Lead Separation by Anodic Deposition and Isotope Ratio Mass Spectrometry of Microgram and Smaller Samples I. L. Barnes, 1.J. Murphy, J. W. Gramlich, and W. I?.Shields Analytical Chemistry Division, lnsfifute for Materials Research, National Bureau of Standards, Washington, D.C. 20234
A method is reported for the separation by anodic deposition and subsequent analyses by isotope ratio mass spectrometry of small samples of lead from a variety of matrices. The combined procedure is applicable to samples containing from 1 0 p g to less than 10 ng of lead and the electrodeposition is more than 95% efficient at these levels. Only a few elements interfere with the deposition, most notably iron and cerium, and procedures for removing the interfering elements are given. The optimum conditions for the anodic deposition of lead as PbO2 were studied. The mass spectrometric procedure described permits a precision of 0.1% (95% limit of error) or better for the measured isotopic ratios.
Because of the great interest of geochronologists, environmental chemists, clinical chemists and other workers in the precise and accurate analysis of lead, considerable effort has been devoted to the efficient separation of this element from cellulose filters, silicate and other rocks, minerals, and a wide variety of other matrices. Doe ( I ) recently has given a general review of eight methods for the separation and an equal number of procedures for the purification of lead which, though surveyed for the analysis of this element in rocks and minerals, should be widely applicable to other materials as well. Because of the increasing demands for analyses of improved accuracy and precision relative to isotopic and assay analyses and the need for handling smaller sample sizes, we have carefully investigated the most promising techniques for the separation, purification, and subsequent mass spectrometric analysis of this element. Of the many methods in use only two, vacuum volatilization with subsequent chemical purification [Tatsumoto (2, 3 ) ] and dissolution in acid followed by electrodeposition [Shields ( 4 ) ]seemed both to be generally applicable to a wide variety of samples and to show promise of being applicable to samples containing 0.5 pg or less of lead. Inasmuch as the vacuum volatilization technique, while quite useful for isotopic measurements, is difficult, if not impossible, to apply to isotope dilution concentration measurements, we decided for this reason to investigate the technique of separation of lead as lead dioxide by anodic deposition. Although the efficiency of separation could be examined through the use of a radioactive tracer, as will be shown, development a t the same time of an effective mass spectrometric analytical procedure useful for isotope ratio and isotope dilution analysis seemed desirable which would serve as a final verification of the separation method. Thus this paper reports both the chemical and mass spectrometric methods used for the determination of lead in samples containing microgram and smaller quantities of the element. ( 1 ) 6.R . Doe, "Lead Isotopes," Springer-Verlag, New York, N . Y . , 1970. ( 2 ) M . Tatsumoto, Science, 153, 1094 (1966). (3) M . Tatsumoto,J. Geophys. Res., 71, 1721 (1966). ( 4 ) W. R. Shields, Ed., "Analytical Mass Spectrometry Section: Summa-
ry of Activities, J u l y 1966 to June 1967," Naf. Bur. Sfand. (U.S.), Tech. Note. 426, 53 pages (Sept. 1967)
EXPERIMENTAL Reagents. Water. Water that had been passed through a fivestage ion exchange system, a commercial metal-lined still, and a laboratory still with quartz condenser was then further purified with one pass through a quartz sub-boiling distillation apparatus. A sample of this water was analyzed for 17 elements by isotope dilution spark source mass spectrometry which showed that the total for all 17 impurities was 0.52 ng/g with lead present a t a concentration of 0.008 ng/g. A detailed description of the apparatus, procedure, and analytical method has been published (5). Nitric Acid. Commercial ACS reagent-grade nitric acid was purified by the sub-boiling distillation technique as above. The resulting 70% (wt) " 0 3 containing 0.02 ng/g of lead was diluted with water to the appropriate concentration. Perchloric Acid. Commercial ACS reagent-grade perchloric acid was purified by the sub-boiling distillation technique as above. The resulting 70% (wt) acid containing 0.2 ng/g lead was diluted with water to the required concentration. Nitric Acid-Hydrogen Peroxide Solution. A solution was prepared to contain 2% (wt) nitric acid and 0.3% hydrogen peroxide in water. The resulting solution contained 0.014 ng/g lead. Standard Lead Solutions. These were prepared by dissolving weighed amounts of pure lead wires (SRM 981, 982 and 983, Office of Standard Reference Materials, NBS) in 6 N "03. These were then diluted to give a final concentration of 200 pg/ml in 2% ( v o ~ ") 0 3 . Lead-210 Solutions. A stock solution containing 21'JPb was diluted with 2% (vol) nitric acid so that 0.1 ml of the final solution would contain 20 ng of lead and a 210Pb activity of 1.64 X 103 cps. Silica Gel. Thirty grams of sodium metasilicate (Na2SiOa. 9Hz0) were dissolved in 200 ml of nitric acid solution (1 19). The solution was evaporated nearly to dryness a t 160 "C. This procedure was repeated once and the evaporation was then carried t o dryness. About 200 ml of nitric acid solution (1 + 99) was added to the residue and the solution allowed to boil for 10 minutes. The hot solution was filtered through a medium porosity filter paper. The residue was washed back into a beaker, diluted with 200 ml of redistilled water, boiled for 10 minutes, and allowed to cool and settle for approximately hour. The water was carefully decanted and the washing repeated 3 or 4 more times. After the final washing and decanting, the residue was diluted with 10 times its volume with redistilled water and transferred to a Teflon (Du Pont) bottle. the solution was shaken thoroughly 2 or 3 times a day for 3 days and then, after a 3-hour settling time, the upper liquid layer was carefully decanted into a second Teflon bottle. This colloidal suspension is the "silica gel" referred to in all subsequent operations. During the preparation of this report, an alternate method for the preparation of a suitable silica gel suspension has been found. A quantity of a commercial silica gel powder used for thin layer chromatography (Silica gel, 60 HR, extra pure, E. Merck, Darmstadt, obtained from Brinkmann Instruments, Inc., Westbury, N.Y.) was prepared in the following manner. Approximately 1 g of this powder was ground in a new agate mortar for 2 to 3 hr to produce an extremely fine particle size. This material was then mixed with 15 ml of pure water and shaken thoroughly for a few minutes at %hour intervals during the first day. The solution was then allowed to settle for 3 days after which the supernatant was carefully decanted and used as the "silica gel." We estimate that the particle size in either preparation is less than 0.4 micron and believe that silica gel particles of this size or smaller are required for successful use in this application. Both preparations performed equally well in the lead determinations giving identical results, with the ground silica gel giving more consistent performance between individual preparations of new silica gel.
+
(5) E. C. Kuehner, R. Alvarez, P. J . Paulsen, and T . J . M u r p h y , Anal.
Chern.. 44, 2050 (1972).
ANALYTICAL CHEMISTRY, VOL. 45, NO. 11, SEPTEMBER 1973
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Table I. Effect of Acidity on Anodic Deposition of Lead Pb remaining in 20 ml solution,a electrolysis time, 16 h r Acid, N 0.025 0.050 0.10 0.50 1 .o 2.0 a
"03, W
0.00 0.00 0.00 0.22 4.28
HCIO4,
w
0.00 0.00 0.00 0.04 0.10 0.42
Table It. Effect of Time on Anodic Deposition of Lead Lead remaining in 20 ml of 0.025N HC104 pg Applied potential 1.4 V 1.6V l.8V 2.0 v 2.2 v Time, h r 5.00 5.00 0 5.00 5.00 5.00 2.36 2.19 2.94 1 1.83 2.48 1.28 1.69 1.23 1.21 2 0.79 0.53 0.31 0.34 4 0.20 0.25 0.07 0.08 0.03 8 0.03 0.03 20 0.00 0.00 0.00 0.00 0.00
Initial lead, 5.00 Fg,
Phosphoric Acid. A phosphoric acid solution was prepared by carefully dissolving "ultra pure grade" resublimed phosphorous pentoxide in redistilled water in a Teflon bottle and diluting to make a solution 0.75N in phosphoric acid. Equipment. Electrodeposition Apparatus. All plating was carried out using a stab1e;regulated power supply (Hewlett-Packard Model 6113A) connected to two platinum wire (0.020-in. diameter by 3 inches long) electrodes. Commercially available "pure" platinum wire contained small amounts of lead as an impurity which, under the conditions employed, may diffuse from the electrodes, so that only certified platinum wire (SRM 680, OSRM, NBS) was used in these experiments. The solutions were stirred magnetically during electrodeposition. Radioactiue Counting Apparatus. A 2-inch by 2-inch NaI(T1) solid detector in a shielded well was used with a single-channel analyzer to count the aliquots of solution containing the zloPb tracer. Mass Spectrometry. Each of the several mass spectrometers used in this work was a 12-inch radius, 90" magnetic sector instrument designed for high-precision isotopic ratio measurements and constructed at NBS. Details of the instruments and the component parts including ion sources, collectors, and high-precision measuring circuits have been published (4).
RESULTS AND DISCUSSION General Procedure for the Anodic Deposition of PbO2. The initial procedure for the anodic deposition of lead as lead dioxide (Pb02) was based on a method suggested by Muller (6) whereby trace quantities of lead are separated electrolytically as PbO2 from a solution 0.005N in "03 containing added copper and ammonium nitrate. The copper is added to convert the platinum cathode to a copper cathode which reduces nitrate ion directly to ammonium ion preventing the formation of nitrous acid which interferes with the deposition of PbO2. However, because later experiments showed that good recoveries of lead could be obtained without added copper or ammonium ions, use of these additives was discontinued because of the attendant possibilities for contamination from these salts. The anodic deposition of lead as PbO2 rather than the cathodic plating as P b metal is possible because nitrate ion functions as a cathodic depolarizer because it is reduced easier than plumbous ion (Pb2+) and maintains the cathode potential below the value required for the reduction of P b 2 + . The use of perchloric acid as a cathodic depolarizer was also investigated because the dissolution of a glass or silicate rock furnishes a solution containing perchlorate ion. A preliminary investigation showed that good recoveries were obtained by anodic deposition of PbO2 from dilute perchloric acid solutions. A series of experiments was designed to elucidate the optimum conditions for the anodic deposition of lead as PbO2. Small known quantities of lead, usually about 5 pg, were spiked with a known quantity (20 ng) of radioactive 210Pb containing 1.4 X lo3 cps of activity. Lead was electrodeposited under preselected conditions onto a 0.020-in. (6) H . Muller, Z. A n a / . Chern.. 113, 161 (1938).
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Table Ill. Optimum Conditions for Anodic Deposition of Lead as PbOz Applied potential Acid concentration, HC104 or " 0 3 Volume Time
1.8-2.0V 0.025N 20 ml 16 hr
platinum wire anode. A similar platinum wire served as the cathode. At selected times, aliquots of the solution were withdrawn and counted for a t least 140-sec periods to determine the amount of lead remaining in solution. (210Pb has a daughter, ZlOBi, with a half life of 15 days. Electrochemically, Bi can be deposited both on the anode and cathode during the deposition of Pb02. Counts from the cathode in these experiments were shown to be due to 210Bifrom the decay ratio.) The results of the experiment to determine the effect of nitric acid and perchloric acid concentrations on the efficiency of lead removal are shown in Table I. After plating for 16 hours, the recovery of lead was complete from both nitric and perchloric acid solutions a t acid concentrations of 0.1N or less. The recoveries from more concentrated nitric acid solutions were, however, much poorer than the recoveries of lead from perchloric acid of corresponding normality. Practically no lead was recovered from 1.ON "03 while the recovery was about 95% from 1NHC104. The results of the experiment to determine an optimum time of deposition are shown in Table 11. This table shows the amount of lead remaining in solution after plating a t various applied potentials (range 1.4 to 2.2 V) from 20 ml of 0.025N HC104 for various time intervals. In each case, the deposition was nearly 100% complete after 20 hours and only 30-80 ng remained after 8 hours. We estimate that the recovery is greater than 98% a t 1.8 to 2.0 V after 16 hours which is a convenient time (overnight). A t 2.0 V, the current density was 0.5 mA/cm2. The effect of volume of solution was investigated. From a solution initially containing 5 wg of lead, recovery of 100% was effected from 50 ml of solution and about 99% from 100 ml a t 2.0 V while about 90% was removed from 5 0 ml and only 70% from 100 ml at 1.6 V. All solutions were electrolyzed for 16 hours. The effect of temperature was also studied. While the deposition rate increased during the first hour with increased temperature, a t higher temperatures the final solution contained more lead than when the plating was carried out a t lower temperatures; thus there seems to be little benefit in using elevated temperatures. All of these effects may be combined to give a set of optimum conditions for the electrodeposition of lead. These conditions are shown in Table 111. Interferences. The optimum conditions in Table 111 were employed while the effects of other elements were investigated using 5 p g of lead and the 2lOPb tracer as in t h e previous experiments. No interference was found from thc alkali metals, the alkaline earths, zinc, cadmium, alumi.
ANALYTICAL CHEMISTRY, VOL. 45, NO. 1 1 , SEPTEMBER 1973
num, nickel, chromium, ammonium ion, chlorate ion, or sulfate ion when these were individually present a t concentrations of 500 mg in 20 ml of solution. Titanium, zirconium, and tin hydrolyzed from solution, but the lead was still recovered. Copper, silver, and mercury deposit on the cathode but do not interfere when individually present at a concentration of 50 mg/20 ml of solution. A few elements and ions interfered. Bismuth, manganese, and thallium can co-deposit even when present in low amounts while arsenic, antimony, and cobalt co-deposit when present in large amounts (500 mg per 20 ml was the level studied). Chloride ion, fluoride ion, phosphate ion, and other complex-forming ions such as tartrate and oxalate completely prevent the deposition of lead as Pb02, but a t higher applied potentials lead is deposited onto the cathode as lead metal. Iron and cerium prevent the deposition of PbOz when present in large amounts. However, small amounts of iron (less than 10 mg/20 ml of solution) can be tolerated. The recovery of added lead was 86% complete from a solution containing 10 mg Fe per 20 ml of solution but only 30% from a solution containing 30 mg Fe per 20 ml of solution; thus the more iron present, the poorer the recovery of lead. What apparently happens is that ferrous ion is oxidized a t the anode and ferric ion is reduced a t the cathode. When the iron concentration is high enough, this ferrous-ferric system behaves as a redox buffer preventing the oxidation of lead at the anode. In cases where interfering ions are present, separation or pre-concentration of the lead prior to electrodeposition is necessary. Ion-exchange has been useful for this purpose; typically, lead in 1.5 to 3.OM HC1 solution can be adsorbed on a strongly basic anion exchange column and eluted either with water or 8M HCl. This procedure is effective for separating lead from the interfering ions. After evaporation and conversion to nitrate or perchlorate, the lead is separated as PbO2 by electrodeposition to free it from organic matter and other elements that may have eluted with it. This separation from interfering ions ensures a high recovery of lead during the subsequent electrodeposition, thus making the blank correction more meaningful for small samples (less than 1pg). Mass Spectrometric Analysis. Extensive experience with the mass spectrometric procedure for the analysis of lead using silica gel-phosphoric acid as an ionization enhancing agent has shown that great care must be exercised in all steps of the procedure for a successful analysis. The most useful method of mounting the sample on the filament and the subsequent instrumental analysis is given here. The lead sample, which has been stripped from the platinum electrode with 0.2 ml of the nitric acidhydrogen peroxide solution, may be presented for mass spectrometric analysis in one of several forms. If sufficient sample is available, the above solution is evaporated to dryness and then taken up in 2% (vol) nitric acid to give a solution with a concentration of 200 pg/ml. Where the lead sample is very small (Le., less than 2 pg), the solution is evaporated to a spot in the botton of a Teflon beaker. Just prior to analysis, a single drop of the 2% nitric acid solution is added to the spot with a micropipet and one half of this drop is withdrawn for analysis. A single drop (-5 ~ 1 of ) the silica gel suspension is placed on a 0.001-in. by 0.030-in. wide rhenium filament and dried for 5 min with a current of 1 A and with a heat lamp placed about 8 in. above the filament. A drop (-5 pl, 0.2 to 2 pg Pb) of the sample solution is placed on the filament and dried for 5 min, also with a current of 1 A and with the heat lamp as above. A drop of the phosphoric acid solution is added and then dried with a current of
1.5 A and with the heat lamp for 5 min and an additional 5 min at 2 A with the heat lamp. With the heat lamp turned off, the current is slowly increased until the first appearance of vapors from the filament is noticed (about 2.0-2.15 A). The filament is allowed to remain at this current level until it appears completely dry and a uniform, white deposit appears. The filament is then heated to a dull red heat, the current is turned off, and the filament is immediately placed in the mass spectrometer for analysis. When a pressure of 1 X 10-6 Torr is reached in the source chamber, liquid nitrogen is added to the source cold finger which reduces the pressure to -1 X 10-7 Torr. The following steps are then taken in the mass spectrometric analysis procedure: Time, min
Procedure
The filament is heated to 1100 "C. 1-2 A lead peak is located and the beam focused. 5 The filament temperature is raised to 1150 "C. 10 The filament temperature is raised to 1200 "C. 20 A spectrum is obtained to determine base-line values. 30 Data taking begins. For an isotope dilution analysis, ten 208Pb/206Pb ratios are measured. Isotopic compositions are obtained by measuring four 208Pb/206Pb,four 207?b/206Pb,eight 2Q4Pb/206Pb,four 207Pb/206?b,and four 208Pb/206Pbratios. Each peak is monitored for 30 sec and switching between peaks is done magnetically. Decaying signals are always observed at 1200 "C. The signal decays quite rapidly at first but the rate of decay decreases a t about 20 min into the procedure to give slowly decaying signals. A 1-pg sample of common lead will normally give a 2osPb ion current of -3 X 10-10A a t 30 min. This signal level is reproducible to *lo%. Should this desired signal level not be obtained, the sample must be reprocessed chemically by converting the lead nitrate solution to a lead chloride solution in 1.5N HC1 and passing this solution through a small anion exchange column. The eluate is evaporated and replated as described above. Only a small number (-5%) of samples have required reprocessing and, in our experience, no sample has failed to give a satisfactory signal after this reprocessing step. Forcing a dirty sample to run in the mass spectrometer results in an indeterminate amount of fractionation which can cause errors of as much as 1% in the observed ratio. To obtain signals of this size, however, very careful attention must be given to the process of drying the sample on the filament. Our experience indicates that the temperature of the filament during the drying stages is important and that the temperature as measured a t the filament with a thermometer must be kept in the range of 50 f. 3 "C. To maintain this temperature tolerance range, a variable transformer was connected to the heat lamp and the temperature a t the filament is checked daily. Failure to control this step results in the silica gel forming a small ball in the center of the filament rather than forming a thin uniform layer which results in an uneven signal of greatly reduced intensity. As an indication of the accuracy and precision that may be obtained with this procedure, the results of 32 analyses of SRM 981 are shown in Table IV. These analyses were performed by four persons over a one-year period and represent the combined data. Comparable results were also obtained with SRMs 982 and 983. Also shown in Table IV are the results of a series of 12 analyses using zone refined rhenium as the filament material. The zone refined material invariably gives smoother signals but results in a wider data spread. We believe that this spread is due to the presence of a high but variable preferred (0001) orientation in zone refined rhenium metal as has been found by 0-1
ANALYTICAL CHEMISTRY, VOL. 45, NO. 11, SEPTEMBER 1973
1883
Table I V . Analysis of SRM 981 (Common Lead) Isotope ratio
2081206
2071206
2041206
Certified value
2.1681 f 0.0008
0.91464f 0.00033
0.059042f 0.000037
Silica gel-zone refined rhenium
2.1715 f 0.0061
0.915181 0.0016
0.058940f 0.00014
Silica gel-regular rhenium
2.1668 f 0.0022
0.91441f 0.0006
0.059051 f 0.00006
Table V. Method lntercomparison of Analysis of Lead in Eight Brass and Bronze Samples Ratio of ratios
2081206
2071206
2041206
Silica gel-zone refined rhenium
Triple filament Silica gel 95% L.E.
0.9988 10.30%
0.9990 f0.22%
1.0005
f0.32%
Silica gel-regular rhenium
Triple filament Silica gel 95% L.E.
1.0002 f0.12%
0.9997 10.09%
1.0014 f0.22%
McHugh (7). This crystal face has a higher work function than the other crystal faces. In Table V are shown the results of the analyses of eight “real” samples of lead extracted from various archaeological brasses and bronzes in which sufficient lead was pres(7) J. A. McHugh, Int. J. MassSpectrorn. /on Phys., 3, 267 (1969).
ent to permit analyses by both the silica gel method and the precise triple filament method of Catanzaro et al. (8). Because each sample had a different set of isotopic ratios, the results are presented as the ratio of triple filament ratios to those obtained by the silica gel procedure. Data showing the wider variation obtained using zone-refined rhenium filament material is also given in Table V. Since the development of these combined analytical procedures, they have been successfully applied to a . wide range of sample matrices. For example, we have used them for lunar materials (0.4 to 10 ppm lead), silicate glasses (0.1 to 500 ppm), air particulate filters (0.01 to 8 ppm), tuna fish (0.4 to 0.7 ppm), human blood (0.4 to 0.8 ppm), and commercial beer (0.001 ppm). The ability to analyze small lead samples spectrometrically combined with the plating procedure make a particularly valuable technique because the combination reduces the number of chemical steps, minimizes the amount of reagents required thus allowing the processing of smaller total samples and/or samples with much lower concentrations of lead. The combined procedures have enabled us to reduce the overall laboratory lead blank for silicates from about 0.5 microgram to 2-3 nanograms and to analyze with good aaccuracy and precision samples which could not be analyzed by other techniques. Received for review January 26, 1973. Accepted April 16, 1973. In this paper, certain commercial products have been identified by name in the interest of brevity. This is not intended to imply that these are the only suitable products available or even that they are necessarily the best available for the application. (8) E. J. Catanzaro, T. J. Murphy, W. R. Shields, and E. L. Garner, J , Res. Nat. Bur. Stand.. Sect. A,. 72 (3), 261 (1968).
Electrohydrodynamic Ionization Mass Spectrometry B. N. Colby and C. A.
Evans, Jr.
Materials Research Laboratory, University of Illinois, Urbana, 111. 67807
Electrohydrodynamic ionization mass spectrometry is based on an ionization mechanism resulting from the interaction of a conducting liquid meniscus with a strong electrostatic field. The resultant intense ion beam is then mass spectrometrically analyzed to qualitatively and quantitatively characterize the liquid sample. Routine operation at constant ion emission is attained by the use of an emitter self-biasing resistance and feedback control of the emitter current. The time and temperature dependence of ion yields and ionization stability have been investigated. With constant operating conditions, the production of singly charged monoatomic ions from liquid metals was constant within 5% relative standard deviation. Without the use of relative sensitivity factors, apparent concentrations deviated by no more than a factor of 2 from the known values over the range of several hundred ppm up to 70%. 1884
Currently, spark source mass spectrometry is widely used for minor and trace element analysis of metallic materials. Generally, this technique requires minimal sample handling prior to analysis. However, when analysts have encountered either liquid samples or materials with a low melting point, they have turned to special handling techniques. Wolstenholme ( I ) used an electrode chilling ‘technique to analyze gallium. Nalbantoglu and Cherrier (2-4) used similar methods to analyze impurities in gallium, mercury, and pastes of various acids with graphite powder. Berkey and Hickam (5) have formed self-electrodes from molten sodium and then used the rf spark source and electrode chilling to obtain an analysis. Recently they (1) W. A . Wolstenholme. Appl. Spectrosc., 17, 51 (1963). (2) M. Nalbantoglu, Advan. MassSpectrorn., 3, 183 (1966). (3) M. Nalbantoglu, Chirn. Anal. (Paris), 48, 148 (1966). ( 4 ) C. Cherrierand M. Nalbantoglu, Anal. Chern., 39, 1640 (1967). ( 5 ) E. Berkey and W. M. Hickam, Advan. Mass Specfrom., 5, 543 (1971).
ANALYTICAL CHEMISTRY, VOL. 45, NO. 1 1 , SEPTEMBER 1973
