Electron optics and microanalysis

Scanning electron microscopy

GTK has four scanning electron microscopes (SEMs): two in Espoo, two in Outokumpu (see table). Both locations feature one “traditional” SEM with tungsten filaments, and one more modern SEM using field emission (FE-SEM). The traditional SEM can produce images at 10,000X magnification in optimal conditions, and it can produce images of particles with a size of less than one micron. The FE-SEM can reach a magnification of several hundred thousands, and it can produce nanometre-scale images.

All GTK’s SEMs are connected to an energy-dispersive X-ray spectrometer (EDS/EDX), which means that in addition to imaging, they can be used to analyse the chemical element composition of samples. The equipment enables us to analyse a variety of mineral-based materials and other inorganic materials.

Some SEMs have an LV feature, which enables samples to be analysed in a pre-set low vacuum. This means samples do not need to be separately prepared for analysis. However, in low vacuum conditions, the analysis accuracy is not as good as in high vacuum conditions.

The results of EDS analysis are semi-quantitative. The measuring accuracy of chemical element contents varies depending on sample but is usually around 0.3–0.5 weight percent (wt%). Samples with a grain size less than 1–3 μm cannot be analysed reliably. Nor is exact identification of mineral phases from an EDS spectrum always possible. In addition, phases with similar or identical chemical composition cannot be distinguished from each other.

EDS systems are operated with a set of software that enables analyses to be conducted largely automatically. Point or line analyses can be conducted on samples, their modal mineralogy can be analysed, and automatic searches related to them can be performed. In addition, dispersal maps on chemical elements can be prepared, and data can be produced on grain size distribution and mineral associations and liberations for use in mineral processing.

GTK’s SEM and EDS equipment

Microanalysis

Microanalysis is an important mineralogical analysis method that is often needed to supplement SEM and EDS analyses, for example. The GTK Research Laboratory in Espoo has an electron probe microanalyser (EPMA), which enables us to do quantitative analyses of nearly all elements on the periodic table (Be–U). The Cameca SX100 EPMA equipment in Espoo features five wavelength-dispersive spectrometers (WDS) that are used in quantitative analysis. In routine analyses, the chemical-element-specific detection limits are around 0.01%, or as low as 10 ppm if a long measurement time is used in trace element analysis. The chemical elements to be analysed are selected beforehand. The microanalyser also features an EDS, as well as BSE, SEI and CL detectors for imaging.

Point, profile or grid analysis can be conducted with the microanalyser. The smallest area that can be analysed is an area of around 1–2 microns in diameter. Usually, mineral structures and other targets to be analysed must be identified with microscopy or SEM beforehand.