Methods for the separation of rhenium, osmium and molybdenum applicable to isotope geochemistry

~3~91~~91 $3 00 + 0 00 Fergamon Press plc

T~~fa,Vol ~&NO 3,pp259-265, 1991 Pnnted m Great Bntam

METHODS FOR THE SEPARATION OF RHENIUM, OSMIUM AND MOLYBDENUM APPLICABLE TO ISOTOPE GEOCHEMISTRY J. W MORGAN,*D. W. G~LIGHTLY~and A. F. D~RRZAPF,JR. U S Geologmal Survey, 981 Natronal Center, Reston, VA 22092, U S A (ipecerwd 5 Juae 1990 Accepted 27 Judy1990) Stunmary-Effkcttve methods are described for the chemmal separatton of rhemnm, osmnnn and molybdenum The methods are based on Qstdlatron and anion-exchange chromatography, and have been the basis for rhen~um-osmmm isotope studres of ore depostts and meteontes Successful amen-exchange separation of osmmm requires both recogmuon and careful control of the osmmm specres m sol&ton; thus, dtstdiatron of osmmm tetroxrde from a rmxture of sulfiuxc and and hydrogen peroxrde 1s preferred to amon-exchange Drstnbutton coefficmnts measured for perrhenate m sulfunc actd me&a are sutbcrently lugh (& > 500) for rhenium to be duectly loaded onto an Ion-exchange column from a drstrllatton resrdue and subsequently eluted with mtrrc acrd. Polymermatron of molybdenum specres dunng elutton 1s prevented by use of a solutron that IS 1M m hydrochlonc actd and 1M m sodmm chlonde

The rhemum- 187-osnnum-187 decay scheme provides the basis for a powerful geochemical and geochronologrcal tool that has been known for more than 30 years. ‘*’The widespread application of this isotopic system, however, has been hampered by the lack of suitable mass-spectrometric techniques and appropriate chemical separations. Renewed interest m rhenium-osmium isotopes has been created by the recent demonstration of the ability of secondary ionization mass s~trometry (SIMS) to measure precisely and sensrtively isotopes of these two elements.3;J At least four other techniques now may become competmve with SIMS by virtue of sensitivity, speed of measurement, and precision; these techniques are laser microprobe mass analysis, inductively coupled plasma mass spectrometry (ICP-MS), tandem accelerator mass spectrometry, and resonance ionization mass spectrometry (RIMS). Each of these techmques has been applied to measurements of osmium isotopes.54 Previous analyttcal methods reported for the rhenium-osmium isotopic system have generally depended on the acid dissolutton of a metal,

*Author for correspondence tPresent address Ross Laboratories, 625 Cleveland Avenue, Columbus, OH 43215, U S A $Any use of trade names or trademarks IS for descrrptwe purposes only and does not constitute endorsement by the U S Geologxal Survey 259

sulfide or silicate, followed by distillation of osmium tetroxide from a sulfuric acid medium contaimng Cr(VI)2*3*9or Ce(IV).* In another approach, fusion with sodium peroxide was used to dissolve Creta~ou~Te~i~y boundary material before distillation of osmium tetroxide from perchloric acid.6 In a related area, the quantitative distillation of osmium tetroxrde from a mixture of sulfuric acid and hydrogen peroxide, after an alkaline fusion,iO has been used successfully m the carrrer-free separation of osmium from platinum metal ore for neutron-activation analysis.” After osmium separation, rhenium may be recovered by distillation at a higher temperature from a dichromate-sulfuric acid solution2” or from perchloric acid-r2 In most recent studies, solvent extraction of rhenium has been preferred. Ahquat-336$ and other quaternary amines are widely favored,‘3*14but the ease of back-extraction into alkaline or ammoniacal media has made tertiary amines” particularly attractive.38 Approaches to the separation of rhenium, osmium and molybdenum reported here have formed the basis for the development of effective rapid methods for numerous studies of rhenium-osmmm isotope geochemistry and geochronology. 16-i9Materials analyzed contain from less than 1 rig/g to more than 10 pg/g rhenium and osmium and vary widely in place of origin, geological age and bulk composition. Samples include iron meteorites, carbonaceous

J W MORGANet al

260

and enstatite chondrites, chromitites, gold-rich ores and many types of sulfides. Chemically, the sulfides ranged from molybdenite (molybdenum disulfide) to ores containing up to 19% copper and up to 49% nickel. The separations described in detail in this account of research provide data on anion-exchange and chemical species that are applicable to many chemical investigations of rhenium and osmium in rocks, minerals and meteorites. EXPERIMENTAL

Apparatus The elution of osmium, rhemum and molybdenum from ion-exchange columns was monitored by inductively coupled plasma atomic emission spectrometry (ICP-AES). Aerosols produced from eluent solutions by a fixed crossflow nebulizer were inJected into an argon plasma sustained by 1.l kW of forward power from a 27.12-MHz generator. Flows of argon through each of the plasma-torch channels were: injector, 0.64; intermediate, 0; and outer, 17 l./min. A constant flow (0.8 l./min) of each eluent solution to the nebulizer was provided by a peristaltic pump (Gilson Minipuls 2). Background-corrected relative intensities of the Mo(I1) 202.030 nm, Os(I1) 228.226 nm, and Re(1) 197.3 13 nm lines were measured at 16-mm observation height with a direct-reading polychromator (Jarrell-Ash model 1160) having a focal length of 0.75 m and a bandpass of 0.036 nm. The polychromator was equipped with a separate 0.5-m Ebert monochromator (bandpass 0 04 nm) that functioned as a tunable channel.” Measurements for osmium and rhenium were made with the 0.5-m Ebert monochromator, which, unlike the polychromator, has no motor-driven quartz refractor plate at the entrance slit to enable automatic correction of the spectral background. Thus, the signal generated by a blank solution was subtracted from the gross hne signal to provide the background correction, which always constituted less than 5% of the gross signal. Concentrations of osmium, rhenium and molybdenum m the eluent solutions were kept well above the detection limits” (by a factor of at least 50) to enable relative standard deviations of less than 1% for four replicate measurements. Reagents Osmium Much of the fascmation, and most of the exasperation, related to osmium chemistry

lies in the multiplicity of oxidation states and complex species that can exist, and often coexist, in solution. Thus, preparation of a recognizable single species is an important first step because both the ion-exchange behavior and the response of the ICP-AES are affected by the chemical form of osmium.“s23 Three 250-ml stock solutions [OS-I, OS-~ and OS-~] were prepared from 0.25 g each of osmium tetroxide (Fisher purified osmic acid) contained in sealed glass ampules. Each solution was treated so that it contained a species of osmium different from that in the others. Three dilutions of each stock solution were made, and each dilute solution was treated to form or maintain a distinct osmium species. A fourth solution was prepared from ammonium hexachloro-osmate(IV). Details concerning the preparation of these solutions are given below. OS- 1. The osmium tetroxide dissolved rapidly in a solution of 10.5 g of sodium hydroxide in 100 ml of water to give a brown solution of Na,[OsO,(OH),]. Ethanol (5 ml) was added, and the solution turned very dark brown. The solution was warmed for 20 min, during which the brown color changed to purple. Another 5 ml of ethanol were then added, and the solution was gently heated to complete the color transformation to purple. This solution was diluted to 250 ml with 1M sodium hydroxide to provide a final concentration of 624 pg/ml OS as Na,[OsO,(OH),]. Portions of 30 ml each were treated as follows and diluted to give 100 ml of solution containing 187 pg/ml osmmm. OS-IA. Addition of 2.8 g of sodium hydroxide to a portion of OS-1 solution and subsequent dilution gave a purple solution of Na,[OsO,(OH),] in 1M sodium hydroxide. OS-IB. Addition of 32.5 ml of 4iU nitric acid to a portion of OS-1 solution caused the solution to become opaque black and to appear on the point of precipitating hydrated 0~0,. The solution became colorless after sturing for 20 min. After dilution, this solution contained OsO., in IM nitric acid. OS- IC Addition of 11 ml of 12M hydrochloric acid to an OS-1 solution gave, after solution dilution, a pale brownish-yellow (unlike solution OS-~, described later) of Na,(OsO,Cl,) m 1M hydrochloric acid. OS-Z. Osmium tetroxide was dissolved m sodium hydroxide solution and diluted to a final volume of 250 ml to give a solution containing 603 pg/ml osmium as Na,[Os0,(OH)2] in lit4 sodium hydroxide. Portions of 30 ml were

Methods for the separation of rhemum, osmmm and molybdenum

diluted to 100 ml after the following treatments, to give solutions containing 181 fig/ml osmium. OS-ZA. Addition of 2.8 g of sodium hydroxide to a portion of OS-~ solution gave the same light brown solution as OS-~, which contained Na,[OsO,(OH),] in 1M sodium hydroxide. OS-2B. Addition of 32.5 ml of 4M nitric acid to a portion of OS-~ solution immediately produced a colorless solution without the intermediate blackemng observed previously for OS-1B. OS-2C Addition of 11 ml of 12M hydrochloric acid to a portion of OS-~ solution and subsequent dilution immediately gave a colorless solution that contained 0~0, in 1M hydrochloric acid. The solution gradually became colored, and after one week it was light yellow, presumably because of the presence of H,(OsO,Cl,). OS-~. Osmium tetroxide was dissolved in 100 ml of 2M nitric acid but went mto solution very slowly. The solution was diluted to 200 ml to contain 720 pgg/ml osmium as 0~0, in lit4 nitric acid. Portions of 25 ml were treated as described below and diluted to 100 ml each to contain 180 hg/ml osmium. OS-3A. Addition of 18.8 ml of 4M nitric acid to a portion of OS-~ solution and subsequent dilution gave a colorless solution that contained 0~0, m 1M nitric acid. OS-3B. Addition of 5.2 g of sodium hydroxide to a portion of OS-~ solution and subsequent dilution gave the same light-brown solution obtamed as OS-~; the resulting solution contained Na,[OsO,(OH),] in 1M sodium hydroxide. OS-3C Sodium hydroxide (5.2 g) and 1 ml of ethanol were added to a portion of OS-~ solution. As this mixture was warmed, the dark brown color that developed on addition of ethanol changed to purple, as with OS-~. After dilution with water, the solution contained Na,[OsO,(OH),] in 1M sodium hydroxide OS-4. Ammonium hexachloro-osmate [(NH,),OsCl,] was heated to constant weight at 150” and then dissolved m 250 ml of 4M hydrochloric acid to give a reddish-yellow solutton contammg 320 pg/ml osmium. A 25-ml portion of the resulting solution was diluted to 200 ml m 3M hydrochloric acid to give a solution that contained 40.0 pgg/ml osmium Rhenzum. High-purity rhenmm ribbon (H B. Cross, 0.03-mm thick, 7.6-mm wide) weighing 0.1034 g was dissolved in 10 ml of concentrated nitric acid (J. T. Baker, Ultrex), and the result-

261

ing colorless solution was diluted to 250 ml to give a solution containing 414 p g/ml rhenium as HReG, in 1.4M nitric acid. This solution was used for the elution experiments. In the batch equilibration experiments to determine distribution coefficients in sulfuric acid medium, evaporation of this solution to remove nitric acid gave erratic results, apparently because of incomplete dissolution of the dried residue. A second stock solution was prepared by dissolving KReO., in the appropriate concentration of sulfuric acid (0.25-2.5M) to give solutions containing between 194 and 207 pgg/ml rhenium. Molybdenum. Two stock solutions were prepared, one from ammonium heptamolybdate tetrahydrate, (NH,),Mo,0,~4Hz0 and one from sodium molybdate. A 23-g quantity of the heptamolybdate was dissolved in a mixture of 150 ml of water and 20 ml of concentrated ammonia solution, and then diluted to 250 ml to give a solution containing 50 mg/ml molybdenum in 1M ammonia solution. A quantity of 2.52 g of Na,MoO,,.2H,O was dissolved in Hz0 together with 8 g NaOH to give 200 ml a solution containing 5.0 mg/ml molybdenum in 1M sodium hydroxide. Anion -exchange columns. Anion-exchange resin was supported between plugs of glass wool m Pyrex glass columns that were 1 cm m diameter and 15 cm long. Each column had a 200-ml spherical glass reservoir at the top and a Teflon stopcock at the bottom. The columns were packed with 5.5 g wet weight (equivalent to 3.5 g of dry weight) of BioRad AG 1 x 8 resin, 200400 mesh, chloride form, that had been slurried with water, to yield a column 12-13 5 cm long. The columns were washed with 100 ml of water to remove a pink impurity from the resin, and then conditioned with the appropriate acid. RESULTS AND DISCUSSION

Rhenium The anion-exchange of ReO; in chloride and nitrate systems is well known2k26 and, in prmciple, provides a ready means to separate rhenium from molybdenum and other contammants.22~27*28 Most osmium distillation procedures leave a residual solution that is 2-4M in sulfuric acid. Rhenium may be recovered from this solution by solvent extraction with a tertiary amine dissolved in chlorofonn3 but this procedure can be messy and rhenium recoveries may be mexplicably low. An ion-exchange

262

J W MORGANer al.

procedure that could be applied directly to the residual sulfunc acid solution appeared attractive. Although the strong sorption of rhenium by anion-exchange resin from sulfmc acid medium seems well known,*’ this element was not included m the extensive study by Strelow and Bothma3’ and apparently has not been systematically studied in a quantitative way elsewhere. Distribution coefficients (&) were determined by taking l-g (dry weight) batches of resin that had been converted mto the sulfate form and then pre-equilibrated for 4 hr with the appropriate concentration of sulfuric acid. The resin was then equilibrated for 4 hr with 25-ml aliquots of sulfuric acid varying in concentration from 0.25 to 2.5M and each containing 200 pg/ml rhenium. Equilibrations at each concentration were conducted in tnphcate. The concentration of Re remaining in the solution was determmed by ICP-AES; because the K,, values are large (>500), estimation of the amount of rhenium on the resin by difference introduced insignificant error Results are summanzed m Table 1. Over the concentration range studied, rhenium 1s strongly sorbed by the anionic resin. The vanatron of log Kd with log molahty or molarity of sulfuric acid 1s remarkably linear, particularly above 0.5M (Fig 1). The underlying physlcochemlcal reasons for this linearity are not clear, as It 1s not observed with hydrochloric, hydrobromlc, or nitric acids25 (Fig 2) For a wide range of acid concentration, Re occurs almost entirely as the strong monobasic acid HReO,?’ and sorption by the resin depends largely upon competltlon with the major amomc species. In the resin, the sulfuric acid is present predominantly as HSOT.~* The Kd values for rhenium in the vanous acids might be expected to follow inversely the trends determined by acid strength [ClO; > NO; > Br- > Cl- > HSO;], so that

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251 -8

I -6

I -4

I -2

I 0

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Log of H,SO, mololdy Rg 1 Log K,, for rhemum us log (molahty of sulfurc acid) WoRad AG 1 x 8 resin, 20@400 mesh

Kd values for the sulfuric acid systems would be larger than those for hydrochloric acid systems, as is the case for solvent extraction with tertiary amines.ls In fact, the reverse, 1s true, as can be seen from Figs. 1 and 2, the X;, values for sulfunc acid solutions are uniformly lower than those for hydrochloric acid solutions. Osmium Osmium can be reversibly sorbed by amonexchange resins from dilute hydrochloric acid, apparently as OSO~C~$-.***~~ This observation was confirmed for 3 ml of solution OS-~, containing 624 pg/ml osmium as the purple Na2[Os0,(OH),]. The solution was acidified, diluted to a final volume of 10 ml m 1M hydrochloric acid, and allowed to stand at room temperature for 10 min before it was loaded onto the column, predommantly as Na,(OsO,Cl,) Elutlon with 0.8M nitric acid appeared to remove osmium from the column, although, for reasons discussed later, the experiment was not quantitative. In a second experiment, the same procedure was followed, except

Table 1 Dlstnbutlon coeffiaents* (IQ for rhenium m sulfmw acid solution I%% Molarlty

Molahty

0 25 0 50 100 1 50 2 00 2 50

0 253 0 510 1 04 1 59 2 16 2 76

&it 4180 f 2510 f 1300 f 866 + 677 f 523 +

150 70 25 8 7 7

*BloRad AG 1 x 8 rean, 200-400 mesh tMean f standard dewatlon of tnphcate measurements

-l

1

-75

-5

-25

0

25

50

75

1

Log of acid molollty Rg 2 Log Kd rhenium vs log (molahty of hydrochlonc, hydrobromlc and nitric acids) BioRad AG 1 x 8 resm, 200-400 mesh

Methods for the separation of rhemum, osmmm and molybdenum

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100

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125

150

175

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Rg 3 Elutlon of osmmm with IM hydrochloric acid and 0.8M mtnc acid from BloRad AG 1 x 8 resin, 200-400 mesh, chlonde form, m a column 10 mm m diameter and 125 mm long

that the solution m 1M hydrochloric acid was gently warmed for 10 min before being cooled quickly in ice and loaded onto the column. This experiment also could not be shown to be quantitative, and the recovery was lower, possibly because of the formation of some Na,OsCl,, which is strongly retained by the resin (Fig. 3). The elution experiments were not continued because the easy sorption of rhenium from the residual liquid from the osmium distillation offered an attractive alternative means of separation. In addition, Wee33 has confirmed that osmium and rhenium can be separated cleanly and m good yield by elution with nitric acid. The elution experiments for osmium were not quantitative because of a peculiarity in the determination of this element by ICP-AES. Very large differences m response were observed for the various chemical forms of osmium (Table 2). The results confirm the marked increase of sensitivity for osmmm when present as 0~0, in non-alkaline solution.23*” The signal observed for 0~0, m hydrochloric acid and

263

nitric actd solution is approximately 20 times that of corresponding solutions containing Os(VII1) or Os(V1) in sodium hydroxide or Os(V1) and Os(IV) in hydrochloric acid. Mallet et al 34 found the sensitivity for Os(VII1) as 0~0, to be 10 times that for Os(V1) as hexachloro-osmate. Summerhays et LzZ.*~ reported an enhancement of a factor of 50 for 0~0, in water relative to “OsC13” in 10% v/v hydrochloric acid, and a 15-fold increase m sensitivity if the “OsC13” were oxidized in 9M nitric acid for 4 hr. Given these large variations and the uncertainty concermng the chemical form of osmium (originally present as OsO,Cl:-) that is eluted with 0.8M nitric acid, accurate standardization was not possible. For osmium, radioactive tracer experiments would clearly be a better choice for quantitative elution studies. Molybdenum

The anion-exchange behavior of molybdenum has been studied for nitric acid, hydrochloric acid,26*3ssulfuric acid3’ and sodium hydroxide so1utions,36 and also for hydrochloric acid-hydrofluoric acid and hydrochloric acid-ammonium thiocyanate mixtures.35*37The marked minimum in K,, for molybdenum in approximately 1M hydrochloric acid*’ should afford an easy separation from rhenium, which has a & of 1000,28 or more,*’ at this acidity. Most previous studies were made at very low concentrations of molybdenum, however, and many investigators used radioactive tracers. The chemistry of molybdenum is complicated by a tendency towards condensation polymerization reactions that are strongly dependent on molybdenum concentration 38In an initial experiment, a solution made from ammonium heptamolybdate contammg 150 mg of molybdenum in 25 ml of 2 5M hydrochloric acid formed a green band on the resin. The band separated into several components that were difficult to elute cleanly. Suspecting that the problem arose from the

Table 2 Relative sensltlvltles of ICP-AES for osmmm species Solution 4 1A 3c 1c 2A 3B 1B 2c

Oxldatlon state IV VI VI VI VIII VIII VIII VIII

OsCliOsO,(OH):OsO,(OH):oso,c1:OsO,(OH);OsO,(OH):oso, oso,

Medmm

Color

Relative sensitlvlty*

3M HCl 1M NaOH 1M NaOH 1M HCl 1M NaOH 1M NaOH IM HNO, 1M HCl

Reddlsh-yellow Purple Purple Yellow Light brown Light brown None None

110 110 90 120 130 100 1800 2700

*Relative sensltlvlty for OS 1s measured for the OS (II) 228 226 nm lme, m counts set-’ pg-’

264

J W MORGANet al

original heptamolybdate form, we fused the salt with sodium hydroxide at 625 K for 1 hr; this fusion produced no significant improvement. The starting material was changed to sodium molybdate, and the elution experiments were repeated at much lower concentration (1.5 mg of MO in 10 ml of 1M hydrochloric acid loaded onto the column). There was no sign of polymerization, and tailing was insignificant. At higher concentrations, polymerization became more apparent, even with sodium molybdate. Tailing was significant when 15 mg of molybdenum m 10 ml of 1M hydrochloric acid was loaded onto the column; at higher concentrations, polymerization was indicated by the fractionation of colored bands on the resin, and multiple elution peaks. These problems were easily circumvented, however, by replacing the 1M hydrochloric acid medium with a 1M hydrochloric acid-N sodmm chloride mixture. Tailing was reduced to very low levels to give almost symmetrical elution peaks (Fig. 4), and even with a solution containing 150 mg of molybdenum m 50 ml, no polymerization was visible. These results provide a simple means for the study of rhenium and osmium m molybdenite. In this mineral, osmium is entirely radiogenie [‘870s formed from the decay of ‘*‘Re ($ = 4.23 x 10” years)39]. Even for the oldest molybdenites, the sample size needed is governed by the sensitivity for osmmm, because rhenium is present in large excess. For sample sizes up to 500 mg, after sodium hydroxidesodium peroxide fusion, an aliquot (5 or 10% of total volume) could be taken for the rhenium analysis, and a solution suitable for ICP-MS 45

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Anion-exchange studies of rhenium, molybdenum and osmium suggest that these elements may be separated from each other Rhenium is readily held on the column from sulfuric acid of concentration up to 2.5M or hydrochloric acid up to at least 5M, and from dilute (< 1M) nitric acid. Elution by higher nitric acid concentrations (> 3M) is essentially quantitative. The strong sorption from sulfuric acid solution is particularly useful if osmium is first distilled from sulfuric acid-hydrogen peroxide because the residue can then be passed directly through an anion-exchange column for recovery of rhenium. Separation of rhenium from substantial amounts of molybdenum (important because molybdenite is the major rhenium-rich mineral) is possible in one or two steps when polymerization of molybdenum is curtailed by using hydrochloric acid-sodium chloride solution. Osmium can be reversibly sorbed as OsO,Cliby an anion-exchange resin from dilute hydrochloric acid and eluted with nitric acid, but distillation may be preferable. The sensitivity of determination of osmium by ICP-AES is very dependent on the presence of osmium tetroxide, and for quantitative elution experiments, use of radioactive tracers may be a more useful technique for this element. REFERENCES

W Herr and E. Merz, 2 Naturfirsch , 1955, 100, 613 B Hut, W Herr and W Hoffmaster, Radwacttue p 35. Vienna, Austna, J -M Ph.D Drssertatron, of Pans 1982 J-M and C Allegre, Nature, 302, 130 Lmdner, D Letch, R Borg, G Russ, J Bazan, D Stmons and R. Date, 1986, 320, 6 F E S M R R R D J Holzbecher D E rbld, 322, 816 I Fehn, R D Elmore P W rbld,

E z

CONCLUSIONS

,

n

; :: 2

or RIMS obtained in a single anion-exchange step. Alternatively, sample sizes of up to 250 mg of molybdenite (containing 150 mg of molybdenum) could be processed by two anionexchange steps.

t

1986, 707 8 R Walker, S

00% Volume,

ml

Rg 4 Elutton of 1 $15 and 150 mg of molybdenum by 1M hydrochlortc actd-IM sodmm chlortde solutton from BtoRad AG 1 x 8 resm, 200-400 mesh, chlonde form, m a column 10 mm m dtameter and 125 mm long

Shtrey and Stecher, Earth Scl Lea, 87, 1 9 Herr, W B Hut, Gerss and G Houtermans, Naturforsch, 1961, 1053 10 K Chung and E Beamrsh, 1968, 15, 11 Idem, Anal

Acta, 1968,

351

Methods for the separation of rhemum, osmmm and molybdenum 12 M W Solt, J S Wahlberg and A. T. Meyer, Talunta, 1969, 16, 37 13 W J Maeck, G L Booman, M E Kussy and J E Rem, Anal. Chem, 1961, 33, 1775 14 E V Elhott, K R Stever and H H Heady, At Abs Newsl, 1974, 113 15 V YatlraJam and L R Kakkar, Anal Chun Acta, 1970, 52, 555 16 J W Morgan and R J Walker, lbld, 1989,222, 291 17 R J Walker and J W Morgan, Sczence, 1989,243,519 18 D D Lambert, J W Morgan, R J Walker, S B Shxey, R W Carlson, M L Zlentek and M S Koskl, Sctence, 1989, 244, 1169 19 J W Morgan, R J Walker and J N Grossman, Lunar Planet Set , 1990, 21, 809 20 J J Leary, A E Brooks, A F Dorrzapf and D. W Gohghtly, Appl. S’ctrosc, 1982, 36, 37 21 R K. Wmge, V A Fassel, V J Peterson and M A Floyd, Inductively Coupled Plasma Atomic Emlssron Spectroscopy; An Atlas of Spectral Information, Elsevler, New York, 1985 22. J W. Morgan, Ph D. Dzssertatron, Australian National Umverslty, 1965 23 R D Summerhays, P J. Lamothe and T L. Fries, Appl Spectrosc , 1983, 37, 25 24 K A Kraus and F Nelson, Proc Fust Intern Conf Peaceful Uses Atomu Energy, 1955, 7, 113 Umted Nations, New York, 1956

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25 B Chu and R M Dmmond, J Phys Chem , 1959,63, 2021 26 J P Fans and R. F. Buchanan, Anal Chem , 1964,X, 1157. 27 V. W Meloche and A. F. Preuss, lbtd, 1954,X, 1911. 28 E H Huffman, R L Oswalt and L A Wdhams, J Inorg. Nucl Chem , 1956, 3, 49 29 R N Se.n Sarma, E Anders and J M Mdler, J Phys Chem , 1959, 63, 559 30 F W E Strelow and C J C Bothma, Anal Chem, 1967, 39, 559 31 J E Early, D Fortnum, A WoJclckl and J E Edwards, J Am. Chem Sot , 1959, 81, 1295 32 F Nelson and K A Kraus, zbrd, 1958, 80, 4154 33 P S -L Wee, MS Dwertatton, University of Toronto, 1986 34. R. C Mallet, S. J. Royal and T. W Steele, Anal. Chem., 1979, 51, 1617 35. K. A. Kraus, F. Nelson and G E. Moore, J Am. Chem. Sot , 1955, 77, 3972 36. S. A. Rsher and V. W Meloche, Anal. Chem , 1952,24, 1100 37. H. Hamaguchl, K. Kawabuchl and R Kuroda, rbrd., 1964, 36, 1654 38 A I Busev, Analytrcal Chemrstry of Molybdenum, Humphrey Science Publishers, Ann Arbor, 1969 39 M Lmdner, D A Lelch, G P. Russ, J M Bazan and R J Borg, Geochzm Cosmochtm Acta, 1989,53, 1597