Anal. Chem. 1994,66, 4186-4189
r7sHf/177Hf Determination in Small Samples by a High-Temperature SlMS Technique Vincent J. M. Salters* Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York 10964
This paper describes a new method for the analysis of the Hf isotopic composition in geological materials. This technique requires the separation and puri6cation of Hf out of a complex matrix. The resulting Hf separate contains less than 0.2%Zr and less than 0.01%Ti. In the new mass spectrometric technique, called “hotSIMS”, the sample is held at a high temperature while being bombarded by a primary beam. The hot-SIMS technique has several orders of magnitude better ionization efficiency than conventional thermal ionization. Also, due to the high temperature at which the sample is held, the mass spectrum of the sputtered secondary ions contains only a limited amount of molecular ions; the formation of hydrides, especially, is inhibited by the high temperatures. This new technique now allows routine high-precision 17%f/17’Hf determination on 50 ng of hafnium separate. Samples down to 15 ng can be analyzed at a lower precision level. 176Ludecays to 176Hfwith a half-life of 35.7 Ga. The natural variation in 176Hf/177Hf is thus dependent on the time-integrated 176L~/177Hf history of the sample. This makes Hf isotopes an important tool in dating events on a geological time scale. However, the difticulty of the analysis has limited the use of Hf isotopes. The problems with analyzing Hf isotopes are 2-fold: First, Hf, in nature, and in the laboratory, behaves very similar to Zr, which makes it difficult to separate. Most materials still have the same Hf/Zr ratio as the initial Hf/Zr ratio of the earth 4.55 Ga ago. The purity of the Hf separate affects the ionization efficiency of Hf, the purer the Hf the higher the ionization efficiency. Second, the ionization potential of Hf is relatively high, resulting in poor ionization efficiencies for thermal ionization. Previous techniques were able to measure 176Hf/177Hfon Hf separates down to 1 pg of Hf with a precision between 0.007% and 0.04%.1-3 Natural levels of Hf in basalts range from 1 to 5 ppm, which requires 1 g of material to be processed for thermal ionization mass spectrometric measurements. This paper presents a new and improved chemical separation technique for Hf and an improved mass spectrometric technique for high-precision determination of 176Hf/177Hf.This new technique is an enormous improvement over the old thermal ionization technique in that ionization efficiencies are -2 orders of magnitude better than the thermal ionization technique. This now allows determination of Hf isotopic composition on Hf separates down * Present addrsss: National High Magnetic Field Laboratory and Department of Geology, Florida State University. Tallahassee, FL, 32310.4005. (1) Corfu, F.; Noble, S.R. Geochim. Cosmochim. Acta 1992,56,2081-2097.
(2) Patchett, P. J.;Tatsumoto, M. Contrib. Mineral. Petrol. 1980,75,263-267. (3) Salters, V. J. M.; Hart, S. R Earth Planet. Sci. Lett. 1991,104, 364-380.
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Analytical Chemisfry, Vol. 66,No. 23,December 7, 7994
to 15 ng in size, which was heretofore beyond our analytical capabilities. The mass spectrometry is done on the Lamont Isolab4 by the “hot-SIMS” technique, where the sample is held at a high temperature while being bombarded by 15 kV Ar+ions. However, the increased ionization efficiency of the hot-SIMS technique compared to thermal ionization is not restricted to Hf alone. All other species present on the filament ionize better too, thus requiring a purer Hf separate and improved Hf chemistry. Specifically, isobaric interferences of 90Zr12C8at mass 176 and @W2Cll at mass 180 need to be avoided. The chemistry described above is designed to produce these higher degrees of purity of the Hf separate. The new separation chemistry, not much more complicated than previous technique^,^,^ achieves an almost 100% effective separation between Hf and Zr and Hf and Ti, while using smaller volumes of concentrated acids. Hf separates as small as 50 ng can now be analyzed for 176Hf/177Hf with a routine precision between 0.007% and 0.015%. This paper describes the new technique and reports reproducibility for both standards and natural samples. DISSOLUTION AND SEPARATION CHEMISTRY
The separation of Hf out of the complex matrix of a rock powder was investigated by addition of radioactive 181Hf to the rock powder. Is1Hf was produced at the nuclear reactor at MIT by neutron addition through irradiation of preconcentrated lsoHf (>95% IS0Hf). lslHf activity in the different fractions was determined using a NaI detector. This method requires only small additions ( < l o ng) of Hf to the sample, and Hf concentrations can be determined in both liquids and solids independent of matrix complexity. Other analytical techniques (atomic absorption or inductive coupled plasma mass spectrometry) are less sensitive (higher doping levels are needed) and require longer time for analyses. Throughout the chemistry only ultrapure reagents and always double subboiling distilled acids are used. (CAUTION: Mineral acids, such as HN03, HC1, HF, and HCIOl are irritants to the eyes, skin, and mucous membranes. Perchlorates can form explosive mixtures in combination with organic materials. In addition, efects of direct exposure to HF can go unnoticed for several hours. Proper precaution is required for handling of any of the above materials.) Theoretically, separation of Hf and Zr from the sample matrix is easily achieved with a AG 50W-X8 cation exchange column. However, dissolved Hf and Zr associate strongly with fluoride ions (4) England, J. G.; et al. Int. J. Mass Spectrom. Ion Processes 1992,121, 201240.
0003-2700/94/0366-4186$04.50/0 0 1994 American Chemical Society
Column I
Samole in 4N HF 5.4v; 4N HF 4.0 cv IN HFIlN HCI 5 . 4 INHFIINHCI ~ ~ ..............................................................................................
&!Id Hf.Zr.Ti
L Column 11
Sample in 2.5 MiCI 2.5 cv 2.5 MiCI .................................................................................. Ti 3.0 cv 6.0 NHCI ................................................................................ Hf.21
Table I. Ion Exchange Columns
column
resin type
h (cm)
vol (cm3)
I I1 I11
AG l-X8,200-400 mesh (anion) AG50-X8,200-400 mesh (cation) HDEHP on Teflon beads (anion) HDEHP on Teflon beads (anion)
19 10
3.7
IV
12
2.0 2.4
12
2.4
L Column 111
Sample in 6.0 NHCIIO. IS M i F 4 cv 6.0 NHCIIO.15 NHF ........................................... 2 CY 2.5 NHCI10.3NHF ................................................
.Zr HI
L
Figure 1. Elution scheme.
and form a Hf-F, complex (or Zr-F,). The distribution coefficient (D = {concentration of element in resin}/{concentration of element in solution}) for this complex on AG 50W-X8 is close to zero, while Hf in a chloride solution will adhere to the resin. Also, in theory, Hf and Zr are easily separated from the sample matrix precipitation of zirconium tetramandelate.5*6However, the presence of fluoride inhibits the formation of mandelate salts. Dissolution of the rock powder with concentrated HF obviously introduces a large amount of flioride ions. The bulk and complexity of the matrix inhibits the effective and complete evaporation of HF even at high temperatures (150-200 "C) in the presence perchloric or sulfuric acid. Therefore, cation exchange resins can only be used after Hf is separated from the bulk of the matrix and the fluoride can be evaporated quantitatively. An alternative way of bringing rock powder in solution is by fusing a sample with flux, thereby making the sample soluble in hydrochloric acid. A minimum sample to flux ratio of 1:5 is needed to allow dissolution and then a large volume of hydrochloric acid is required (50-100 mL of 2.5 N HCl), which introduces high blank levels. With the present technique, 100 mg of rock powder is dissoived in 4 mL of concentrated hydrofluoric acid in a clean aidow box as follows: concentrated HF is added to the sample and allowed to react at room temperature in an open 15 mL Savillex Teflon reaction vessel. After 12 h the vessel is closed, and the temperature is raised to 100 "C, and the rest of the sample is completely dissolved in the next 24 h. At that time the vessel is opened and the sample is dried at 100 "C. The precipitated salts are partly dissolved in 0.2 mL of 4 N HF, which is added at room temperature and allowed to sit for at least 2 h. The sample is ultrasonically agitated, centrifuged, and decanted; the residue is repeatedly treated with 0.1 mL batches of 4 N HF until a total of 0.6 mL is collected. If the temperature of the sample was over 100 "C during the dry-down stage, some Hf will stay in the residue. The Hf is separated from the bulk sample and purified by three different ion exchange columns (see Figure 1 and Table 1). The first column separates a Hf-Zr-Ti fraction from the bulk sample. The 0.6 mL sample solution is loaded on a Dowexl-X8 (200-400 mesh) anion exchange column 19 cm long and 0.5 cm in diameter. The sample is rinsed on the column with another 2 x 0.2 mL 4 N HF after which the bulk of the matrix is eluted with first 20 mL of 4 N HF followed by 20 mL of 1.0 N HCl/1.0 N HF. The Hf (5) Belcher, C.; Sykes, A; Tatlow, J. C. Anal. Chim.Acta 1954, 10,34-47. (6) Hahn, R B.; Baginski, E. S. Anal. Chim.Acta 1956, 14, 45-47.
concentrate is eluted with 15 mL of 1.0 N HCV1.0 N HF. (The column is cleaned with 12 mL of concentrated HF, twice with 12 mL of 8 N HN03, backwashed with 8 mL of 4 N HF, and conditioned once with 8 mL 4 N HF.) Concentrated perchloric acid (0.1 mL) is added to the Hf fraction from the tint column, and the sample is dried to a small drop at 100 "C in a clean airflow box. At this point the temperature is raised to 150 O C , and the rest of the perchloric acid is slowly evaporated. When almost dry,a second 0.1 mL of perchloric acid is added and the sample is again heated till almost dry. This step is repeated once more. It is important to not dry the sample completely because once dry it will not redissolve in hydrochloric acid. The fluoride ions are evaporated during the repetitive drying of the sample, and after the last dry down the sample cooled and dissolved in 0.3 mL of 2.5 N HCl. Incomplete dissolution in HCl at this stage indicates the presence of HfOz and a drop of HF is added the dissolve the HfOz, after which the dry-down procedure has to be repeated from the beginning. Drying the sample in a PFA-type Teflon beaker creates unusually large samples which sometimes do not dissolve in HCl. I attribute this to leaching of organic material out of the PFA by the perchloric acid, resulting in an increase in the bulk of the sample. In addition, organic material is not effectively separated from the Hf on the ion exchange columns and the columns themselves can be a source for organic material, both resulting in impure Hf separates. Therefore the Ti-Hf-Zr fraction from the first column needs to be dried in PTFE-type Teflon, which is more resistant to perchloric acid. The resin bed of the second column is 10 cm long and 0.5 cm in diameter and is used to separate Ti from Hf and Zr. Ti needs to be separated from Hf quantitatively because 4qi12C11is an isobaric interference on IS0Hf. The column material is AG 50WX8 cation resin. A 30 ,uL aliquot of Ultrex concentrated hydrogen peroxide is added to the sample solution. The hydrogen peroxide will form an orange-brown complex with titanium.2 The formation of this orange-brown complex (exact color is dependent on the amount of Ti present) is a qualitative check of the chemistry up until this point. Incomplete expulsion of the fluoride ions will result in only a pale yellow or even a colorless solution. If this occurs, the dry-down sequence with perchloric acid has to be repeated. This second column is essentially similar to the technique described by Patchett and Tatsumoto.2 The Hf separate is loaded on the column in 0.3 mL of 2.5 N HCl and rinsed on the column with another 0.4 mL of hydrochloric acid. Ti is eluted as an orange Ti-(H202), complex in 5 mL of 2.5 N HCl. Hf and Zr are eluted in 5 mL of 2.5 N HC1/ 0.30 N HF and dried on a hot plate in a clean airflow box at 100 "C. This column step ensures * T P O H fsignal intensity ratios of less than 2 in the mass spectrometer. The 4Ti/180Hfratio in geological materials varies from 6900 to 12 000, which combined with the 10 times higher ionization efficiency of Ti over Hf indicates that more than 99.998%of the Ti is routinely Analytical Chemistry, Vol. 66, No. 23, December 7, 1994
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backwashed with 8 mL of 2.5 N HCl.) The third column procedure is adapted in part from Fujimaki et aL7and is used to separate Hf from Zr. The third column bed is 12 cm long and 0.5 cm in diameter. The column has a long stem to ensure a pressure head of at least 20 cm. The pressure is kept relatively constant by a reservoir at the top of the column. A hole, -1 cm above the resin, is used to load the sample and to drain the excess acid out of the reservoir. During sample elution and cleaning, the hole is closed by Teflon tape wrapped around the column. The column material is (bis(2ethylhexyl)orthophosphoric acid (HDEHP) coated on Teflon beads. A description of the material can be found in Zindler.* The column is packed to a flow rate of 10 mL of HzO/h with a 20 cm pressure head. The pressure head assures a faster flow rate, but consequently, the column needs to be calibrated by time instead volume. The drieddown Hf-Zr separate from the second column is redissolved in 0.2 mL of 6.0 N HCV0.15 N HF, which is loaded on the column and rinsed on the resin bed with 3 x 0.2 mL of 6.0 N HCV0.15 N HF. This column is calibrated by time instead of volume. The sample is eluted with 6.0 N HCV0.15 N HF until just before the Hf comes off the column (after 2-3 h). The excess acid is drained through the hole just above the resin, and the hole is closed by wrapping Teflon tape around the column. The Hf fraction is eluted and collected in -60 min with 2.5 N HCV0.30 N HF. me column is cleaned with 10 mL of 4 N HF and 2 x 10 mL 6.0 N HC1.) The exact HF normality is not important in this separation step as long as the Hf stays on the column for at least 2 h. The behavior of Hf and Zr is extremely sensitive to flow rate and HF normality. The HF-HCl mixture is made up in a 20 L batch, and the columns are calibrated with each new batch. After collection, the Hf fraction is dried down in a clean airflow box at 100 "C. This column step is repeated once to achieve optimum separation of Hf and Zr. It has been found that the ion exchange procedure with the HDEHP-Teflon columns produces a more complete separation of Hf from Zr than either the technique described by Patchett and Tatsumoto2using cation columns or the separation technique described by Strelow and Bothmagusing anion columns. Both Hf and Zr have a strong affinity for fluoride, which makes it difficult to assure complete removal of fluoride from the samples. In contrast to the old separation methods, this new technique does not require the removal of fluoride ions before the sample is loaded on the third ion exchange column. This makes the new technique more robust and allows a more reproducible Hf-Zr separation. The procedure results in wZr/180Hfintensity ratios in the mass spectrometer that are consistently less than 0.14, indicating 99.7% of the Zr is routinely removed from the Hf fraction (wZr/180Hf is -55 in nature). 4T1/180Hfratios in nature are -9250, while the Ti/180Hf intensity ratios in the mass spectrometer are consistently less than 0.3. The total procedural blank is in the order of several picograms (always less than 10 pg) and does not affect the isotope ratio. Based on tests with radioactive lS1Hf,less than 1 ppm of (7) Fujimaki, H.; Tatsumoto, M.; ~ o k iI,L J Ceophys. R ~ 1984, ~ . 89, 662-672. (8) Zindler, A Geochemical processes in the earth's mantle and the nature of crust-mantle interactions: Evidence from studies of Nd and Sr isotope ratios in mantle derived igneous rocks and lherzolite nodules. Massachusetts Institute of Technolom. Cambridge. MA. 1980. (9) Strelow, F. W. E.; Bothma, C. J. Anal. Chem. 1967,39, 595-599.
e.
4188 Analytical Chemistry, Vol. 66, No. 23, December 1 , 1994
Faraday cup position
Hi1
Hi2
Hi3
nominal m u collected position 1 position 2 position 3 position 2 position 1
175 176 177 176 175
176 177 178 177 176
177 178 179 178 177
the loaded sample is present on any of the ion exchange columns after the first cleaning step and the largest blank contribution is from the reagents. Yield of the total chemistry as measured both by isotope dilution analyses and as determined with radioactive ls1Hf range from 80 to 95%. MASS SPECTROMETRY Hf is loaded on a five pass zone refined Re filament 30 pm thick and 760 pm wide. The loading procedure is as follows: first two stripes of Parafilm are melted on the filament -400 p apart. The area between the two stripes is squared by melting one more stripe of parafilm. The sample is dissolved in 1pL of 0.15 N HF/ 0.3 pL of colloidal graphite (20 mg/mL), and 0.5 pL of 0.25 N H3P04 is added. The mixed sample solution is deposited onto the filament between the Parafilm stripes in small increments. Each increment is dried down slowly at a current of -1.0 k After the complete sample is loaded, the filament is slowly brought to a dull red glow. The filament is kept at this temperature for 2 s. The addition of colloidal graphite results in the formation of hafnium carbide when the filament is heated. The hafnium carbide is extremely refractory, and it avoids the evaporation of the Hf before the high temperatures required for optimum ionization efficiency are attained (-1700-1900 "C during data acquisition). The mass spectrometric measurement of the isotopic composition of Hf is performed on the Lamont Is01ab.~This is a doublefocusing forward geometry mass spectrometer with secondary ionization capability. The Ar+ primary beam is produced by a cold cathode duoplasmatron and is focused to an -100 pm diameter by two sets of Einzel lenses. Impurities in the primary beam are reduced by a Wienfilter which limits the mass spectrum of the primary beam to between 36 and 44. The determination of Hf isotopes is done by the hot-SIMS (secondary ionization mass spectrometry) technique. Under vacuum the sample on the filament is heated to 1900'C while being bombarded by 15 kV Art ion beam. The sputtered secondary ions are accelerated and focused by a lens system similar to that of a conventional thermal ionization mass spectrometer. The ion beam is energy filtered by an electrostatic analyzer, and a 20 eV wide energy spread of the ions passes through. The electrostatic analyzer is followed by a conventional magnetic sector, and the ion beams of the different isotopes are collected simultaneously in Faraday cups according to a multidynamic collection scheme (see Table 2). There are several advantages of the high temperatures at which the sample is kept. First, the high temperature reduces the number of molecular ions and eliminates the formation of hydrides. since hydride correction is difficult for the Hf mass spectrum, the successful elimination of hydrides is the key to obtaining the correct isotope ratios. Second, the
T
t
Figure 2. Standard values obtained on JMC 475. Over the first 3 years the r76Hf/177Hf averaged 0.282 207. After repair of the collector block the value for the standard changed to 0.282 236 for 176Hf/'77Hf.
high-temperature SIMS increases the ionization efficiency of Hf by a factor of 3 as compared to cold SIMS. The achieved ionization efficiency of the hot-SIMS technique is 1 ion in 500 atoms, which is a marked improvement over previous methods of 1 ion in 40 000 atoms.2 Total Hf ion beams of 0.5-1.0 nA for 2-3 h are routine for 150 ng of Hf standard JMC 475, which is 2 orders of magnitude better than thermal ionization techniques.lS2 The ion beams of the three individual isotopes are simultaneously collected, and a multidynamic peak switching routine (see Table 2) is used in which Faraday cup, ampl8er bias, and signal instabilities are canceled out. 176Hf/177Hfis corrected for fractionation using 177Hf/178Hf = 0.6816, which corresponds to 179Hf/177Hf = 0.7325 as used by Patchett.2 The resulting, in-run precision, is consistently better than 0.008% (20 level, whereby u signiiies the standard deviation of the mean). External precision of the standard JMC-475 is better than 0.01% at the 20 level, while the average of the standard (176Hf/177Hf = 0.282 207) is very close to the accepted value (176Hf/177Hf= 0.282 200) before 1993 (see Figure 2). In 1993, some repairs and modiiications were performed on the collector block of the mass spectrometer and after these repairs the standard value changed to 176Hf/177Hf= 0.282 237. The shift in the standard value is expected after changes to the mass spectrometer. The reproducibility of the internal standard (AUG7; see Table 3) is still excellent after correction with the appropriate standard value. The last two AUG7 values were obtained after the shift in the standard. Duplicates and triplicates of samples (see Table 3) are all within the 2 0 internal run precision of each other. The new technique
Table 3. Reproducibility and Comparison with Other Laboratories'
sample no.
176Hf/177Hf
absolute 2a x lo6
lab
AUG7 AUG7 AUG7 AUG7 AUG7 AUG7 2392-9 2392-9 W H 57W KbH 57W 77SL13 77SL13 WIC 020 WIC 020 WIC 020
0.283 270 0.283 281 0.283 259 0.283 259 0.283 257 0.283 273 0.283 215 0.283 207 0.283 58 0.283 41 0.284 96 0.284 66 0.283 121 0.283 100 0.283 070 0.282 389 0.282 361
h26 124 517 f20 h23 120 122 h14 1170 f202
LDEO LDEO LDEO LDEO LDEO LDEO LDEO LDEO LDEO LDEO LDEO LDEO LDEO MPI MPI MIT LDEO
"E'-8
NTP-8
f170
f268 h35 f60 f40 128 h18
Results of multiple anal sis of a Aleutian arc basalt (AUG7), which is used as an in-house r o d standard, and duplicate analysis of midocean ridge basalt 2392-9 and of peridotites (KbH57W and 77SL13), both with the hotSIMS techni ue. Also samples analyzed with the thermal ionization technique at &e Massachusetts Institute of Technology NIT) and at Max-Planck Institute (MPI), Maim, and with the new technique at LDEO (Lamont Doherty Earth Observatory) are compared. also reproduces 176Hf/177Hfof samples analyzed by thermal ionization in other laboratories (see Table 3). In summary, the new technique allows high-precision determination of Hf isotope ratios on samples as small as 50 ng of Hf (2a
