CONCLUSIONS The utility of the tungsten bronze electrode in potentiometric titrations with EDTA has been demonstrated. It is believed that this technique can be extended to systems involving other complexing agents and probably precipitating agents as well. Because of the extremely versatile behavior of these electrodes, demonstrated here and elsewhere ( I, %), it appears that in order to perform a successful metal ion titration, only one of the following criteria need be met: the metal ion be titrable in basic solution either directly or indirectly, the ion be easily reducible
directly a t the electrode surface, or a metal redox couple be present which undergoes a change in concentration ratio a t the end point.
ACKNOWLEDGMENT The authors are indepted to Howard R. Shanks of the Ames Laboratory for providing the tungsten bronze crystals used in this work, and to Dennis C. Johnson, Chemistry Department, Iowa State University, for his advice. Received for review September 11, 1972. Accepted December 18, 1972.
Measurement of Zirconium-Hafnium Ratios in Geological Samples by Electron Impact Mass Spectrometry J. J. Leary, S. Tsuge, and T. L. lsenhour Department of Chemistry. University of North Carolina, Chapel Hill, N.C. 27574
dium borate (decahydrate) was obtained from J. T . Baker Chemical Company. Reagent grade carbon tetrachloride was dried with freshly activated molecular sieve 5 A (50-60 mesh). Specpure zirconium oxide and hafnium oxide were obtained from Johnson Matthey Chemicals Ltd. Canadian Association for Applied Spectroscopy Standard SY-1 and zircons were kindly supplied by P. D. Fullagar (Geology Department, University of North Carolina, Chapel Hill) and were used after grinding to a fine powder in agate and boron carbide mortars. Preparation of S t a n d a r d Chelates. Pure Zr(fod)c and Hf(fod)4 were prepared using a modification of a previously published procedure (9). About 50 mg of the Specpure metal oxide and 100 *1 of cc14 were sealed in a Pyrex tube (110 X 4 mm i.d.). This tube was then placed in a stainless steel explosion shield and heated a t 450 "C overnight in an electric oven. The resulting chloride was then converted to the corresponding chelate compound by adding a slight excess of H(fod) dissolved in c c l 4 , and heating gently. Purification of the chelate compounds was necessary due to the excess H(fod) and by-products of the carbon tetrachloride reaction (principally CZCls). Sublimations were performed at 2-3 mm. The first fraction was discarded but that which condensed on the cold finger a t 135 "C was saved for the preparation of standard solutions. The uncorrected melting points of both Zr(fod)r and Hf(fod)r were 170.5-171.5 "C. Only the solvent and chelate peaks were observed in the temperature programmed gas chromatograms of the CC14 solutions of the pure compounds. There was no indication of impurity in the mass spectra of these compounds. The standard solutions of the pure compounds were prepared by dissolving 90 mg of Zr(fod)4 and 25 mg of Hf(fod)4 in 10 ml and 25 ml of CC11, respectively. By mixing these two solutions, in the proper proportions, the following series of mixture solutions were prepared (Hf/ZR)100 = 0.6, 1,2, 1.8, 2.4. Chelate Synthesis from Minerals. The chelate compounds were prepared from mineral samples using a previously published EXPERIMENTAL procedure (9) with the following modifications. After the borax Reagents. 1,1.1.%,%.3,3-heptafluoro-'i,'i-dimethyl-4,6-octane-fusion, it was desirable to remove the excess borate because there was such an abundance of this reagent. It was also desirable to d i m e [H(f'od)]was obtained from Pierce Chemical Company and remove iron and aluminum a t this point. The separation of ions was used after distillation at reduced pressure. Reagent grade soother than Zr and Hf was accomplished with a small column (6 x ( 1 ) N B. Priceand G . R . Angell.Ana/. Chem.. 40,660 (1968). 20 mm i.d.1 of cation exchange resin ( I O ) , AG 50W-X8 (100-200 (2) C.K . Brooks. Geochim. Cosmochim. Acta. 33, 357 (1969) mesh). This column was unique in that there were two glass wool ( 3 ) C.K Brooks. Geochim. Cosmoch/m. Acta. 34,411 (1970). plugs a t its upper end. The digest from the borax fusion was (4) E. Merz, Geochim. Cosmochim. Acta. 2 6 , 347 (1962). washed onto the column where the excess acid digested borax ( 5 ) W. D. Ehrnann and J. L. Setser. Science. 139, 594 (1963). precipitated on the upper plug of glass wool. The column was (61 G. E. Gordon, R Randle. G . C. Goles. J. B. Gorlis, H . M . Beeson, washed with 2 ml of 2M HC1 after which the upper glass wool and S S. Oxley. Geochfm.Cosmochim. Acta. 32, 369 (1968). ( 7 ) M. K Horn and J. A . S. Adarns. Geochim. Cosmochim. Acta. 30, 279 (1966) (9) S. Tsuge, J. J. Leary, and T. L. Isenhour, Anal. Chem., 45, 198 (81 A . J. Erlank and J. P. Willis. Geochim. Cosmochim. Acta. 28, 1715 (1973) (1964). (10,) F. W. E . Strelow, A n a / . Chem., 3 1 , 1974 (1959).
Measurement of the zirconium-hafnium ratio in rocks and minerals is of fundamental geological interest. Zirconium and hafnium, which have nearly the same ionic radii (Zr: 0.74 A, Hf: 0.75 A) due to the lanthanide contraction, have almost identical chemistries and occur in nature as a coherent geochemical pair. The Zr/Hf ratio has usually been determined by X-ray fluorescence (1-3) or neutron activation analysis (4-6). The former is capable of giving results accurate to a few per cent but is limited by the much lower hafnium concentration. The latter often gives extremely poor results for zirconium, relative standard deviations of 50% not being uncommon. According to one of the best estimates, that calculated by Horn and Adams ( 7 ) ,the overall Zr/Hf ratio should be 41; however, measured ratios range between 30 and 70. The wide variation in measured ratios is in part due to true natural variations but, as Erlank and Willis (8) point out, is also strongly influenced by erroneous experimental results. A more sensitive and accurate technique is therefore needed. This work describes a new method for measuring the Zr/Hf ratio in rocks and minerals. The steps in the analysis include dissolution of the sample, conversion of the metals to volatile chelates, and finally mass spectral analysis with a double focusing mass spectrometer equipped with peak-switching facilities and a computerized integrator. By this method the Zr/Hf ratio can be determined with a precision of about 2%, measured as the relative standard deviation of the mean.
ANALYTICAL CHEMISTRY, VOL. 45, NO. 7, JUNE 1973
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+
S o Z r ( f od )3
m/e Figure 1. Partial mass spectrum covering Zr(fod)s-, Hf(fod)s-clusters (sample synthesized from zircon)
Table I. Mass Spectrometer Operating Conditions for Zr/Hf Ratio Measurements
Ion chamber temperature Electron accelerating voltage Ionizing current Ion accelerating voltage Electron multiplier Magnet current setting Resolving power Band width Source amplitude controls L, H Drift circuitry Peak width Ratio of mass of H f ( f ~ d ) s + / Z r ( f o d ) ~ + of Zrgl (fod),+/ZrgO (fod)3 f of Zrg*(fod)3+/Zrgo(fod)3f
225 "C 70 V 100 ,uA 6 kV
1.7-2.2 kV
- 68 2000 50 Hz
10,lO Off
Minimum 1.092344
1.001026 1.002051
TIME i m i n ~ l e 6 )
Figure 2. Signal intensity vs. time curve for Zr(fod)s' at 225 "C RESULTS AND DISCUSSION
plug, with the borate precipitate, was removed. Then the column was eluted with an additional 3 ml of 2M HCI and finally with 8 ml of 5M HC1. The last 8-ml fraction was collected, dried, and converted to the chelate compounds by treating with CC14 and H(fod) as mentioned above. This 20-mm column was sufficient for the separation of ions with extremely different chemical properties but no observable fractionation occurred between zirconium and hafnium. Mass Spectral Measurements. The mass spectra of the standards and mineral samples were taken on an Associated Electronics MS-902 mas spectrometer equipped with a direct insertion probe and peak-switching facility. At the beginning of the experiment the source was tuned to give maximum signal and good peak shape, using the g0Zr(fod)3+ peak. About 4 wl of the CCl, solution of the sample chelate, containing about 2 wg of zirconium and between 10 and 40 ng of hafnium, was deposited on the glass capillary of the direct insertion probe which was then dried under a stream of nitrogen and introduced into the mass spectrometer. Table I lists the mass spectrometer operating conditions. A Digital Equipment Corporation PDP-8 computer was used for the data acquisition. The computer program used was basically that described by Frew and Isenhour ( Z I ) with the following modifications. The program always began integration with the high mass peak and took sets of about 20 points. The sampling frequency was approximately tripled to improve the accuracy of measuring the much smaller hafnium peak. (11) N. M. Frew and T. L. Isenhour, Anal. Chern., 44, 659 (1972).
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As natural zirconium and hafnium have five and six stable isotopes, respectively, the associated chelate compounds exhibit very characteristic mass spectra. Figure 1 illustrates portions of a typical mass spectrum, of a sample synthesized from the mineral zircon, covering the Zr(fod)3+ base peak region. As might be anticipated, the hafium base peak, 180Hf(fod)3+, is also present a t about one hundredth the intensity of the zirconium base peak, goZr(fod)3+. In the ratio determination, only the two base peaks were utilized; therefore, the mass ratio 180Hf(fod)3+/ gOZr(fod)3+ = 1.092344 was dialed into the peak switching circuitry. In an attempt to obtain a strong signal and a reasonably flat portion in the peak intensity us. time curve, source temperatures ranging from 175 to 275 "C were tried. At the low end of this temperature range, 175 to 200 "C, the signal was constant but weak. Signal intensity a t temperatures between 250 and 275 "C was high but there were serious fluctuations. Rapid fluctuations in signal intensity were a serious problem because peak matching was involved in the data acquisition process. Figure 2 shows a portion of the signal intensity us. time curve taken a t 225 " C . The evaporation characteristics a t 225 "C had the following advantages. First, the high intensity within the first 15 sec after the probe tip was extended allowed al-
most immediate confirmation that there was no instrumental problem. Second, during the next 2 to 3 min, the Zr(fod)3+ peak was easily located on the oscilloscope and its identity confirmed; the sharply changing intensity during this period causes no difficulty. Finally, when signal intensity becomes approximately constant, after about 3.5 min, data acquisition was begun. Following each analysis, the source was baked out until the Zr(fod)3+ base peak was no longer observable even a t much higher multiplier voltages. The source was then cooled to 225 "C. When this procedure was used. the average instrumental time required per sample was about 45 min. Figure 3 shows the relationships between the true Hf/Zr peak ratio of the standard samples and the observed Hf/Zr peak ratio a t multiplier voltages of 1.7 and 1.8 kV. I t is interesting to note that the slope of the curves, which corresponds to the apparent relative sensitivity for both chelate compounds, is changing as a function of the multiplier voltage. The apparent relative sensitivities Hf(fod)4 to Z r ( f ~ d )calculated ~, from the linear portion of the calibration curve, are 0.159:l and 0.327:l a t multiplier voltages of 1.7 and 1.8 kV, respectively. The calibration curve obtained a t a multiplier voltage of 1.8 kV is superior to the other in both precision and linearity. Using linear least-squares calibration curves, the Zr/Hf ratios were determined for: a zircon, Canadian Association for Applied Spectroscopy Standard SY-1, and V.S. Geological Survey Standard GSP-1. The ratios for these samples were 64.9, 44.8, and 49.5, respectively. The zircon used was from the Green River Basin of Henderson Country, North Carolina, but its geological history was unknown. Very high values of the Zr/Hf ratio are not uncommon for zircons. This is because zircon tends to exclude Hf as long as that element can be accommodated in ferromagnesium silicates (12) or other basic matrices while, in the absence of such host sites, zircons becomes progressively richer in Hf and approach the Zr/Hf ratio of the total rock. T o date, no data have been published for the Zr/Hf ratio of SY-1; therefore, no comparison was possible. According to the average of all compiled data reported by Flanagan (13), the Zr/Hf ratio of the GSP-1 standard is 44.1. This is 'in good agreement with the value of 49.5 obtained in this workparticularly when it is noted that there is a relative standard deviation of 20%' in the ratio calculated from Flanagan's compilation. It should also be mentioned that the precision measured as the relative standard deviation of the mean for three consecutive analyses of the same zircon sample was around 170,and, furthermore, the slope of the linear least-squares calibration curves has changes only about 1% from night to night (when the same multiplier voltage was used). . Among the ,advantages of this method of analysis are: first, both elements are carried through exactly the same chemical preparation steps with the final step being a simultaneous determination; second, the instrument required is the reasonably common midresolution organic mass spectrometer; finally, there are no matrix effects to contend with. Possible disadvantages are the many chemical steps involved in the analysis. These steps decrease the absolute amount of zirconium and hafnium; however, (12) H. Degenhart, Geochim. Cosmochim. Acta. 28, 279 (1957). (13) F. J. Flanagan, Geochem. Cosmochim. Acta. 33, 81 (1969).
6.0
50
nfbz, lo3 iobs) 4 0
3.0
2.0
10
0
05
15
10
20
2 5
Hf/Z, x l o 2
Figure 3.
Hf/Zr calibration curves showing effects of multiplier
voltage. they have no effect on the Zr/Hf ratio. H(fod) forms very stable chelates with zirconium and hafnium compared to the volatile P-diketonates of trifluoroacetylactone (14), hexfluoroacetylacetone ( 1 5 ) , and benzoyltrifluoroacetylacetone (16); however, there are indications of catalytic decomposition in the presence of excess ligand and trace quantities of other contaminants. This instability requires that the analysis be performed as soon as possible after the sample has been prepared. Finally, the processes occurring within the mass spectrometer, fractional evaporation, changes in relative ionization efficiency, differential pumping, etc., eliminate the possibility of an absolute analysis, but these problems are easily overcome by using a calibration curve. We feel this technique has now been developed to the point where it can be applied to the determination of other elemental ratios of geological interest such as uranium/lead or thorium/uranium.
ACKNOWLEDGMENT The authors greatfully acknowledge the assistance of David Rosenthal for his cooperation and for the use of the facilities of the Research Triangle Center for mass spectrometry which is supported by the Biotechnology Resources Branch of the NIH under Grant Number PR-330. Received for review August 31, 1972. Accepted January 11, 1973. Work supported by Materials Research Center, University of North Carolina, under contract Number DAHC15-67-(2-0223 with the Advanced Research Projects Agency, and the National Aeronautics and Space Administration. ( 1 4 ) R. E. Sievers, B. W. Ponder, M . L. Morris, and R. W. Moshier, inor0. Chem.. 2. 693 11963). (15) s. Chattoraj: C. T. Lynch, and K. S. Mazdiyasni, inorg. Chem., 7, 2501 (1968). (16) M. G. Allcock, R. Belcher, J. R. Majer, and R . Perry, Ana/. Chem.. 42, 776 (1 970).
6.
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