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 measurements. These instruments have provided new and/or improved analytical opportunities for measuring radioactive, radiogenic and stable isotope ratios of elements ranging from Li to U. PCIGR houses three MC-ICP-MS by Nu Instruments. The information below discusses 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 detectors with 3 on each mass side, and 3 ion-counting multipliers interspersed on the low mass side. The high-resolution feature comprises a continuously adjustable source slit, alpha/beta slits before the electric sector analyzer (ESA), and slits adjustable in width and central position before each collector that allows 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 highly precise and accurate measurements and minimal loss in sensitivity.
Other key features: 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 DSN-100 or Aridus II desolvating nebulizer systems, 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 such as 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 two-standard-deviation (2σ) for δ56Fe 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 has been upgraded and greatly shortens 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 and an externally mounted sample introduction system. As for sample introduction, our Nu Plasma II is equipped with both a direct solution aspiration system and a DSN-100 desolvating nebulizer system, the latter of which increases sensitivity by 5–10 times, depending on the elements, and decreases oxide formation.
In addition to these general design improvements, our Nu Plasma II is also equipped with high-sensitivity cones that increase overall sensitivity without compromising isotope ratio accuracy and precision. The cones work with both DSN-100 or Aridus II desolvating nebulizer systems. This set-up has gone through extensive testing with elemental standards and real-life samples, and is now regularly used for radiogenic isotopic analyses of low-concentration samples. Ideal results (i.e., without loss of reproducibility) can be achieved with analyte quantities of 10, 20 and 25 ng for Pb, Hf and Nd, respectively, for isotope ratio measurement.
Although mainly used for radiogenic isotopic measurements in unit mass resolution, our Nu Plasma II is also capable of pseudo-high-resolution measurements when mass 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 analyze a wide range of radiogenic (Nd, Hf, Pb) and stable (Li, Fe, Cu, Zn, Mo, Cd) isotopes, and serves as both 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 25-cm 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 was improved in 2003 by the addition of an 80-L primary pump that significantly improves pumping at the interface. Sensitivity is further enhanced by use of a DSN-100 desolvating nebuliser system 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 isobaric interferences of 40Ar14N+ on 54Fe and 40Ar16O+ on 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 in both instrument techniques and troubleshooting and the ability to make critical evaluations of data quality.
Cd isotope analyses have been performed on oyster tissues in order to evaluate heavy metal contamination along the North American coast (Shiel et al., 2012), and in products of the smelting and refining processes of metal ores (Shiel et al., 2010). Cd isotope analyses use a standard-sample-bracketing technique. We use the PCIGR-1 in-house standard, which matches the JMC Cd Muenster standard (Wombacher et al., 2003). For δ114/110Cd ratios, internal precision is <1.5ppm and external precision is typically <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 is cross-checked with the PCIGR-2 Zn standard. External precision is typically <0.1‰ (2sd) for δ66/64Zn ratios. During an analytical session, a value of δ66/64Zn = 0.00±0.04‰ was achieved for the standard PCIGR-1 Zn. Analyses of duplicate samples, including separate sample processing with purification, are carried out routinely and have shown to be identical within analytical uncertainties.
High–precision Mo isotope ratios are prepared with the double-spike technique where a 100Mo/97Mo double spike is added to the samples prior to MC-ICP-MS analyses. This ensures that any mass-dependent fractionation induced by lab handling and processing can be accounted for. After adding the spike and removing 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 DSN-100 desolvating nebulizer is used. Blank correction is carried out by on-peak zero measurements in between sample analyses. Mo isotope ratio values are reported against 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) was achieved using this method. Details of the method are described in Skierszkan et al. (2015).
Li isotopes are analyzed with 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 (Jeffcoate et al., 2004), which involves two ion-exchange column steps to provide a pure Li fraction. Care is taken to recover close to 100% of the Li fraction (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 that spanned over three days and ~240 analytical runs, we achieved an average 7Li/6Li ratio of 13.95±0.03‰ (2sd) for L-SVEC. For low-level samples, we use a DSN-100 desolvating nebulizer system.
Lu-Hf isotope analysis and geochronology
Lu-Hf isotope analysis and geochronology allows for 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 is part of the LA-(MC-)ICP-MS infrastructure, which is currently under development. The facility, which also includes the Finnigan Element2 HR-ICP-MS 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.