Anal. Chem. 1992, 64, 469-475
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Three-Color Resonance Ionization of Titanium Sputtered from Metal and Oxides for Cosmochemical Analyses: Measurements of Selectivity and Isotope Anomalies D. R. Spiegel,**tp*W.F.Calaway,' A. M.Davis,t J. W.Burnett,' M.J. Pellin: S.R. Coon,* C.E. Young,* R.N. Clayton,' and D. M. Gruen*
Enrico Fermi Institute, The University of Chicago, Chicago, Illinois 60637, and Materials SciencelChemical TechnologylChemistry Divisions, Argonne National Laboratory, Argonne, Illinois 60439
l h r e resonance lonlzatbn mass spectrometry (RIMS) measurements of sputtered TI have been carrled out on several oxklee contslnlng both TI and (isobarically htetfedng) Ca In order to evaluate mkrobeam RIMS as a tool for Isotopic analyses of meteoritic sampler. The elemental selectlvitles of Ti over Ca, aR measured at saturating laser lnterwlties, are In the range of 100 f 20 to 500 f 150, with the largest selectivity obtained In hibonite. We have also carrled out experhents on TiO, and Ti metal. RIMS measurementsof Ti isotope r a m are reported for each material. We Hnd that the odd ratios (47148,49148) are conststentty enhanced over standard values by as much as 48%, whlle the even ratlor (46/48,50/48) are much darer to their standard values. Thls odd-even effect probably a r k s from differences In laser lonltatlon effkkncles due to hyperfine structure and should not hinder RIMS searches for Mope anomalles if lsotoplc abundance8 are measured In both an unknown and a standard. Finally, we have tested the precldon and sensitivity of the SARISA apparatus for meteorltk appllcatbns by udng a threecolor RIMS scheme to measure Ti isotope anomalks In 664,a hlbonite-rkh Inclusion taken from the Murchlson meteorite. A '"Ti detlclt (previously observed In SIMS studies) and a neutron actlvatlon-imluced'on e x c o are readHy measurable by the SARISA technique. Less than 5 X 1O1O TI atoms were removed from the 66-5 sample during the acquldtlon of the RIMS data.
INTRODUCTION Recently there has been increased interest in the use of resonance ionization mass spectrometry (RIMS) for isotopic analysis.'-'O Due to its elemental selectivity, RIMS has the potential to eliminate many of the isobaric interferences that often plague conventional mass spectrometry without necessitating difficult chemical separation procedures or the use of low transmission, high mass resolution instruments." Meteorites comprise one of the most intriguing potential applications of RIMS,12 where anomalies in the isotopic abundance5 of many elements can serve as tracers of stellar n~cleosynthesis.'~Such anomalies are often difficult to measure using conventional techniques because of ubiquitous isobaric overlaps in chemically complex samples. The elemental selectivity of RIMS offers a possible solution to the interference problem; there are, however, several additional criteria which a RIMS apparatus must fulfill if it is to prove useful in this endeavor. These include (i) microprobe capaThe University of Chicago. *SPreaent Argonne National Laboratory. address: Department of Physics, Trinity University, San
Antonio, TX 78212.
0003-2700/92/0364-0469$03.00/0
bilitiea, since many of the largeat anomaliea are localized within grain^ or inclusions with sizes leas than 100 Mm; (ii) the ability to analyze insulating materials, (iii) sufficient sensitivity for isotopic analyses of minor elements; (iv) sufficient mass resolution to resolve the isotopes of the element of interest; and (v) good precision. Meteoritic isotopic anomalies rarely exceed a few percent,13J4although anomalies of tens of percent and larger have been reported in recent studies of interstellar grains.', The present writing is intended to demonshate that all of these criteria can be met in a properly designed resonance ionization experiment and that isotopic anomalies can indeed be measured using a RIMS apparatus. We present here the results of RIMS measurements of titanium atoms sputtered from pure Ti metal as well as from several naturally occurring oxides: perovskite ((Na,Ca,Fe)(Ti,Nb)03,[Ti] = 15.4at. %, [Ca] = 18.9 at. %I, from Magnet Cove, AR, sphene (CaTiSiO,, [Ti] = 12.0 at. %, [Ca] = 12.4 at. %) from the Tilly Foster Mine, Brewster, NY;hibonite (CaAllz_zlTixMgx019, [Ti] = 1.2 at. %, [Ca] = 2.6 at. %) from Esiva, Madagascar;and rutile (Ti02) from Our0 Preto, Minae Geraes, Brazil. We have chosen Ti because of the considerable importance of Ti isotope measurements to cosmochemistry: Ti isotopic anomalies of nucleosynthetic origin occur in refractory oxide inclusions within stony rneteorite~.'~J'j-~~ These inclusions invariably also contain calcium, which interferes with Ti at mass numbers 46 and 48. (The Wa/46Ti and 4aCa/"LBTimass differences are approximately 23 and 96 ppm, respectively). The work was carried out using the Surface Analysis by Resonance Ionization of Sputtered Atoms (SARISA) apparatus at Argonne National Laboratory.lg The SARISA instrument employs tunable lasers to resonantly ionize neutral atoms of a preselected element sputtered by a 3.6-keV primary ion beam, and detects the photoions with an energy-and-angle-refocussing time-of-flight (EARTOF) mass spectrometer. To maximize the selectivity of Ti over Ca, two separate resonance ionization schemes were investigated, each of which used three tunable lasers to pump Ti atoms into an autoionizing resonance. After using the terrestrial samples to quantify systematic effects in RIMS measurements of Ti isotopes, we employed a three-color RIMS scheme to measure the deficit in BB-6, a hibonite-rich inclusion ([Ti] = 0.5 at. %) taken from the Murchison meteorite. To quantify the utility of a RIMS experiment for a particular isotopic analysis, there are several parameters which must be explored and understood." Special consideration should be given to the inherent trade-offs involved in optimizing these parameters. (a) Laser Ionization Efficiency. The precision of a RIMS measurement depends on the sensitivity of the apparatus to the analyte, which in turn depends on the degree of ionization efficiency provided by the lasers. A saturated ionization signal is highly desirable, since in this case the sensitivity is max0 1992 Amerlcan Chemlcal Society
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ANALYTICAL CHEMISTRY. VOL. 64. NO. 5. MARCH 1, 1992
imized and the signal is relatively insensitive to fluctuations in laser power." Increasing the laser intensity to drive the analyte ionization into saturation,however, results in increased nonresonant ionization of other species.21,*2We will present results of saturation studies on each of the three steps employed for Ti ionization. (b) Elemental Selectivity. Resonance ionimtion schemes are designed to ionize only a single analyte. However, nonresonant ionization of other species, including isobars, invariably enters as a background noise souree.n The selectivity S(X1Y) for the detection of an analyte X in the presence of an isobarically interfering species Y is defined as S(X1Y) =
(signal of
.........................
< .......... Sample : : Chambsr:
Threc-color ~
~
ExCimer-Pumped Dye tam, system
...........................
I Y \
X)/[X]
(signal of Y)/[Y]
(1)
where [XI and [Y] are atomic concentrations. The selectivity depends on the species X and Y and their chemical state within the sample, and on the resonance ionization scheme Selectivities are generally observed to increase when the number of resonant steps used to produce the ionization is increased, since the lower energy photons are in general less likely to cause nonresonant ionization of the interfering species.",n,u We have measured the selectivity of Ti over Ca in perovskite, sphene, and hibonite. (c) Precision and Accuracy of Isotope Ratio Measurements. We have measured Ti isotope ratios on each oxide and Ti metal. The precision relevant for meteoritic isotopic analyses has been tested through measurements of isotopic anomalies in a meteoritic inclusion. Several authors have reported accurate isotope abundance measurements using In these measurements, either the isotope ratios ohtained using RIMS agree with accepted values determined from thermal ionization techniques or differences between measured and standard ratios follow a simple mass fractionation pattern. Others, however, observe a biasing in ratice obtained by RIMS attributed to differenma in the laser ionization cross sections for different isotopes. Fairbank et d.,3 for example, have shown that odd-even isotope biasing may persist in tin and molybdenum a t saturating laser powers and were able to invert the sign of the biasing in Mo hy changing their dye laser wavelength. Miller et aL8 observed wavelength-dependent biasing on the order of 5 1 0 % in Lu, and Young et a l ? O observed dd-even effecta in Nd. Theoretical explanations of isotopic biasing due to resonance ionization have been given recently by Lambropoulos and Lyras" and hy Whitten and Ramsey." It is clear that one cannot assume a priori that a RIMS measurement of isotope ratios will be accurate. Systematic isotopic biasing, however, is not necessarily detrimental to RIMS measurementa of isotopic anomalies since a standard material can be employed as a reference. We will present our RIMS measurements of sputtered Ti in the context of the parameters outlined ahove. Titanium is particularly well-suited for an application of RIMS to isctopic analysis, since, in addition to its importance in cosmochemistry, Ti has five adjacent stable isotopes,and no isotope is less abundant than - 5 % , so that the relative peak intensities in the m w spectra can be studied over a convenient dynamic range. EXPERIMENTAL DETAILS Apparatus. The SARISA IV instrument employed for this thus, we emphasize here work has been described previo~sly;'~~~~ the details most relevant for the present study. The apparatus, shown in a schematic diagram in Figure 1, consists of four major components: the sample chamber, the primary ion source, the three-color laser system, and the time-of-flightdetection system. (i) Sample Chamber. The ultra-high vacuum chamber employed for these studies has a base pressure of less than 5 X 10-"O
~
~
Enarw-and-Anol. Relocusslnp Time-ol-FIIohl Oeta~tor
(EARTOF) .................................. ~
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Flgure 1. Schemalk diagram of UW Surface Analysis by Resonance Imizalbn Of Sputtered A t m (SARISA) apparaw used fw UW RIMS studies reported in this paper.
Torr. The samples reside on an XYZ-transhtion/rotation stage; the stage is also equipped with a Faraday cup to facilitate alignment and size measurement of the primary ion beam. (ii) Pulsed Primary Ion S o w . The primary 'OAit ions are generated at an initial energy of 5 keV by a Colutron ion source mass-filteredwith a Wien velocity selector. A set of deflection plates is used to direct the beam through a series of additional apertures and Einzel lenses onto the sample, with a reaultant iinal spot size of 1W200 pm (FWHM) at a current of 1.7 pA. A 0.5or l.0-r~pulse of primary ions is created by pubing the voltages applied to two seta of deflection plates, thereby sweeping the primary ion beam acrw the apertures. The alignment, foeuasing, and pulsing controls of the primary ion beam are interfaced to a computer. (iii)ThreeGolorExcimer-Pumped Dye Laser System. The laser system consisted of a 3Wnm XeCI excimer laser (Questek 2660) with an output puke energy between 140 and 2M) mJ. The excimer beam was split into three portions to pump three dye lasers: two manufactured by Lumonics (HD300) and one by Molectron (DL18/18P). The bandwidth of the Lumonica lasers was measured at 0.07 cn-', while that of the Moleetron is speeifed at 0.3 cn-'. The larger-bandwidth Moledron dye h r was used for the final (ionization)step since the autoionizing absorption bands are expected to be wider than the bound-state resonance lines." The laser pulse lengths were about 20 ns. Optical delay lines were added when necessary 80 that the three pulses arrived simultaneouslyat the sample chamber to within 5 118. The phaw of the excimer laser pulse timing was locked to within 20 ns by using an optical feedback circuit (Questek 9200) to automatidy adjust the laser trigger whenever required for compensation of temporal drift. The achievement of an optimal laser ionization volume is an important criterion for a RIMS devi~e.'~,~ Large laser volumes are desirable from the standpoint of maximizing the number of photoionized atom and insuring that the RIMS efficiency is not velocity- or angle-discriminative. On the other hand, focussing to a small volume makes it easier to achieve saturation and improves the mass resolution by minimizing the peak-broadening due to the accelerationvoltage gradient a c r m the volume. We uaed cylindricallenses and redangular apertures to shape the three laser beams to a cross section of approximately 1mm X 3 mm, as shown in Figure 2. The distance from the sample to the center of the laser beams is about 1 mm. Operating voltages and placement of photoion lens element8have been chosen to minimize the voltage drop across the laser volume without sacrificing transmission efficiency, in order to minimize the energy spread of the accelerated photoions. It has been estimated that for this geometry,19" given a Sigmund-Thompson distribution of sputtered neutrals,= approximately15% of the atom sputtered during a 500-ns ion pulse are contained within the laser volume at an
ANALYTICAL CHEMISTRY, VOL. 64, NO. 5. MARCH 1. 1992
649.88 nm
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Flgure 2. Orientation of the laser lmlzatlon volume wHh respect to the sample surface and the primary ion beam In the SARISA lnstru. ment. The &&n of laser popagaijm is pspn6cular to the page. optimum laser firing time. The correct laser timing was determined hy maximizing the Ti signal from Ti metal. The dye laser wavelengths were calibrated over the 501-703-nm region as follows. First, a Perkin-Elmer spectrometer (Model El, 450 nm blaze) was calibrated using the tahulated strong Ne and Ti lines of a Ti hollow cathode discharge lamp. The spectrometer was then used to measure the wavelengths emitted from the dye lasers. This calibration procedure allowed wavelength measurements with an estimated uncertainty of *1 em-'. (iv) Energy-and-Angle-Refocussing Time-of-Flight (EARTOF) Detection System. The design of the energyand-anglerefocussingtimeof-flight (EARTOF) detection system of the SARISA apparatus (Figure 1)permits a photoion transmission on the order of 5-50%'2b.22bwhile also providing highly efficient rejection of secondary ions. The use of two resistive disk analyzers, which function as high-transmission l/r-energy fdters, generates a focussing of the incidentenergy spread without serious loss of signal. The correct timing of the primary ion pulse, laser pulse, and target potential are crucial to the operation of the EARMF. During the t i e that the primary ions strike the tarpet, the latter is held at 1370 V; thus the primary ions are decelerated from 5 keV to 3630 eV. Secondary ions, created at the target potential of 1370 eV, are rejected by the resistive disk analyzers which are tuned to a handpass of 1.0 0.1 keV. At the end of the primary ion pulse, the target potential is lowered to 1070 V, which sets the potential in the laser volume (Figure 2) to about 1kV. The lasers are fired approximately 200 ns after the end of the primary ion pulse, and the photoions are accelerated down the time-of-flight path and focussed by Einzel lenses onto the resistive disk analyzers. The total photoion flight path length is approximately 1m. T h e current pulse induced on the channel plates is preamplified and recorded temporally (5- or IO-ns bin width) with a LeCroy 8828C or 8828D 8-hit transient digitizer. The LeCroy digitizer has a guaranteed input bandwidth of 120 MHz and a minimum of 5.9 effective hits for the digitization of a IO-ns sine wave.= The temporal response is transferred to a microcomputer, and the timing sequence is repeated for the next laser pulse. The software allows collection and averaging over an arbitrary number of laser pulses. Samples. In order to avoid sample charging it is necessary that the target surface be electricallyconducting. Two methods have been used to mount insulatingsamples. In the first method, chips of the four oxides with lateral dimensions of several millimeters and a piece of high-purity Ti metal were potted in a UHV-compatible epoxy in a '/8-in.-thick, 5/lcin.-diameter stainleas steel ring, polished using standard techniques, and coated with a ZOO-A layer of carbon to give a conducting surface. The sample was then glued to a stainless steel post using conducting epoxy, and this post was mounted within the SARISA chamber. In the second method, a piece of gold foil was pressed onto the roughened surface of a stainless steel post for mounting in the chamber, A grain of hibonite with approximate dimensions of 250 x 350 pm was pressed directly into the gold foil; no conducting coating was applied in this case. A second hibonite grain (300 X 350 pm) and the meteoritic sample BB-5 (80 X 110 pm) were ala0 preased into gold foil. Finally, we employed a 5/,6-in.-diameterdisk of pure Ti metal for saturation studies and as a control sample to tune the SARISA instrument (laser alignment and wavelength adjustment) at the beginning of each day. Design of Laser Excitation Schemes. Two resonance ionization schemeshave been investigated for titanium and are shown schematically in Figure 3. The wavelengths of the hound-to-
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hound resonances were obtained from standard spectroscopic tables?" The autoionizing states, however, are in general not known and were therefore obtained experimentally. In both schemes, the first laser ('step 1")was used to excite sputtered Ti atoms from the a3F2ground state to the z3F2"state at 19323 The second laser ('step 2") was employed to populate one of two excited hound states: g3F2at 41872 em-' in scheme I or e3F2at 37539 ur-' in scheme n. A third laser ("step 3") was then used to pump the excited populations into autoionizing resonanca above the ionization potential (55010 m-'). By scanning the third laser and monitoring the Ti signal, a number of autoionizing transitions in the energy regions 57231-57688 em-' and 55052-55871 em-' were obtained. The energies, widths, and relative intensities of these resonances have been described elsewhere?l The strongest lines in each region appeared at 57255 & 1and 55192 i 1em-' and subsequently were used as ionizing steps in the two resonance schemes shown in Figure 3. We note that since Ca has an ionization potential of 49306 cm-'?2 nonresonant ionization of ground-state Ca atom requires a minimum of three photons of any color for either scheme. Before implementing either scheme I or scheme 11, we undertook a careful search of the relevant spectrmcopic tables to confirm that none of the wavelengths employed could excite near-resonances in C a RESULTS AND DISCUSSION We ohserved that the sektivity of Ti over Ca in perovskita was 10 1for scheme I and 100 10 for scheme 11. From these results it was concluded that the second scheme was superior, and scheme I1 was employed for all subsequent studies. Thus, the results detailed helow were all obtained using scheme 11. Figure 4 shows a typical timeof-flight RIMS maas spectrum for Ti sputtered from carhon-coated Ti metal embedded in epoxy. The spectrum has been normalized to 4 T i = 1. All
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Flguro 5. Laser energy saturation curves for Ti for each of the three steps employed in resonance ionization scheme I 1 of Figure 3. (a) 48Ti signal level as a function of the pulse energy. (b) Signal level for each of the minor Ti isotopes (mass 46, 47, 49, and 50). The unRs used for the signal level are the same for each of the plots in a and b. signals were obtained at different channel plate gain settings; five isotopes are clearly resolved; the FWHM mass resolution thus, the signal has in each case been normalized to the signal at mass 48 is about MIAM = 130. The peaks of the timeobtained in Ti metal (Figure 4) by dividing by the appropriate of-flight spectra in the relevant mass region have a full-width gain factor. That is, the absolute signal levels in Figure 6 are at 5% maximum of approximately 110-120 ns. Thus for all referenced to the value '@TimeM = 1. No corrections were made time-of-flight spectra, 23 channels at 5 ns/channel were for differing sputtering yields or Ti concentrations. The essummed to obtain peak integrals. The postsignal base line timated errors in the normalization procedure, due to the consistently displays a negative offset, relative to the presignal uncertainty of the channel plate gain calibration, are f20% base line, equal to about 0.5% of the maximum signal. This (perovskite and sphene), *45% (hibonite), and f10% (rutile). offset, probably due to capacitive overshoot in the preamFor each of the oxides, the nonresonant signal taken with plifier, is reproducible from run-to-runand day-to-day. The step 1 detuned (AX1 = -0.1 nm) was also obtained. The offset has a negligible effect on measurements of selectivity resonant and nonresonant spectra can be used together to and laser saturation but is important for the determination measure the selectivity. We have confiied that the Ca signal of Ti isotope ratios. A base-line correction was therefore does not change for a detuning of f0.5 nm about each of the obtained by interpolating linearly between the pre- and three resonances employed. Therefore, for the perovskite and posts&al base lines in all isotope ratio measurements. Fitting sphene samples, the Ti signal was obtained from the resonance the base line to higher order had negligible effects on comspectrum (AX1 = 0), while the Ca signal was obtained from putation of the isotope ratios. the off-resonance spectrum (AX1 = -0.1 nm) taken a t higher Saturation Studies. The saturation curves for each of the gain. The main source of error for the selectivity measurecolors used in scheme 11are shown in Figure 5. An uncoated menta in the perovskite and sphene is the uncertainty in disk of Ti metal was employed as the sample for these meachannel plate gain calibration. In the carbon-coated hibonite, surements. The laser pulse energy in each case was attenuated the gain required to measure the nonresonant signal resulted using neutral density filters. While the laser powers were being in partial saturation of the channel plates. In this case, the readjusted between data acquisitions, the Ti sample was selectivity was therefore measured by obtaining both the Ti raster-cleaned over a 1-mm2area. To insure that the satuand Ca signals from the resonance spectrum (Figure 612). The ration curves were not affected by instrumental drift, the measured selectivities were 100 f 20 (perovskite), 200 f 40 profile for each color was measured twice: fvst in the direction of decreasing power, and then with increasing power. The (sphene), and 500 f 150 (hibonite). Isotope Ratios. Ti isotope ratios for each of the four oxides saturation curves for increasing and decreasing powers were and Ti metal were determined by measuring the ratios obnot measurably different. It is clear that the bound-to-bound tained from several consecutive spectra. Typically, we obresonance steps 1 and 2 are easily saturated for all five Ti tained five consecutive RIMS spectra, with each spectrum in isotopes. The autoionizing step (step 3) ale0 saturates, albeit turn consisting of an average over 200 laser pulses. Thus at at a somewhat higher laser power. We conclude that the Ti ionization can be saturated without difficulty using the the laser repetition rate of 23 Hz, the acquisition of these data takes less than 1 min. Our results are summarized in Table threetunablecolor laser system. Saturation powers were used I. The precision reported is 2umem,and the accuracy of each for each of the three colors in all measurements reported ratio is given as a deviation from the expected value taken below. Selectivity. Figure 6 displays the RIMS (scheme 11)mass from Niederer et al.33 All samples were assumed to have this isotopic composition; no variations in Ti isotopic compositions spectra obtained from the four Ti-containing oxides. The
ANALYTICAL CHEMISTRY, VOL. 64,
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NO. 5, MARCH 1, 1992 473
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Table I. Ti Isotope Ratios in Several Materials (*2a-) material
6rsTi"
6"Ti"
6'gTi"
66oTi"
354 f 23 303 f 13 240f 26 285 f 17 325 f 4
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perovekiteb 39 f 11 spheneb -2 f 21 -5 f 29 hibonite' -29 f 8 rutileb Timetald -15 f 9
10 f 20 48 f 41 43 f 19 -17 f 8
"$Ti(%) = [cTi/qi)-/cTi/rsTi)n,,m - 11 X 1000. The (iTi/ qi),, values, taken from Niederer et al.," are (rsTi/qi) = 0.10855, ("Ti/qi) = 0.099315, (.reTi/'BTi) = 0.074463, and ( W T i / q i ) = 0.072418. bFivoconsecutive spectra taken with 200 laser pulses per spectrum. 'Four consecutive spectra taken with 2000 laser pulses per spectrum. dThreeconsecutive spectra taken with 2000 laser pulses per spectrum.
in terntrial rocks have been reported. A clear odd-even effect is evident from Table I, in which large enhancements are seen in the odd isotope signals when normalized to the 4eTi signal. The 48TiI48Ti and 5oTi/48Ti ratios, on the other hand, are generally much closer to the expected value. As discussed in the Introduction, odd-even biasing of isotopes has been reported previously in RIMS, and is generally attributed to differences in laser ionization efficiencies due to hyperfine structure in the odd-mass-numberisotopes. We believe this to be a plausible explanation of the large odd-even biasing displayed by our data;positive identification of this cause must be confiied by future experiments. It should be pointed out that the isotopic biasing displayed by our RIMS apparatus and others does not prevent the use of resonance ionization in the search for isotopic anomalies, so long as an isotopic standard is employed as a reference in such experiments, and the bias effect is the same in the standards and the samples. Clearly the appropriate isotopic standard in a RIMS measurement will have a chemical composition which is as close as possible to the meteoritic sample. We have found that the signal level from hibonite is substantially stronger for samples which are pressed into gold substrates rather than mounted in epoxy, polished, and car-
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Figure 7. Resonance ionkatbn mass spectrum obtained from a hibonite grain (250 X 350 pm) pressed into gold.
bon-coated. Figure 7 shows the signal obtained from a hibonite grain approximately 250 X 350 pm in diameter pressed into gold foil. The signal has again been nonnalized to % = 1;the uncertainty in the normalization is f30%. We note that the normalized signal level is equal within error to the atomic Ti concentration in the hibonite. (The 39Kevident in Figure 7 was introduced as a filament contaminant after installation of a new ionization source.) The selectivity of Ti over Ca in the gold-substrate sample is 500 f 150, the same as that obtained for the carbon-coated hibonite, but the signal level is greater by approximately a factor of 10. The reason for the difference in signal level is not known;however, it is possible that the carbon-coated samples are susceptible to carbon-induced surface chemistry. Sample charging is an unlikely explanation for the observed difference since (1)the amount of insulating surface area exposed during ion bombardment of the carbon-coated hibonite is leas than the surface area of the uncoated 250 X 350 pm hibonite grain pressed into gold, and (2) the optimum extraction voltages for an uncoated disk of Ti metal and a test sample of carbon-coated perovskite embedded in epoxy were not significantly different. As a teet of the SARISA apparatus for Ti isotopic analysis of meteoritic samples, we analyzed a sample of the hibonite-rich inclusion BB-5 from the Murchison meteorite using the three-color scheme I1 shown in Figure 3. Previous SIMS
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ANALYTICAL CHEMISTRY, VOL. 64, NO. 5, MARCH 1, 1992
Table 11. Ti Isotope Ratios in the Meteoritic Sample BB-5, Normalized to the Ratios in Madagascar Hibonite" (*2umsan) 6'8Ti"
SIMS result 0 SIMS result, +1.4 correctedb SARISA I-10.2 RIMS
647Ti0
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* 7.5
* 1.9 * 1.9
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I-2.3 f 6.9 +16.0 f 9.6 -57.6 f 7.4
"The SIMS data are taken from Fahey et al.34 The SARISA data were collected using RIMS scheme I1 of Figure 3 with no corrections for mass fractionation. All numbers are in parts per thousand. GiTi(%)= [('Ti/4~i),,,,/(iTi/4BTi)ler,BV- 11 X lo00 as described in the text. bThe 6 values have been corrected by calculating the changes in the BB-5 isotope ratios due to a thermal neutron fluence of 2 X 1020neutrons/cm2received during instrumental neutron activation analysis. analyses have shown that this inclusion has a 5oTi deficit of - 7 0 5 ~ 0 . ~ Our ~ 8 ~BB-5 ~ sample was elliptical and had dimensions of approximately 80 X 110 pm; the Madagascar hibonite standard was roughly triangular with dimensions of approximately 300 X 350 pm. BB-5 and the Madagascar hibonite were pressed into a piece of gold foil such that they were separated by a center-tocenter distance of about 750 pm. The experiment consisted of a series of measurements of mass spectra obtained alternately from the standard and BB-5 by translating the sample stage so that the ion beam probed either the sample or the reference. The comparison between an isotope ratio iTi/48Tiin BB-5 and the standard was then expressed using the relation
where ('Ti/48Ti),ef,aV = 1/z[('Ti/48Ti)before + ('Ti/48Ti)dk,] is the average of the ratio iTi/48Ti in the Madagascar hibonite reference obtained immediately before and after collection of data from the BB-5 sample. Each spectrum was obtained from the average of signals generated by 20 000-40 OOO pulses at a repetition rate of 67 Hz. For these studies, the primary ion beam (1.7 pA) was focussed to a diameter of about 100 pm. From a comparison of signal levels obtained from BB-5 and the Madagascar reference, we estimate that approximately three-fourths of the ion beam cross section was incident on the BB-5 sample. We collected 11 mass spectra from the reference, interleaved with 10 mass spectra from the sample, for a total of 10 measurements of 6('Ti/48Ti) (i = 46,47,49, 50). During the 9th and 10th measurements, the laser power was low due to dye degradation, resulting in a lower signal level for these last trials. The channel plate gain was raised in an attempt to compensate for this signal level decrease; however, subsequent analysis revealed that 6 values obtained during these final two trials were well outside the spread (~IJ,,,) of the 6 values obtained during the first eight trials. The observed variation could be due to the loss of saturating laser intensities because of dye degradation, or to nonlinear channel-plate response introduced a t the higher gain, or both. Thus only the first eight trials have been included in the averaging. Error bars (2umem)are deduced from the variance of the 6 values over these eight trials. Our results for BB-5 are presented in Table 11, along with SIMS analyses. Our sample of BB-5 had been previously used for neutron activation analyses and received a neutron fluence of -2 X lozoneutrons/cmz. The Ti isotopic composition reported by Fahey et al.%was corrected for this neutron dose using thermal neutron capture cross sections from the litera t ~ r e The . ~ ~uncertainties in the neutron cross sections are not known but are believed to be minor compared to uncer-
tainties in the SIMS data. The main effect of the neutron irradiation was to raise 649Tiby -20%. The 50Ti deficit originally in BB-5, evidenced as a -15um,, anomaly, is clearly displayed in the data collected on the SARISA instrument. The smaller i4@ 'I excess caused by neutron irradiation is also evident in the SARISA data. One of the more impressive features of a sputter-based RIMS system is the small amount of sample removed for the measurement of isotope ratios. In order to estimate the amount of meteoritic sample consumed in our measurements of isotopic anomalies in BB-5 during data acquisition, we will assume that the sputtering yield (Y) of hibonite-rich BB-5 under 3.6-keV Ar+ bombardment is close to the sputtering yield of A120,under 5-keV Ar+ b ~ m b a r d m e n t :Y(A1203) ~~ = 0.9 (AZO3units/incident ion). Given the three-fourths of the SARISA primary ion pulse (1.7 PA, 1ps) incident on the BB-5 sample, this corresponds to a removal of about 3.7 x lo7atoms from BB-5 per pulse. Thus for the 2 X lo5 pulses used to acquire data from BB-5, a total of approximately 8 X 10l2 atoms (hence about 4 X 1O'O Ti atoms) were removed for data acquisition. It seems clear from these studies that sputterbased RIMS will prove most valuable in meteoritic applications in which very efficient sample usage is m a n d a t ~ r y . ~ ~ ! ~ ~
OUTLOOK AND CONCLUSIONS Our studies indicate that RIMS can be useful in the analysis of Ti atoms sputtered from oxides. The major results may be summarized as follows: (A) Selectivities of Ti over Ca in excess of 100 (approximately 500 in hibonite) can be obtained with the proper choice of laser excitation scheme and should prove useful for measurements of low-abundance 'I'i in geochemical or cosmochemical analyses. Given the selectivity of 500 f 150 demonstrated here for hibonite and assuming normal isotopic compositions for Ti and Ca, the *Ca/48Ti interference will not reach the 10% level until the ratio [Ti]/[Ca] of atomic concentrations is reduced to (5.0 f 1.5) X In evaluating the usefulness of the measured selectivities it is helpful to consider a "worst-case scenario": Melilite (Caz[MgSi,A12]Si07) probably has the lowest [Ti]/[Ca] ratio among minerals of cosmochemical interest, with about 25% Ca and 100 ppm Ti by weight. Assuming terrestrial isotopic abundances, selectivities S[TilCa] of 100 and 500 would reduce the *Ca/48Ti isobaric interference to about 8% and 1.5%, respectively. By contrast, since the sputtered ion yield of Ca from oxides under 0- bombardment is about a factor of 10 greater than that of the *Ca/48Ti signal level in melilite will be about 80 in a typical SIMS measurement. (B) The Ti ionization can be saturated without difficulty in a three-color autoionizing laser scheme, and the selectivities reported were all obtained at saturating laser intensities. The saturation and selectivity data taken together demonstrate the power of multiple-color resonance ionization: the laser ionization of the analyte is saturated; at the same time, competing species are efficiently rejected. (C) The Ti isotope ratios display a strong odd-even effect. While it is crucial that further work be undertaken to understand isotopic biasing effects in RIMS, it is clear that standards can and must be employed in RIMS searches for isotopic anomalies. (D) Ti isotopic anomalies have been measured in a hibonite-rich meteoritic inclusion (BB-5) using three-color RIMS. A SOTi-deficit observed in previous SIMS measurements and a neutron activation-induced 49'i excess have been measured by the SARISA apparatus through repeated comparison of the Ti isotope ratios in the meteoritic sample to ratios in a terrestrial hibonite standard. Precisions of 1%or better were obtained for all ratios. The measurement was carried out with removal of less than 5 X 1O'O Ti atoms and a total removal
ANALYTICAL CHEMISTRY, VOL. 64, NO. 5, MARCH 1, 1992
of about 8 X 10l2atoms from the inclusion during data acquisition.
ACKNOWLEDGMENT We thank James Whitten and George Lamich for invaluable assistance. This work was supported by NASA grants NAG 9-51 and NAG 9-111 and U.S. Department of Energy BES Materials Sciences contract W-31-109-ENG-38. S.R.C.acknowledges a graduate fellowship from the National Science Foundation.
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RECEIVED for review June 3, 1991. Revised manuscript received November 25, 1991. Accepted December 3,1991.
