complex formation is present, these depressions are usually slight (14). In this case, the effect is quite large. The curves begin to flatten out at the higher MSCN mole fractions. The equivalent conductance appears to be changing from the conductance of a system as TPN+ SCN- to that of (TPN+),-* [Cd(NCS),Z-" or TPN+ SCN- to (TPN+),- [Ag(SCN),]l-". When Figure 1is compared to Figure 2, the system Cd(NCS)2TPNSCN shows a much more pronounced effect than the system AgSCN-TPNSCN. This is not unexpected as Cd2+ can form species of coordination number four or higher whereas Ag" usually displays two-fold coordination (15). The solubility limitations of the two systems indicate a higher coordination number for cadmium. The system Cd(NCS)r TPNSCN near saturation at 25.5 M Z Cd(SCN)z, whereas the system AgSCN-TPNSCN does not approach saturation until 47.7 M%'. A system which contained only TPN+ and Cd(SCN)bs- would be 25 M % in Cd(SCN)2. A system which contained only TPN-and Ag(SCN)2- would be 5OMZ AgSCN. Since Cd(SCN)2 can be in solution at greater than 25 M Z , the average coordination number must be less than 5. However, Cd*+ is known to form a series of successive complexes
with thiocyanate in various media, (16, 17) so that a nonintegral coordination number is not unusual. The solubility of AgSCN points to the formation of only one silver species, Ag(SCN)Z-. Gordon (18) predicts that inorganic salts will have a low solubility in molten quaternary ammonium salts on the basis of cohesive energy densities. As is evident in this investigation, his statement does not apply to all cases. BaSCN and KSCN do, however, have a low solubility in molten TPNSCN, indicating that coordination is necessary for the dissolution of inorganic salts in molten quaternary ammonium salts. Figures 3 and 4 show plots of log A us. 1jT. As can be seen, these plots are not linear. This is not unexpected since the log A 6s. 1jT plot for TPNSCN is not linear. However, if "activation energies" of equivalent conductance are calculated from the slope of the curve at any point, the activation energy increases as the mole fraction of MSCN increases. This type of behavior in molten salt systems has also been explained as evidence for complex formation (14).
(14) H. Bloom and E. Heyman, Proc. Roy. SOC.,Ser. A . , 188, 825 (1947). (15) F. A. Cotton and G. Wilkinson, "Advanced Inorganic Chemistry," Interscience, New York, N.Y., 1962, p 864.
(16) T. V. Long and R. A. Plane,J. Chem. Phys., 47 (l), 138 (1967). (17) Hsiao-shu Hsiung, Ph.D. Thesis, University of Cincinnati, Cincinnati, Ohio, 1960, and references therein. (18) J. E. Gordon, J. Amer. Chem. Soc., 87, 4347 (1965).
RECEIVED for review November 4, 1968. Accepted December 6, 1968.
Mass Spectrometry of Nanogram-Size Samples of Lead A. E. Cameron, D. H. Smith, and R. L. Walker Oak Ridge National Laboratory, P.O. Box Y , Oak Ridge, Tenrt. 37830 THEAPPLICATION of the lead-uranium method of geochronology could be extended if one were able to make a precise determination of the isotopic composition of a few nanograms of lead. There is also interest in lead in biological specimens where the isotopic composition might be a clue to the source of the element. Determinations of the isotopic composition of lead have required samples ranging in size from several micrograms to several milligrams, depending upon the compound used and the mode of ionization. Thermal ionization is usually preferred for small sample use inasmuch as the problem with background from residual gases and hydrocarbons in the mass spectrometer and impurities in the sample is minimized. Lead ions are produced thermally in rather low yield from lead sulfide on hot tantalum filaments, but the volatility of the compound makes it impossible to raise the filament temperature sufficiently to increase the ionization efficiency. Akishin et al. (1) have reported that lead ions were readily produced thermally when the sample was deposited on a silica-zirconia gel. This was investigated for small sample use, but we found that it was very difficult to prepare this gel in sufficiently pure form to avoid lead background and a generally dirty spectrum caused by the presence of hydrocarbons. The observation was made that silica gel alone was fully as effective as the mixture with zirconia and was much easier to prepare in sufficiently pure form. Reagent quality sodium metasilicate, Na2SiOs, was dis(1) P. A. Akishin, 0. T. Nikitin, and G. M. Panchenkov, Geochemistry (USSR) No. 5, 500 (1957).
solved in water and enough redistilled concentrated nitric acid added to make the solution approximately 5 % acid by volume. This solution was evaporated to dryness on a sand bath and the procedure was repeated by adding more 5 % nitric acid and re-evaporating. A 1 Z nitric acid solution was added to the salts and the gel, heated to boiling, and filtered through Whatman #40 paper. The gel was washed repeatedly with boiling water for about 4 hours. It was stored in plastic vials to prevent complete drying. Phosphoric acid was purified by passing 0.15N orthophosphoric acid through a Dowex-50 cation resin column, 10 mm in diameter and 60 mm long. This effectively removed lead contamination. The 0.15N solution was concentrated by evaporation to 0.75N. The mass spectrometer used in this investigation is of the single-focusing, tandem magnet type described by White and Collins (2). Ions are detected by a secondary electron multiplier behind the receiver slit. The pulses which result from the arrival of individual ions at the first dynode are amplified and counted. This instrument with 30-cm radius and 90" deflection in each magnet gives a spectrum at high mass which is very clean because of the reduction in intensity of scattered ions. The determination of an isotopic ratio of 106 at adjacent mass positions is readily possible. This is not required for work with lead, but the fact that there is no contribution to a given peak from one adjacent to it is a distinct advantage. The method of spectrum scanning and (2) F. A. White and T. L. Collins, Appl. Spectrosc., 8, 17 (1954). VOL. 41, NO. 3, MARCH 1969
525
Table I. Isotopic Composition of NBS-200 Lead Sample Atom per cent No. 204 206 1lQ 1.53 i 0.006 22.60 i 0.04 1l Q 1.53 k 0.02 22.63 0.17 8Q 1.53 i d 22.46 i. 0.02 36b 1.54 i 0.002 22.48 k 0.01 16b 1.553 i. 0.006 22.573 L- 0.020 60 1.539 k 0.002 22.517 k 0.008
Laboratory LaMont, Columbia Univ. (5) Natl. Bureau of Stds. (5) Univ. of British Columbia (6) Cal. Tech. (5) This Measurement, ORNL NBS, Absolute Measurement (7) Pb(CH,)4, electron ionization. i~ Thermal ionization. c 0.03 added to 208 to make composition add to 100.00. 0‘ Publication gives errors of iO.01 on ratios of 206/204, 207/204, and 208/204. The error calculated for the atom per cent 204 becomes meaningless.
207 22.56 i. 0.02 22.68 i 0.21 22.65 5 0.02 22.62 k 0.01 22.645 L- 0.041 22.637 rt: 0.012
208 53.31 i 0.03 53.16 i 0.19~ 53.36 k 0.02 53.36 0.02 53.229 k 0.096 53.307 i 0.031
Q
recording of the data from our modification of the instrument has been described (3). The accelerating voltage is repetitively swept with a sawtooth wave-form derived from the horizontal deflection voltage of the oscilloscope in the 400-channel analyzer which is used as a multi-channel memory. Two corrections are necessary in the calculation of the data. One is the “voltage correction” per mass unit, and the other is the “count loss” or dead-time correction. For the measurements reported here, a gravimetric uranium standard with z35U/23W ratio of 1.0564 and 233U/235U ratio of 0.0466 was used. The voltage correction derived from the observed and known 235/238 ratio was extrapolated to the lead accelerating voltage. The count-loss correction was gotten from the known 233/235 ratio and that which was observed. We generally use the single “V” filament of rhenium. The sample handling device which permits mounting and installing five of these filaments at once has been described by Christie and Cameron ( 4 ) . About 1 mg of the moist silica gel was placed in the “V” filament. The amount used was not critical. The lead sample in the form of nitrate solution was pipetted on the gel and dried under a n infrared lamp. One microliter of the purified 0.75N phosphoric acid was then added. This may form lead phosphate, but in any case is necessary for the production of ions. Lead ions were produced at a filament temperature of 11001300 “C depending somewhat upon the sample size. A loading of 10 ng was more than enough to allow ten “runs,” each consisting of 200 sweeps of the spectrum from mass 204 through 208 at a counting rate of 5 X IO5 cps of mass 208. Each sweep took 0.5 second over 200 channels. Emission of lead ions was thus obtained at a level of approximately IO-13 anip for at least 20 minutes. Below about 1100 “C the hydrocarbon background was slow in burning away and measurements of z04Pb were in doubt. This effectively limited the sample size to less than 500 ng, because larger samples emitted lead ions copiously at lower temperatures. The sample size used for the measurements on the standard reported here was 100 ng as a compromise between cleanliness of spectrum and over-riding any lead background. The latter was determined to be 0.06 ng for a typical loading of gel and phosphoric acid, by loading a known amount of very pure zogPb and z04Pb, 99.75z and 99.7 respectively, and estimating the contamination from the isotopic composition observed.
z,
(3) H. S. McKown, A. E. Cameron, W. H. Christie, and A. S. Trundle, E-14 Committee, A. S. T. M., St. Louis Meeting, May 16-21 (1965). (4) W. H. Christie and A. E. Cameron, Rev. Sei. Instrum., 37, 336 (1966). 526
ANALYTICAL CHEMISTRY
The National Bureau of Standards lead isotopic reference was run repetitively, using a new filament for each measurement. This sample is the galena from Ivigtut, Greenland. The results of the measurements are given in Table I and compared t o some results reported from other laboratories. Three results quoted were obtained using lead tetra methyl and electron ionization of the vapor. This requires a fairly large sample, but the results are considered to be more precise than those obtained by thermal ionization. When galenas are being analyzed isotopically, the sample size is unimportant. Only when the attempt is made t o use a single crystal of zircon or sphene or small samples of whole rock for geochronological purposes does the necessity arise for the minimum in sample size. Uranium is readily determined with the KAPL-type instrument, both quantitatively and isotopically, by isotope dilution with very high-purity 233U. Nanogram-size samples are handled easily because the clean chemistry for uranium is well developed. Clean chemistry for lead samples at the nanogram level is not so easy at present. Since these measurements were made, the National Bureau of Standards has made available three isotopic standards for lead (8). These were carefully calibrated against gravimetric standards prepared from electromagnetically-separated zo6Pb and 2ogPb. Catanzaro has used these standards to calibrate the mass spectrometers and has determined the absolute isotopic composition of three “standards” which have been used for interlaboratory comparison (7). The result of this calibration of the NBS 200 (Ivigtut) material is given in line 6 of Table I. The results which we obtained on the NBS-200 sample compare quite favorably to the NBS absolute measurement and to the results reported from the other laboratories quoted in the table. Our results could be improved by the calibration of the mass spectrometer with the lead standard SRM-982 in which the 208/206 ratio is 1.00016 and the 204/206 ratio is 0.027211. The technique described has also been applied successfully to small samples of tellurium, thallium, bismuth, and polonium. RECEIVED for review November 6, 1968. Accepted November 29, 1968. Research sponsored by the U.S. Atomic Energy Commission under contract with the Union Carbide Corp. (5) F. L. Mohler, National Bureau of Standards Technical Note 51, May 1960. Isotopic Abundance Ratios Reported for Reference Samples stocked by the National Bureau of Standards. (6) E. R. Kanasewich and W. F. Slawson, Geochim. et Cosmochim. Acta, 28, 541 (1963). (7) E. J. Catanzaro, Earth and Planetary Sci. Letters, 3, 343 (1968). (8) E. J. Catanzaro, T. J. Murphy, W. R. Shields, and E. L. Garner, J. Res. Nut. Bur. Sfand., 72A, 261 (1968).
