2.5 Applications of MC-ICP-MS to radiogenic isotopes

MC-ICP-MS is a more expensive technology than conventional TIMS, both to build and to operate. This is because the instruments have several complex engineering systems, involving the RF generator, the torch assembly with its argon supply, and the ion extraction system with its pumps, in addition to the magnetic sector analyser. Hence, MC-ICP-MS is most effectively applied to isotope systems where TIMS has encountered technical difficulties. A few of these systems will be described.

2.5.1 Hf–W

¹⁸²W is the decay product of the short-lived nuclide ¹⁸²Hf (t_1/2 = 9 Myr). As an extinct nuclide ¹⁸²Hf can place important constraints on the origin and early history of the Solar System, but the very high ionisation potential of tungsten (7.98 eV, similar to osmium) precludes its analysis as a positive ion by TIMS. The first high-precision W isotope measurements were made by negative ion TIMS, but MC-ICP-MS proved ideally suited to the analysis of this element. Following the establishment of the method (Lee and Halliday, 1995), these authors made numerous W isotope analyses of meteoritic and planetary samples, throwing new light on the timing of terrestrial core formation and the formation of the solar nebula (section 15.5.4).

2.5.2 Lu–Hf

The Lu–Hf isotope system was first exploited using positive TIMS analysis (section 9.1), but the high ionisation potential of hafnium (6.65 eV) has always rendered TIMS Hf analysis difficult, requiring large sample size and high purity ion exchange separation in order to achieve good results. The ability of MC-ICP-MS to achieve excellent ionisation on impure sample solutions makes the method ideal for Hf isotope analysis. Whole-rock samples containing as little as 100 ng Hf can be analysed after pre-concentration only (Blichert-Toft et al., 1997), and zircons can be analysed in situ using a laser probe MC-ICP-MS (Thirlwall and Walder, 1995). Previously, TIMS analysis of Hf also required a very demanding chemical separation method (Patchett and Tatsumoto, 1980), Hence, for all of these reasons, MC-ICP-MS has effectively rendered TIMS analysis of hafnium obsolete.

2.5.3 U–Th

Thorium is another element with a high ionisation potential (6.97 eV). It is routinely analysed by TIMS, but the poor ionisation efficiency is a significant problem, particularly for samples with large common thorium (²³²Th) contents (section 13.1). Such samples are well suited to simultaneous analysis on a Faraday and Daly collector. In this approach the small ²³⁰Th beam is analysed on a Daly or multiplier collector, while the large ²³²Th beam is analysed simultaneously on a Faraday collector (e.g. Luo et al., 1997). In order to calibrate the relative gain of the Daly and Faraday collectors, Luo et al. added uranium of known isotopic composition directly to the Th sample. The ²³⁵U/²³⁸U ratio measured on two Faraday collectors is used for mass fractionation correction of both U and Th, while the ²³⁴U/²³⁵U ratio is used to calibrate the Daly/Faraday collector gain. Results for the Table Mountain Latite standard showed excellent external (between run) precision, as well as internal (within run) precision. In more recent work, the method was used successfully to duplicate TIMS analyses of carbonate samples (Shen et al., 2002) and igneous rock analysis (Pietruszka et al., 2002).

2.5.4 Pb–Pb

Pb isotope analysis has long been performed by TIMS, but the existence of only one stable non-radiogenic isotope has always made mass fractionation the principal source of analytical error. Mass fractionation occurs in the plasma source, but it is almost completely independent of the chemistry of the species involved. Therefore, the two stable isotopes of thallium (202 and 205) can be used to make accurate fractionation corrections for Pb (Walder and Furuta, 1993).

More detailed experiments (White et al., 2000) appeared to show small differences in the mass fractionation behaviour of Pb and Tl, although these could be corrected for. Some minor problems were also found by Thirlwall (2002) and attributed to subtle solution chemistry effects in the nebuliser. However, Collerson et al. (2002) observed coherent fractionation behaviour between Pb and Tl, demonstrated by plotting interference-corrected raw ratios for both elements on a log-log plot. Hence, they were able to obtain accurate fractionation-corrected Pb isotope data on several different types of sample, with an average reproducibility of ca. 300 ppm.

It is concluded that MC-ICP-MS can achieve accuracies of Pb isotope analysis approaching those produced by the much more experimentally demanding double spiking methods using TIMS (section 2.4.2). Therefore it can be expected that MC-ICP-MS will take over from TIMS as the preferred method for Pb isotope analysis where large samples suites must be analysed to high precision. This would apply to studies of igneous petrogenesis, and especially to Pb isotope analysis as a environmental tracer, such as in the study of seawater evolution from the analysis of ferromanganese crusts (Christensen et al., 1997).

2.5.5 U–Pb

Another area where MC-ICP-MS has been successfully applied is the in situ U–Pb analysis of zircons by laser ablation, for applications in geochronology. An infra-red or (more usually) an ultra-violet laser is used to ablate the sample inside a small chamber which is at atmospheric pressure. The atomic sample cloud in the chamber is carried to the plasma by the argon ‘sampler’ gas flow, and is then ionised in the plasma in the usual way (Fig. 2.25).

Fig. 2.25. Schematic illustration of the procedure for feeding laser ablated material into the ICP source. After Halliday et al. (1998).

Walder et al. (1993a) used a glass standard to calculate the efficiency of laser ablation MC-ICP-MS for ion production. They found that the efficiency was comparable with ion microprobe analysis using the SHRIMP (section 5.2.3). However, because the volume of sample excavated by laser ablation can be larger than with the SHRIMP, laser ablation MC-ICP-MS can generate ion beams large enough for analysis by multiple Faraday collectors (Halliday et al., 1998). Hence the instrument is potentially capable of generating ages with higher precision than the SHRIMP. The instrument used by Halliday et al. (1998) was fitted with a wide flight tube to permit simultaneous analysis of U and Pb.

Some problems remain in the accurate calibration of U/Pb ratios measured during the laser ablation process. However, Horn et al. (2000) proposed that ²⁰²Tl, ²⁰⁵Tl and ²³⁵U spike could be added by solution nebulization to the gas stream from laser ablation of the sample. Use of a zircon standard showed that elemental U/Pb fractionation during ablation could be quantified for a given laser power, spot size, and depth of excavation. This calibration procedure then allowed U/Pb fractionation during sample ablation to be calibrated. Pb mass fractionation during beam extraction from the plasma was also corrected. Horn et al. applied this procedure on a quadrupole ICP–MS, whose ultimate precision is limited to ca. " 1% (2F) by the instability of the plasma. However, the use of this spiking technique in combination with MC-ICP-MS offers the possibility of matching the dating precision of ion microprobes such as the SHRIMP (section 5.2.3).

2.5.6 Sm–Nd

Experiments to test the effectiveness of MC-ICP-MS for Nd isotope analysis were reported by Walder et al. (1993b) and Luais et al. (1997). Most analyses were made in static multi-collection mode, but a few multi-dynamic analyses gave similar results to static analysis. It was shown that Sm interference corrections could be made very accurately, so that Nd isotope ratio determinations could be made on bulk REE separates from a cation column, without secondary clean-up to remove Sm. In addition, the typical analysis time was only 20 minutes per sample.

Disadvantages of the ICP method were the longer instrument setup times, the need to analyse more standards, and the slightly lower external (between run) reproducibility. This was found to be ca. 30 ppm over short periods, but 60 ppm over a period of a year. However, a more recent study (Vance and Thirlwall, 2002) suggests that this lower reproducibility was due to the inadequacy of the exponential law to correct for the large fractionation factors encountered with ICP, which are more consistent than TIMS fractionation effects, but usually more than an order of magnitude larger. Vance and Thirlwall showed that these problems could be solved using information from the fractionation of additional non-radiogenic ratios, therefore allowing MC-ICP-MS to generate reproducibilities comparable with TIMS. However, TIMS Nd analysis is simpler and cheaper, making it accessible to a wider range of users. Therefore, it appears that Nd isotope analysis will be a niche where TIMS will be applied for some time to come.

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