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Anal. Chem. 1982, 5 4 , 2158-2161
very narrow range. Below 1100 "C there is copious smoke during atomization which cannot be accommodated by the background corrector. A significant amount of Mg is lost a t char temperatures above 1200 O C . By using the "Max Power" feature of the HGA 500 furnace which provides very rapid ramping to the atomization temperature, we obtained better precision and better sensitivity than by using a 1.0-s ramp time. The faster heating rate reduces interferences by rapidly producing a constant temperature environment in the tube during the atomization (33).
LITERATURE CITED Dotson, R. L. US. Patent 3793 163, 1974. Seko, M. Japan Patent Applic. Kokai 51-88100, 1978. Seko, M. US. Patent 4 178218, 1979. Suhara, M.; Arai, T. U.K. Patent Applic. 2 047 271. Oda, Y.; Suhara, M.; Goto, S.;Hukushlma, T.; Miura, K.; Hamano, T. German Patent 2 819 527, 1978. Oda, Y.; Suhara, M.; Goto, S.; Hukushima, T.; Miura, K.; Hamano, T. U.S. Patent 4 202 743, 1980. "Hypochlorite Generator for Treatment of Comblned Sewer Overflows"; Water Pollution Control Research Series; U S . Environmental Protection Agency, PB 211 243, 1972. Sturgeon, R. E.; Berman, S. S.;Desaulniers, A.; Russell, D. S. Talanta 1880, 2 7 , 85. Jaslm, F.; Barbooti, M. M. Talanta 1981, 2 8 , 353. Epstein, M. S.;Zander, A. T. Anal. Chem. 1979, 5 1 , 915. Berggren, P.-0.; Berglund, 0.; Hellman, B. Anal. 6iochem. 1978, 8 4 , 393.
(30) (31) (32) (33)
Parker, J. C.; Gitelman, H. J.; Glosson, P. S.;Leonard, D. L. J . Gen. Physiol. 1975, 6 5 , 84. Bek, F.; Janouskova, J.; Moldan, B. At. Absorpt. Newsleft. 1974, 13, 47. Suzuki, M.; Ohta, K. Talanta 1981, 2 8 , 177. Cragin, J. H.; Herron, M. M. At. Absorpt. Newsl. 1973, 12, 37. Smith, M. R.; Cochran, H. B. At. Spectrosc. 1981, 2 , 97. Smith, M. R.; Cochran, H. B. US. Patent 4308030, 1981. Ediger, R. D.; Peterson, G. E.; Kerber, J. D. At. Absorpt. Newsl. 1974, 13, 61. Segar, D. A.; Cantillo, A. Y. Anal. Chem. 1980, 5 2 , 1766. Sturgeon, R. E.; Berman, S. S.; Desaulniers, A.; Russell, D. S.Anal. Chem. 1979, 5 1 , 2364. Churella, D. J.; Copeland, T. R. Anal. Chem. 1978, 5 0 , 309. Hydes, D. J. Anal. Chem. 1980, 5 2 , 959. Guevremont, R. Anal. Chem. 1980, 5 2 , 1574. Ediger, R. D. At. Absorpt. Newslett. 1975, 14, 127. L'vov, B. V. Spectrochlm. Acta, Part 8 1978, 336, 153. Slavln, W.; Manning, D. C. Anal. Chem. 1979, 5 1 , 281. Carnrick, G. R.; Slavin, W.; Manning, D. C. Anal. Chem. 1981, 5 3 , 1866. Laxen, D. P. H.; Harrlson, R. M. Anal. Chem. 1981, 5 3 , 345. Slavin, W.; Manning, D. C.; Carnrick, G. R. Anal. Chem. 1981, 5 3 , 1504. Peters, D. G.; Hayes, J. M.; Hieftje, G. M. "Chemical Separations and Measurements"; Saunders: Philadelphia, PA, 1974; Chapter 2. Frech, W.; Cedergren, A. Anal. Chim. Acta 1980, 113, 227. Muller-Vogt, G.; Wendl, W. Anal. Chem. 1981, 5 3 , 651. Siavin, W.; Manning, D. C. Spectrochim. Acta, Part 6 1980, 356, 701.
RECEIVED for review May 7, 1982. Accepted July 29, 1982.
Determination of Lithium Isotopes at Natural Abundance Levels by Atomic Absorption Spectrometry Allen L. Meler U S . Geological Survey, MS 973, Box 25046, Federal Center, Denver, Colorado 80225
The relationships of the absorption of 'Ll and 7Li hollow cathode lamp emlsslons are used to determine ilthlum Isotopic composition In the natural abundance range of geologic materlais. Absorption was found to have a nonlinear dependence upon total ilthlum concentratlon and Isotopic composltlon. A method using nonlinear equatlons to describe the relatlonshlp of the absorption of 'Li and 7Li lamp radlation Is proposed as a means of caicuiatlng Isotopic composition that Is Independent of total llthium concentration.
The determination of the occurrence and relative concentration of stable lithium isotopes in natural materials is of interest in geochemical exploration as a guide to hydrothermal alteration. It may also have useful application in the exploration for ore deposits and geothermal reservoirs (I). Lithium-6 is apparently enriched in rocks associated with hydrothermal alteration (I). Isakov et al. (2) reported an increase in lithium-6 as the degree of rock alteration increased. The average abundance of lithium in the earth's crust is about 20 ppm, ranging from 5 to 200 ppm in soils and averaging 10 ppm in basalt and 60 ppm in shale (3). Some lithium minerals may contain as much as 4.7% lithium. With the exception of these minerals and highly anomalous samples, the expected lithium concentration range for most geologic samples is about 10-200 ppm. The average natural isotopic abundance of lithium is 7.42% and 92.58% 7Li (4). Some controversy exists over how much natural variation in isotopic composition exists. Svec and Anderson (5) summarized published values, determined
by mass spectrometry, which showed isotopic abundance variation in lithium reagents and lithium separated from various minerals to be from 6.90% to 7.98% 6Li. They attributed this variation to instrumental factors as well as natural variation. In their study of lithium-bearing minerals, variations of only 7.34% to 7.61% 6Li were found. Isakov et al(2) reported a range of 6.77% to 9.28% 6Li in mica samples analyzed by mass spectrometry. Natural water samples analyzed by atomic absorption were reported to have as much as 13.38% 6Li (6). Divis and Clark (I) reported values, determined by atomic absorption, of 7.30% to 27.4% "i in hydrothermally altered rocks and an average of 9.38% 6Li in 24 unaltered volcanic rocks, with a standard deviation of 1.94. The values for the hydrothermally altered rocks have the greatest reported variation in natural lithium isotopic abundance that is readily found in the literature. The possibility of determining isotopes by atomic absorption was suggested by Walsh (7). The basis for the atomic absorption determination of lithium isotopes is that each isotope emits a doublet a t the 670.8-nm resonance line of lithium. The two peaks of each doublet are separated by 0.015 nm, and the doublet of one isotope is shifted by 0.015 nm with respect to the other. The upper wavelength of the 7Li doublet overlaps the lower wavelength of the 6Li doublet as a result of the isotopic shift and doublet separation being equal. The separate wavelength components cannot be resolved by conventional atomic absorption spectrophotometers; therefore, the isotopes cannot be determined independently. However, the difference in absorption of emissions of monoisotopic lamps by the two isotopes provides a means of estimating lithium isotopic composition (I, 6-12). The most commonly
This article not subJect to U.S. Copyright. Published 1982 by the American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 54, NO. 13, NOVEMBER 1982
2159
--
Table I. Mean and Relative Standard Deviation of A , for Five Replicate Analyses of Solutions of Mixed Isotopic Composition total Li.
0% 6Li concn, _______ WgmL-’ X % RSD I
2.0 1.5 1.0 0.5 0.2
0.7214 0.5503 0.8677 0.11873 0.0720
0.22 0.71 0.92 0.85 1.39
-
5% 6Li
X
0.7174 0.5479 0.3669 0.1826 0.0682
10% 6Li
-
% RSD 0.52 0.60 0.65 0.49 1.32
X
0.7119 0.5447 0.3641 0.1810 0.0697
reported way of relating the absorption of 6Li and 7Li lamp radiation to isotopic composition is to plot the ratio of the absorbance measured using a 6Li lamp to absorbance measured using a 7Li lamp vs. percentage 6Li (1, 6, 10, 11, 13). The resultant curve is nearly linear over the entire isotopic composition range. Some investigators (1,13)have reported the ratio to be insensitive to total Li concentration. However, the absorbance ratio was found by others (6, 10, 11) to be dependent upon the total Li concentration; therefore, standards and samples must be matched to make a valid estimate of isotope composition. In this study, the total Li concentration was found to have a significant affect on t,he absorbance ratio. Atomic absorption has been applied to the determination of lithium isotopes in material used in the nuclear industry (10, ll),lithium reagents (IO), natural water (6), and rocks (1). The use of atomic absorption for the determination of lithium isotopes in geologic materials is attractive when compared to the conventional method of mass spectrometry in regard to cost, speed, ease of sample preparation, and availability to most laboratories involved with geochemical analysis. Techniques for the dissolution of geologic materials for atomic absorption determination of lithium are applicable to isotopic determinations. These techniques usually result in solutions that contain 1%or less of sample by weight per volume of solution. The expected lithium concentration range of these solutions is about 0.1-2.0 pg mL-l with isotopic composition of about 5% to 20% 6Li by vveight. In this study the relationship of the absorption of 6Li and 7Li lamp radiation to isotopic composition is examined, and a method is proposed for estimating isotopic composition within the expected range for geologic materials.
EXPERIMENTAL SECTION Apparatus. An Instrumentation Laboratories Model 951 atomic absorption spectrophotometer equipped with a Model 254 autosampler manufactured by the same company was used in this study. This instrument has two double beam channels allowing simultaneous measurements of a single solution with individual 6Li and ‘Li hollow cathode lamps. A single-dot 10-cm burner was used with an air-acetylene flame. Reagents. Lithium-6 and lithium-7 stock solutions of 1000 pg mL-l were prepared firom monoisotopic metal obtained from Oak Ridge National Laboratory by dissolving 1.000 g of freshly cut metal in approximately 50 mL of distilled-deionized water. Twenty milliliters of concentrated HC1 was added and diluted to a volume of 1L,with distilled-deionized water. Mixed isotopic solutions of 100 hhg mL-l total Li and 0, 5, 10, 15, and 20% %i by weight were prepared by mixing and diluting appropriate aliquots of the 1000 yg 1nL-l 6Li and 7Li stock solutions. Mixed isotopic working solutions were prepared by diluting aliquots of each mixed isotopic solution to give solutions with total Li concentrations of 0.1-2.0 p g mL-’ in 0.1 yg mL-l increments. For samples with unknown isotopic composition and various total Li concentrations, solutions of reagent grade LiC1, LiNOB, Li2S04,and Li2C03were prepared to contain approximately 1.5, 1.0, 0.5, and 0.2 pg mL-l total Li. Procedure. The atomic absorption spectrophotometer was set up with a 6Li lamp in one channel and a ‘‘Li lamp in the other. Each lamp was operated at approximately 4 mA. The wavelength
-
X
% RSD
0.7133 0.5381 0.3570 0.1773 0.0675
0.67 0.54 0.98 0.39 1.33
% RSD 0..0.41 0.50 0.66 0.61 1.00
15% 6Li
-
20% ‘jLi X % RSD 0.7035 0.5329 0.3538 0.1743 0.0661
0.45 0.79 1.10 0.63 1.21
._
of each monochromator was peaked at 670.8 nm, with a bandwidth of 0.5 nm. An oxidizing air-acetylene flame was used, adjusted to give maximum absorbance when a lithium solution was aspirated. The working solutions and reagent solutions were placed in the autosampler, and a simultaneous 10-s integration of the absorbances expanded 10 times was taken for each solution. The instrument was automatically zeroed after each four solutions. Five consecutive readings were made on the set of solutions over a 6-h period. The flame conditions were then readjusted and another five readings were taken in the same manner.
RESULTS AND DISCUSSION The individual isotopic lines could not be resolved by this instrument even when a bandwidth of 0.04 nm was used. The use of bandwidths of less than 0.5 nm did little to improve the signal but did increase the noise level. A nearby neon line at 671.7 nm did not interfere with the 670.8-nm lithium line a t a bandwidth of 0.5 nm. The dual-channel capability of the instrument made it ideally suited to this work because simultaneous measurements of each solution could be made by using individual hollow cathode lamps. This capability not only increases the speed a t which data are collected but helps to match the measurement conditions for each lamp. However, flame conditions have a significant affect on the sensitivity of the isotopes. In this study the flame conditions gradually changed over a 6-h period, resulting in a decrease in sensitivity of about 18% for the 7Li lamp and 13% for the 6Li lamp. This unequal change in sensitivity invalidates comparisons of measurements taken within the first few hours of operation. After the first 6 h of operation the flame was readjusted, and the sensitivity of both lamps then remained relatively constant for the next 6 h of operation. Simultaneous absorbance values using the 6Li lamp (A6) and absorbance values using the 7Li lamp (A7) for each of the mixed isotopic working solutions and reagent solutions were taken in five consecutive runs after the flame was readjusted. The mean and relative standard deviations of the two absorbance values for each solution were calculated. Tables I and I1 list the means and relative standard deviations of A7 and A6 for five different total Li concentrations of the mixed isotopic working solutions. These five concentrations are representative of the concentration range of interest. However, all 20 concentrations for each isotopic composition were used in the comparisons and calculations made in this study. Table I11 shows the ratio of the A, to A7 of five total Li concentrations for each isotopic composition studied. The table shows that the absorbance ratio (A6/A7)increases with decreasing total Li concentration. Thus, use of the absorbance ratio alone, without matching the sample and standard concentration, would lead to serious errors in the estimate of isotopic composition. The relationship between As and A , for solutions of the same isotopic composition is defined by eq 1. The coefficients A6 = u + bA7 c A ~ * (1)
+
a, b, and c are determined by solving the normal equations
for the least-squares parabola of A , vs. A , for each set of
2160
ANALYTICAL CHEMISTRY, VOL. 54,
NO. 13, NOVEMBER 1982
Table 11. Mean and Relative Standard Deviation of A , for Five Replicate Analyses of Solutions of Mixed Isotopic Composition total Li concn, pgmL-*
X
% RSD
x
% RSD
x
% RSD
x
% RSD
x
% RSD
2.0 1.5 1.0 0.5 0.2
0.2540 0.2041 0.1440 0.0769 0.0313
0.51 0.93 0.76 0.78 1.60
0.2775 0.2219 0.1563 0.0820 0.0324
0.36 0.68 0.90 0.49 1.54
0.3022 0.2402 0.1680 0.0876 0.0357
0.53 0.50 0.42 1.03 1.40
0.3265 0.2554 0.1783 0.0934 0.0370
0.37 0.74 0.79 0.75 1.35
0.3495 0.2749 0.1905 0.0982 0.0389
0.86 0.62 1.26 0.51 1.29
0% 6Li
5% 6Li
10% 6Li
15% 6Li
20% 6Li
Table 111. Mean and Relative Standard Deviation ofthe Absorbance Ratio A , / A , for Five Replicate Analyses of Mixed Isotopic Solutions total Li concn, pgmL-'
X
2.0 1.5 1.0 0.5 0.2
0.3521 0.3710 0.3915 0.4108 0.4342
0% 6Li
5% 6Li
10% 6Li
15% 6Li
% RSD
x
% RSD
x
% RSD
x
% RSD
x
% RSD
0.60 0.35 0.51 0.11 0.64
0.3839 0.4049 0.4260 0.4491 0.4747
0.58 0.56 0.31 0.17 0.27
0.4244 0.4411 0.4614 0.4842 0.5122
0.55 0.21 0.64 0.70 0.96
0.4578 0.4764 0.4994 0.5265 0.5472
0.44 0.56 0.51 0.36 0.26
0.4967 0.5159 0.5384 0.5635 0.5886
0.46 0.47 0.30 0.34 0.30
Blank absorbance and standard absorbance have a nonlinear dependence on A,, as defined by eq 1. Therefore, the formula becomes
04
(az
w
u
z
+
+~OA~~)(Z)
+ ~ Z A ~ C ~- A(ao, ~+ )boA7 + ~oA7') (4)
where AB = absorbance of the sample measured with a lamp, A7 = absorbance of the sample measured with a 7Li lamp, ao, bo, and co = coefficients for 0% 6Li, and aZ,bZ, and cz = coefficients for 2% 6Li, where 2 = percent 6Li in standards. The coefficients for each set of mixed isotopic working solutions were substituted into the formula and reduced to derive the following equations:
2
802 v)
9
i 01
02
04
06
5% 6Li = As - 0.0011
08
'LI ABSORBANCE
Figure 1. Best-fit parabolic curves of A Bvs. A , for mixed Li isotopic solutions containing from 0.1 to 2.0 pg mL-' total Li and 0, 5, 10, 15, and 20% 'Li by weight.
solutions with the same isotopic composition. The coefficients were determined for each set of mixed isotopic working solutions used in this study and are given in the following equations: 0% %i A6 = 0.0011 0.42644, - 0.1051A7' A6
= 0.0016
+ 0.426447 - 0.1051A72 0.0001 + 0.0066A7 + 0.0001A72
A6 = 0.0020 -!- 0.4921A7- 0.1007A7'
15% 6Li
A, = 0.0006 -!- 0.5381A7- 0.1138A7' A6 = 0.0014 + 0.57604, - 0.1156A7'
for 0 and 5% 6Li
(5) % 6Li =
+ 0.4264A7 - 0.1051A72 0.0001 + 0.0066A7 + 0.0004A72
A6 - 0.0011
for 0
and 10% 6Li (6) % 6Li =
A6 - 0.0011
+ 0.426447 - 0.1051A,'
0.0074~47- 0.0006A7'
+ 0.45934, - 0.1044A7'
10% 6Li 20% 'Li
(A6 - ao
% 6Li =
03
5% 'Xi
20% 6Li
for 0
and 15% 6Li (7)
5% 6Li =
A6 - 0.0011
+ 0.4264A7 - 0.1051A7'
0.0075A7 - 0.0005A72
and 20% 6Li (8)
(2)
Figure 1 shows the best-fit parabolas for these equations. Each pair of absorbance values has a corresponding unique isotopic composition, thus providing a means of determining the isotopic composition of unknowns by using A6 and A7. The standard formula for calculating elemental concentration in a sample is (sample abs - blank abs)(std concn) sample concn = (std abs - blank abs)
(3) This formula can be used to calculate isotopic composition by substituting the appropriate equations into the formula.
for 0
The differences in the coefficients for each of the isotopic compositions are due to experimental error and a difference in curvature as the percent 6Li increases. By averaging the coefficients, one can derive a compromise equation which can be used to calculate an estimate of percent 6Li over the entire range. % 6Li =
A6 - 0.0011
0.426447 - 0.1051A72
o.oo70A7- 0.0002A~~
(9)
The A6 and A7 values for the mixed isotopic working solution were used in eq 9 to calculate the estimated percent 6Li of each solution. Table IV, which lists the mean and
ANALYTICAL CHEMISTRY, VOL. 54, NO. 13, NOVEMBER 1982
Table IV. Mean and Standard Deviation, of the Estimated $LiPercentage of Mixed Isotopic Working Solutions with Total Li Concentration of 0.1-2.0 wg mL-' actual % 6Li 0 5 mean of -0.101 4.91 estimated % 6Li std dev 0.11 0.14
10 9.90
15 15.04
20 20.45
0.13
0.18
0.22
Table V. Mean and Standard Deviation of the Estimated 'jLi Pe:rcentage for Five Replicate Analyses of Lithium Reagents LiCl total Li -__ concn, - std dev pgmL-' X 1.5 1.0 0.5 0.2 0.2-1.5
4.06 4.03 4.25 3.74 4.02
LiNO, std dev
x
10.20 7.23 0.17 7.50 10.30 7.63 0.36 7.47 0.31 7.46
0.32 0.28 0.19 0.69 0.41
Li,SO, -
Li,CO, std dev
X
std dev
x
6.92 7.35 7.61 7.63 7.38
0.27 10.18 10.29 1.04 0.60
7.25 7.44 7.44 7.99 7.53
-
0.37 0.19 0.40 0.90 0.57
standard deviation of the estimated percent 6Li, shows that the equation fits the data and the estimated percent 6Li is in close agreement with the actual percent 6Li of the solutions. The estimated 6Li percentages for the Eiolutions of lithium reagents were calculated by using eq 9. Talble V lists the mean and standard deviation of five analyses of reagent for different total Li concentrations. The mean and 13tandarddeviation for all analyses of each reagent are also shown. The data show that the precision of tho estimate decreases with decreasing total Li concentration. The estimates of isotopic composition are in close agreement at all concentrations, indicating that the effects of total Li concentration are adequately accounted for in the calculation. lJnfortunately, no statement can be made about the accuracy because no previously analyzed reference material was available. For greater precision and accuracy, coefficients for standards that bracket the concentration and isotopic composition of interest may be used in the formula. ]For example, if the concentration range of interest is 0.5-1.0 yg mL-l Li with isotopic composition of 5% to 10% "i, then standards of 0.5-1.0 yg mL-l total Li containing 5% and 10% 6Li are used to determine the coefficients. The coefficients for 5% 6Li are used in place of the 0% 6Li coefficients and the coefficients for 10% 6Li are used in the denominator. Because 5% 6Li
2161
is the base line, 5% is used as the standard concentration and 5% is added to the result to give the estimated 6Li% in the sample. The findings of this study show that atomic absorption spectrophotometry is applicable to the determination of lithium isotopes in the natural abundance range of geologic materials. The use of nonlinear equations to describe the relationship of the absorption of "i and 7Li lamp radiation provides a means of calculating isotopic composition that is independent of total lithium concentration. The precision and ability to resolve small differences in isotopic composition decrease with decreasing total lithium concentration. Precision and accuracy are improved by keeping the total lithium concentration as high as possible and by using standards that bracket the sample concentration and isotopic composition.
ACKNOWLEDGMENT The author thanks J. G. Viets, R. W. Leinz, and T. T. Chao of the U.S. Geological Survey and R. K. Glanzman of Chevron Oil Co. for helpful discussions.
LITERATURE CITED Divis, A. F.; Clark, J. R. Geochemical Exploration 1978: Proceedings of the Seventh Internatlonal Geochemical Exploratlon Symposium, Golden, Colorado; The Association of Exploration Geochemists: Rexdale, Ontario, Canada, 1978; pp 233. Isakov, Y. A.; Plyusin, G. S.;Brandt, S. D. Geochem. Int. 1969, 6 , 598. Levenson, A. A. "Introductlon to Exploration Geochemistry"; Applied Publishing Ltd.: Maywood, IL, 1974; p 43. Weast, R. C., Ed. "Handbook of Chemistry and Physics", 53rd ed.; The Chemlcal Rubber Co.: Cleveland, OH, 1972-1973; p 8-247. Svec, H. J.; Anderson, H. J., Jr. Geochim. Cosmochlm. Acta 1965, 29, 833. Taylor, H. E.; Scholder, L. J. 29th Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, 1978; p 63. Walsh, A. Spectrochim. Acta 1955, 7 , 108. Zaldel, A. N.;Korennol, E. P. Opt. Spectrosc. 1961, I O , 299. Mannlng, D. C.; Slavin, W. At. Absorpt. Newsl. 1962, 1 , 39. Goleb, J. A.; Yokoyama, Y. Anal. Chim. Acta 1964, 30, 213. Wheat, J. A. Appl. Spectrosc. 1971, 25, 328. Rade, H. S. At. Absorpt. Newsl. 1974, 13, 81. Chapman, J. F.; Dale, L. S.; Fraser, H. J. Anal. Chim. Acta 1980, 116, 427.
RECEIVED for review September 9, 1981. Resubmitted July 26,1982. Accepted July 26,1982. This study was conducted in the laboratories of the U S . Geological Survey. The use of brand or manufacturer's names is for descriptive purposes only and does not constitute endorsement by the U S . Geological Survey.
