A linear time-of-flight mass analyzer for thermal ionization cavity mass spectrometry

Spectrochimica Acta Part B 56 Ž2001. 1175᎐1194

A linear time-of-flight mass analyzer for thermal ionization cavity mass spectrometry 夽 David M. Wayne a,U , Wei Hang b, Diane K. McDaniel a,1, Robert E. Fields b, Eddie Rios c , Vahid Majidi b a

Nuclear Materials and Technology Di¨ ision, Pit Disassembly and Sur¨ eillance Technologies, NMT-15, Los Alamos National Laboratory, Los Alamos, NM 87545, USA b Chemistry Di¨ ision, Analytical Chemistry Sciences, C-ACS, Los Alamos National Laboratory, Los Alamos, NM 87545, USA c Nuclear Materials and Technology Di¨ ision, Actinide Ceramics and Fabrication Group, NMT-9, Los Alamos National Laboratory, Los Alamos, NM 87545, USA Received 5 September 2000; accepted 14 March 2001

Abstract In this paper we report the basic design characteristics, typical operating parameters, and isotope ratio performance of an orthogonal acceleration linear thermal ionization cavity time-of-flight mass spectrometer ŽTIC-TOFMS.. The present system is capable of mass resolution of 750᎐850 ŽFWHM. over a wide range of masses, and can generate and analyze multi-element spectra from sub-␮g samples Žsolids and solution residues. in - 30᎐45 min. The optimum precision Ž1␴ . of isotope ratios determined from 60᎐80 spectra Žeach the average of 600 individual spectra. is 0.2᎐0.4% R.S.D., and is limited by the instrument drift, dead time and the data acquisition and processing capabilities of the 8-bit digital oscilloscope used to collect the data. Isotope ratio accuracy Ž1␴, per mass unit. for major isotopes is typically - "1.0%. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Thermal ionization cavity; Thermal ionization mass spectrometry; Time-of-flight mass spectrometry ŽTOFMS; Isotope ratio; Precision; Accuracy

夽

This paper is published in the Special Issue of Spectrochimica Acta Part B dedicated to the late Professor Velmer A. Fassel. Corresponding author. Tel.: q1-505-665-7552; fax: q1-505-665-5982. E-mail address: [email protected] ŽD.M. Wayne.. 1 Present address: University of Maryland, Department of Geology, College Park, MD 20742-4211, USA.

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0584-8547r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 5 8 4 - 8 5 4 7 Ž 0 1 . 0 0 2 0 7 - 5

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1. Introduction Thermal ionization mass spectrometry ŽTIMS. is the definitive analytical method for precise and accurate determinations of isotopic compositions w1x. Combined with isotope dilution, TIMS also provides precise and accurate quantification of ultra-trace Ž- ppb. analytes. Thermal ionization is most efficient for elements with low to moderate first ionization energies Ž- 7 eV., which can be volatilized at the source operating temperature Žtypically 1500᎐2000⬚C.. Thermally generated ion beams are extremely stable and long-lived. Ion beam stability permits the acquisition of highly precise and accurate data Ž- 0.01%. by magnetic sector mass spectrometry. Thermal ionization MS is exceptionally sensitive, and precise measurements can be performed on exceedingly small samples Ž- 10 pg.. As a result, TIMS is the method of choice for geochronology, environmental tracer studies, and various nuclear applications. There are limitations to TIMS analysis. Not all elements in the periodic table are thermally ionizable, although some Že.g. Mo, Re, Os, etc.. are thermionizable through the use of demanding sample preparation procedures, or by negative ion mass spectrometry w2᎐5x. Although thermal ion beams are easily and reproducibly obtainable from certain raw particulate samples Že.g. zircon. w6᎐8x, most samples intended for TIMS analysis must undergo painstaking preparation, dissolution and purification in a clean environment to reduce contamination of the mineral-borne isotopic signature. Isotope ratio accuracy and precision is optimal if data are collected in low-resolution mode, which may lead to potential spectral interference from isobaric species. Therefore, single elements, or groups of chemically similar elements Žsuch as the rare earths., must be extracted via ion exchange using specially prepared clean reagents and ion exchange resins. Several sequential extractions may be required for the determination of multiple elements from a single sample Že.g. U, Th, and Pb from zircon.. Recent advances in high-resolution, double-focusing magnetic sector inductively-coupled mass spectrometry ŽICPMS. w9,10x, glow discharge mass spectrometry

ŽGDMS. w11,12x, and multi-collector ICPMS w13᎐15x have addressed some of these complications. Although the precision of isotope ratios obtained using multi-collector magnetic sector ICPMS approaches that of TIMS w13᎐15x, severe Ž; 1%. mass bias due to space᎐charge effects w16,17x must be corrected using fractionation factors determined from an internal standard of similar mass Že.g. Tl for Pb measurements. w13᎐15x. Another limitation to TIMS analysis, for some elements, is low ionization efficiency. Conventional TIMS ion sources utilize a flat metal Žusually Re, Ta or W. ribbon which can be resistively heated to ) 2000⬚C in a vacuum. As the sample vaporizes, gaseous atoms interact with the hot metal surface and are ionized. Elements that readily volatilize, and have low first ionization potentials, are ionized most efficiently. For refractory elements, such as Zr, Hf, Th, and U, on tantalum, tungsten or polycrystalline rhenium surfaces, ionization efficiency is typically less than 0.25% unless additional measures are taken. These consist of special chemical preparation and filament modification techniques aimed at reducing sample volatility, increasing the work function of the ionizing surface, or increasing the number of opportunities for ionization following sample vaporization. Established techniques include the use of multiple filament assemblies w18x, point source loading w19x, resin beads w20x, carburized filament materials w21,22x, and the application of a thin Pt film over the sample w23x. In general, these techniques produce significant enhancements in ion yield and signal-to-noise ratio, although ionization efficiency is enhanced by less than an order of magnitude in most cases. Thermal ionization cavities ŽTICs. have been used for over three decades as ion sources for large-scale isotope separators, which produce mg quantities of high-purity single isotopes w24᎐26x. Early studies of these devices showed that ionization efficiencies were enhanced by one᎐two orders of magnitude over those obtained from ribbon-type ion sources for many elements w25,27x. The theoretical basis for thermal ionization from a single surface, and from within a volume, is thoroughly described and discussed by Kirchner

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w28᎐30x and others w27,31x. Cesario and co-workers w32x first adapted the TIC for use as an analytical ion source in TIMS, and their preliminary study showed an enhancement of 5᎐30-fold for the ionization efficiency of U, relative to a filamenttype ion source fitted to the same mass spectrometer. Using a simpler TIC source, Duan and co-workers w33,34x documented enhanced ionization efficiencies for several lanthanides, uranium, and thorium relative to published ionization efficiencies obtained for the same elements using standard Ži.e. filament-type. thermal ionization sources. It is important to note that some of the ionization efficiency data presented by Duan Že.g. Th. were collected using the TIC source in an isotope separator, while other data were collected using an analytical quadrupole mass filter Že.g. Eu. w33x. We have coupled a TIC ion source to a timeof-flight mass spectrometer ŽTOFMS.. Although the coupling of a continuous thermal ion source to a pulsed TOFMS is unusual, it is not unprecedented w35x. The ability of TOFMS to collect data from numerous ionic species created simultaneously in the source region makes it a potentially useful technique for the rapid determination of isotope ratios. Myers et al. w36x have collected isotope ratio data using an axial acceleration reflectron TOFMS coupled to a continuous ICP source. They report analytical precision of ; 1.0᎐0.3% R.S.D. for the major isotopes of Zr, Ag, Cd, Sb, Nd and Pb. Using similar instrumentation, Vanhaecke et al. w37x obtained even greater isotope ratio precision Ž; 0.025% R.S.D. for 107 Agr 109Ag, and F 0.05% for Cu, Zn, Sr, Ba, and Pb isotopes. from concentrated solutions Žup to 500 ␮grl.. Further work using similar instrumentation by Tian et al. w38x and Emteborg et al. w39x yielded similar results. Sturgeon et al. w40x characterized the isotope ratio analysis capability of a commercial orthogonal acceleration ICPTOFMS ŽOptimass 8000 ICP-TOFMS., and showed that isotope ratio precision varied from 0.04 ŽMg. to 0.55% R.S.D. ŽU. in solutions at the 50-␮grl concentration level. A further advantage of the TIC source is its ability to generate ion beams directly from solid particles Žas well as from dried solutions., thereby

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reducing the time and resources needed for sample preparation. Direct isotope ratio analysis of solid particles is also readily accomplished via laser ablation ICPMS, glow discharge mass spectrometry ŽGDMS. w41x, secondary ion mass spectrometry ŽSIMS. w42x and, under certain conditions, via unconventional TIMS techniques w6᎐8x. Implementation of the TIC would expand such TIMS applications, and could readily provide highly precise isotope ratio data in a relatively short time compared to traditional TIMS techniques. For example, Duan et al. w33x demonstrated isotope ratio precision of 0.21᎐- 0.005% R.S.D. for Eu and Sm isotopes in a rare earth oxide mixture using the TIC source and a quadrupole-based mass analysis system. In this paper, we describe the fundamental characteristics and properties of the orthogonal acceleration linear TIC-TOFMS system, and present preliminary data on the precision and accuracy of isotope ratios obtained using this apparatus. The ionization efficiency performance of the TIC-TOFMS system will be the topic of a subsequent paper.

2. Materials and equipment 2.1. Inlet system All mass spectrometer parts, except where specified, were fabricated in-house from 304 stainless steel. The inlet system and ion source ŽFig. 1. are on opposite faces of the adapter flange Ž9.9 cm o.d.., which also contains four coaxial high-voltage ŽHV. feedthroughs, and four additional HV feedthroughs Žall on the inlet side, not shown in Fig. 1. to accommodate cables from the high-current power supply. The inlet accommodates a direct insertion probe Ž0.635 cm o.d.., used to introduce the TIC into the ion source region. For analysis, the TIC is loaded and fastened to the TIC holder ŽFig. 1.. The TIC holder is fastened to the DIP by a threaded alumina coupling, which prevents conductive heat loss to the rest of the system. During operation, the DIP, TIC holder and TIC are held at ground relative to the ion source Žsee below..

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Fig. 1. Schematic diagram of the TIC-TOFMS system. The electronic feedthroughs and vacuum ports are not shown.

The present sample inlet design is simpler, safer, and more efficient than that described by Duan and co-workers w33,34x. The smaller diameter of the DIP, and the use of an alumina Žrather than boron nitride. coupling, reduces conductive heat loss to the rest of the system. Although the housing surrounding the source region and the adapter flange become warm during operation, there are no burn hazards and no need for an external cooling system. We have added two spring-loaded vacuum seals to the inlet, one behind the roughing valve, and one at the base of the adapter flange, which serve to center the DIP relative to the ion source shielding can. In this way, reproducible sample placement inside the thermal ion source is ensured, and there is no need for a mechanical stage for physical adjustment of the DIP during beam optimization w33,34x. The ion extraction optics of the TOFMS described below are sufficient to draw ions out of

the cavity, thus obviating the need for a small Ž- 0.1 kV. potential on the direct insertion probe w33x. 2.2. Thermal ion source The thermal ion source for the TOFMS studies was adapted and simplified from an earlier design w33,34x. It is comprised of 50% fewer parts than the previous version, and is approximately 50% smaller in size. The filament assembly ŽFig. 1. consists of seven parts: the filament; two filament posts; the electron shielding can; a base plate; and two focusing plates. Filaments are hand-made loops Ž; 1.0 cm diameter. of narrow-gauge Ž0.0381 cm o.d.. tantalum wire ŽWhitmor Wirenetics Co., Valencia, CA., which are centered on two posts that protrude through the base plate into the electron shielding can. The shielding can is a hollow cylinder Ž2.54 cm diameter, 2.54 cm

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length. with a central ion exit orifice Ž0.25 cm diameter. at one end. During operation, a high negative voltage Žy2.5 kV. is applied to the can to repel electrons. The ion beam is focused using two lens plates mounted in front of the can on the same assembly, on alumina-jacketed posts. The first plate has a circular orifice Ž0.25 cm diameter. for ion transport, and the second has a rectangular slit orifice Ž0.13 cm wide= 0.51 cm long. to give the ion beam a rectangular crosssection. Ion source components must be cleaned periodically using an abrasive powder Žor sandpaper., followed by sonication in 30% HNO3 , rinsing in deionized distilled water, and rinsing again in acetone. Alumina spacers and insulators must also be cleaned or replaced periodically to remove conductive, vapor-deposited metal films. Electron emission filaments can last for ) 70 h before they become unstable due to thinning. Filaments were routinely replaced when the ion source was removed and cleaned, regardless of their condition. Ion source cleaning was performed every 7᎐10 days, and therefore individual filaments are rarely used for more than ; 40 h. 2.3. Thermal ionization ca¨ ities Tungsten cavities were fabricated by Dan-Lin Products ŽAlbuquerque, NM, USA. from highpurity Ž) 98.5%. tungsten wire stock Žo.d.s 1.2 mm, Whitmor Wirenetics Inc., Valencia, CA, USA. drilled to specifications Ž0.2᎐0.4 mm i.d., 6᎐20 mm deep. using electrical discharge machining ŽEDM. techniques. After sonication in dilute nitric acid, distilled water and acetone to remove surface debris, new cavities were oven-dried, inserted into the ion source region of the TICTOFMS, and heated to ) 2200⬚C for ; 30 min prior to use. Cavities can withstand numerous mass spectrometry runs with little or no damage, although they may melt or weld shut when subjected to excessive temperatures w33x. However, if previously used TICs are recycled, or if the ion source components are not thoroughly cleaned or exchanged between runs, contamination via crossover may result.

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2.4. Power supplies and other electronics Filament current is provided by a power supply Žmodel 6641, Hewlett-Packard Co., Rockville, MD, USA. that is floated at y2.5 kV DC, and can deliver up to 25 A of current to the emission filament. The power supply is operated through an isolation transformer. Both the floating and can potentials are generated by a Matsusada HV power supply Žmodel AU-5R60, Matsusada Precision Inc., Japan.. At typical run conditions, ; 20᎐30 W of power are delivered to the filament. The system can be run at filament emission currents in excess of 60 mA, if necessary. At present, emission current is manually controlled and is typically stable to within "1 mA at typical operating conditions Ž10᎐40 mA.. 2.5. Time-of-flight mass spectrometer (TOFMS) The linear time-of-flight mass spectrometer Žmodel C-677, R.M. Jordan Co., Grass Valley, CA. has a total flight length of 1 m, and two acceleration stages for spatial focusing ŽFig. 1.. The total acceleration potential was either 2 or 3 kV. Vacuum within the TOFMS is maintained by a turbo pump ŽTHP 240, Balzers, Germany., and is backed by a mechanical roughing pump. The system vacuum is ; 4 = 10y8 torr at rest, and increases to between 8 = 10y8 and 1 = 10y6 torr during operation. This ensures a mean free path length of ) 50 m. Orthogonal ion extraction was used to minimize the effect on the resolution of the energy distribution of thermal ions emitted from the source. The x᎐y steering plates are mounted immediately behind the acceleration stages to guide the ions to the detector. The detector consists of two chevron-mounted, 25-mm, high-gain Ž; 10 6 . micro-channel plates with subns response time. The duty cycle of the TOFMS for Th ions is ; 5.6%, assuming a maximum ion energy of 1 eV w43x, a 4-kHz repeller frequency, and a 1.27-cm TOFMS aperture diameter. Mass spectra were collected using a Tektronix oscilloscope ŽTDS 520C, Tektronix Measurement Group, Portland, OR, USA.. An IBM-compatible computer with an Intel Pentium microprocessor and LabView software ŽVersion 5.0 for Windows 95,

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Table 1 Linear TIC-TOFMS operating conditions TIC source Repeller frequency Repeller voltage Floating potential Emission current ŽmA.

4 kHz q230᎐310 V y2500 V K: 2᎐3 Mg: 10᎐12 Zr Žmetal.: 28᎐35 Ž3-kV acceleration . Zr Žoxide.: 30᎐33 Ž2-kV acceleration . Zr Žoxide.: 27᎐29 Ž3-kV acceleration . Nd Žoxide.: 13.5᎐15.5

Mass spectrometer Ion optics: 3-kV acceleration Lens 1 Lens 2 X Y Grid

y150 to y400 V y500 " 50 V y2850" 50 V y3200" 100 V 0 to y8 V

Ion optics: 2-kV acceleration Lens 1 Lens 2 X Y Grid MCP detector Detector gain Flight tube pressure

y600 to y1100 V y650 " 50 V y1820" 20 V y2050" 50 V 0᎐4 V y2000 V 106 4 = 10y8 ᎐5 = 10y7 torr

National Instruments Inc., Austin, TX, USA. was used for acquiring and processing the spectra. Typical operating parameters are summarized in Table 1. 2.6. Ca¨ ity temperature Unlike the TIC quadrupole MS w33,34x, the orthogonal acceleration TIC-TOFMS permits direct temperature monitoring. Cavity temperature was monitored using a rack-mounted optical pyrometer ŽQuantum Logic model QL1200C, Quantum Logic Corp., Westport, CT, USA. focused on the ion exit aperture of the TIC. Heating varies with the size, condition, and position of the emission filament ŽFig. 2., although the overall trend of increasing temperature with increasing emission current is similar. For Ta filaments with

-; 40 h of use, temperatures in excess of 2300⬚C are easily and reproducibly obtained. As the filament ages and thins, more current is required to support high temperatures. After ; 80 h of use ŽFig. 2., the filament may not be able to support temperatures above ; 2200⬚C. Filaments are replaced whenever the ion source is cleaned, and thus filaments are very rarely used for more than 40᎐50 h. Isotope ratio data were collected only when the most intense peak Že.g. 90 Zr, 142 Nd16 O, etc.. reached 100 mV on the oscilloscope viewing screen. Neodymium oxide peaks first appear at ; 10 mA, and NdOq isotope ratio data were collected at 15.0" 1.0 mA Ž; 1525⬚C.. The collection of Zrq data required a significantly higher operating temperature ŽG 2200⬚C. than ZrOq Ž1850 " 50⬚C.. For all experiments discussed here, the potential Žy2.50" 0.01 kV. on the TIC was equal to that on the electron shielding can. Electron bombardment efficiency may be increased somewhat by raising the can potential relative to the TIC Žwhich remains at ground potential.. However, we observed that a y0.5-kV increase in can potential increased the TIC temperature by only 30᎐50⬚C. 2.7. Sample preparation All data for this study were obtained from solid particulate samples. Neodymium isotope data were acquired using - 1.0 ␮g of high purity Ž) 99%, AlfarAesar. Nd 2 O 3 powder, and Mg isotope data were acquired using - 1.0 ␮g of reagent grade MgŽOH . 2 powder Ž; 95%, Aldrich.. Potassium data were collected at low emission currents Ž2.0᎐3.0 mA. during degassing of ubiquitous surface contamination prior to the acquisition of data at higher emission currents. All other spectra were acquired using - 1.0 ␮g of natural zircon ŽZrSiO4 ., which was powdered using a tungsten mortar-and-pestle. Magnetic impurity phases were extracted from the crushed zircon crystals using a hand magnet, and the powder was acid-washed for several hours to remove impurities. Sub-␮g quantities of the powders were loaded into the TIC, either by tamping the cavity directly into the powdered material, or by

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Fig. 2. Temperature at the ion exit aperture of the tungsten thermal ionization cavity vs. emission current Žin mA.. Filled circles plot the heating trend for the initial use of a Ta emission filament. Filled triangles and squares represent data from Ta filaments that have been used for more than 80 h.

Fig. 3. Elemental and oxide peaks generated during the ‘step heating’ of a - 1 ␮g zircon sample Žno carbon. in the TIC-TOFMS. Only Zrq and ZrOq peaks remain at 2200⬚C.

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wetting the end of a thin tungsten wire with a colloidal graphite solution Ž‘Aquadag’., dipping it into the powder, and drying the resulting bead under a heat lamp. A final coating of graphite was then applied by passing the sample through a wire loop containing a thin film of the liquid suspension. The sample was then re-dried and inserted into the cavity. Intense Ž; 1 V. Naq and Kq peaks were observed in all runs, though their intensity was reduced to - 2 mV at temperatures required for the observation of refractory metal and metal oxide peaks Žtypically ) 1500⬚C.. Lowto moderate-intensity Ž; 1᎐50 mV. tungsten peaks, routinely observed by Duan and co-workers w33x, are also observed in TIC-TOFMS runs at temperatures in excess of ; 2200⬚C. During several runs, we also observed low intensity ŽF 1 mV. peaks at mrz 91.5, 93, 95 and 97 at very high temperatures Ž; 2400⬚C.. These peaks were likely due to doubly charged tungsten ŽW 2q ., and possibly Moq ions Žpresent as an impurity in W metal., sputtered directly from the TIC as a result of intense electron bombardment.

3. Results and discussion 3.1. Multi-element TIMS spectra An advantage of direct ionization from solid particles using TOFMS is that isotopes from all species ionized at a particular temperature can be detected and measured concurrently. An illustrative example of this capability is shown in Fig. 3. Complicating factors include the potential for spectral interference from a variety of sources, including molecular and multiply charged isobars. Some molecular ions, particularly the metal oxides, can be eliminated if carbon is added to the sample load. In the case of zircon, heating the powder in the presence of carbon ŽAquadag. yielded only Zrq metal peaks, which first appeared between 1900 and 2000⬚C ŽFig. 4.. Peaks for other elements Žtungsten, rare earths, etc.. were F 2 mV at the conditions used for Zrq isotope data collection Ž2050᎐2200⬚C.. Without carbon ŽFigs. 3 and 4., zircon yielded a complex multi-element mass spectrum dominated by ZrOq peaks Žat mrz 106, 107, 108, 110 and 112., with a

Fig. 4. Mass spectra Žzircon. collected at ; 2000⬚C using 2- and 3-kV TOF acceleration potentials, with and without added carbon. Zirconium oxide ŽZrOq. peaks are most intense in spectra obtained from samples without carbon, while zirconium metal peaks ŽZrq. are most intense Žand oxide peaks are absent . in spectra obtained from runs with carbon. Offset along the y-axis is for purposes of clarity only.

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ZrqrZrOq intensity ratio between 0.05 and 0.1. Data were also collected using two different ion acceleration potentials: 2 and 3 kV ŽFig. 4.. The data collected at 2 kV are characterized by a flatter, more stable background, while the 3-kV spectra are 5᎐10-fold more intense. Relatively intense ŽG 5 mV. peaks for several species, including Feq, Srq, YOq, Baq, CeOq and several rare earth elements and oxides, appeared between 1000 and 1500⬚C. It is not clear whether these elements are present as trace impurities in the zircon itself, or as separate, microscopic included phases. Tungsten oxide peaks may represent admixed impurities inherited as a consequence of grinding the zircon sample in a tungsten crucible. Zirconium oxide, rare earth metals ŽSm, Gd, Yb, etc.., tungsten oxide, and tantalum

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oxide peaks appeared or intensified at ; 1700⬚C, as Feq, Srq, Baq and Euq peaks diminished. At higher temperatures Žup to 2000⬚C., Zrq, ZrOq and ZrOq 2 peaks intensified. Although some of the rare earth oxide Že.g. CeOq and LaOq . peaks diminish, others Že.g. Smq and Ybq . persist to higher temperatures. Above 2100⬚C, Zrq and ZrOq comprise the only significant peaks in the zircon mass spectrum ŽFig. 3.. During a single run Ž; 8.2 min. at 27.0" 0.5 mA emission Ž; 1750⬚C., we observed numerous peaks corresponding to rare earth elements ŽREEs. and oxides ŽREE oxides. emanating from the powdered zircon sample ŽFig. 5.. Peak intensities at several masses diminish precipitously Ž mrz 151, 153, 155᎐160, 162. during the run, while other peaks remain almost constant Ž mrz 144,

Fig. 5. Block diagram of trace element and oxide peaks emitted by zircon powder during a single 8-min TIC-TOFMS run at ; 1750⬚C.

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147᎐150, 152, 154, 161, 170᎐174, 176.. Fig. 6 depicts intensity and isotope ratio trends vs. time for the data in Fig. 5 at several masses. Although several of the peaks which ‘burn off’ during the course of the run correspond to metals Že.g. 151 Euq and 153 Euq . others, such as the intense peaks at mrz 156 and 158, likely correspond to REE oxides Že.g. 140 CeOq and 142 CeOq .. The intensity ratio of the peaks at mrz 156 and 158 varies from ; 0.5 at the outset to ; 1.6 at the end of the run. It is possible that the systematic change in isotope ratio ŽFig. 6. corresponds to the burn-off of CeOq Ž 140 Cer 142 Ce s 0.125., while the more refractory Gd isotopes remain Ž 158 Gdr 156 Gds 1.21.. The excess at mrz 158 may be due to the presence of an additional REE oxide component, such as 142 NdOq. Many of the remaining peaks observed between mrz 140 and 180 ŽFig. 5. are combinations of two or more isobars, such as 176 Ybq and 148 SmOq at mrz 176. However, peaks at mrz 141 ŽPrq. , 144, 147᎐150, 152, 154 Žall Smq. , 151, and 153 ŽEuq. appear to represent pure metal isotope peaks. Samarium isotope ratios determined directly from the spectra Ž 152 Sm peak height ; 1 mV. depicted

in Fig. 5 yield data Ž n s 57. that are accurate to y1.9 to y5.6% Žwith the exception of 144 Smr 152 Sm, q40.8%., and precise to - 4.7% R.S.D. Although the Sm data were not collected using the optimum oscilloscope settings for isotope ratio determinations, they provide an example of the versatility and speed of the TIC-TOFMS approach. 3.2. Isotope ratio precision For isotope ratio Žor peak ratio. determinations, - 1 ␮g of fine particulate oxide sample was loaded into the TIC, degassed, and heated to drive off impurities ŽNaq, Kq, Caq, etc... Cavity temperature was increased until spectra that represent ion emission from a single element Že.g. Zrq and Mgq . or oxide Že.g. ZrOq and NdOq . of interest were identified. Mass resolution Ž mr⌬ m, measured as full width at half-maximum, FWHM. at the most intense peak of the element of interest Že.g. 90 Zr., was between 750 and 850 for isotope ratio determinations. Oscilloscope parameters were adjusted in order to maximize the dynamic range for data collection. Prior to

Fig. 6. Intensity trends for several peaks and peak ratios from the data in Fig. 5.

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Fig. 7. Standard deviation Ž1␴, represented as %R.S.D.. for ; 100 isotope ratio measurements vs. number of averages pre-selected for data collection using an oscilloscope.

Fig. 8. ZrOq spectrum generated from - 1 ␮g zircon powder at ; 1850⬚C. The peak at mrz 105 corresponds to YOq. Mass resolving power Ž mr⌬ m. is between 750 and 800.

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Table 2 Isotope data from - 1 ␮g of zircon powder 90

Reference values

ZrOr94 ZrO

91

2.9603

ZrOr94 ZrO

92

ZrOr94 ZrO

96

ZrOr94 ZrO

0.6456

0.9868

0.1611

[ZrO]q: 3-kV acceleration potentialb 3 May, n s 70 2.996Ž8. n s 89 2.982Ž8. 4 May, n s 69 2.955Ž7. n s 86 3.012Ž9. n s 69 2.990Ž8. n s 69 2.973Ž7. n s 69 2.970Ž7.

0.659Ž6. 0.635Ž3. 0.644Ž3. 0.643Ž2. 0.644Ž2. 0.642Ž2. 0.642Ž2.

0.990Ž4. 0.951Ž4. 0.982Ž7. 0.990Ž5. 0.979Ž6. 0.976Ž2. 0.987Ž3.

0.159Ž2. 0.159Ž2. 0.157Ž2. 0.159Ž2. 0.159Ž2. 0.158Ž2. 0.157Ž1.

[ZrO]q: 2-kV acceleration potentialb 3 May, n s 69 3.062Ž9. n s 69 3.042Ž9. n s 69 2.999Ž7. n s 70 3.022Ž9. n s 72 3.011Ž10.

0.667Ž3. 0.667Ž3. 0.644Ž3. 0.669Ž3. 0.653Ž4.

0.990Ž5. 1.005Ž3. 0.982Ž3. 0.997Ž3. 0.985Ž4.

0.161Ž1. 0.160Ž2. 0.157Ž2. 0.157Ž2. 0.157Ž2.

[Zr]q: 3-kV acceleration potential 28 June, n s 70 3.007Ž12. n s 73 3.022Ž8. n s 70 3.003Ž6. n s 70 2.995Ž5. n s 70 3.020Ž9.

0.634Ž2. 0.636Ž2. 0.634Ž4. 0.651Ž3. 0.662Ž3.

0.983Ž2. 0.996Ž2. 0.992Ž3. 0.989Ž2. 1.006Ž4.

0.164Ž2. 0.165Ž1. 0.158Ž2. 0.161Ž2. 0.158Ž1.

% biasa 90 ZrOr94 ZrO q [ZrO] : 3-kV acceleration potentialb 3 May, n s 70 0.30 n s 89 0.19 4 May, n s 69 y0.04 n s 86 0.44 n s 69 0.25 n s 69 0.11 n s 69 0.08

91

% biasa ZrOr94 ZrO

92

% biasa ZrOr94 ZrO

96

0.71 y0.54 y0.06 y0.12 y0.09 y0.19 y0.17

0.17 y0.29 y0.22 0.16 y0.39 y0.54 y0.01

y0.62 y0.69 y1.27 y0.74 y0.74 y1.10 y1.27

[ZrO] q : 2-kV acceleration potentialb 3 May, n s 69 0.86 n s 69 0.69 n s 69 0.33 n s 70 0.52 n s 72 0.43

1.09 1.22 y0.06 1.20 0.38

0.17 0.90 y0.25 0.51 y0.08

y0.06 y0.44 y1.14 y1.16 y1.43

[Zr]q: 3-kV acceleration potential 28 June, n s 70 0.40 n s 73 0.52 n s 70 0.37 n s 70 0.29 n s 70 0.50

y0.62 y0.47 y0.62 0.28 0.87

y0.22 0.46 0.27 0.10 0.95

0.90 1.14 y0.93 y0.05 y1.01

Numbers in parentheses are 1␴ standard deviations. a % biasrmass unit s 100 = wŽmeasured mass y known mass.rŽknown mass.xrmass difference. b Ratios corrected for M 17 O and M 18 O contributions.

% bias a ZrOr 94 ZrO

Table 3 wNd18 Oxq isotope data from- 1 ␮g Nd 2 O 3 at ; 1550⬚C 142

NdOr146 NdO

143

NdOr146 NdO

144

NdOr146 NdO

145

NdOr146 NdO

148

NdOr146 NdO

150

NdOr 146 NdO

0.7086

1.3845

0.4828

0.3351

0.3281

0.719Ž4. 0.722Ž3. 0.721Ž2. 0.710Ž2. 0.721Ž2. 0.715Ž3. 0.704Ž4. 0.712Ž3.

1.413Ž5. 1.398Ž4. 1.395Ž4. 1.381Ž3. 1.398Ž4. 1.396Ž4. 1.389Ž6. 1.390Ž3.

0.495Ž3. 0.496Ž2. 0.496Ž2. 0.495Ž2. 0.497Ž2. 0.481Ž3. 0.480Ž3. 0.481Ž2.

0.333Ž2. 0.323Ž2. 0.330Ž2. 0.330Ž2. 0.329Ž2. 0.333Ž2. 0.335Ž3. 0.332Ž1.

0.327Ž2. 0.316Ž2. 0.320Ž2. 0.325Ž2. 0.323Ž2. 0.323Ž2. 0.323Ž3. 0.325Ž2.

[NdO]q: 3-kV acceleration potentialb 27 April, n s 71 1.583Ž5. n s 78 1.557Ž6. n s 70 1.577Ž3. n s 69 1.582Ž3. n s 80 1.578Ž4.

0.696Ž3. 0.691Ž3. 0.985Ž2. 0.691Ž2. 0.688Ž2.

1.373Ž4. 1.368Ž4. 1.373Ž3. 1.375Ž3. 1.373Ž3.

0.477Ž2. 0.478Ž2. 0.468Ž1. 0.469Ž2. 0.470Ž2.

0.338Ž2. 0.338Ž2. 0.344Ž2. 0.340Ž2. 0.341Ž2.

0.340Ž2. 0.335Ž2. 0.334Ž2. 0.335Ž2. 0.334Ž1.

% biasa 142 NdOr146 NdO [NdO]q: 3-kV acceleration potentialb 27 April, n s 70 0.36 n s 71 0.19 n s 71 0.32 n s 70 0.34 n s 71 0.26 24 April, n s 201 0.38 n s 88 0.21 n s 140 0.24 [NdO]q: 3-kV acceleration potentialb 27 April, n s 71 0.08 n s 78 y0.34 n s 70 y0.02 n s 69 0.06 n s 80 0.00

143

% biasa NdOr146 NdO

144

% biasa NdOr146 NdO

% biasa 145 NdOr146 NdO

148

% biasa NdOr146 NdO

% biasa 150 NdOr146 NdO

0.51 0.65 0.58 0.07 0.58 0.32 y0.24 0.16

1.01 0.50 0.38 y0.12 0.48 0.42 0.15 0.21

2.57 2.70 2.67 2.60 2.85 y0.39 y0.52 y0.46

y0.36 y1.77 y0.76 y0.72 y0.93 y0.32 y0.04 y0.53

y0.12 y0.95 y0.58 y0.26 y0.37 y0.42 y0.38 y0.25

y0.61 y0.81 y1.09 y0.84 y0.95

y0.41 y0.59 y0.42 y0.36 y0.41

y1.13 y0.92 y3.09 y2.89 y2.59

0.48 0.38 1.35 0.76 0.93

0.91 0.56 0.46 0.54 0.44

1187

Numbers in parentheses are 1␴ standard deviations. a % biasrmass unit s 100 = wŽmeasured mass y known mass.rŽknown mass.xrmass difference. b Ratios corrected for M 17 O and M 18 O contributions.

D.M. Wayne et al. r Spectrochimica Acta Part B: Atomic Spectroscopy 56 (2001) 1175᎐1194

Reference values 1.5782 s [NdO]q: 3-kV acceleration potentialb 27 April, n s 70 1.601Ž5. n s 71 1.590Ž5. n s 71 1.598Ž4. n s 70 1.600Ž4. n s 71 1.595Ž4. 24 April, n s 201 1.602Ž7. n s 88 1.592Ž6. n s 140 1.593Ž4.

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D.M. Wayne et al. r Spectrochimica Acta Part B: Atomic Spectroscopy 56 (2001) 1175᎐1194

Table 4 Mg and K isotope data and fractionation factors Žexponential law. 25

Mgr24 Mg

26

Mgr24 Mg

% biasa Mgr24 Mg

25

Reference values

0.12660

0.13938

3 July, n s 72 n s 70 n s 71 n s 84 n s 76

0.1236Ž6. 0.1240Ž6. 0.1244Ž4. 0.1264Ž10. 0.1278Ž6.

0.1367Ž8. 0.1382Ž6. 0.1408Ž4. 0.1431Ž11. 0.1433Ž6.

41

Reference value

Kr39 K 0.07217

28 June, n s 120 n s 69 n s 74

0.0720Ž5. 0.0721Ž4. 0.0737Ž8.

y2.37 y2.04 y1.77 y0.18 0.94

% biasa Mgr24 Mg

F Ž25 Mgr24 Mg.

F Ž25 Mgr24 Mg.

y0.95 y0.42 0.49 1.32 1.41

0.024 0.021 0.018 0.002 y0.010

0.010 0.004 y0.005 y0.013 y0.014

26

% biasa 41 Kr39 K

F Ž41 Kr39 K.

y0.14 y0.08 1.09

0.002 0.001 y0.011

Numbers in parentheses are 1␴ standard deviations. Exponential fractionation law: F s wlnŽ R true rR meas .rm1Žln m 2rm1 4.x for m 2 ) m1 w46᎐48x. a % Biasrmass unit s 100 = wŽmeasured mass y known mass.rŽknown mass.xrmass difference.

data collection, oscilloscope parameters Žnumber of averages, screen resolution, etc.. were optimized to achieve the greatest isotope ratio precision and accuracy, while keeping the timeranalysis to - 10 s ŽFigs. 7 and 8.. The oscilloscope which was used for these studies ŽTektronix TDS 520C. is capable of acquiring isotopic data Ži.e. for a single element. from the TOFMS at frequencies of ; 50᎐75 Hz. Thus, many spectra are not recorded by the oscilloscope, as the TIC-TOFMS is set up to produce spectra at 4 kHz. Additional data collection time is lost during the data transfer from the GPIB board to LabView. Each such transfer requires ; 0.5 s, and therefore all signal averaging and processing was performed on the oscilloscope. The averaging algorithm utilized by the oscilloscope w44x is: A n s Ž AŽ ny1. . q w X n y AŽ ny1. x rn

Ž1.

where n is the current acquisition number of a set of N averages, A n is the current running average of n spectra, Ž AŽ ny1. . is the previous running average, and X n is the most recent spectrum collected by the oscilloscope. Thus, the n-th

acquisition has a 1rn effect on the running average. In practice, this equation effectively approximates a true average. However, to obtain the average of n spectra, the first spectrum collected is divided and multiplied n y 1 times, the second is divided n y 2 times, and so on w44x. Thus, for n s 3: A 3 s Ž A 2 = 2r3. q X 3r3

Ž2.

Unlike a conventional average, which has only one round-off error at the final normalization, the final oscilloscope average will have Ž N y 1. round-off errors. Although we cannot say precisely how much error the averaging algorithm will introduce into the final averaged spectrum, such calculations and their associated rounding errors would add a noise component to the spectrum thus obtained, relative to one obtained using simple averaging. The averaging parameter, N, can be manually set to average up to 10 4 individual spectra. For Zr and Nd isotopes, the time required to process 10 3 spectra using a low screen resolution Ž50 pointsrdivision. is ) 20 s. Furthermore, if the number of averages n exceeds the manually set

D.M. Wayne et al. r Spectrochimica Acta Part B: Atomic Spectroscopy 56 (2001) 1175᎐1194

‘averaging’ parameter N, the algorithm used by the oscilloscope for data averaging is altered to one that results in the re-use of data obtained during previous runs w44x, resulting in an artificially low standard deviation. The LabView-based software had to be written so that the oscilloscope would restart automatically once N, the manually set number of averages, was exceeded and the previous spectrum was sent to the computer. For oscilloscope optimization, we performed numerous isotope ratio determinations by collecting averaged spectral data for several values of n ŽFig. 7.. We also varied the screen resolution from 1500 to 50 pointsrdivision. By gathering data at lower screen resolutions, the time needed to process spectra is minimized. The data in Tables 2᎐4 were collected at the optimal oscilloscope settings: 600 averages at a screen resolution of 1000 pointsr10 divisions. Under these conditions, averaged Zr and Nd spectra can be obtained every 9.0" 0.3 s. Note that the size of the window required for the collection of simultaneous isotope data depends on the time resolution required to observe the entire spectrum of a given element. Typical runs consist of 60᎐80 spectral observations Žeach of which comprise 600 averaged individual spectra., for a total analysis time of 10᎐12 min. Isotope ratio data were also collected using two different ion acceleration potentials: 2 and 3 kV. Isotope ratio precision for major isotopes of Zr and Nd in Nd 2 O 3 powder and ZrSiO4 powder varied from 0.2 to 0.4% R.S.D. ŽTables 2 and 3., which is at or near the theoretical maximum for data collected using an 8-bit digital oscilloscope Žaveraging increases the practical resolution to 11 bits w45x.. It is also worth noting that the trend in precision vs. the number of averaged spectra ŽFig. 7. follows the limits set by counting statistics. In other words, a three-fold increase in the number of spectra improves the precision Ž% R.S.D.. by a factor of ; 63. Measurements of 96 Zrr 94 Zr are less precise Ž; 1.0% R.S.D.. than for the other Zr isotopes, due to the low abundance Ž2.8%. of 96 Zr. Similarly, precision is slightly degraded Ž; 0.4᎐0.8% R.S.D.. for the less abundant Nd isotopes Ž 148 Nd

1189

and 150 Nd.. The precision of Mg and K isotope ratios ŽTable 4. varies from approximately 0.3 to 1.0% R.S.D. Precision does not vary with the species used to determine isotope ratio Že.g. Zrq vs. ZrOq. , although all oxide data had to be corrected for contributions from 17 O and 18 O. The abundance of x M 17 O and x M 18 O were assumed to be proportional to the abundance of the minor O isotopes, and were subtracted from the signal observed at the appropriate mrz ratios. Isotope data were not otherwise normalized or corrected for instrumental mass bias. 3.3. Mass bias Extensive studies of mass bias in sector TIMS indicate that it is related to both time- and massdependent thermal fractionation, and to static instrumental artifacts w46x. In thermal mass fractionation, the lighter of two isotopes is preferentially vaporized relative to the heavier, and the net mass bias is a function of time, the amount of sample remaining after ion emission is initiated, the relative mass difference between the two isotopes in question, and temperature. Time- and mass-dependent thermally generated bias is predictable and can be approximated using a variety of empirical fractionation laws w46᎐48x. For single collector magnetic sector TIMS instruments, the magnitude of thermally induced mass bias is usually well below 0.06%, even for lighter elements such as Fe w49x. Few data exist for mass bias due to thermal Žor other. processes in the TIC source. Using the quadrupole TIC-TIMS, Duan et al. w33x reported isotope ratios that deviated significantly from known values Žy2.2%rmass unit for 154 Smr 1 5 2 Sm to q 6.3% rmass unit for 144 Smr 152 Sm.. Although mass bias in plasmasourced MS originates from space᎐charge effects w15᎐17,46,50x and not from time-dependent thermal effects, we refer to several recent studies of mass bias in commercial reflectron ICP-TOFMS instruments w36᎐40x as a basis for comparison to the results from the linear TIC-TOFMS. Recent investigations of mass bias effects in axial acceleration ICP-TOFMS w38,39x and orthogonal acceleration ICP-TOFMS w40x demonstrate that isotope ratio accuracy is mass-depen-

1190

D.M. Wayne et al. r Spectrochimica Acta Part B: Atomic Spectroscopy 56 (2001) 1175᎐1194

dent and varies from ; 10%rmass unit for Li to ; 0.2%rmass unit for Pb when the instrument is tuned to the middle of the mass range Že.g. 103 Rhq .. Sturgeon et al. w40x point out that mass bias at low masses Že.g. mrz 24. is reduced to ; 1%rmass unit if the instrument tuning mass is shifted to the low mass end. More significantly, these authors w40x establish that the orthogonal acceleration TOF configuration produces no additional mass bias effects relative to instrumentation having the axial TOF configuration w36᎐39x. A plot of Zr isotope ratios measured using TIC-TOFMS ŽFig. 9. illustrates their accuracy Žand precision. relative to those measured using conventional sector TIMS techniques w51x. The accuracy of Zr and Nd peak ratios ŽTables 2 and 3. measured using TIC-TOFMS varies from ; 3 Že.g. 145 Nd16 Or 146 Nd16 O. to - 0.1%rmass unit. The magnitude of mass bias Žrmass unit. obtained using the TIC-TOFMS is comparable to that obtained using reflectron ICP-TOFMS w38᎐40x. Mass bias for lighter elements, such as K and Mg ŽTable 4., varies from 0.2% to approximately 2%. We observed no deleterious mass bias effects specific to the lower masses, although the beam was focused at a low mass position for these runs.

Fractionation factors are used routinely in geochemistry to normalize variable isotope ratio data Že.g. 87 Srr 86 Sr. based on the near-simultaneous measurement of a constant isotope ratio for the same element Že.g. 88 Srr 86 Sr s 0.1194. w48x. Normalization to a constant isotope ratio also improves within-run isotope ratio precision from ; 1.0 to - 0.01% w48x. Fractionation factors, F, for Zr metal isotopes were obtained using the linear fractionation law w46᎐48x: F s wŽ R obs y R true . y 1 x r⌬ mass

Ž3.

where R obs is the observed isotope ratio, R true is an accepted standard value for that ratio, and ⌬ mass is the mass difference of the two isotopes in question. Fractionation factors obtained for Zr isotopes measured using the TIC-TOFMS ŽTable 5. do not vary predictably as a function of mass difference Ž ⌬ mass ., nor do they vary systematically during a single run. Although 90 Zrr 94 Zr measurements show strong positive departures from the accepted values ŽTable 5., as predicted by thermal fractionation, 91 Zrr 94 Zr measurements do not. Neither the sign nor the magnitude of the F values for Zr isotopes changes in a systematic way within a single run. Thus, any attempt to

Fig. 9. Zirconium isotope ratios Žand associated errors. measured using the current TIC-TOFMS setup. Reference values for Zr isotope ratios Žbased on metal peaks. are from Sahoo and Masuda w51x.

D.M. Wayne et al. r Spectrochimica Acta Part B: Atomic Spectroscopy 56 (2001) 1175᎐1194

normalize using F values obtained from one isotope pair would only introduce greater uncertainty into the other Zr isotope ratio measurements. Therefore, mass bias in the Zr Žand Nd. measurements is probably not the result of thermal fractionation. Mass bias for Zrq, ZrOq and NdOq does not change systematically in response to cavity temperature, or time spent at temperature. Rather, it is likely that the mass bias observed in these trials results largely from baseline effects and instrumental drift, rather than from any physical processes occurring inside the ion source or mass spectrometer. Ringing noise on the highmass side of intense peaks Že.g. 90 Zr. was observed in several runs, and would certainly contribute a noise component to the background. The Mg and K data, however, do show some evidence of thermal fractionation. A single sample for each element was run at progressively higher temperatures in order to maintain a constant peak height Že.g. 24 Mg) 200 mV.. The emission current was increased, in increments of 0.5 mA, from 10 to 12 mA for each successive run ŽTable 4.. The mass bias and fractionation data ŽTable 4. indicate that 24 Mg was depleted relative to 25 Mg and 26 Mg with increasing temperature. Similarly, the mass bias for potassium isotopes increases dramatically in the final run, which was set at a higher emission current Ž3.0 mA. than the first two Ž2.0 mA..

4. Conclusions Unlike resistively heated filament-type thermal ion sources, the thermal ionization cavity ŽTIC. is

heated via electron bombardment and can sustain temperatures ) 2300⬚C for several hours. Although dried solutions can be ionized in the TIC source, sub-␮m Žand greater. quantities of refractory Že.g. oxide and silicate . particulates can be loaded directly for thermal ionization mass analysis without dissolution or pretreatment of any kind. Samples can be treated with colloidal carbon to eliminate numerous metal oxide peaks, if so desired. A time-of-flight mass spectrometer ŽTOFMS. interfaced to the TIC source acquires a complete mass spectrum from each ion packet extraction. Isotope ratios of multiple elements in a single sample can then be monitored through a range of temperatures. Previous work on the TIC source for mass spectrometry w32᎐34x suggests that the cylindrical geometry of the TIC source may promote greater ionization efficiency, which would permit the analysis of smaller samples. Our results indicate that the TIC source has several other potential advantages. These include: Ž1. rapid sample loading for solids and dried solutions; Ž2. the availability of rapid, direct isotopic analysis of carefully chosen solid particles; Ž3. concurrent multi-element isotope ratio analysis of a single sample due to the pseudo-simultaneous detection offered by TOFMS; and Ž4. collection of multi-element isotope ratios over a thermal gradient. As an illustrative example of the potential multi-element isotope ratio capabilities of the present TICTOFMS system, we identified ion signals corresponding to a multitude of elements ŽSrq, Baq, Zrq, Smq, Euq, etc.. and oxides ŽZrOq, YOq, CeOq, LaOq, etc.. during a ‘step-heating’ experiment from ; 1600 to ) 2200⬚C on powdered natural zircon ŽZrSiO4 . crystals.

Table 5 Linear fractionation factors obtained for Zrq isotope ratios Sample

F Ž90 Zrr94 Zr.

F Ž91 Zrr94 Zr.

F Ž92 Zrr94 Zr.

F Ž96 Zrr94 Zr.

1 2 3 4 5

0.004 0.005 0.004 0.003 0.005

y0.006 y0.005 y0.006 0.003 y0.003

y0.002 0.005 0.003 0.001 0.009

0.009 0.011 y0.009 y0.001 y0.01

3 kV, data from 28 June only.

1191

1192

D.M. Wayne et al. r Spectrochimica Acta Part B: Atomic Spectroscopy 56 (2001) 1175᎐1194

Clearly, further work must be carried out to determine if the required thermal and temporal resolution of various elemental and oxide peaks is feasible for a wide range of materials. At this point, it seems possible that the potential for isobaric interference between many Žbut not all. elemental and oxide peaks could be reduced greatly, or eliminated entirely, by carefully controlled heating, and by the use of reagents Žsuch as colloidal carbon. which regulate the thermal ion emission of oxide and metal species. Many techniques for the acquisition of thermally resolved elemental and isotopic data have already been developed for conventional TIMS analysis, as well as for electro-thermal vaporization ŽETV. and graphite furnace atomic absorption spectroscopy ŽGFAAS., and it may be a trivial matter to adapt them for use with the TIC source. The precision of isotope ratio data Ž0.2᎐0.4% RSD. collected using the current system is approximately one order of magnitude worse than is achievable with commercially available reflectron ICP-TOFMS w36᎐40x. Far greater precision Žand accuracy. is achieved by magnetic sector, multicollector TIMS and ICPMS instrumentation w2᎐5,51᎐53x. Using a linear TOFMS, we have not yet attained the level of precision required to determine the nature of time-dependent thermal isotopic fractionation imposed by the TIC source. Our results suggest that the precision achievable by the present TIC-TOFMS system is limited by instrument drift, the length of time needed for data processing and transfer, and the use of a digital oscilloscope to acquire simultaneous, time-resolved mass spectra. Thus, the maximum capabilities of the TIC-TOFMS for isotope ratio data have not yet been ascertained. The implementation of a multi-channel boxcar averaging system for data acquisition and transfer will increase data acquisition efficiency by several orders of magnitude, thereby improving isotope ratio accuracy and precision.

Acknowledgements Joe Hauser, Art Montoya, and Bob Mier ŽCST Division Machine Shop, Los Alamos National

Laboratory. fabricated the TIC ion source assembly. Dan-Lin Industries, Albuquerque, NM, provided EDM support. E. Phil Chamberlin ŽChamberlin Enterprises ., Juan Cuadrado ŽUS Army and LANL., Joseph Banar ŽLANL., Yixiang Duan ŽLANL., and Xaiomei Yan ŽLANL. provided valuable technical support. The comments of Steven Schubert ŽDOE-ORE. on an early version of this manuscript contributed greatly to its clarity and readability. The paper also benefited from comments by R. Samuel Houk, Douglas Duckworth, and an unidentified reviewer. The United States Department of Energy supplied funding for this research through the Office of Research and Engineering Žformerly the DOE Office of National Security and Non-proliferation, NN-20., and through the Nuclear Materials Stockpile Support Capability Development Program at Los Alamos National Laboratory. The University of California operates Los Alamos National Laboratory for the US Department of Energy under contract W-7406-ENG-36. This paper is LAUR003806. References w1x K. Haßfast, in: I.T. Platzner ŽEd.., Modern Isotope Ratio Mass Spectrometry, John Wiley & Sons Ltd, West Sussex, England, 1997, pp. 11᎐82. w2x S.B. Shirey, R.J. Walker, The Re᎐Os isotope system in cosmochemistry and high-temperature geochemistry, Annu. Rev. Earth Planet. Sci. 26 Ž1998. 423᎐500. w3x A. Kawashima, K. Takahashi, A. Masuda, Positive thermal ionization mass spectrometry of molybdenum, Int. J. Mass Spectrom. Ion Processes 128 Ž1993. 115᎐121. w4x J. Volkening, M. Koppe, K.G. Heumann, Tungsten isotope ratio determinations by negative thermal ionization mass spectrometry, Int. J. Mass Spectrom. Ion Processes 107 Ž1991. 361᎐368. w5x J. Volkening, T. Walczyk, K.G. Heumann, Osmium isotope ratio determinations by negative thermal ionization mass spectrometry, Int. J. Mass Spectrom. Ion Processes 105 Ž1991. 147᎐159. w6x B. Kober, Whole-grain evaporation for 207 Pbr 206 Pb age investigations on single zircons using a double-filament thermal ion-source, Contrib. Mineral. Petrol. 93 Ž1986. 482᎐490. w7x B. Kober, Single-zircon evaporation combined with Pbq emitter bedding for 207 Pbr 206 Pb age investigations using thermal ion mass-spectrometry, and implications to zirconology, Contrib. Mineral. Petrol. 96 Ž1987. 63᎐71. w8x P. Karabinos, An evaluation of the single-grain zircon

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