Introduction MC-ICP-MS instruments combine a plasma source capable of ionizing most of the elements in the periodic table with a mass spectrometer capable of producing the flat top peaks necessary for precise isotope ratio measurement. These instruments have provided new and/or improved analytical opportunities for the measurement of radioactive, radiogenic and stable isotope ratios of elements ranging from Li to U. PCIGR houses three multi-collector ICP-MS by Nu Instruments. See below for more information about the specifications and uses of each instrument.
1. Nu Plasma 1700 The Nu Plasma 1700 is a double focusing MC-ICP-MS with a large geometry/mass dispersion and unique collector slit design. The collector array consists of a fixed core of 10 Faraday detectors, 6 movable ones with 3 on each mass side, and 3 ion-counting multiplier interspersed in the low mass side. The high resolution feature comprises of a continuously adjustable source slit, alpha/beta slits before ESA and slits adjustable in width and central position before each collector allowing independent variable resolutions on each collector. With these features, our Nu Plasma 1700 can provide resolutions of >5,000 (10% valley) while maintaining fully separated flat top peaks for high precision and accuracy measurements with minimal loss in sensitivity.
Other key features include: 1) Enhanced variable zoom optics for maximum flexibility; 2) High abundance deceleration filters available for each ion counting detector; 3) Compact torch box design with an externally mounted sample introduction system; 4) Enhanced pumping configuration to provide maximum protection of vacuum integrity and pump life time; 5) State-of-the-art electronics and software. Analyses are performed in dry or wet plasma mode, with a DSN-100 desolvating nebulizer system or Aridus II, respectively. Long runs are possible with an ASX-100 Cetac autosampler to minimize downtime.
Our Nu 1700 was installed in PCIGR’s nUBC lab in July 2013. The instrument has been used routinely for radiogenic isotope measurement, e.g. Pb, Hf and Nd with very high precision and accuracy. Stable and reliable results are also achieved in challenging isotopic systems like Fe, Cr and Si that require high resolution. For example, measurements of the small fractionation of Fe isotopes in igneous rocks are regularly performed with an average twice standard deviation for 56Fe of just 0.04‰ (n = 3-7; standard-sample bracketing). Isobaric interferences of 40Ar14N+ on 54Fe, 40Ar16O+ on 56Fe, and 40Ar16O1H+ on 57Fe are fully resolved.
This opens avenues for new isotopic tracer studies and collaborative interdisciplinary research, and marks a significant addition to PCIGR’s analytical capacity.
2. Nu Plasma II (#214) The Nu Plasma II is an evolution of the Nu Plasma and Nu Plasma HR. It is a double focusing magnetic sector instrument that is fitted with 16 Faraday detectors and 5 full size ion-counting multipliers that allows multiple isotope systems to be studied. The zoom optics system is also upgraded that greatly shortens the response time to enable instant changes in dispersion and peak coincidence. Additional changes to the Plasma II include enhancements to the electronics, pumping system and software, as well as a more compact torch box design with an externally mounted sample introduction system. In the sample introduction part, our Nu Plasma II is equipped with both direct solution aspiration system and a desolvating nebulizer system (DSN-100), the latter of which entails a sensitivity increase in the range of 5 – 10 X depending on the elements and a decrease in oxides levels.
In addition to the improvement in the aforementioned general design, our Nu Plasma II is also equipped with high sensitivity cones that increase the overall sensitivity without compromise of isotope ratio accuracy and precision. The cones work with DSN-100 or Aridus II. This set-up has gone through extensive tests with elemental standards and real life samples in our nUBC facility. It is now regularly used for radiogenic isotopic analyses of low concentration samples. Ideal (i.e. without loss of reproducibility) results can be achieved with the amount of analyte at 10, 20 and 25ng for Pb, Hf and Nd, respectively for isotope ratio measurement.
Although mainly used for radiogenic isotopic measurement in unit mass resolution, our Nu Plasma II is also capable of pseudo high resolution measurement when interferences are on the same side of the peak. This is achieved by reducing the width of the source defining slit using a selectable slit mechanism and then reducing the width of the alpha slit located before the ESA to enhance the peak shape by reducing any image aberration.
3. Nu Plasma (#021) The Nu Plasma fulfills multiple roles: it is used to analyse a wide range of radiogenic (Nd, Hf and Pb) and stable (Li, Fe, Cu, Zn, Mo & Cd) isotopes serving both as a research tool and an educational resource. The Nu Plasma is equipped with a plasma source capable of ionizing most elements. An electrostatic analyzer, a 25cm radius magnet and two zoom lens stacks are used to focus the ion beam into a fixed array of 12 Faraday cups and 3 ion-counting multipliers. Initial instrument sensitivity has been improved in 2003 by the addition of an 80-litre primary pump that significantly improves pumping at the interface. Sensitivity is further enhanced by use of a Nu Plasma desolvating nebuliser system (DSN-100) along with matching cones to operate in dry plasma conditions. The entrance slit is adjustable and can be used to increase instrument resolution sufficiently to partially resolve 40Ar14N and 40Ar16O from 54Fe and 56Fe and enable measurement of Fe isotope ratios under “pseudo high resolution” conditions. Depending on the project, samples may be run by the PCIGR team or by visitors after the instrument has been tuned. All students/PDFs however, are trained to operate the instrument independently from fire up to shutdown, to assess instrument performance and to process their own data. In doing so they gain valuable experience of both instrumental techniques and troubleshooting and the ability to make critical evaluations of data quality.
Cd isotope analyses were carried out on oyster tissues in order to evaluate the heavy metal contamination along North American coastal regions (Shiel et al., 2012) as well as in products of the smelting and refining process of metal ores (Shiel et al., 2010). Cd isotope analyses are carried out using a standard-sample-bracketing technique. We use the PCIGR-1 in-house standard, which matches the JMC Cd Muenster standard (Wombacher et al., 2003). The internal precision on the 114Cd/110Cd ratios is <1.5ppm and we achieve an external precision of typically d114/110Cd <0.30‰ (Shiel et al., 2012).
For Zn isotope analyses, the purified Zn fraction is doped with a well-defined Cu amount for internal standardization, and run on the MC-ICP-MS in static mode. Sample runs are bracketed by runs of our in-house standard PCIGR-1 Zn (Shiel et al., 2009). The bracketing standard PCIGR-1 Zn was crosschecked with the PCIGR-2 Zn standard. External precision is typically below 0.1‰ (2sd) for 66Zn/64Zn ratios. During an analytical session, a value of d66/64Zn = 0.00±0.04‰ is achieved for the standard PCIGR-1 Zn. Duplicates of samples including a separate sample processing with purification and analyses are carried out routinely and have shown to be identical within analytical uncertainties.
High–precision Mo isotope ratios are analyzed by the double-spike technique. A 100Mo-97Mo double spike is added to the samples prior to any preparation for MC-ICP-MS analyses. This ensures that any mass-dependent fractionation induced by lab handling and processing can be accounted for. After the addition of the spike and removal of matrix elements via column chemistry, the Mo isotopes 92Mo, 94Mo, 95Mo, 96Mo, 97Mo, 98Mo, and 100Mo are collected in static mode. For low level samples the desolvating unit DSN-100 is used. Blank correction is carried out by on-peak zero measurements in between sample analyses. Mo isotope ratios are referred to those of an in-house Mo standard solution (Mo(UBC)), that has been cross-calibrated to internationally available Mo standards, such as NIST SRM 3134. A long-term external precision of 0.05‰ (2sd) is achieved using this method. Details of the method are described in Skierszkan et al. (2015).
PCIGR also carries out Li isotope analyses by the standard-sample-bracketing technique, using the internationally accepted lithium carbonate standard L-SVEC from the National Institute for Standardization and Technology (NIST). Samples are either dissolved or leached and undergo a sample purification method after Jeffcoate et al. (2004), which involves two ion-exchange column steps to provide a pure Li fraction. Care is taken that the recovery yield of the Li fraction is close to 100% (cf., Harrison et al., 2015 for full description). Li isotope signals are collected in static mode and reported relative to L-SVEC that is analyzed alternately. Each sample is measured at least three times. During an analytical session of over three days with around 240 analytical runs, we achieve an average 7Li/6Li ratio of 13.95±0.03‰ (2sd) for L-SVEC. For low sample materials, we also use a desolvating sample introduction system such as the DSN-100 (Nu instruments, Instruments, Wrexham UK).
The Nu Plasma (#021) MC-ICPMS also provides a platform for Lu-Hf isotope analysis and geochronology. This method allows high-precision dating of garnet, an important indicator mineral for crust and mantle processes. At PCIGR, this method is applied to investigate a variety of topics, such as the timing and rates of subduction, the long-term evolution of the cratonic mantle, and the tectonic reconstruction of mountain belts. The Nu Plasma Nu#021 will additionally become part of a new LA-(MC-)ICPMS infrastructure, which is currently under development. The facility, which also includes the Finnigan Element2 HR-ICPMS and an ESI/New Wave Research NWR 193 ArF Excimer laser ablation system, will be used for simultaneous in-situ U-Pb + Hf isotope analysis of zircon and U-Th-Pb + Nd isotope analysis of phosphate minerals. These methods serve to uncover source characteristics of magmatic rocks from various stages of Earth’s history.
Mainly, we focus on stable isotopes and isotope systems that require a spike during analysis, while radiogenic isotope analyses such as Nd, Hf and Pb are assigned to our MC-ICP-MS Nu Plasma II that is described above.