MC-ICPMS

Accuracy for all thorium measurements by TIMS is limited by the absence of an appropriate normalization isotope ratio for internal correction of instrumental mass fractionation. However, external mass fractionation correction factors may be obtained via analysis of suitable thorium standards, such as the UC-Santa Cruz and IRMM standards (Raptis et al. 1998) for °Th/ Th, and these corrections are usually small but significant (< few %o/amu). For very high precision analysis, the inability to perform an internal mass fractionation correction is probably the major limitation of all of the methods for thorium isotope analysis discussed above. For this reason, MC-ICPMS techniques where various methods for external mass fractionation correction are available, provide improved accuracy and precision for Th isotope determinations (Luo et al. 1997 Pietruszka et al. 2002). 

Multiple-collector inductively coupled plasma mass spectrometry (MC-ICPMS) combines sector-field ICPMS with a multiple collector detector system and has recently emerged as an alternative to TIMS for precise U-Th isotope measurement. The full potential of MC-ICPMS has yet to be realized. Yet despite this, its performance in high precision isotope measurement already challenges and, in some cases, surpasses that ever achieved by TIMS (e.g., Lee and Halliday 1995 Blichert-Toft and Albarede 1997). 

The weakness of MC-ICPMS lies in the inefficiency by which ions are transferred from the plasma source into the mass spectrometer. Therefore, despite very high ionization efficiencies for nearly all elements, the overall sensitivity (defined as ionization plus transmission efficiencies) of first generation MC-ICPMS instruments is of the order of one to a few permil for the U-series nuclides. For most, this is comparable to what can be attained using TIMS. 

All MC-ICPMS instruments are equipped with a multiple Faraday collector array oriented perpendicular to the optic axis, enabling the simultaneous static or multi-static measurement of up to twelve ion beams. Most instruments use Faraday cups mounted on motorized detector carriers that can be adjusted independently to alter the mass dispersion and obtain coincident ion beams, as is the approach adopted for MC-TIMS measurement. However, some instruments instead employ a fixed collector array and zoom optics to achieve the required mass dispersion and peak coincidences (e.g., Belshaw et al. 1998). 

Multiple-collection techniques. Uranium. Table 1 shows a typical protocol used by multi-collector instruments (equipped with one ion counting channel) both in MC-TIMS, MC-ICPMS and LA-MC-ICPMS (e.g., Cohen et al. 1992 Stirling et al. 1995 Luo et al. 1997 Stirling et al. 2000 Pietruszka et al. 2002). A first sequence monitors the atomic ratios between and by aligning Faraday collectors for masses (10 ... 

Figure 8. Schematic outline of a second-generation MC-ICPMS instrument (Nu Instalments Nu Plasma), equipped with a multiple-Faraday collector block for the simultaneous measurement of up to 12 ion beams, and three electron multipliers (one operating at high-abundance sensitivity) for simultaneous low-intensity isotope measurement. This instmment uses zoom optics to obtain the required mass dispersion and peak coincidences in place of motorized detector carriers. [Used with permission of Nu Instruments Ltd.]...

Figure 9. Schematic diagram showing a second-generation MC-ICPMS instrament (ThermoFinnigan Neptune). This instrument utilizes double-focusing and is equipped with a motorized multiple-Faraday collector block with two channels that can be operated in high-resolution mode. Optional multiple-ion counting channels are also available for the simultaneous measurement of low-intensity ion beams. [Used with permission of Thermo Finnigan.]...

For solution nebulization MC-ICPMS and MC-TIMS, typical runs consist of up to 100 ratios collected over 30 minutes to 1 hour. This includes time for background measurement (typically monitored at half mass on either side of the peak) and peak centering, but excludes filament warm-up time for TIMS and nebulizer cleaning for MC-ICPMS. Sample sizes of-1000-10 ng of total U for TIMS and MC-ICPMS are required to attain 2a measurement uncertainties of a permil or better on (e.g., Stirling et... 

Uranium Multi-Static, Internal Normalization using MC-TIMS or MC-ICPMS... 

Thorium Multi-Static, External Normalization using MC-ICPMS ... 

U-Th Multi-Static, internai Normaiization using MC-iCPMS or LA-MC-iCPMS ... 

Faraday collector, simultaneously with U, U and U during the first sequence. This shortens the analysis routine, consuming less sample. Ion beam intensities are typically larger in MC-ICPMS than in TIMS due to the ease with which signal size can be increased by introducing a more concentrated solution. While this yields more precise data, non-linearity of the low-level detector response and uncertainties in its dead-time correction become more important for larger beam intensities, and must be carefully monitored (Cheng et al. 2000 Richter et al. 2001). 

However, thorium has only two naturally occurring long-lived isotopes, and all Th measurements by TIMS are limited by the absence of a well-constrained isotope ratio that can be used for internal normalization purposes to correct for instrumental mass fractionation. In this regard, one of the most important advantages of MC-ICPMS over MC-TIMS is the ability to admix two elements with overlapping mass ranges and use the... 

Without isotope dilution, the simultaneous measurement of U and Th is essential in LA-MC-ICPMS, although large (10-100% level) elemental, matrix-dependent fractionation effects can still be observed between U and Th (e g., Stirling et al. 2000). As a result, Th/U ratios can be systematically lower, and apparent U- Th-ages... 

In the past three years, MC-ICPMS has emerged as an alternative to TIMS for precise measurement of the U-series isotopes with comparable or better precision. U-Th isotopes can now be routinely measured at the sub-permil level. Previously, this had only been demonstrated using charge-collection TIMS applied to thorium isotope measurement. Data collection efficiency, sample size requirements, and detection limits can also be greatly improved over TIMS. For the U- U- Th system applied to carbonate samples, this has extended the dating range beyond 600,000 years, and °Th-age uncertainties of 2000 years are now attainable on 300,000 year-old samples (e g., Stirling et al. 2001). 

Alpha Spec. TIMS MC-TIMS ICPMS MC-ICPMS LA-ICPMS LA-MC- ICPMS SIMS... 

Notes HR = High Resolution HR-ICPMS, Q = Quadrupole Q-ICPMS, 1 = First generation MC-ICPMS, 2" = Second generation MC-ICPMS. 

None of the above-mentioned ICPMS techniques can rival MC-TIMS and MC-ICPMS in terms of analytical precision, but the advantage of conventional ICPMS lies in the speed and ease with which data can be acquired. Analysis times are typically less than 10 minutes, and results can be obtained on solid, liquid or gas samples directly, without chemical preparation. Direct analysis will, however, give rise to high levels of molecular... 

SIMS techniques have occupied somewhat of a narrower niche in uranium-series analysis, but have significantly improved Th isotope analysis relative to TIMS for chemically separated samples. The major improvement relative to TIMS is an improvement by about an order of magnitude in efficiency or sample size requirements for silicates. For uranium and/or thorium rich minerals such as carbonates and zircons, both SIMS and laser-ablation MC-ICPMS have been used for the direct in situ analysis of U and Th isotopes (Reid et al. 1997 Stirling et al. 2000) on very small (pg to ng levels of total U and Th) samples, at 10-100 pm scale resolution. 

For both TIMS and MC-ICPMS, improvements in sources are likely to play a major role in enhancing measurement sensitivity. This includes further development of cavity sources for TIMS and newer ICP sources and nebulizers with reduced isobaric... 

Rauscher T, Heger A, Hoffman RD, Woosley SE (2002) Nucleosynthesis in massive stars with improved nuclear and stellar physics. Astrophys J 576 323-348 Rayet M (1995) The p-process in type II supemovae. Astron Astrophys 298 517-532 Rayet M, Prantzos N, Amould M (1990) The p-process revisited. Astron Astrophys 227 271-281 Reedy RC, Arnold JR, Lai D (1983) Cosmic-ray record in solar system matter. Science 219 127-135 Rehkamper M, Halliday AN (1999) The precise measurement of T1 isotopic compositions by MC-ICPMS application to the analysis of geological materials and meteorites. Geochim Cosmochim Acta 63 935-944... 

Young ED, Ash RD, Galy A, Belshaw NS (2002) Mg isotope heterogeneity in the Allende meteorite measured by UV laser ablation-MC-ICPMS and comparisons with O isotopes. Geochim Cosmochim Acta 66 683-698... 

---