Mineral extraction and their processes have a huge impact on the environment, energy consumption and safety. With the ever-increasing demand for high grade mineral sources, there is a need for improved productivity and value addition while meeting the environmental standards and quality. This demand calls for efficient analytical techniques to help mining operations so that faster decisions can be made, and more value-added products can be processed.
Efficient analytical techniques in the lab, such as ICP-OES, ICP-MS, EDXRF and WDXRF play a crucial role in assisting mining operations and laboratories. These advanced techniques offer enhanced sensitivity, selectivity, and expanded analytical capabilities, empowering faster decision-making and enabling the processing of value-added products.
ICP-OES
Inductively coupled plasma optical emission spectroscopy (ICP-OES), sometimes referred to as ICP-atomic emission spectroscopy, is the technique of choice for many applications that require analyzing a sample for its elemental content. Typical samples include those in the environmental, metallurgical, geological, petrochemical, pharmaceutical, materials, and food safety arenas. It can be applied to varying sample types such as aqueous and organic liquids and solids. Some of these sample types need specific sample preparation techniques or the use of specific accessories to allow the sample to be introduced into the ICP-OES instrument.
A variety of sample types can be analyzed by inductively coupled plasma optical emission spectroscopy (ICP-OES), including aqueous and organic liquid and solid samples. These have to be brought into a state that the ICP-OES instrument as a whole can process for elemental analysis. The most typical sample form is a liquid. A liquid sample is introduced using a peristaltic pump to ensure constant, stable flow. Commonly, a nebulizer uses a high-speed flow of gas (usually argon) to shatter small droplets of liquid into an aerosol. This aerosol is then introduced into a spray chamber which removes the larger droplets. Only the aerosol is then transported to the plasma. Solid samples are typically digested into liquid form using acid hot plate digestion or microwave digestion or ablated into small particles either using a laser or spark ablation system and then transported directly to the plasma by a carrier gas.
The advantages of using ICP-OES over other elemental analysis techniques such as atomic absorption spectrometry (AAS) include its wide linear dynamic range, high matrix tolerance, and the enhanced speed of multi-elemental analysis that can be achieved.
ICP-MS
Inductively coupled plasma mass spectrometry (ICP-MS) is an elemental analysis technology capable of detecting most of the periodic table of elements at milligram to nanogram levels per liter. It is used in a variety of industries including, but not limited to, environmental monitoring, geochemical analysis, metallurgy, pharmaceutical analysis, and clinical research.
The Inductively Coupled Plasma (ICP) is an ionization source that fully decomposes a sample into its constituent elements and transforms those elements into ions. It is typically composed of argon gas, and energy is “coupled” to it using an induction coil to form the plasma. In ICP-MS, the ions generated by the ICP are introduced into a mass spectrometer, where they are separated based on their mass-to-charge ratios and detected, allowing for highly sensitive and precise elemental analysis.
To be processed efficiently in the plasma, samples must be in either gas or vapor (aerosol) form. So, while gases can be analyzed directly by the plasma (e.g., when separated by gas chromatography), solids and liquids have to be converted to aerosol form using either a nebulizer (for liquids) or an ablation device (for solids). In addition, solid samples in ICP-MS are typically introduced through sample digestion techniques. The solid sample is first dissolved or digested using acid digestion methods, such as hot plate digestion or microwave digestion, to convert it into a liquid form. Once in liquid form, the sample can be introduced into the ICP-MS instrument using a nebulizer or other sample introduction system, where it undergoes ionization and subsequent analysis.
ICP-MS has advantages over ICP-OES including higher sensitivity for trace multi-element detection, isotopic analysis capability, lower spectral interferences, wider dynamic range, and faster analysis times.
ICP-OES and ICP-MS for Trace Elemental Analysis
As mentioned above, ICP-OES and ICP-MS are powerful analytical techniques used in the mining and geological industries to assess elemental composition. While some samples require dual analysis, with ICP-OES for higher concentration elements and ICP-MS for trace elements, a single analysis would enhance productivity and reduce costs per sample.
ICP-OES is widely employed for mining processes, purity control, and analysis of rocks and rare earth elements. It ensures the purity of extracted ores and evaluates the amount of metal that can be recovered from electronic waste. With its ability to handle a wide range of elements, including rare earth elements, ICP-OES plays a crucial role in quality control, process optimization, impurity detection, and ensuring batch-to-batch consistency.
ICP-OES systems are even capable of analyzing Platinum Group Element (PGE) analytes in high concentration metal matrices with excellent accuracy by effectively handling complex matrices and achieving full separation of matrix lines, such as the Pd 340.458 nm line from the neighboring Co 340.512 nm matrix line.
ICP-MS systems, on the other hand, offer superior detection limits, making it suitable for measuring trace levels of elements. When equipped with AGD (argon gas dilution), they offer a robust solution for precise and dependable elemental analysis, even in challenging sample types such as rocks and ores. For even better performance, triple quadrupole ICP-MS can be used as a powerful solution for analyzing ultra-trace levels of noble metals (Rh, Pd, Pt, Au). It effectively eliminates interferences and enables accurate quantification of noble metals even at extremely low concentrations.
EDXRF Analysis
Energy-dispersive X-ray spectroscopy (EDXRF) an analytical technique that enables the chemical characterization/elemental analysis of materials. A sample excited by an energy source (such as the electron beam of an electron microscope) dissipates some of the absorbed energy by ejecting a core-shell electron. A higher energy outer-shell electron then proceeds to fill its place, releasing the difference in energy as an X-ray that has a characteristic spectrum based on its atom of origin. This allows for the compositional analysis of a given sample volume that has been excited by the energy source. The position of the peaks in the spectrum identifies the element, whereas the intensity of the signal corresponds to the concentration of the element.
A convenient front-end analysis tool, EDXRF instruments enable quick and easy analysis of even irregular samples with little-to-no sample preparation. In EDXRF, sensitivity and precision are achieved by targeted excitation of the sample to fluoresce only the elements of interest. EDXRF technology is engineered to simultaneously process whole groups of elements for qualitative or quantitative analysis and can be used in portable and laboratory-based formats. As a result, EDXRF can accommodate samples of almost any shape and size.
WDXRF Analysis
Wavelength Dispersive X-ray Fluorescence (WDXRF) is preferred to EDXRF for high resolution applications (~15-150 eV) and analysis of lower atomic mass elements and rare earths. Rather than process a complete spectrum, WDXRF separates fluorescent signals into individual wavelengths using crystals and a series of optical components (collimator, optical encoders, detectors, etc.).
The WDXRF Spectrometer uses crystals to disperse the fluorescence spectrum into individual wavelengths of each element, providing high resolution and low background spectra for accurate determination of elemental concentrations. Two types of detectors can be used in WDXRF instruments. Sealed or flow gas detectors are best for measuring lower energies (light elements, below iron [Fe]), while scintillation detectors are better for measuring higher energies. Both have poor resolution, which is compensated for by the crystals.
XRF Mapping is a great tool to measure the homogeneity of a sample in a sub-millimeter size range. This can help validate sample preparation or indicate problems in a process. Geologists use XRF elemental mapping to select or screen samples for more in-depth analysis with a scanning electron microscope (SEM), which requires highly controlled sample preparation and provides information in the sub-micrometer size range.
Conclusion
These technologies help enable better sensitivities across the periodic table and improved precision and detection limits. Alternatively, the counting time can be reduced, increasing sample throughput significantly. Improved identification coupled with increased throughput helps reduce costs and improve productivity in the mining lab so mining operators can come to faster and better decision making.
Additional Resources:
- Application Note: Analysis of nickel ore with the ARL OPTIM’X WDXRF Spectrometer
- Application Note: EDXRF Analysis of Nickel Ore as Pressed Powders in an Air Environment
- The Suitability of Using EDXRF and WDXRF to Analyze Nickel Ore
- Analysis of noble metals at low levels in geological reference materials and ores using ICP-MS
- Analysis of platinum group elements using ICP-OES
- Robust and accurate analysis of refined Nickel using ICP-MS
- Ebook: XRF Technology for Non-Scientists
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