Measurement of Lead Isotopes by Differential Atomic Absorption Willis H. Brimhall Department of Geology, Brigham Young Unioersity, Prouo, Utah
ONLY A FEW INVESTIGATIONS have been reported on the effects of isotopic differences in atomic absorption spectrometry, although Walsh ( I ) noted the possibility that isotopic differences might in some instances be measurable. Zaidel and Korennoi (2) and Manning and Slavin (3) applied atomic absorption techniques to the study of Li isotopes. Goleb ( 4 ) applied the technique to the measurement of U isotopes. In 1968, experiments on differential atomic absorption of Pb indicated that sufficient differences exist in the hyperfine spectrum of Pb, attributable to isotopes, to form the basis of a potentially useful technique for their rapid and convenient measurement of Pb isotopes. While it was recognized at the outset that the measurements could not be as precise as those obtainable from mass spectrometric methods, the method nevertheless gave promise of application to certain classes of geological materials for coarse screening, or in instances wherein a large number of measurements of moderate accuracy would be more desirable than a few of high accuracy.
_--
PbZOB lamp PbZ07 lamp
. , ,. .., .. Pb206 lamp
.......
EXPERIMENTAL
The instrument used in the experiments is a Model 303 Perkin-Elmer atomic absorption spectrophotometer equipped with a strip chart recorder and three hollow cathode source lamps in all respects similar to commercially available lamps except that each contains a hollow cathode prepared from Pb metal strongly enriched in one of the three isotopes Pb206, 207, and 208. In addition, three aqueous standards, each likewise enriched in one of the three isotopes, and containing a total of 10 pg per ml are utilized. Three aqueous “controls,” labeled Control A , B and C are blends of the first three standards, and are used as comparisons, run as “unknowns,” against which experimental results could be evaluated. Table I summarizes the compositions of the standards and controls. Metal enriched in one of the three isotopes of Pb was obtained either from the U. S. Atomic Energy Commission or from George Tilton of the University of California at Santa Barbara. The hollow cathodes were prepared by the author, and are of standard design except that only a thin, cup-like insert of Pb metal is placed in the hollow cathode in order to conserve material and reduce cost. The remainder of the hollow cathode lamps were prepared by the Westinghouse Electric Corp. The intensity of light produced by the special lamps is only slightly less than that of the standard lamp. Instrumental parameters are the same as those given by Perkin-Elmer (5) for the analysis of Pb, with the exceptions that a wavelength setting of 2833 A, the scale expansion 3, noise suppression 4, and a three-slot burner were employed. Lamp current was 10 mA, as specified by the manufacturer.
2
Pb107
Figure 1. Instrument response to Standards 1, 2, and 3 with respect to the Pb206,207, and 208 lamps Composition of the standards is given in Table I RESULTS AND DISCUSSIONS
Instrumental responses, given as per cent absorption and peak height in millimeters, are given in Table I1 for each of the standard and control solutions with respect to the three source lamps. Clearly, the instrumental response is dependent upon isotopic differences although they are not independent of one another. It is assumed that the peak height for any one standard with respect to a given lamp is the sum of three terms, each of which
Table I.
Isotopic Compositions of Three Aqueous Standards and Three Controls
Corresponding instrument responses are shown when the instrument is equipped with a hollow cathode made of ordinary Pb Inst.
resp., Pb207, pg/ml
Pb208, pg/ml
Total Pb,
pg/ml
pg/ml
absorption
9.739 0.216 0.012 2.494 2.546 4.928
0.069 9.240 0.041 2.348 4.648 2.354
0.192 0.548 9.947 5.158 2.808 2.720
10.000 10.004
8.06 8.55 12.40 10.52 9.48 9.30
Pb206, (1) A. Walsh, Aria/. Chim. Acta, 34, 135 (1955). (2) A. N. Zaidel and E. P. Korennoi, Opt. Spectry (USSR, English translation) 10, 299 (1961). (3) D. C. Manning and W. Slavin, Afomic Absorption Newsleffer, 8, I (1962). (4) J. A. Goleb, A i d . Chim. Acta, 34, 135 (1966). ( 5 ) Perkin-Elmer Corporation, Norwalk, Conn., Analytical Methods for Atomic Absorption Spectrophotometry (1969).
Standard 1 Standard2 Standard 3 ControlA
ControlB Control C
1O.OOO 1O.ooO 10.002 10.002
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1349
is the product of the concentration of a given isotope in the standard times a constant associated with the isotope. The three constants associated with the Pb206 lamp can be determined by the use of Standards 1 , 2 , and 3 in the following format (three unknowns in three simultaneous linear equations) :
Table 111. Instrument Response to Three Aqueous Standards and Three Controls The instrument is equipped with lamps of enriched Pb206,207 and 208 Pb206 Lamp Peak height Absorption in millimeters Standard 1 14.13 109.8 Standard 2 6.04 47.0 Standard 3 15.29 118.8 Control A 13.00 101.o Control B 10.75 83.5 Control C 12.61 98.0
z
Pb207 Lamp Peak height in millimeters 27.1 71.2 43.8 46.3 53.5 43.0
zAbsorption Standard 1 Standard 2 Standard 3 Control A Control B Control C
3.58 9.20 5.63 5.95 6.82 5.35 Pb208 Lamp
zAbsorption Standard 1 Standard 2 Standard 3 Control A Control B Control C
7.98 10.30 15.72 12.64 11.26 10.68
Peak height in millimeters 62.0 80.0 122.2 98.2 87.8 83.0
Table 111. Constants of Proportional Responses to Pb206, 207 and 208 for Each of the Special Lamps Pb206 Pb207 Pb208 11.011 4.123 11.018 Lamp 206 2.644 4.534 Lamp 207 4.370 Lamp 208 6,070 7.790 12.246 Table IV. Comparison of Experimentally Derived Concentrations of Isotopes in Three Controls, and Their Theoretical Values Concn. in pg/ml Pb206, Pb207, Pb208, Total, ccg/ml ccm/gl ccg/ml ccg/ml Control A Experimental 2.546 2.180 5.370 10.09 2,494 2.348 5.158 10.00 Theoretical Deviation 2.1 7.2 4.1 0.9 Control B Experimental 2,693 4.534 2.950 10.17 Theoretical 2,546 4.648 2.808 10.00 Deviation 5,7 2.7 5.0 1.7 Control C 4.995 2.387 Experimental 2.783 10.16 Theoretical 4,928 2.354 2.720 10.00 1.4 2.8 1.6 1.4 Deviation
z
z
z
1350
ANALYTICAL CHEMISTRY
R L ~ ~to?BM. OA ~= 109.8 mm = 9.739k6
+ 0.069k7 + 0.192k8
(1)
47.0 mm = O.2l6ks
+ 9.240k7 + 0.548ks
(2)
R L ~ ~ ~ ? O c = ~ ~118.8 ~ S mm M .= 0.012k6
+ 0 . 0 4 +~ ~9.947k8
(3)
RLamp 206 to std. B
=
where R L 206 to~ 8td.~A is~ the response of the instrument, utilizing the lamp enriched in Pb206, to the isotopic concentrations in Standard A . Similar responses are given to Standards B and C. The figures of the last three columns are the corresponding numbers given in Table I for Standards 1, 2, and 3. Assuming linearity of response in the range 0 to 20% absorption, and the constancy of inter-isotope effects, the three equations can be solved for the constants k6, k7 and kg which are the proportional responses of the Pb206 lamp to the isotopes Pb206, 207, and 208. A similar procedure can be followed on the Pb207 and 208 lamps to derive the constants given in Table 111. (In our laboratory, these calculations are conveniently accomplished by means of a programmable desk computer.) Having determined the foregoing constants, it is now possible to formulate another set of simultaneous linear equations to solve for the concentrations of P b isotopes in an unknown, provided that the samples are measured under the same experimental conditions as the standards. The Controls A , B , and C are utilized for the purpose of comparing experimental results with known isotopic concentrations. The format for Control A is as follows: RconA to POBLamp = 101.O mm = 11.011c206 4.123czo7 11.913c208 (4)
+
ReonA ta 207Lamp R c o nt ~o 2 m a m p
=
=
46.3 mm = 2.644c?os
+
+ 7.385C207 + 4.370c208
98.2 mm = 6.070c206 $. 7.79OC207
(5)
+ 12.246C2~ (6)
where R is instrument response with the given lamp to Control A , and C is the concentration of the given isotope in the unknown. The solution of these equations for the concentrations of Pb isotopes in Control A is given in Table IV, together with those for Controls B and C, similarly derived. For convenience of comparison, the actual concentrations and ratios of experimentally determined and theoretical values are also given. Total P b is overestimated in the three controls by a factor of less than 2 Z, the average of which is 1.4 % for the three. I n regard to each isotope, the largest difference between the experimental results and the control is obtained for Pb207 in Control A , which amounts to underestimation of the isotope by 7.2 % deviation from the control value. Agreement is best in Control C in which the experimental values are no more than 2.8 deviation from the control values. On the whole, remarkable agreement is obtained, and it is best in the control having relatively more Pb206 than 207 and 208, a situation which would be expected to be present in some geological materials enriched in U238 compared to Th232. A possible improvement might be obtained by employing source lamps with higher intensity, such as improved hollow cathode lamps or electrodeless discharge lamps described by Dagnall, Thompson, and West (6). It is possible that the (6) R. M. Dagnall, K. C. Thompson, and T. S. West, Atouric AbSorptiou Newsletter, 6 , 117 (1967).
signal to noise ratio could be increased, linearity of response improved, and the amount of sample decreased. The measurements herein reported were completed in 3 hours which included 1li2hours for lamp warmup, 1 hour for analysis, and an additional 1 hour for data reduction. Thus, the method is relatively rapid. The current experiment consumed approximately 30 ml of sample in which the concentration was 10.00 pg per ml. Consequently, only a few hundred micrograms of Pb is required, and refinements could reduce the amount considerably. Application of the method to geochronology will probably necessitate preconcentration procedures and other techniques t o increase sensitivity. The geochronological promise of the method equals o r exceeds that of the Larsen (Pb-alpha) method ( 7 , 8 ) whereby only total Pb is measured. As of yet no experiments have been performed o n Pb204, but it is expected that differential atomic absorption occurs (7) E. S. Larsen, N. B. Keevil, and H. C. Harrison, Bull. Geol. SOC.Amer., 63, 1045 (1952). (8) H. W. Jaffe et a / . Lead Alpha age determinations of accessory minerals of igneous rocks. Bull. U. S. Geol. Surcey, 1097-B, (1959).
for this isotope as well. Its high cost has prevented investigation thus far, but it is hoped that experimental data can be obtained o n it in the near future. It should be noted from the data of Table I that the measurement of chemical Pb by atomic absorption can be in significant error when the proportions of isotopes in standards and unknowns are markedly different. Because such differences are not likely to occur in most P b analyses, no significant error would be expected o n this account. But AAS measurement of chemical P b in samples isotopically different from ordinary Pb by reason of geological or technological processes would be accurate only if, among other things, the proportions of isotopes in samples and standards are similar. RECEIVED for review February 14, 1969. Accepted May 28, 1969. The Robert A. Welch Foundation, Grant C-9 t o John A. S. Adams and John J. W. Rogers, made possible post doctoral studies in atomic absorption spectrometry at Rice University. Further financial assistance was provided by a grant of the Research Division, Brigham Young University, to the author for study in geochronology, 1967-69. W. K. Hamblin, Department of Geology, Brigham Young University made some additional funds available to the author.
Chemical Separation of Cerium Fission Products from Microgram Quantities of Uranium Ana Albu-Yaron,’ D. W. Mueller, and A. D. Suttle, Jr. Department o j Chemistry, Texas A&M Unioersity, College Station, Tex. 77843
THERE ARE a number of problems in earth sciences which may be elucidated if a n accurate method for determining very small quantities of uranium is developed. This paper describes an accurate, fast and reliable method for the separation of cerium which may subsequently be used to determine nanogram quantities of uranium in raw cores. It involves the fission of a portion of uranium, separation of an abundant fission product, in this case cerium which can be cleanly and quantitatively separated, and then counting the cerium activity. Data in the literature o n the radiochemical determination of cerium in fission product mixtures involves methods based on the insolubility of some cerous and ceric compounds, on ion exchanges processes and on solvent extraction. If precipitation alone is used, as many as nine precipitations may be required t o ensure a radiochemically pure product ( I ) . Because appreciable material is lost in these steps, the final oxalate precipitate must be weighed to obtain a chemical yield. Jain and Singh ( 2 ) reported a more rapid gravimetric determination of Ce(II1) by precipitation at p H 3-6 with diammonium-5, 5-indigo disulfonate. However, Th also forms a n insoluble complex in the same p H range. Recently many articles have appeared describing methods based on ion exchange processes for replacing a number of the On leave from the Hebrew University of Jerusalem, Department of Inorganic and Analytical Chemistry, Jerusalem, Israel. (1) C. D. Coryell and N. Sugarman, Eds., “Radiochemical studies: The Fission Products,” Nat. Nucl. Energy Series, Div. IV, Vol. 9, Book 3, McGraw Hill, New York, N. Y.,1951. (2) B. D. Jain and J. J. Singh, Talaritn, 8, 648 (1961).
precipitation steps (3-6). A large variety of ion exchange media as well as experimental conditions, have been studied for a greater specificity in separation of individual lanthanides. Separation was incomplete and repeated oxalate precipitations were required. Extraction methods developed to date for cerium are generally tedious ( 7 , 8 ) . Glendenin et al. (9) extracted cerium (IV) into methyl isobutyl ketone from strong nitric acid, forming a potentially explosive mixture. Separation from Zr, Nb, Th, and N p was incomplete, however, and it was necessary to effect final decontamination by several oxalate precipitations. Smith and Moore (10) report greater than 98% extraction of tracer level of radiocerium with 0.5M 2thenoyltrifluoro acetonexylene; however, Cl-, F-, and P04ainterfere. McCown and Larsen (ZZ) report the quantitative extraction of Ce(IV) from nitric acid as the chelate of 2ethylhexyl phosphoric acid into n-heptane followed by a
(3) F. H. Spedding, A. F. Voigt, E. M. Gladrow, and N. R. Sleight, J . Amer. Chem. Soc., 69, 2777 (1947). (4) E. C. Freiling and L. R. Bunney, ibid., 76, 1021 (1954). (5) A. P. Baerg and R. M. Bartholomew, Can. J. Cliem., 35, 980
(1957). ( 6 ) J. Alstad and A. C. Pappas, J . Zuorg. Nucl. Chenz., 15, 222
(1960). (7) C. F. Metz, G. M. Matlack, and G. R. Waterburg, Proc. Znterli Conf. Peaceful Uses A t . Energy, Geneca, 28, 441 (1958). (8) M. P. Menon and P. K. Kuroda, Nucl. Sci. Eng., 10,70 (1961). (9) L. E. Glendenin, K. F. Flynn, R. F. Buchanan, and E. P. Steinberg, ANAL.CHEM.,27, 59 (1955). (10) G. W. Smith and F. L. Moore, ibid., 29,448 (1957). (1 1) J. J. McCown and R. P. Larsen, ibid., 32, 597 (1960). VOL. 41, NO. 10,AUGUST 1969
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