Anal. Chem. 1994,66, 1044-1049
Determination of Femtogram Quantities of Protactinium in Geologic Samples by Thermal Ionization Mass Spectrometry David A. Pickett,' Michael T. Murreii, and Ross W. Willlamst Isotope Sciences, J5 14, Los Aiamos National Laboratoty, Los Alamos, New Mexico 87545
We describe a procedure for measurement of nlPa in geologic samples by isotope dilution thermal ionization mass spectrometry, using z33Paas a spike isotope, which provides marked improvementsin precision and sample size relative to established decay countingtechniques. This method allows determination of as little as a few tens of femtograms of 231Pa (- 108 atoms) with a conservative estimated uncertainty of *l% (95% confidence level). Applications of 231Pa-*35Usystematics to uranium-series geochemistry and geochronology should be greatly enhanced by this approach. Protactinium-23 1 is the longest-lived intermediate daughter nuclide in the 235Udecay series, with a half-life ( f 1 p ) of 32 760 a.l.2 Although Pa and U are both actinides, differences in valence (+5 for Pa, +4 and +6 for U) and chemical affinities can result in substantial chemical fractionations between the two during geologic proce~ses.~ The resultant isotopic disequilibrium between 231Paand 235U(Le., activity ratio # 1) can therefore provide a means for dating these processes on a time scale of up to 150 ka. (The intermediate nuclide, 231Th,has a 26-h half-life and is thus negligible.) Examples of applications of this chronometer include studies of marine carbonates4and phosphorite^,^ bones,6and sedimentation and scavenging rates in deep-sea sediments.' In carbonate rocks, 23'Pa dating can provide an important independent check on the assumptions governing 230Th/234U dating. Observations of 231Pa/235U disequilibrium in volcanic rocks8-10 point to potential utility as a chronometer and geochemical tracer in young volcanic systems. Thus, the 231Pa/235U system holds promise for application in fields as diverse as paleoanthropology, volcanology, and paleoclimatology. Despite its potential, however, applications of this system are less common than 230Th/234U due to analytical difficulties with Pa chemical separation and decay ~ o u n t i n g .Protac~ tinium-23 1 is often measured indirectly via a spectrometry
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+ Present address: CompuChem Environmental Corp., P.O. Box 14998,Research Triangle Park, NC 27709. (1) Robert, J.; Miranda, C. F.; Muxart, R. Radiochim. Acta 1969.11, 104-108. (2) Schmorak, M. R. Nucl. Data Sheers 1977, 21.91-200. ( 3 ) Ivanovich, M., Harmon, R. S., Eds. Uranium-Series Disequilibrium: Applicarions to Earth, Marine, and Environmental Sciences, 2nd ed.;Clarendon Press: Oxford, UK, 1992. (4) Ku, T.-L. J . Geophys. Res. 1968, 73, 2271-2276. ( 5 ) Veeh, H. H. Earth Planet. Sci. Lett. 1982, 57, 278-284. (6) Bischoff, J. L.; Rosenbauer, R. J. Science 1981, 213, 1003-1005. (7) Lao,Y.;Anderson,R. F.; Broecker,W. S.;Hofmann,H. J.; Wolfli, W. Geochim. Cosmochim. Acta 1993, 57. 205-217. (8) Williams, R. W.; Perrin, R. E. EOS Trans. Am. Geophys. Union 1989, 70, 1398. (9) Goldstein, S. J.; Murrell, M. T.; Williams, R. W. Earrh Planet. Sci. Lett. 1993, 115, 151-159. (10) Pickett, D. A.; Murrell, M. T. Geol. SOC.Am. Abstr. Prog. 1993,25 (6). A-99.
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of its granddaughter 227Th;1 the required assumption of isotopic equilibrium among these nuclides and the intermediate 2 2 7 Ais~also a potential source of uncertainty. This paper describes a method of 231Padetermination by isotope dilution mass spectrometry which surmounts these difficulties and allows measurement at femtogram levelsto precisionsof better than 1%. Compared to a spectrometric methods, detection limits are -50 times lower and uncertainties are better than 10 times lower. This method offers advantages in precision and sample size analogous to those afforded by the mass spectrometric methods for 230Th-234U-238U12J3 and 226Ra228Ra.14J5This will allow more precise 231Pa/235U dating on much smaller samples than previously possible, but it may also lead to application in fields of study, such as environmental monitoring, where very low levels of protactinium have been prohibitive.
EXPERIMENTAL SECTION Reagents and Materials. Labware. All procedures were carried out using containers of PFA Teflon, FEP Teflon, or low-densitypolyethylene, cleaned in acid baths. No glassware was used. Reagents. Deionized water was obtained using a Milli-Q purification system and then distilled in a subboiling, twoTeflon-bottle apparatus. Ultrapure hydrochloric, nitric, hydrofluoric, perchloric, and sulfuric acids and ammonium hydroxide (Seastar) and hydrogen peroxide (Baker Ultrex) were obtained commercially and used throughout the procedure, except in the initial Np-Pa separation for spike preparation. High-purity boric acid was obtained from Johnson Matthey. Baker Analyzed practical grade diisobutyl ketone (2,6-dimethyl-4-heptanone) was purified by distillation under vacuum. Resins. Ion exchange resins utilized were Bio-Rad AG MP-1, chloride form, in both 100-200 and 200-400 mesh. They were cleaned by successive rinses with HNO3, HCl, and HzO. Columns. Resins were loaded on disposable Bio-Rad 10mL polypropylene columns with polypropylene frits (Catalog No. 73 1-1550). Columns manufactured by Bio-Rad after 1991 have Kynar frits and were avoided due to higher retention (1 1) Gascoyne, M. Geochim. Cosmochim. Acta 1985.49, 1165-1 171. (12) Edwards, R. L.; Chen, J. H.; Wasserburg, G. J. Earrh Planer. Sci. Lett. 1987. 81, 175-192. (13) Goldstein, S. J.; Murrell, M.T.; Janecky, D. R. Earth Planet. Sci. Lett. 1989, 96, 134146. (14) Volpe,A. M.;Olivares, J. A.;Murrell.M. T. Anal. Chem. 1991.63.913-916. (15) Cohen, A. S.; ONions, R. K. Anal. Chem. 1991, 63, 2705-2708.
0003-2700/94/03661044$04.50/0
0 1994 American Chemical Soclety
of Pa during HC1-HF elution. Disposable pipet tips plugged with quartz wool are a possible substitute. Dissolution of the quartz wool was not observed when 0.05 M H F was used but if a problem is suspected, Teflon wool may be suitable. Radionuclide Tracers. Column calibrations and separation factors were determined using silicate rock samples subjected to intense neutron irradiation and containing the short-lived nuclides 233Pa,95Zr,95Nb,“Ba, 9 9 M ~129Te, , 141Ce, 147Nd, and “La. These tracers were measured by y spectrometry in eluates, resins, and extractants following separations experiments. Reference Materials. A tracer solution of 231Pain HC1H F was utilized for mass spectrometric development (filament loading technique, warm-up procedure, determination of ionization efficiency). A mixture of this solution with a 233Pa spike (described below), with an initial 231Pa/233Paof 0.180 (July 9, 1992) and 231Paconcentration of -1.0 pg/g, was utilized for investigation of instrumental reproducibility and mass fractionation effects. Spikecalibrations (see below) were determined using solutionsof the rock sample Table Mountain Latite (TML), which has been used for interlaboratory mass spectrometric 23“Th/232Th~ o m p a r i s o n s . ~ 3 JThis ~ ~ * rock is > 106 a old and thus, since it has been chemically undisturbed for a time much longer than the half-life of 231Pa,secular equilibrium between 231Paand 235Ucan be assumed (i.e., (23*Pa/235U)= 1, where parentheses denote activity ratio, or atomic 231Pa/235U= 4.655 X l e 5 ) . This assumption is supported by the observation of secular equilibrium for 238U234U-230Thfor TML (234Ut l p = 244 ka, 23”Tht1p = 75 ka). Any rock sample in secular equilibrium would serve equally well for calibration. Spike. A spike solution of 233Pa(tip = 26.967 f 0.004 days;lg all uncertainties herein are estimated 2a values) was milked from a solution of its 237Npparent as described below. The 233Paspike was ct counted after separation to ensure that the concentration of 237Np was low enough to produce a negligible effect on 233Padecay over several half-lives of 233Pa. The spike was stored in a solution 4 M in HCl and 0.5 M in HF; the H F is critical for maintaining Pa in solution. In addition to its use as the isotope dilution spike in mass spectrometry, 233Pa was also used as a yield tracer for development of chemical procedures and mass spectrometer filament loading. The concentrations of the 233Pa spike solutions were determined in two ways. First, a 5-mL aliquot of the 233Pa solution was measured for 1 h on a y spectrometer (see below), and the 233Paconcentration was calculated using the photopeak at 31 1.89 keV with a branching ratio of 0.386.20 Second, the concentration was determined by mass spectrometry via isotope dilution with 231Pain the TML reference sample, wherein (1) U concentration was measured, (2) 23lPa concentration was calculated based on the aforementioned assumption of secular equilibrium, and (3) 233Paspike concentration was calculated (16) Gill, J . B.; F‘yle, D. M.; Williams, R. W. In Uranium-Series Disequilibrium: Applicationr to Earth, Marine, andEnuironmentalSciences,2nd ed.;Ivanovich, M., Harmon, R. S.,Eds.; Clarendon Press: Oxford, UK, 1992; Chapter 7. (17) Palacz, Z. A.; Freedman, P. A.; Walder, A. J. Chem. Geol. 1992, 101, 157-
165. (18) McDcrmott, F.; Elliott, T. R.; van Calsteren, P.; Hawkesworth, C. J. Chem. Geol. 1993, 103, 283-292. (19) Jon-, R. T.; Merritt, J. S.; Okazaki, A. Nucl. Sci. Eng. 1986,93, 171-180. (20) Gehrke, R. J.; Helmer, R. G . ;Reich, C. W. Nucl. Sci. Eng. 19’19.70.298-306.
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Julian d a t e , 1993 Figure 1. Results of multiple calibrations of a *=Pa spike solution; ail values corrected for dilution and *=Pa decay to date 139.0000. Open symbols represent direct y counting; uncertainties (-4Yo) include those from counting, the efficiency of the counter, and the branching ratio for the 312keV peak. Closed symbols are determinations by isotope dilution thermal Ionization mass spectrometry (ID-TIMS) uslng the TML reference: uncertainties are 2u errors of the mean for the indhdduai analyses. The dashed lines represent the mean of the ID TIMS values &l%; agreement among the ID-TIMS values is well within these bounds. The square is the original spike solutbn. Circles are for the first dilution of the spike; the two ID-TIMS values represent analysesof the same Pa sample. Diamonds are for the second dilution; the two IDTIMS values are for separate TML determinations. Both dilutions had 239Paconcentrations of 1.4 X loe atoms/g when first made.
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based on the Pa isotopic ratio determined on a spiked aliquot of the sample. The agreement between the two methods was consistently 12.5%, which is within the uncertainty of the y-counting measurement (Figure 1). The more precise mass spectrometric spike calibration was utilized in isotope dilution calculations. When a 233Pasolution was first prepared, the 233Pa/231Pa ratio was sufficiently great that a correction in the isotope dilution calculations for 231Pawas not required. After -6 months of use, when the 233Paconcentration was much lower, such correction was sometimes necessary and was typically on the order of a few percent by the time the spike was retired. Because the same 237Npsource was continually reused for spike production, and since it was cleansed of most of its Pa each time, successive generations of the 233Pa spike had progressively lower levels of 23lPa contamination. Filaments. Zone-refined, 0.76 mm X 0.025 mm, rhenium ribbon (Cross) was cleaned in HNO3 and rinsed in H2O and acetone. The ribbon was welded to filament posts in the NBSstyle standard configuration (ionization surface 6.4 mm long) and degassed in a vacuum at 4.2 A (- 1950 ‘C) for 45 min with an applied potential of -90 V dc. Graphite. Thermal ionization of Pa was promoted by loading on a bed of fine graphite powder. The graphite was obtained from Ted Pella, Inc. (Aquadag, Catalog No. 16051) and cleaned by successive leaching in CCld, acetone, HzO, HCl, HF, HNO3, and H2O. Care must be taken that all organic solvent traces are removed before HNO3 leaching. Instrumentation. y Spectrometry. Radionuclide tracer measurements for technique development and initial 233Pa spike calibration were performed on a Canberra Industries y spectrometer with a Ge(Li) detector (closed end coaxial, 48 mm length X 53 mm depth, 20.7% efficiency relative to NaI). Protactinium-233 yields during chemical separations were monitored using a Packard Auto-Gamma Model 5530 y counter with a NaI detector. AnalyticalChemlstty, Vol. 66. No. 7, April 1, 1994
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Mass Spectrometry. Isotope dilution measurements of Pa Np breakthrough in the Pa fraction due to variable oxidation were obtained on a NBS-designed, 30.5-cm-radius, 90° states (e.g., Np(1V) rather than Np(V)). deflection magnetic sector, thermal ionization mass specChemicalSeparation. All chemical separation procedures trometer equipped with a Balzer Model 3 17 electron multiplier were performed on a clean bench under class 100 conditions; connected to an ion-counting detection system.21922The open-beaker evaporations took place in a class 10 laminardetection system has a dark current of 0.05 counts/s, a deadflow hood. An amount of silicate sample preferably sufficient time correction of 8 ns and is linear and accurate to 0 . 0 6 % ~ ~ ~ to provide 1100 fg of 231Pa(usually 0.1-5 g) was dissolved using conventional HF-HCl-HN03-HC104 techniques, eiSpecialsafetyConsiderations. Solutions containing HC104 ther in Teflon vials on a hot plate or in sealed vessels in a should be evaporated in hoods certified for such use, and microwave oven (CEM Model MDS-81D), with saturated general procedures for safe handling of this and other reagents boric acid utilized to prevent formation of insoluble fluorides should be followed.24 Levels of radioactivity due to the which could remove Pa from solution. Final dissolution was @-decaying233Paspike in routine analyses are very small (95%. The second anion column affords further removal of these contaminants and of 233U,which is the daughter of /3 decay of the 233Pa spike nuclide. The short half-life of 233Pa(27 days) requires that final separation take place soon before mass spectrometric analysis to minimize ingrowth of 233U, preferably within a few days (see below). The column consisted of 0.5 cm3 of MP- 1,200-400-mesh resin, C1- form. All acids in this elution included -0.06% H202 (2 drops of 10% H202 per 10 mL of acid) to ensure all U as U(V1). The resin bed was conditioned with 1 cv of HzO and 4 cv of concentrated HCl. The Pa fraction from the previous column was evaporated to dryness, dissolved and dried down in concentrated HN03, and loaded on the column in 1 cv of
concentrated HC1+ 2 drops saturated H3BO3. The column was sequentially washed with 4 cv of concentrated HCl, 2 cv of 10 M HCl, 4 cv of 8 M HCl, and 2 cv of 6 M HCl. Protactinium was eluted with 3.5 cv of 8 M HCl 0.05 M HF. Tracer experiments indicated a column recovery of >99% for Pa, with no Mo, Nb, or Zr detected in the Pa fraction. As much as 99% of the Nb was removed in the concentrated, 10 M, and 8 M HCl steps. The 6 M HCl wash was intended to remove as much of the remaining Nb as possible before Pa elution; a few tenths of a percent of the Nb remained bound to the resin. Taking into consideration the detection limits of they spectrometer for tracer detection, the Zr/Pa, Nb/Pa, and Mo/Pa of the solution were improved by this elution by a factor of at least lo3. Thus, total procedural separation factors for Zr and Nb were found to be at least 108 and 104, respectively. Repeating this final column did not measurably improve the mass spectrometer analysis, suggesting that this level of separation was sufficient. The DIBK extraction and both ion exchange columns should afford effective separation of Th and U from Pa;26we determined a U-Pa separation factor of IO3 for the HCl column. Total procedural yield for Pa was -85% or better; yields were routinely checked after each chemical procedure by y counting of 233Pa. Mass Spectrometry. Several filament loading techniques were investigated for Pa analysis by thermal ionization, including direct stippling, silica gel, triple filament, Pt overplate, and graphite loads. Ionization efficiencies were calculated by integrating ion count rates over the course of a run and dividing by the number of atoms loaded. The graphite method (also useful for U and Th analysis1*),involving the sandwiching of the Pa solution (0.05 M HN03) between thin graphite layers, was found to yield the highest ionization efficiencies-w 0.7%-for 200-fg loads of our 23IPa reference solution (a 13-fg load yielded an efficiency of 0.9%). In contrast, the Pt overplate method,22 which affords the advantage of lower running temperature, gave efficiencies of only -0.2%. Isobaric interferences of 1-20 counts/s, presumably due to organic species, at all masses in the vicinity of Pa are a potential problem and must be monitored, particularly in light of the small Pa signals (100-1000 counts/ s). We found that limiting the load to the center third of the filament helped suppress these peaks, and that patient warmup to running conditions eliminated them. The performance of this configurationsuggested that a shorter (1.5 mm) filament ionization surfacez8 would be advantageous; however, this configuration yielded somewhat lower ionization efficiencies ( 0.4%). The effect of impurities on ionization efficiencies was checked for Zr and Nb by adding variable amounts to pure Pa and analyzing in the mass spectrometer using the adopted graphite load. A Zr/Pa weight ratio of lo7 and a Nb/Pa ratio of lo6resulted ina considerablereduction in Pa ionization efficiency from 0.6-0.8% (pure Pa) to 0.06%. However, a Zr/Pa of los and Nb/Pa of lo4 had little effect in lowering efficiency (0.54%). In a typical silicate rock, Zr/Pa =lo9 and Nb/Pa =lo8, implying that total chemistry separation factors of only lo4 for Zr and Nb would be sufficient for satisfactory Pa analysis. Ionization efficiencies for most runs
(26) Kluge, E.; Licser, K. H. Radiochim. Acto 1980, 27, 161-171. (27) Anderson, R. F.;Fleer, A. P Anal. Chem. 1982, 54, 1142-1147.
(28) Dixon, P. R.; Perrin, R. E.; Rokop, D. J.; Maeck, R.; Janecky, D.R.; Banar, J. P. Anal. Chem. 1993.65, 2125-2130.
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Ana&ticalChemistry, Vol. 66,No. 7, April 1, 1994
1047
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of natural samples (for which Zr and N b separation factors were >lo4) ranged from 0.4 to 0.6%. Since, as mentioned above, ionization efficiency was not apparently improved by repeating the final chemical step, Zr and N b must not be responsible for these somewhat lower efficiencies relative to pure Pa. A typical run began with gradual filament warm-up to 1700 "C (as measured on a Mikron Instrument Co. Model M77S optical pyrometer) over a 45-min period. A multisample turret afforded ramping in a preheat position during analysis of another sample, which then allowed more rapid warm-up in the ionization position. Heating from 1700 "Cto running conditions (1850-1920 "C) was performed over 15-20 min, with frequent monitoring of isobars and 238U;much of this temperature rise did not require raising the filament current. Analysis began when desired intensities were obtained, isobars were barely above or in the background noise, and the 23821 signal was 1 is nearly ubiquitous among young volcanic rocks, suggesting that Pa is generally more incompatible than U in magmatic systems. This points to potential utility in studies of the timing and character of processes of melt production, transport, and evolution. We are also exploring the possibility of age dating of volcanic rocks via a 231Pa-235Uisochron. Lacking a stable isotope of Pa, this method would require the identification of an element that behaves analogously to Pa in a crystal-melt system. This approach has been successful for 22aRausing Ba as an analogue, as first suggested by Williams et al.31
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ACKNOWLEDGMENT This work has benefitted from our interactions with S. J. Goldstein, R. E. Perrin, and D. J. Rokop. The samples in Table 1 were provided by N. Sturchio. G.G. Miller assisted with 237Npmeasurements. This work was supported by the Department of Energy's Office of Basic Energy Sciences. Received for review October 22, 1993. Accepted January 11,
1994." *Abstract published in Aduance ACS Abstracrs, February IS, 1994.
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