are expected generally with nitrogen because of the absence of oxygen. Also, PABA gave the largest quantum yield values of the three unknown samples investigated. Interestingly, the R T F quantum yield of 5,6-BQ (0.1 M HBr) on filter paper is greater than the solution (0.1 M HC1 or 0.1 M HBr) RTF quantum yield of 5,6-BQ (see Table I). On calculation of the amount of radiation absorbed by 4-PP and 5,6-BQ adsorbed on PAA-NaBr and filter paper, it was necessary to correct for the absorption due to the solid supports. As discussed earlier, the correction was made by one of three means: (a) difference in the area of reflectance bands, (b) difference in relative reflectance bands with BaS04 as a reference, (c) the difference in the Kubelka-Munk function values of the sample and the blank with BaS04as a reference. Kortum (17) has discussed in some detail the use of BaS04 as a reflectance standard and the use of the Kubelka-Munk function in correcting for absorption by substrates. In this work, essentially identical quantum yield values were obtained by using correction method$ (a) and (b) with or without nitrogen flowing over the surface (Table 111). By use of the Kubelka-Munk function for correction due to substrate absorption, similar quantum yield values were obtained for all three correction methods for nitrogen gas flowing over the
surface as shown in Table 111. However, without nitrogen gas somewhat different results were obtained using method c vs. methods a and b. These results indicate that nitrogen gas should be flowing over the surface for good correspondence between correction methods a, b, and c. Additional work is needed to explain the differences in Table I11 by method c without nitrogen compared to methods a and b. In conclusion, it has been shown that the approach developed is applicable to a number of compounds adsorbed on a variety of surfaces with quite different characteristics. The method, at present, cannot be applied to very weakly luminescent components adsorbed on solid surfaces. The approximate minimum quantum yield value for a sample would be 0.001. However, by the calibration of the gain settings of the instrumental system, it would be possible to determine even lower quantum yield values. LITERATURE CITED H. J . Opt. SOC. Am. 1964, 5 4 , 183.
Meihuish, W. Demas, J. N.; Crosby, G. A. J . Phys. Chem. 1971, 7 5 , 991. Britten, A.; Archer-Hall, J.; Lockwood, G. Analyst (London) 1978, 103, 928. Rhys Williams, A. T.; Winfield, S. A,; Miller, J. N. Analyst (London) 1983. 108. 1471. Demas, J. N. I n "Optical Radiation Measurements: Measurement of Photoluminescence"; Mielenz, K. D., Ed.; Academic Press: New York, 1982; Vol. 3, Chapter 6. Kristianpoller, N. J . Opt. SOC.Am. 1964, 5 4 , 1285. Wrighton, M. S.; Ginley, D. S.: Morse, D. L. J . Phys. Chem. 1974, 7 8 , 2229. Adams, M. J.; Highfield, J. G.; Kirkbright, G. F. Anal. Chem. 1980, 5 2 , 1260. Adams, M. J; Highfield, J. R.; Kirkbright, G. F. Analyst (London) 1981, 106, 850. Kirkbright. G. F.; Spillane, D. E. M.; Anthony, K.; Brown, R. G.; Hepworth, J. D.; Hodgson, K. W.; West, M. A. Anal. Chem. 1984, 5 6 , 1644. Hurtubise, R. J. "Solid Surface Luminescence Analysis"; Marcel Dekker: New York, 1981. Vo-Dinh, T. "Room Temperature Phosphorimetry for Chemical Analysis:'; Wiley: New York, 1984. Dalterio, R. A.; Hurtubise, R. J. Anal. Chem. 1982, 5 4 , 224. Donkerbroek, J. J.; Elzas, J. J.; Gooijer, C.; Frei, R. W.; Velthorst, N. H. Talanta 1981, 2 8 , 717. Rhvs Williams, A. T.; Winfield, S. A,; Miller, J. N. Analyst (London) 1983, 108, 1067. Renschler, C . L.; Harrah, L. A. Anal. Chem. 1983, 5 5 , 798. Kortum, G. "Reflectance Spectroscopy"; Springer-Verlag: New York, 1969.
RECEIVEDfor review September 9, 1985. Accepted October 31,1985. Financial support for this project was provided by the Department of Energy, Division of Basic Energy Sciences, Contract No. DE-AC02-80ER10624.
Determination of Platinum and Iridium in Marine Waters, Sediments, and Organisms Vern Hodge, Martha Stallard, Minoru Koide, and Edward D. Goldberg* Scripps Institution of Oceanography, L a Jolla, California 92093 The determination of platinum and Iridium at picogram levels in marine samples is based upon an isolation of anionic forms of these elements upon appropriate resins with a subsequent purification by uptake on a single ion exchange bead. ~ 1 1 steps are followed by radiotracers, and yields vary between 35 and 90 %. Graphite furnace atomic absorption spectrometry is employed as the determinatlve step.
The six platinum metals constitute the last group of ele-
ments in the periodic table to have escaped systematic study by marine scientists, yet their comparative chemistries in the laboratory indicate that their oceanic chemistries will be most revealing as to the nature Of enVironmenta1chemical reactions. For example, platinum is markedly enriched over palladium in seawater compared to cosmic 01 crustal values (1). This can be explained by the softer acidic character of Pt(I1) relative to Pd(I1) and the consequential stronger stabilization in seawater through complex formation of platinum with such ligands as chloride and bromide. Also, iridium and platinum
0 1986 American Chemical Society 0003-2700/86/0358-0616$01.50/0
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are both enriched in some ferromanganese minerals compared to palladium. Platinum and iridium are probably oxidized during mineral formation from t h e divalent and trivalent states, respectively, to the tetravalent ones. By this mechanism, platinum and iridium can be accumulated over palladium, which only exists in the divalent state in the marine environment. Our chemical analyses involve the isolation of the platinum metals from solution upon an anion exchange column (1-3). The elements are stripped and then incorporated onto a single anion exchange bead and subsequently analyzed on a single-beam atomic absorption spectrometer utilizing a graphite furnace. All analyses are followed with radioactive tracers to determine overall yields, which have ranged between 35 and 90%. Herein we present our detailed techniques for platinum and iridium in marine waters, sediments, and organisms. EXPERIMENTAL SECTION Reagents. All solutions were prepared with doubly distilled water. Redistilled nitric, hydrochloric, and perchloric acids were purchased from G. Frederick Smith Chemical Co. (Columbus, OH). The iridium contents of these acids were below detection, Le., less than 20 pg/sample blank. Some redistilled nitric acids contained greater than 1ng of platinum/50 mL, the volume used in a single analysis. Sample blanks greater than 40 pg of platinum were unacceptable and such acids were not used. Platinum Analyses. lglPtRadioactive Tracer Solution. lglPt (tl = 3.0 days) was prepared by neutron activation of enriched l?#t metal powder (4.19 atom % compared to natural abundance of 0.0127 atom %). The enriched metal (purchased from Oak Ridge National Laboratory, Union Carbide Corp., Nuclear Division, Oak Ridge, TN) was irradiated for 160 h at a flux of 3 X 1014(n/cm2)/s in the University of Missouri Reactor Facility. The metal was dissolved in aqua regia, and an aliquot was received at our laboratory 5 days later. This solution was diluted with 1 M HCl to give a tracer solution with a specific activity of about 5 cpm/pg. Seawater samples were spiked with 5 pL of this solution, which contained 175 pg of platinum. This amount of spike gave 900cpm initially with a 2 in. X 2 in. NaI (TI)well crystal coupled to a Nuclear Data multichannel pulse height analyzer. lglPtwas monitored at 64.90 keV. Seawater Collection. Seawater was collected in 30-L modified Go Flo bottles (General Oceanic), coated with Teflon, on Kevlar line. The water was immediately pressure filtered through 0.4-pm Nucleopore filters. The filtered water was acidified with 10 mL of 6 M HCl/L and stored in 8-L acid-cleaned polyethylene bottles. Preconcentration of Platinum by Anion Exchange ( 4 ) . Preliminary experiments demonstrated that the lglPttracer could be quantitatively removed from 2 L of acidified seawater (0.0145 M HCI) by passage through a 0.8 cm X 4 cm column of AG 1 x 2 resin (Dowex-l,50-100 mesh, BioRad Laboratories, Richmond, CA). The resin was contained in a BioRad polypropylene Econo-Column. The resin column was prepared by transferring approximately 1.8 mL of the AG 1 x 2 to a water-filled column. An acid-washed circular polypropylene frit (35-pm porosity, cut with a no. 8 cork borer) (Cole Parmer, Chicago, IL) was fitted into the column just above the resin bed. Precautions must be taken to avoid air bubbles in the resin column and the area just beneath the frit. The resin column was sequentially washed with 50 mL of 12 M HNO, at 90 "C, 20 mL of H20, 10 mL of 6 M HCl, and 20 mL of 0.5 M HCl. Two liters of seawater was transferred from the storage bottles into a clean 2-L polyethylene bottle (Nalgene) and spiked with 5 PL of the lelPttracer solution (dissolved in 10 mL of 0.5 M HCI to facilitate the transfer). The spiked sample was mixed thoroughly and allowed to stand overnight. The next day, the anion exchange resin column was attached to the mouth of the bottle with an adapter made of Teflon. The bottle was inverted and placed on a Lucite rack situated in a laminar flow hood. Seawater passed through the column at a rate of 1-2 mL/min. When the seawater had completely drained, the column was detached.
617
Elution of Platinum from the Resin. Each resin column was washed with 50 mL of 0.5 M HC1 and 50 mL of distilled water to remove sea salts and nonreactive metal ions. Platinum was quantitatively eluted into a 50-mL beaker, made of Teflon, with 45 mL of 12 M HNO, at 83 "C. Purification of Platinum by Concentration on a Single Anion Exchange Bead. Twenty drops of 70% HC104were added to the 45 mL of 12 M "OB eluant and the solution evaporated to dryness. Then the residue was treated 3 times with 3 drops of 70% HCIOl and 0.5 mL of 16 M "OB, going to dryness each time. The beaker walls were washed with 2 mL of 6 M HCl and the acid evaporated to dryness. Finally the beaker was washed with 2 mL of 6 M HC1 and the solution evaporated to approximately 50 pL. The 50-pL drop of sample was dissolved in 500 pL of 0.5 M HCI with warming. The sample was transferred with an Eppendorf pipet from the beaker to a 0.5-mL polypropylene snap-top micro centrifuge tube. Five microliters of 25% w/v ascorbic acid and 25 MLof 4% w/v KSCN were added. One or two -1-mm IRA-900 resin bead(s) (Sigma Chemical Co., St. Louis, MO) (prewashed with 6 M HC1 and 0.5 M HC1) were added to the sample. The tube was rotated at 30 rpm overnight. Preparation of the Sample for Graphite Furnace Atomic Absorption Analysis. After the bead had tumbled overnight it was removed from the solution and washed in distilled water. Three methods were utilized to assay the Pt on the bead by graphite furnace AA: (1)direct insertion of the bead into the furnace, (2) wet digestion of the bead and injection of the resulting solution into the furnace, and (3) stripping the Pt from the bead with HCl/thiourea and injecting this solution into the furnace. While method 1is sometimes successful,over 50% of the beads exploded in the graphite furnace when the temperature reached about 300 "C. Probably traces of HC104and/or HNOBcombined with the resin to produce an explosive material. Method 1 was abandoned and methods 2 and 3 were developed. Method 3 is preferred because it is less time-consuming. In method 2 the single resin bead was placed into a 200-pL beaker made of Teflon. The beaker was fabricated from an 11-mm section of 9-mm-diameter rod (Teflon) that had been bored with a 5-mm drill. The bead was covered with 10 pL of 16 M HNOB and 5 pL of HC104. Several small beakers were sealed in a glass jar covered with a lid made of Teflon. The jar was placed inside a microwave oven (Sears Kenmore Model 566). The oven was turned on high power for 10 min, after which the jar was removed from the oven and opened in a hood to vent acid fumes. This procedure was repeated until the sample was dry. In general 4-6 10-mincycles were necessary,depending on the number of samples and size of beads. The dry residue was dissolved in 10 pL of 16 M HNOBand taken to dryness in a microwave oven. Then 10 pL of 6 M HCl was added, and again the solution was taken to dryness. The residue was dissolved in 10 pL of 6 M HC1 and injected into the graphite furnace. In method 3, a thiourea extraction is employed. Crocket et al. have reported that Pt can be eluted from Rexyn 201 anion resin by 0.1 M HCl/O.l M thiourea (5). It was found that 25 pL of 0.1 M HC1/0.01 M thiourea stripped >95% of the lglPtfrom a single IRA-900bead in 15 min. Therefore, the sample beads were placed in 0.5-mL micro centrifuge tubes and covered with 25 pL of 0.1 M HCI/O.Ol M thiourea solution. The tube was placed on the rotator for one or more hours. The resulting solution was removed from the bead and gamma counted to determine the overall yield. The 25-pL sample was ready for P t analysis. In practice, platinum was found to strip off consistently in high yields when the platinum was derived from samples of seawater or algae. However, the stripping efficiency was not always high when the platinum had come from sediment samples. Therefore for sediments, method 2 is preferred. Sediments. One-halfto one gram of finely ground dry sediment was weighed into a 50-mL beaker made of Teflon (Nalgene), covered with 20 mL of HNO,, and spiked with 25 p L of 19*Pttracer solution. Ten milliliters of HC104was added and the sample taken to dryness on a hotplate at 190 OC. Successively,20 mL each of "0, and 6 M HCl was added to the sample and evaporated to dryness. The residue was covered with 20 mL of 6 M HCl and heated with stirring to about 90 OC. The contents of the beaker
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 3, MARCH 1986
were transferred to a 50-mL polypropylene centrifuge tube and centrifuged for 10 min. The liquid was poured into a 50-mL beaker made of Teflon and the solid leached with another 20 mL of 6 M HCl. The combined leachates were taken to dryness and the solid transferred back into the original beaker with 20 mL of 6 M HC1. Twenty milliliters of HF was added and the sample allowed to evaporate to dryness with occasional stirring. The residue after H F treatment was wet ashed with 5 mL of HNO, and 5 mL of HC104, evaporated to dryness, and converted to the chlorides with 6 M HCl. Twenty milliliters of 6 M HCl was added, and any solid that did not dissolve separated by centrifugation. The 6 M HCl solution was poured into the beaker that contained the residue from the two hot 6 M HC1 leaches. Any solid that remained in the centrifuge tube was transferred to a 30-mL zirconium crucible with water and dried at 100 "C. The residue was fused with 0.25 g of NazOzand the resulting mass dissolved in 6 M HCl, heated to destroy any HzOz,and added to the sample beaker made of Teflon. The beaker contents were taken to dryness, dissolved in 20 mL of 6 M HC1, and evaporated to about 10 mL. This solution was diluted to 100 mL with water and passed through an AGlX2 anion exchange column. The procedure from this point follows that of seawater. Manganese Nodules. Some nodules dissolve completely in hot 6 M HCl, and most dissolve almost completely leaving only a small amount of silica-like material. To ensure complete dissolution, 0.1 g of nodule was weighed into a 50-mL beaker made of Teflon and treated in the same way as sediment samples but with reduced volumes of reagents. For those nodules containing greater than 30 ppb Pt, purification by means of the single IRA-900 bead was unnecessary. Instead, the residue from the column strip, after wet oxidation with HN03 and 0.1 mL of 30% H20z,was diluted to 100 pL and analyzed directly by graphite furnace AA. Additional dilution was sometimes necessary. Use of HC104 in the oxidation step should be avoided because any residual HC104 in the sample will lead to fuming a t the ash temperature and significant loss of analyte. Organisms. For platinum assays of microalgae, 3-5 g of the oven-dried (110 "C) material was placed in a 150-mLbeaker made of Teflon and covered with 30 mL of HNO,. After any visible reaction had subsided, the radioactive tracer was added and the beaker was placed on a hotplate at 190 "C. The sample was heated for 4-6 h and 20 mL of 70% HCIOl added. Any partially digested matter adhering to the beaker walls was washed into the acid mixture with 20 mL of HNO, delivered from a 250-mL wash bottle made of Teflon. The sample was allowed to go to dryness overnight. Two or three treatments of the residue with 10 mL of HN03/10 mL of HC104produces a pale yellow to white salt. The salt was converted to the chloride form and dissolved in 50 mL of 6 M HC1. This solution was added to 1 L of doubly distilled water and the resulting sample run according to the seawater procedure. Graphite Furnace AA Analysis. A Varian AA-6 atomic absorption unit with CRA-90 carbon rod analyzer and background corrector was used for all analyses. The monochromater was set on the 265.9-nm line of Pt. A maximum of 10 fiL of sample (dissolved bead or thiourea strip) was injected into the furnace at one time. The drop was dried at 100 "C for 90 s and the temperature slowly raised to 600 "C over 30 s. A small amount of smoke often appeared a t 300 "C. The tube was cooled and another 10 pL of sample injected and the drying/ashing procedures repeated until the whole sample had been loaded into the tube. The sample was then analyzed for Pt a t an atomization temperature of 2450 "C. The sample signal was compared to a standard prepared from diluting 1000 ppm Pt standard solution (VWR Scientific) to the appropriate concentration (generally 10-100 ppb) with 0.5 M HC1. The instrumental detection limit, defined as a signal double that of background was equal to 15 pg. Blanks. A reagent/resin column/tracer blank was run for every six samples. For seawater samples the platinum blank was 1 4 0 pg/sample. For sediments, including manganese nodules, the blank was always less than 5% of the sample value.
Iridium Analyses. lg21rRadioactive Tracer Solution. Ig2Ir
(tip = 74.2 days) was prepared by neutron activation of a natural KJrCI, solution. It was irradiated for 160 h also at the University of Missouri Reactor Facility. The solution was diluted to give an activity of about 48 counts/pL. The stable iridium content of this solution was at or below the detection limit of the AA, thus the tracer solution was essentially carrier free. All samples were spiked with 25 pL of the Ig2Irtracer solution. Ir activity was monitored a t 320 keV. Ceric Sulfate Solution. Twenty-eight grams of Ce(S04)2was dissolved in 100 mL of a 1.4 N HzS04solution. Seawater. Twenty liters of seawater was filtered through 0.45-pm Millipore filters into a 20-L polyethylene carboy with spigot (Nalgene). The filtered water was immediately acidified with 20 mL of 6 M HCI, spiked with lgzIrtracer solution, and allowed to stand overnight. Chlorine gas (Matheson high purity) was vigorously bubbled into the seawater for 2 min to oxidize any reduced iridium to the tetravalent state. Soon after chlorination a Biorad Econo-Column containing 2 mL of Biorad AG 1x8was attached to the spigot. (The resin column was prepared as the AG 1 x 2 column used in the platinum procedure.) Seawater was allowed to flow through the column at rates from 7 to 20 mL/min. For 100-Lsamples, 5-20-L samples were collected and run through two columns (40 L through one and 60 L through the other). Five microliters lg21rspike (1/5 of the total spike) was introduced into each seawater sample. The 1921ractivity was quantitatively retained on the resin even at the highest flow rates. The column was removed from the spigot and washed with 50 mL of 0.1 M HCl (into which Clz had been bubbled for 30 s) and 50 mL of water. More than 95% of the adsorbed iridium activity was stripped from the resin into a 50-mL beaker, made of Teflon, with 45 mL of 12 M HN03at 83 "C. Fifty microliters of HzS04was added and the sample evaporated to H2S04fumes. The walls of the beaker were washed down with HNOBand evaporated. One hundred microliters of 30% Hz02 was added and allowed to react and evaporate. The beaker was again washed down with HNO,. After the HN03had evaporated, the procedure was repeated with the H20zstep. A small 25-fiL drop of clear, colorless HzSO4 remained after the two HzOz treatments. One-half milliliter of 1.2 M HC1 was added to the beaker and the sample quantitatively transferred to a 0.5-mL microcentrifuge tube. Twenty-five microliters of Ce(S04)zsolution was added along with two IRA-900 beads. The beads were allowed to tumble in the solution for 2 days, separated from the solution, washed twice with 1.2 M HCl, and counted to determine the yield up to this point. The beads were then washed with 0.12 M HC1 in ethanol and transferred to a clean 0.5-mL centrifuge tube. Twenty-five microliters of 6 M HCl was added and iridium stripped by placing the tube in a 110 "C oven for 30 min. A second treatment with 25 pL of 6 M HC1 resulted in the overall removal of 95% of the adsorbed iridium activity. The stripping solution was assayed by graphite furnace AA. Sediments and Manganese Nodules. One-half to one gram of finely ground dry material was wet ashed as in the platinum procedure or was fused with 10 times its weight of NazOzin a 50-mL zirconium crucible. In the case of fusion with Naz02,the sediment was thoroughly mixed with the peroxide and fused in a muffle furnace at 600 "C for 30 min. The sample was allowed to cool overnight, and 10-15 mL of water was added to soften the salt cake. The reaction with water was exothermic and sometimes became quite vigorous. Therefore, care was necessary to avoid loss of sample by eruption of the contents of the zirconium crucible. After the salt slurry was transferred to a 250-mL polyethylene bottle, the crucible was warmed with 6 M HCl to remove any adhering sample and added to the sample bottle. The basic sample was acidified with an amount of 6 M HCl calculated to neutralize the NaOH formed on fusion (excluding the 6 M HC1 used to clean out the crucible). The resulting acid solution was spiked with lgZIrand allowed to stand overnight. The acid solution was poured into about 900 mL of water in a 1-L polyethylene bottle, chlorinated, and allowed to pass through an AG 1x8resin column at the rate of about 1 mL/min. From this point on, the procedure is the same as that for Ir in seawater. Graphite Furnace AA Analysis. Iridium was determined with the instrument's monochromater set on 266.5 nm. The drying
ANALYTICAL CHEMISTRY, VOL. 58, NO. 3, MARCH 1986
and ashing procedure was the same as for Pt, however, the atomization temperature was higher, 2600 O C . Standard solutions of iridium were prepared from the dilution of lo00 ppm Ir (Banco, Anderson Laboratories) with 1M HC1 and also by dilution of an approximately 1000 ppm solution prepared from the dissolution of Ir wire by fusion with KOH/KNOB.The instrumental detection limit, defined as twice the background peak, was equivalent to 20 Pg. All analyses were accompanied by measurement of two or more strip solutions from beads on which a known amount of iridium had been adsorbed. From these analyses, it was apparent that the AA signal from a strip solution is only 60% of the expected value. Therefore, all samples were corrected for this signal loss. Blanks. A reagent/resin column/tracer blank accompanies every six samples. For seawater samples, the iridium blank was 95%) of platinum from 2 L of seawater. As a consequence of the high K,, platinum is very difficult to remove from the resin with most acids. However, it can be eluted completely with hot nitric acid. Evaporation of the nitric acid leaves a small residue, which, if dissolved in dilute HC1 and injected in the graphite furnace, causes negative signals during the atomization cycle. Direct monitoring of the background absorbance during this cycle indicated that the sample matrix absorbance was high. The negative sample signal resulted from an oversubstraction by the background corrector. It should be emphasized that the metal levels are near the detection limit of the atomic absorption unit; Le., the platinum and iridium contents of the samples were from a few tens of picograms to a few hundreds of picograms. Detection limits are 15 pg for platinum and 20 pg for iridium. Since the concentrations of platinum in most manganese nodules are quite high, the nitric acid eluant was dissolved in 100-500 pL of dilute HC1, and 5-pL splits were injected directly into the graphite furnace. At these dilutions, matrix interferences were not evident. However, in most other samples, the platinum and iridium concentrations are so low that it is not possible to utilize a fraction of the column eluant. Further, limited amounts of some samples did not permit the use of greater amounts for the assay. Therefore, it is necessary to remove the matrix effects by purification through adsorption onto a single bead. The high matrix absorbance is characteristic of the blanks. Most probably, the hot nitric acid leach removes a small amount of refractory material from the resin. This interferes with the determinations of small amounts of platinum or iridium. Wet ashing the residues with nitric and perchloric acids or nitric acid and hydrogen peroxide does not significantly reduce this interference. Additionally, when the acid is evaporated, platinum is not available for adsorption onto the single IRA-900 resin bead. Only after repeated wet ashing of the residue with nitric and perchloric acids can the metal adsorb onto the single bead. The highest loss in overall yield occurs at this point in the procedure unless great care is taken to vigorously oxidize the residue. Overall platinum yields
619
Table I. Determination of Platinum in U.S. Geological Survey Samplesn peridodite PCC-1 found lit. values (ref) 5.7
5.7 f 0.7 ( 7 )
3.9
5.1 i 1.5 (8) 3.5 f 1.7 (9) 8 (10) 4.8 (12) 5.8 (13)
5.2
5.5 5.1 f 0.6b
dunite DTS-1 found lit. values (ref) 1.7 1.8
10.5 (14) 13.5 (15) 15 (16)
12 (17 )
