MC-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 ... 

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... 

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... 

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

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... 

Figure 4.8 Multiple ion collector array from MC-ICP-MS NEPTUNE and MC-TIMS TRITON (Thermo Fisher Scientific, Bremen). Miniaturized ion counters identical in size to Faraday detectors are mounted in the high mass range to detect, e.g., low-abundance uranium isotopes. (Reproduced by permission of Thermo Fisher Scientific, Bremen.)...

MC-TIMS measurements are relevant for high precision isotope analysis as discussed in Chapter 8. 

Table 8.1 176Hf/177Hf isotope ratio measured by MC-ICP-MS and MC-TIMS. 

The instrumental mass discrimination effects between MC-ICP-MS (VG Axiom and Isoprobe from Micromass) and MC-TIMS (VG Sector 54, Micromass) were compared for 8UB measurements.82 Measurements by MC-ICP-MS yielded more precise isotope ratios than those obtained by TIMS (one magnitude better compared to negative ion TIMS) despite showing the highest mass bias. However, the mass discrimination effects appear to be unaffected for 8nB values.82... 

Isotope abundances which are free from all sources of bias are defined as absolute isotope abundances. The absolute isotope composition of elements can be measured by MC-TIMS and MC-ICP-MS via gravimetric synthetic mixtures or standard solutions from highly enriched isotopes, as demonstrated for neodymium,11 erbium13 and samarium,11 13 99 respectively. 

The results of uranium isotope ratio measurements by MC-TIMS (TRITON Thermo Fisher Scientific) on a single uranium oxide reference particle (10 pan) are illustrated in Figure 8.5.9,144 The high precision of multiple ion counters and high detection power in the MC-TIMS are... 

Figure 8.5 Precision of uranium isotope ratios measured by MC-TIMS (Triton, Thermo Fisher Scientific, Bremen). (]. S. Schwieters etai., Ceochim. Cosmochim. Acta, 68, A60 (2004). Reproduced by permission...

An interesting approach is the application of multiple ion collector mass spectrometry (MC-TIMS and MC-ICP-MS) for the determination of Cd and T1 in high purity zinc metal after trace matrix separation by the certification of reference materials from the Bureau Communie de Reference (BCR).29 Accurate and precise element concentrations in high purity zinc metal have been obtained with both mass spectrometric techniques via precise isotope ratio measurements using the isotope dilution strategy the analytical data. 

High precision determination of Rb/ Rb isotope ratios by MC-ICP-MS (Isoprobe, Micromass) is advantageous in comparison to MC-TIMS (VG Sector 54, Micromass) due to an unproved precision, and fraction capability problems can be easily considered using admixed Zr for mass bias correction via the Zr/ °Zr isotope ratio. Rb isotopic composition in geological materials (basalt, granite and greywacke) and synthetic samples (in house mica standard, NBS 607 K-fsp standard) have been examined with a reproducibility of Rb/ Rb measurements of 0.02-0.05 % (2 O ). No anomalous isotope fractionation was observed for the samples investigated. 

---