Anal. Chem. 1988, 60, 37-39
almost all of the Li, to minimize isotope fractionation effects. Results, Table 111, for replicate analyses of a 50% eLi-enriched material, processed through the two column procedure for each analysis, showed good precision (0.2% RSD) and a small bias of 0.56% relative to results for the same material analysed directly (no columns). Correction for the column bias factor was not required for many applications, since it was reproducible and since only relative changes in isotope ratios were of interest. Otherwise the above correction factor was used. The bias may have been caused by isotope fractionation on the ion exchange columns, although chance of this effect was minimized by recovery of >98% of the Li. Applications. Aqueous solutions of lithium trifluoracetate were equilibrated with a solution of cryptand 221 in chloroform and, in a separate experiment, monobenzyl-15-crown-5 in hexanol. Lithium in the organic phases was released by back extraction into water. Isotope ratio measurements were reproducible within 0.2% (1RSD), and 6Li enrichments between l and 4% were observed for the organic phases. At one stage in the work ultrapure HN03 (Ultrex, J. T. Baker Chemical Co, Phillipsburg, NJ) was used to wet-ash the organic phase and release Li+, but results showed significant contamination with natural Li. Thus, in general, concentrated reagents should be avoided and Li dissolution or liberation steps should be tailored to each type of sample. The Li isotopic analysis method can be readily extended to Li concentration determination by use of the isotope dilution technique. The high precision achieved with a common single-detector mass spectrometer, and the simple steps for purification of samples, make the method attractive for any Li determinations that require high precision.
37
ACKNOWLEDGMENT The authors thank L. Plante for preparation of the isotope exchange samples and some of the standards. Registry No. t i , 14258-72-1;'Li, 13982-05-3;LiF, 7789-24-4; Li2F+,50927-99-6. LITERATURE CITED Lloyd, J. R.; Field, F. H. Blamed. Mass Spechorn. 1981, 8 , 19-24. Svec, H. J.; Anderson, A. R. Geochim. Cosmochim. Acta 1985. 2 9 , 633-641. Hastings, I. J. Canadian Fusion Fuels Technology Project Report CFFTP-B-66003 (AECL 9201); Chalk River Nuclear Laboratories, Chalk River, Ontario, 1986. Jepson, B. E.; Cairns, G. A. Monsanto Research Corp. Report M L M 2622, Mound Facility, Ohio, 1979. Svec, H. J.; Anderson, A. R. J . Sci. Instrum. 1986, 43, 134-137. Brown, H. L.; B l k . C.; Anbar, M. Int. J . Mass Spechom. Ion Phys. 1977, 25, 167-181. Michiels, E.; De Bievre, P. Int. J . Mass Spectrom. Ion Phys. 1983, 48, 369-372. Mlchiels, E.; De Bievre, P. Int. J . Mass Spectrom. Ion Phys. 1983, 49. 265-274. Kanno, H. J. Chem. SOC.Jpn. 1971, 44, 1808-1812. Moore, L. J.; Heald, E. F.; Fllliben, J. J. A&. Mass Spectrom. 1978, 7A, 448-474. Habfast, K. Int. J . Mass Spectrom. Ion Phys. 1983, 51. 165-189. CRC Handbook of Chemisby and Physics; Weast, R. C., Ed.; CRC Press: Boca Raton. FL, 1985. Green, L. W.; Barsczewskl, J. S.; Elliot, N. L. Int. J . Mass Spectrom Ion Processes 1985, 87. 253-265. Friedman, L. J . Chem. Phys. 1955, 23, 477-482. Matsumoto, K.; Kiba, N.; Takenchi, T. Talenta 1975, 2 2 , 695-697. Flesch, G. D.; Anderson, A. R.; Svec, H. J. Int. J . Mass Spec. Ion PhyS. 1973, 12, 265-272. Kent, R., Oak Ridge National Laboratory, Oak Ridge, TN, private communication, 1986.
RECEIVED for review January 12,1987. Resubmitted May 18, 1987. Accepted September 15, 1987.
Isotopic Fractionation of Gallium on an Ion-Exchange Column Lawrence A. Machlan and John W. Gramlich* Inorganic Analytical Research Division, Center for Analytical Chemistry, National Bureau of Standards, Gaithersburg, Maryland 20899
slgnmcant isotopic fractlonatlon of galllum has been observed during elution through an anlon-exchange resln In the thiocyanate form. Two samples of NBS Standard Reference Materlal 994 (certified @@Ga/"Ga= 1.506 76 f 0.000 39) were passed through the lon-exchange columns, and fractbns were analyzed for lsotoplc composltlon and galllum content by uskrg thermal lonlzstkn isotope dMlon mass Spectrometry. The results show a depletion of the ilght isotope In early fractions from the columns, wlth a steady increase In the llght Isotope throughout the elution. A materials balance of the product of the lsotoplc compositlon and galllum content of each fractlon Is In agreement wlth the lsotoplc composltlon of the starting material.
Isotope separation resulting from ion-exchange chromatography has been reported in the literature for nearly 50 years. Taylor and Urey (I),in 1938, found that by passing ratio solutions of potassium salts through zeolites, the 39K/41K could be altered by as much as *lo% from the normal
abundance ratio. Similar studies with lithium showed variations in the 6Li/7Liratio of up to 60% between the leading and trailing edges of the elution (1). Early work in this area was limited in accuracy due to the difficulty in controlling isotopic fractionation in the ion source, an inherent problem in the thermal ionization mass spectrometric process. In 1978, Russell and Papanastassiou (2) reported 40Ca/44Caisotopic variations of over 1% between the first and last fractions from an ion-exchange separation. These authors employed a double spike technique to correct for variations in the mass spectrometric fractionation between analyses. Most reports of isotopic fractionation during ion-exchange chromatography have been observed with light elements, where the mass difference between isotopes is large relative to the average atomic weight of the element. Recently Fujii et al. have published observations of isotopic fractionation of copper, which approaches the gallium mass range, using electromigration through a cation-exchange membrane (3). Isotopic fractionation of gallium by physical processes has been previously observed. Variations of more than 10% have been reported when a continuous electrical current is passed
This article not subject to US. Copyright. Published 1987 by the American Chemical Society
38
ANALYTICAL CHEMISTRY, VOL. 60, NO. 1, JANUARY 1, 1988
through a capillary column containing the metal (4, 5 ) . A survey of commercial high-purity gallium has shown isotopic variations of up to 0.25%, apparently due t o t h e multiple recrystallization steps used for the purification (6). Gallium isotopic fractionation on an anion exchange column in the thiocyanate form was first observed during the research at NBS to redetermine the atomic weight of gallium (7). Tetraphenylarsonium chloride was the reagent chosen for the assay of t h e gallium separated isotopes, and spark source isotope dilution mass spectrometry (SSIDMS) was utilized to determine the level of impurities in the separated isotopes. SSIDMS measurements on t h e separated isotopes revealed t h e presence of sufficient amounts of iron contamination t o interfere with the assay procedure through coprecipitation of iron with the gallium. Iron and gallium are not easily separated by ion-exchange chromatography due t o the similarity of the elution curves of the two elements, although some success has been reported with a cation exchange column in the chloride form (8). In an attempt to improve the separation of small quantities of iron from gallium, a n anion exchange column using a thiocyanate solution for elution was investigated (9, IO). Initial experiments using this separation procedure indicated that significant isotopic fractionation occurs during the elution. This paper presents t h e results of subsequent experiments to confirm the initial observations.
EXPERIMENTAL SECTION Two samples of approximately 200 pg of gallium in 1 mol/L HN03 were converted to the chloride form by evaporating twice with 6 mol/L HC1. Each sample was dissolved in 10 mL of 0.1 mol/L KSCN. Two ion-exchange columns were prepared with AG 1x8 (100-200 mesh) anion-exchange resin to form columns 0.75 cm X 10 cm. The resin was prepared by passing 45 mL of 1 mol/L KSCN through the columns, followed by 10 mL of 0.1 mol/L KSCN. Approximately 0.3 mL of resin from column 2 was transferred to sample 2 and the solution was warmed (50 "C) for 1 h and allowed to cool to room temperature for a period of 1 h with occasional swirling. (This was done to check previous indications that the equilibration on a thiocyanate column might not be rapid.) The sample solutions, including the preequilibrated resin with sample 2, were added to the columns, and the elution was carried out with a solution containing 0.1 mol/L KSCN and 0.05 mol/L HC1. The first 5-mL fraction was collected from each column and then subsequent fractions were collected after delivery of the following additional volumes: 5, 10, 20, 50, 50, 100, 100, 150, 150, 200, and 200 mL. Diluted HCl and diluted H N 0 3 (volumes and acid concentrations are reported in Tables I and 11, fractions 13-15) were then used in an attempt to remove the remaining gallium. The gallium content of each fraction was determined by isotope dilution mass spectrometry by transferring approximately one-third of the fraction by weight to a separate beaker and spiking with 71Ga. The remaining two-thirds of the sample was used to determine the gallium isotopic composition. The gallium in each fraction was purified by using two ionexchange columns. The first column contained 2 mL of AG 50x8 (100-200 mesh) cation-exchange resin in a 3-mL syringe. Each gallium fraction, as it came from the thiocyanate column or after spiking with 71Ga,was loaded onto the cation column and most of the impurities were eluted with 75 g of 0.4 mol/L HC1. The gallium was then eluted with 15 g of 4 mol/L HC1. After evaporation to dryness, the sample was dissolved in 2 mL of 5 mol/L HCl and loaded onto a column of AG 1x8 (100-200 mesh) resin in a plastic dropper (0.5 cm X 2.5 cm). Impurities were eluted with 20 g of 5 mol/L HC1 and the gallium was removed with 10 g of 0.5 mol/L HCl. The two ion-exchange purifications should not have affected the isotopic composition of the gallium, since nearly 100% recovery of the gallium was obtained from each column. Isotope ratio measurements were made on an NBS designed thermal ionization mass spectrometer with a 30-cm radius of curvature, 90" magnetic sector (11). The instrument was equipped with a thin lens "Z" focusing ion source and multicomponent deep-bucket Faraday cage collector. The remainder of the
Table I. Column Fraction Data for Column 1 A, column fraction 1 2
3 4 5 6 7
8 9 10 11 12
13 14 15
B, vol of eluant," mL
C, pg of Ga
5 5 10 20 50 50 100 100 150 150 200 200
1.06 0.52 6.00 1.82 5.08 10.54 24.75 18.10 19.35 20.20 24.15 16.61
0.52 0.25 2.96 0.90 2.50 5.19 12.20 8.92 9.54 9.96 11.90 8.19
1.5053 1.5063 1.5064 1.5028 1.5031 1.5028 1.5049 1.5041 1.5049 1.5063 1.5062 1.5071
lOOb
39.83 13.72 1.17
19.63 6.76 0.58
1.5092 1.5089 1.5101
20c 20c
D, % Ga E, 6gGa/71Ga
A materials balance on the gallium isotopic composition was obtained by summing the products of the fraction of gallium and the isotopic composition in each fraction: sum 6BGa/71Garatio = Z(0.01D "0.1 mol/L KSCN
+ 0.5 mol/L HC1.
X
3' 30
E) = 1.5063 g
of 5 mol/L HC1; 20
g of 10 mol/L HCI; 50 g of 0.5 mol/L HCl. c20 g of 8 mol/L
"0,. Table 11. Column Fraction Data for Column 2 A, column fraction 1 2
3 4 5 6 7 8 9 10
11 12
13 14 15
B, vol of eluant," mL
C, pg of Ga
5 5 10
0.45 0.06 0.03 0.04 1.43 7.37 26.78 11.54 25.03 25.29 22.39 18.09
0.02 0.71 3.64 13.24 5.71 12.37 12.50 11.07 8.94
1.5028 1.5042 1.5044 1.4935 1.4961 1.5009 1.5041 1.5040 1.5052 1.5063 1.5070 1.5074
50.10 13.54 0.17
24.77 6.69 0.08
1.5092 1.5082 1.5092
20 50 50 100 100 150 150 200 200 lOOb
20c 20c
D, % Ga E, 6vGa/71Ga
0.22 0.03 0.01
A materials balance on the gallium isotopic composition was obtained by summing the products of the fraction of gallium and the isotopic composition in each fraction:
sum 6BGa/71Garatio = Z(0.01D
"0.1 mol/L KSCN
+ 0.5 mol/L HC1.
X
E) = 1.5065
b30 g of 5 mol/L HCl; 20
g of 10 mol/L HC1; 50 g of 0.'5 mol/L HCC '20 g of 8 moi/L
"0,. measurement circuitry consisted of a vibrating reed electrometer, voltage-to-frequency converter, scaler, and computer. Timing, magnetic field switching, and data acquisition were controlled by the computer. The sample loading procedure and the mass spectrometric analytical procedure used during this work were identical with those used for the redetermination of the atomic weight of gallium at NBS and the certification of NJ3S Standard Reference Material 994. A detailed description of the mass spectrometric analytical procedure has been published (6, 7). Periodically during this work isotopic measurements were made on SRM 994 and the results were statistically indistinguishable from the observed 69Ga/71Ga ratio obtained during the gallium atomic weight determination (1.528 29 vs. 1.52828 for the atomic weight determination). The same analyst and the same instrumentation were used for both experiments. Since an absolute value for the isotopic ratio of SRM 994 is available (7), the isotopic ratios reported in Tables I and
ANALYTICAL CHEMISTRY, VOL. 60, NO. 1, JANUARY 1, 1988
II have been corrected to absolute valuea by applying a systematic bias correction of 0.98591 to the experimentally observed ratios. RESULTS AND DISCUSSION Several conclusions can be drawn from the data reported in Tables I and 11. The first three column fractions of column 1 show essentially no fractionation and, combined with the relatively large amount of gallium removed with the f i t 1.7% of the eluant, imply slow equilibration of gallium with the thiocyanate column. After the first 20 mL of elution, there is a significant decrease in the 6gGa/71Garatio, followed by an increase in the ratio with successive elution volumes. The data reported in Table I1 reflect an attempt to preequilibrate the gallium with the resin before addition to the column. The data in Table I1 indicate that a significantly greater equilibration was achieved, as reflected in the lower concentrations of gallium and the lower isotopic ratios obtained in the first three fractions from the column. In spite of extensive attempts to obtain quantitative recovery (more than 1 L of eluant per column), some gallium remained on the columns. The recoveries were 93+% and 96+% for columns 1 and 2, respectively. The sum ssGa/71Ga ratios (obtained by summing the products of the fraction of gallium and the isotopic composition in each fraction) is slightly lower than the certified value of the starting material for both columns but within the uncertainty of the certified value for column 2. Since there is a definite trend of increasing isotopic ratio with increased retention on the column, a high 69Ga/71Garatio for the ma-
39
terial remaining on the column is to be expected. A ratio of 1.513 for the gallium remaining on either column would resolve the small differences between the summed isotopic ratios from the columns and the isotopic ratio of the starting material
(SRM 994). Although the thiocyanate column was effective in separating iron from gallium, the long elution times and lack of quantitative recovery limit its usefulness. The important point to make is that when ion exchange separations are used for isotopic analysis, either quantitative recovery must occur or the absence of isotopic fractionation on the column must be confirmed. Registry No. 69Ga,14391-02-7;'lGa, 14391-03-8;Ga, 7440-55-3.
LITERATURE CITED Taylor, T. I.; Urey, H. C. J. Chem. Phys. 1838, 6 , 429-438. Russell, W. A.; Papanastasslou, D. A. Anal. Chem. 1978, 5 0 , 1151-1 154. FuJIl, Y.; Hosoe, M.; Okamoto, M. Z . Naturforsch., A : Phys., Phys. Chem.. Kosmophys. 1986, 41A, 769-770. Nlef, G.; Roth, E. C . R . HeM. Seances Acad. Sci. 1954, 239, 162. Goldman. M.; Nlef, G.; Rooth, E. C . R . Hebd. Seances Acad. Sci. 1958, 243, 1414-1416. Gramllch, J. W.; Machlan, L. A. Anal. Chem. 1985, 57, 1788-1790. Machlan, L. A.; Gramllch, J. W.; Powell. L. J.; Lambert, G. L. J. Res. Natl. Bur. Stand. (US.)1988, 91, 323-331. De Laeter, J. R. Geochlm. Cosmochim. Acta 1972, 3 6 , 735-743. Turner, J. B.; Phllp, R. H.; Day, R. A. Anal. Chlm. Acta 1982, 2 6 , 94-98. Korklsch, J.; Hecht, F. Mikrochlm. Acta 1958, 1230-1237. 1987, No. 426. Shields, W. R., Ed. NBS Tech. Note (US.)
RECEIVED for review June 2,1987. Accepted August 14,1987.
