The NuSapphire uses a plasma to ionise a sample (introduced either in solution or through laser ablation). The ion beam is accelerated through the flight tube, where a combination of an electrostatic analyser and magnetic field focuses ions of similar mass/charge onto different collectors. Because the plasma is generated with argon gas, it has not always been possible to measure isotopes with similar mass to argon (^36-40Ar interferes with ^39-41K and ^40-44Ca), and argon oxides (⁴⁰Ar¹⁶O interferes with ⁵⁶Fe). The NuSapphire is equipped with a collision/reaction cell that uses hydrogen and helium gas to eliminate argon and related isotopes from the ion beam. This allows for the interference-free measurement of K, Ca and Fe isotopes without the need for high resolution slits that minimises signal intensity, opening up new avenues for exciting scientific discoveries. The Wits NuSapphire is equipped with 16 Faraday cups with 10¹¹ Ohm resistors, where six cups can be switched to 10¹² ohm resistors for higher sensitivity at low abundances, and one cup can be switched to 10¹⁰ ohm resistors for measurement of higher abundance isotopes (e.g., ⁴⁰Ca). 

Dual-path of the NuSapphire, showing the collision/reaction cell (low energy path) along with the conventional high-energy path. 

TIMS provides the ability to measure small sample sizes to higher precision compared to most other techniques. To use this instrument, a tiny amount of purified sample is loaded onto a metallic filament, which is then heated up to induce ionisation. The ion beam is accelerated through the flight tube where a magnetic field focuses ions of similar mass/charge onto different collectors.

The Wits NuTIMS is equipped with a Daly detector, an Electron Multiplier, and 16 Faraday cups with 10¹¹ Ohm resistors. Three Faraday cups can be switched to 10¹³ ohm resistors for higher sensitivity at low abundances. The NuTIMS is used for high-precision Sr and Nd isotopic measurements. Future applications include high-precision U-Pb isotopic dating of zircon and baddeleyite, and Re-Os isotopic dating of mantle materials.  

Sector Field Inductively Coupled Plasma Mass Spectrometry (SF-ICP-MS) has emerged as a powerful analytical technique for precise and accurate determination of trace element concentrations and isotopic ratios in geological materials. Particularly valuable in geochemistry and geochronology, this method has enabled major advancements in the study of U-bearing minerals (zircon, monazite, titanite, etc). These minerals are crucial archives of geologic processes, recording both the timing (via U-Pb dating) and conditions (via trace element chemistry and thermometry) of mineral formation and recrystallisation during metamorphism. The high sensitivity, low detection limits, and high mass resolution make it especially well-suited for in situ analyses when coupled with a Laser Ablation (LA) system.

The Thermo Element XR at Wits uses a magnetic sector analyzer combined with an electrostatic analyzer (ESA) to enable high mass resolution and ion transmission. Ions are separated based on mass-to-charge ratios (m/z) by applying magnetic and electric fields, allowing resolution of isobaric interferences and improved accuracy of isotope ratio measurements. The high performance, dual model secondary electron multiplier (SEM) offers a wide linear dynamic range (nine orders of magnitude) for analysis over a wide range of concentrations (from “background noise of <1 cps to a maximum of 5 x 10⁹ cps). This capability is especially important as a major matrix component (e.g., Si, Ca) is used as an internal standard to correct for the quantity of ablated material, allowing simultaneous measurement of matrix elements (wt%) and ultra trace elements (ppq). For signals of <5M cps, the Counting mode is used, while signals of 50 k to 5 G are measured in Analog mode; a large cross-over allows for simultaneous application of both modes, making use of an automatic detector cross calibration. An added Faraday collector extends the dynamic range by another two orders of magnitude (to > 10¹² cps).

Operational modes include Low Resolution (LR) (~300; highest sensitivity, but limited resolution), Medium Resolution (MR) (~4000; resolves some common polyatomic interferences) and High Resolution (HR) (~10,000; resolves complex isobaric overlaps, but with reduced sensitivity). Coupling with Laser Ablation (LA-SF-ICP-MS) facilitates spatially resolved, in situ analysis directly from solid samples, eliminating the need for dissolution and allowing analysis of small mineral domains or zoning patterns.

U-Pb dating is one of the most robust and widely used geochronological methods, making use of the decay of ²³⁸U to ²⁰⁶Pb, ²³⁵U to ²⁰⁷Pb and ²³²Th to ²⁰⁸Pb. U-bearing accessory minerals such as zircon, monazite, and titanite concentrate U, while typically (but not always) excluding initial (common) Pb. At Wits, common Pb-bearing minerals are also being dated using appropriate corrections. Rapid and precise determination of U-Pb ages with spatial resolution as low as 10–20 μm allows analysis of complex internal zoning or overgrowths in minerals. The high sensitivity also ensures that even low concentrations of Pb can be accurately measured.

When compared to SIMS or TIMS, this method offers several advantages, including rapid data acquisition (leading to faster throughput), minimal sample preparation (direct ablation reduces contamination risk and sample loss) and multielement capability (whereby U-Pb ratios are measured simultaneously with trace element concentrations). When compared to quadrupole ICP-MS (Q-ICP-MS), SF-ICP-MS offers higher mass resolution (crucial for resolving polyatomic interferences), improved precision and accuracy (important for isotope ratio measurements), and superior detection limits (capability to analyse low-concentration elements and isotopes).

While techniques like SIMS offer higher spatial resolution and TIMS provides unmatched precision for isotope ratios, SF-ICP-MS balances spatial resolution, precision, and throughput, making it ideal for many geological applications.

Beyond age determination, trace element signatures provide vital information on the crystallization environment, magmatic evolution, and post-crystallization alteration. Elements such as Th, REEs, Y, Ti, and Hf are commonly analyzed alongside U and Pb:

Zircon: REE patterns help distinguish magmatic from metamorphic origins. Ti-in-zircon thermometry provides crystallization temperature estimates.

Monazite: Th and REE contents are useful for petrogenetic studies and understanding metamorphic reactions.

Titanite and Apatite: Trace elements such as Nb, Zr, and Sr offer insights into fluid-rock interaction and magmatic evolution.

At Wits, careful calibration using matrix-matched standards and rigorous data reduction are applied to ensure accurate U-Pb dates and elemental concentrations. Laser-induced elemental fractionation, downhole fractionation, and isobaric interferences are corrected through standardization and data processing tools (Iolite or VizualAge).

Cathodoluminescence images of zircons showing internal growth zonation, and the locations of analytical spots for laser ablation U-Pb isotope measurements. Uranium-Pb isotope compositions are then plotted to calculate upper-intercept ages with the Concordia curve.

Trace element analysis plays a pivotal role in geochemistry, environmental science, biology, materials science, and other disciplines requiring precise and accurate determination of elements at ultra-trace levels. Quadrupole Inductively Coupled Plasma Mass Spectrometry (Q-ICP-MS) has become a cornerstone method due to its high sensitivity, wide dynamic range, multi-element capability, and relatively fast analysis times. When operated in solution mode, Q-ICP-MS allows for the quantitative determination of trace elements in digested rocks and minerals, soils, biological tissues, waters, and synthetic materials.

Q-ICP-MS combines an Inductively Coupled Plasma (ICP) source with a quadrupole mass analyzer. The sample is introduced as an aerosol from a liquid solution via a nebulizer. The aerosol is transported into the high-temperature (~6000–10,000 K) plasma for desolvation, atomisation and ionisation, and ions are then extracted into the mass spectrometer. A quadrupole mass filter, consisting of four parallel rods with oscillating RF and DC voltages, separates ions based on their mass-to-charge ratio (m/z). Only ions of a specific m/z can pass through the quadrupole at a given voltage setting, enabling rapid scanning and detection of multiple elements.

At Wits, for solution mode analysis, samples are first digested or dissolved into a homogeneous liquid phase using acid digestion methods (e.g., using HNO?, HF, or HCl) in open or closed-vessel microwave digestion systems to ensure complete breakdown of sample matrices. The liquid is then aspirated into the iCAP RQ  using a nebuliser, making solution mode especially effective for homogeneous sample analysis with minimal matrix effects (after digestion). Quantitative elemental analysis (stable and consistent sample introduction) and calibration using liquid standards. The solution-based trace element analysis is able to detect elements down to parts-per-trillion (ppt) levels.

In geochemistry and petrology whole-rock and mineral digests are analyzed for trace and rare earth elements (REE); this allows interpretation of magmatic differentiation, mantle-crust evolution, and ore genesis. Lithium metaborate fusion or HF-HNO? digestion are used to prepare silicate samples. For environmental monitoring heavy metals (e.g., Pb, Cd, As, Hg, Cr) are quantified in water, soil, and air particulates to track pollution sources and assessing environmental risk, aiding in regulatory compliance and scientific research. In biological and medical sciences trace metals (e.g., Fe, Zn, Cu, Se) may be analysed in blood, urine, and tissues in the study of nutrition, toxicology and for clinical diagnostics.

The key strengths of Q-ICP-MS in solution mode are high sensitivity (detection of most elements in the periodic table at ppt to ppb levels), multi-element capability (simultaneous analysis of >70 elements in a single run), the wide dynamic range (concentrations from low ppt to high ppm with appropriate dilutions), rapid analysis (for high-throughput as each sample can be analyzed within minutes) and relatively low operating cost compared to high-resolution instruments like SF-ICP-MS or TIMS.

To ensure high quality control and data accuracy at Wits, reliable trace element data from solution-mode Q-ICP-MS undergo rigorous quality control: Use of Certified Reference Materials (CRMs) for accuracy validation, application of internal standards (e.g., In, Rh, Re) to correct for drift and matrix effects; implementation of blanks and duplicates and calibration with multi-element standards, preferably matrix-matched.

Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) has revolutionised the field of geochemical and materials analysis by enabling in situ, high spatial resolution, and multi-elemental analysis directly on solid samples. When coupled with an Inductively Coupled Plasma Mass Spectrometer (ICP-MS), the technique offers high sensitivity, low detection limits, and high mass resolution. This combination is particularly suited for applications requiring precise trace element quantification and isotopic ratio measurements in small mineral domains, glass inclusions, biological tissues, and a wide range of other solid matrices.

At Wits, an Applied Spectra (formerly Australian Scientific Instruments) Resolution SE laser ablation system can be coupled to either MC-ICPMS, SF-HR-ICPMS or quadrupole ICPMS, depending on the analytical setup (isotope ratio analysis, U-Pb dating, trace element spot and image analysis). A pulsed UV laser (193 nm excimer) is focused onto the sample surface, enabling ablation of material from a solid sample in the form of a fine aerosol (particles <1 µm).This aerosol is transported by a stream of inert gas (He, Ar, N) into the plasma of the ICP-MS. Key applications at Wits include trace element analysis in minerals (e.g., zircon, monazite, apatite, garnet, pyroxene), U–Pb dating of any U-bearing mineral (including common Pb-bearing minerals, such as carbonates and apatite) with high precision and spatial resolution, inclusion studies in minerals and volcanic glass, REE patterns and elemental zoning in major and accessory silicates, oxides, phosphates, sulphides and carbonates; and Ti-in-zircon thermometry and Zr-in-rutile thermometry; an additional application is LA-based imaging of trace elements.

Accurate quantification uses a wide range of matrix-matched standards to calibrate against reference materials with known composition (e.g., NIST SRM 610–614 glasses); internal standardization is accomplished using a major element with known concentration (²⁹Si, ⁴³Ca) to correct for ablation and transport efficiencies. Data reduction employs the software Iolite for baseline subtraction, drift correction and quantification. Using rigorous calibration and quantification protocols, relative standard deviations (RSD) of 2–5 % for major elements and 5–10% for trace elements are routinely achieved. Depending on the type of analysis and material, spot size (from 10 to 100 μm) and energy density (up to 20 J/cm^-2) are optimised to enhance ablation efficiency; smaller spots yield higher spatial resolution but lower signal intensity. The LA system uses the Laurin Technic S155 ablation cell, a constant, dual volume cell that enables fast wash out times, superior signal sensitivity, uniform signal across the entire cell and minimal fraction; samples are imaged using a motorised off-axis viewing system and a high resolution camera, all which are controlled by the Geostar software.

The Thermo Nicolet iN10 Fourier transform infrared (FTIR) microscope is capable of transmission, reflection and attenuate total reflection (ATR) analyses of different types of samples. This instrument has a liquid-nitrogen cooled MCT-A detector with improved signal-to-noise ratio for the collection of high-quality absorption spectra, and a dry air purge around the sample stage to remove atmospheric moisture that may negatively impact the quality of the sample spectrum. The motorised stage enables mapping of the distribution of particular IR peaks across a sample. In transmission mode, the FTIR will be used to analyse nitrogen in diamonds, and the water content and fluid inclusion speciation in various minerals. The ATR add-on will be used to measure spectra in rock powders and sediments. 

Infrared spectra of pure diamond (left), and a diamond containing nitrogen as nitrogen pairs (right). Image from Green et al., 2022, Reviews in Mineralogy and Geochemistry. 

Heatmap showing the spatial distribution of the 1282 cm^-1 FTIR peak across a double-sided polished diamond plate.