New Electrochemical Technique for the Separation of Lead at Trace Levels from Natural Silicates J. W. Arden and N. H. Gale Department of Geology and Mineralogy, University ofOxford, Parks Road, Oxford, England
A rapid low blank method is described for the separation of lead by sequential cathodic and anodic deposition from a variety of matrices, especially complex natural silicates. The technique is especially suitable as a separative step for subsequent mass spectrometric isotopic analysis and for lead concentration determination by mass spectrometric isotope dilution; it is applicable to samples containing 10 ng of lead upwards and the combined electroseparation is more than 95% efficient at these levels. Methods have been devised for overcoming all interferences, and the optimum conditions for both the cathodic and the anodic deposition of lead have been studied. A technique has been developed for the dissolution of natural silicates which ensures recovery of more than 99% of the lead which they contain.
There has been much recent interest in the extension of modern geochronological studies based on the uranium, thorium/lead natural radioactive decay chains to the study of the record left by lead present in rocks a t the part per million (and sub-part-per-million) level. (See reference 1 for a general discussion.) Such studies require a rapid, low blank, high yield method for separating lead in microgram and sub-microgram amounts from natural silicates, for subsequent thermal ionization mass spectrometric analysis to obtain lead concentrations by isotope dilution, and the abundance of the z04Pb, 206Pb,207Pb, and 208Pb isotopes present. A further stimulus to the development of such a method has been the recent silica gel activator method for the thermal ionization mass spectrometric analysis of Pb, which allows analysis down to the 10-nanogram level (2-4). The case which at present presents the most extreme challenge to the development of an analytical method is that of meteorite analysis, where the lead may be present at concentrations between 0.01 and 1.0 ppm in a complex matrix and where available sample size is much restricted. The development of the present technique was aimed a t solving the problem of meteorite analysis, while recognizing that a method which met this challenge would also have many other applications in geochronology, such as the analysis of the lead contained in single small zircon or sphene crystals, or lead in ultrabasic rocks. The technique is equally applicable to less complex matrices, such as biological samples. [A preliminary version of the method described here was given in outline a t the second European Colloquium on Geochronology, Brussels, 1971 ( 5 ) ] . Most previous methods used for the extraction of lead from large (>0.5 gram) rock samples do not appear to B. R . Doe, "Lead Isotopes," Springer-Verlag, Berlin, 1970. V. P. Bedrinov, I . V. Chernyshev. L. L. Shanin, and R . N. Shcherbakova, Russ. J Phys. Chem.. 42, 1671 (1968). L. K . Levskiy, A. N. Murin, and V . G. Zaslavskiy, Geokhfmiya. 1969, 751. A. E. Cameron, D. H. Smith, and R. L. Walker, Ana/. Chem.. 41, 525 (1969). J. W. Arden and N . H. Gaie, Ann. SOC. Geoi. Beig., 1.94, 101 (1971).
lend themselves easily to a reduction of the blank per analysis below about 30 to 50 nanograms. Table I summarizes some of these methods together with the lowest blanks achieved by their use so far as is known to the authors. The pyrochemical method, while superficially attractive, suffers from an uncertain yield; the isotopic composition of the extracted lead is not always identical with that in the rock (29, 20) and it is not suitable for isotope dilution concentration measurements. Lunar sample analysis has stimulated improvements in technique, especially by Tatsumoto, who has achieved the lowest previously reported blank for large samples by a combination of coprecipitation and electrodeposition (8). More recently Tera and Wasserburg (15) have achieved still lower blanks for lunar samples using a miniaturized ion exchange separation, but, because of a major interference from iron, their method is directly suitable only for very small weights (
