Precise Isotopic Analysis of Lead in Picomole and Subpicomole Quantities Fouad Tera and G. J. Wasserburg The Lunatic Asylum of the Charles Arms Laboratory, Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, Calif. 9 1125
A mlcroanalytlcal procedure for the separatlon of Pb and U and thelr determlnatlon by isotope dllutlon-mass spectrometry Is described. It Is shown that hlgh preclslon Isotopic analysls of as llttle as 8 X 10’l Pb atoms Is readily achlevable. The level of Pb contamlnatlon Is reduced to as llttle as 4 X 1O1O atoms and the blank fluctuatlon Is controllable. In the speclal case where the chemlcal separatlon could be omitted, 4 X lo1’ Pb atoms were isotoplcally analyzed wlth a blank correction of 6 X l o g Pb atoms. Thls procedure was applled to a 0.2-mg lunar sample contalnlng 2.5 ppm Pb and 1 ppm U and the U-Pb lsotoplc measurements obtalned are fully compatible wlth measurements on large samples. In addltion, Pb on the surface of Individual lunar glass balls of 150-300 pm dlameter was measured. Extenslon of this approach to other heavy metals may be posslble.
We report on a newly developed capability that permits the accurate measurement of both P b and U at the level of from to 10-13 mol in mineral samples on a routine basis and without serious laboratory contamination. The control of the experimental conditions at these levels for both elements represents a step toward the application of the U-Pb dating method a t its full potential. The U-Pb method has the unique advantage of comprising two radiometric clocks. The two daughter isotopes (207Pband 20sPb) are produced by two parents which are and 238U).The interrelated systhemselves isotopes (235U tematics allows a strict examination of the temporal interpretation inferred from a given isotopic composition and/or a given parent-daughter relationship. In practice, however, the method has been hampered until recently by two major experimental difficulties: a) poor thermal ionization efficiency of P b necessitating the use of large samples for mass spectrometric measurements; and b) serious contamination with terrestrial P b during sample processing. This is known as the “Pb blank”. The accurate determination of the amount and isotopic composition of lead is carried out by mass spectrometry using a surface ionization method following the procedure of isotope dilution. Consequently, the development of surface ionization methods with a high ratio of Pb+/Pb for the emitted lead is of great importance. For many years, the amount of P b analyzed by surface ionization of PbS was typically -lod6 gram. A technical breakthrough was indicated by the report by Akishin, Nikitin, and Panchenkov ( I ) who showed that relatively high ion yields could be obtained by mixing P b with a silica-zirconia gel on the spectrometer filament. Later efforts were made to analyze small P b samples by Roubault, Coppens, and Kosztolanyi ( 2 ) . Cameron, Smith, and Walker ( 3 )followed up these reports with a study of the mass spectrometric analysis of nanogram size samples of P b and clearly demonstrated that good quality data were obtainable on samples of 100 X gram of P b by mixing the P b sample with silica gel and phosphoric acid on the filament. These workers reported 2214
that the “background” from a typical loading of silica gel and phosphoric acid was -6 X lo-” gram. The advent of the silica gel-phosphoric acid loading technique suggested to us the need for the development of P b separation methods with a low Pb‘blank and the need for miniaturization. The latter is a prerequisite for establishing good quality internal isochrons on small mineral samples, for checking on detailed isotopic equilibration, and for identification of rare relics of early solar system events in extraterrestrial samples. Using simple anion and cation exchanger columns for the separation of Pb, we established a procedure for analyzing P b in samples of -3050 mg with a total procedural blank of 1-2 X gram (4, 5 ) . Subsequently, by modifying the chemical procedures, we were able to process samples of up to 200 mg in weight gram with a highly reproducible blank of 0.5-0.2 X (6). The quality of the analytical data was essentially independent of the amounts of P b used for analysis. In addition, these procedures also allowed routine measurement of U down to the level of 10-lo gram. The reliable measurement of both elements is, of course, mandatory for chronologic investigations. The ability to produce high purity reagents contributed strongly to the realization of these low blanks. In this report we describe a miniaturization of this procedure that decreases both sample size and procedural contamination by a decade.
EXPERIMENTAL Reagents. The P b contents of reagents that are currently in use in this laboratory are given in Table I. For uniformity, we will give the number of atoms of a given element or isotope throughout the remainder of this paper. Inspection of the entries in Table I shows that if the quantities of reagents used are of the order of 10 grams, then the total procedural blank would be -lo1* atoms. Without advancing to a new generation of high purity reagents, it was evident that the only direct way to decrease the blank was to further miniaturize the chemistry to minimize the amounts of reagents used. Chemical Procedure. The levels of P b contamination contributed by ion exchange columns of different sizes are shown in Table 11. A comparison of the P b contamination contributed by the reagents with that found for the corresponding column procedure indicates that the anion exchange resin itself contributes almost no P b blank, while the cation resin contributes a significant amount of Pb. For this reason, we originally relied principally on the anion resin in our lead separation procedure. In this procedure (4, 6 ) P b is absorbed from CH30H-HN03 solution on the anion exchange resin under a condition where the major rock-forming elements as well as U are not absorbed. Although the P b is well purified in this step, small traces of Fe, Ba, Ca, and T i which are retained with P b cause serious suppression of the P b signal in the mass spectrometer. This requires that the P b be specifically cleaned of these traces in a complementary step through the use of a micro-cation column which also separates P b from Th ( 4 ) . The miniaturization technique reported here is based on the same principles as discussed above with one possible variation: for small samples (51 mg) either the cation or the anion separation step may be omitted. This miniaturization technique using both columns is easily adaptable to large samples with low Fe content. We have used it successfully for the U and P b analysis in plagioclase samples as large as 30 mg. The following is a detailed descrip-
ANALYTICAL CHEMISTRY, VOL. 47, NO. 13, NOVEMBER 1975
Table I. Lead Content of Reagents Reagent
H,On H20b CHSOH' Concd " 0 ;
HFe 41V HClf 1 HClf
Pb content (atoms, g )
8.73 x 1.45 x 1.45 x 6.54 x 4.36 x 2.91 x 2.00 x
109 10" 10" 10" 10" 10"
1010
Twice distilled in Si02 still followed by a cation exchange column containing purified cation exchange resin: AG-5OW-X8, 100200 mesh. Three times distilled in Si02 still. Purified by the cation exchange resin as in a. From NBS (Analytical Chemistry Division); or three times distilled by us. e From NBS (Analytical Chemistry Division). 1 Prepared by bubbling HC1 gas in HzO. a
tion of the procedure for a 1-mg sample. The amount of reagents and resin for different sample sizes should be scaled in proportion to the sample size relative to a 1-mg sample. The sample is mechanically removed from the internal surface of a larger fragment by a miniature tungsten carbide chisel or tungsten tweezers. The sample is digested by soaking in 50 pl of concentrated "03 + 50 pl H F and allowed to stand in a covered beaker until signs of reaction subside. The sample is evaporated to dryness and the residue "dissolved" in 50 p1 of 5N HN03. If T h is being analyzed, this step is repeated because small traces of fluorides lower the T h yield in the leaching step. The Pb, U, and T h are leached from the residue by 50 pl of concentrated "03 and centrifuged. The supernate is evaporated to dryness. The residue from the supernate is dissolved in 200 pl of 94% MeOH-6% concentrated "03 mixed solvent and loaded onto the micro-anion column. Micro-Anion Column. This column is 8 cm long, of high purity silica glass of 0.2-cm i.d., fitted with a quartz wool plug a t the bottom and a 1-ml capacity bulb a t the top. About 150 pl of resin is soaked in -200 p1 of the 94% MeOH-6% concentrated H N 0 3 mixed solvent and loaded onto the column to a height of 4 cm. The resin is conditioned by -300 pl of the same mixed solvent and is allowed to drain. The sample is then loaded onto the column and allowed to drain. The removal of most elements from P b is effected by washing the column by a total of 800 p1 of 70% MeOH30% 3.3N "03 mixed solvent (if U is being analyzed, then this effluent should be saved); then the P b is eluted by 1 ml of 0.5N aqueous "03 and the eluate is evaporated to dryness. The observed blank for this procedure is given in Table I1 and should be compared with the contribution from the reagents used. It is seen from Table I1 that miniaturization, using the conditions described, resulted in about an order of magnitude reduction in the level of the anion column blank. In three separate determinations of the anion column blank, using different column sizes (and proportionally different amounts of liquid reagents), it was established that -90% of the blank in each case was accounted for by the reagents (Table 11).
T o prepare the sample for the micro-cation column step, the residue is dissolved in 50 p1 of 1.5N HC1 and evaporated to dryness; then the residue is dissolved in 25 pl of 0.5N HCl and loaded onto the cation column. Micro-Cation Column. The dimensions of this column are the same as those of the micro-anion column mentioned above. The specially cleaned resin (see Table 11) is soaked in 4N HCl and is loaded onto the column to a height of 2 cm. The resin is washed by a total of 1 ml of 4N HC1 and then conditioned by 300 pl of 1.5N HCl. The sample is loaded and allowed to drain. The P b is eluted by a total of 200 pl of 1.5N HC1 and evaporated to dryness. If T h is also being analyzed, the column is washed by 500 p1 of 4N HCl to remove trace contaminants held with T h on the cation resin, and then T h is eluted by 500 p1 of 4N HzS04. The blanks for this procedure are given in Table 11. Values are given for our "standard" clean resin and the ultra clean resin used in this work. The contribution of reagents to the total observed blank is also tabulated. In three separate determinations of the cation column blank, it was found that in each case the liquid reagents used account for only -20% of the determined blank, indicating that the cation resin is the major contributor. This is further indicated by the fact that the cation column blank level is directly proportional to the amount of resin used (Table 11). U Micro-Chemistry. This is a miniaturization of the procedure published earlier ( 4 ) . The U is separated from most elements through the absorption of its chloride complex on the anion exchange resin. The only major element seriously absorbed under this condition is Fe. The addition of ascorbic acid to the HCl solution complexes this element and prevents its absorption on the anion resin. The following is a detailed description of the U microchemistry for a 1-mg sample. The effluent from the anion column is evaporated to dryness, dissolved in 200 ~1 of 4N HC1 and evaporated to dryness again, and then dissolved in 500 p1 of 4N HCI containing 1 gram of ascorbic acid per 20 ml of 4N HCl. The sample solution is allowed to stand for -10 minutes, until the yellow color of the ferric ion fades away before loading onto the anion column. The resin soaked in 1N HC1 is loaded onto a micro-column (0.2 cm 4) to a height of 6 cm. The column is washed by 2 ml of 1N HCI, followed by 1 ml of 4 N HCl containing ascorbic acid. The sample is then loaded and allowed to drain. The column is washed by 1 ml of 4N HC1-ascorbic acid solution to remove Fe, Co, Cu, etc. Then ascorbic acid is removed by washing the column with 2 ml of 4N HCl. Finally, U is eluted by 1 ml of 1 N HC1 and the eluate evaporated to dryness. The resulting residue is loaded on a flat oxidized T a filament using H3P04 and T a powder as described earlier ( 4 ) .The U blank for this procedure is
