Electron probe microanalysis

There is a long history in the field of electron probe microanalysis at BGR. In 1968, the first microprobe Elmisonde from SIEMENS was installed. A CAMECA Camebax replaced the Elmisonde in 1985, which was followed by a CAMECA SX100 in March 2000. The newest acquisition is a JEOL JXA-8530F Hyperprobe, which was installed in autumn 2015. The fundamental innovations of this microprobe are a Schottky field emitter and a spectrometer specially designed for light element analysis (soft X-ray emission spectrometer - SXES).

Electron microprobe JEOL JXA-8530F Hyperprobe with SXES at BGR Source: BGR

The electron microprobe is a device for non-destructive in situ measurement of element concentrations (detection limits down to about 50 ppm at optimised conditions) in order to determine the chemical composition of solids (e.g. minerals, glass, steel) at high spatial resolution. The Schottky field emitter enables high resolution (resolution down to 3 nm in secondary electron image) because of its significantly smaller beam diameter (about 50 nm) and facilitates microanalysis at low acceleration voltages. All elements from atomic number 3 (lithium) to 92 (uranium) may be detected. For this purpose, a high energy (up to 30 kV) electron beam is focussed on the polished surface of the specimen (which previously needs to be coated with carbon for discharge) and causes physical interactions with the atoms of the respective elements in the specimen, which can be recorded with different detectors.
The excited volume is very small (about 1 μm³) depending on the applied acceleration voltage. Secondary (morphology/topography) and backscattered primary electrons (distribution of the mean atomic number) are used for imagery, whereas characteristic X-ray are measured for qualitative or quantitative element analysis. The wavelength as well as the energy of characteristic X-rays are specific for each chemical element within the specimen. The JEOL JXA-8530F is equipped with an energy-dispersive system (EDX), with wavelength-dispersive spectrometers (WDX) and SXES. The EDX is generally used for quick qualitative element analysis. Quantitative analysis is performed by WDX with various diffracting crystals covering different spectral ranges. The SXES has a high energy resolution and enables simultaneous measurement at low energy levels for light elements like lithium to boron. Optionally, a fifth WDX (TAP, LIF) could be installed in exchange for SXES.

The current configuration of the single channels is a follows:

• channel 1: TAP, PET, LDE1, LDE2
• channel 2: SXES
• channel 3: PETL, LIFL
• channel 4: TAPH, PETH
• channel 5: PETH, LIFH

The basic principle for quantitative WDX analysis is that the measured intensity of characteristic X-rays is proportional to the elemental composition of the specimen. The measured intensity of the specimen is compared to the intensity of a standard with known composition. A large number of standard materials (minerals, metals, glass and synthetic compounds) are available, which are essential for quantitative measurements. Trace element analysis on ppm level is possible with optimised settings (e.g. detection limits of 20 ppm Pd in sulphides).
The setup allows analysis of spots, lines or areas of the sample surface. The measurement of characteristic X-rays from an area is used to generate element maps of single elements either by WDX, EDX or in combination. Element mappings are particularly interesting for zoned mineral phases. A PC-controlled microscope called pointlogger facilitates quick and timesaving identification of the desired coordinates. Additionally, the microprobe is equipped with a so-called specimen navigator.

Current application fields:

• Mineralogy and trace element concentrations in sulphide deposits from the Indian Ocean
• Trace elements (Ga, Ge, In) in sulphides from Cu-Pb-Zn deposits
• Characterisation of platinum group minerals (PGM) in rocks, ores, concentrates and placers
• Genesis, cycle of materials and optimisation of processing of oxidised platinum ores
• Chemical composition of single layers within manganese nodules
• Characterisation and trace element contents of mineral residuals in mine heaps, tailings and slags as resource potential
• Analytical fingerprint of mineral resources, e.g. tantalum, tin and tungsten ores
• Characterisation of standard reference materials