Queen's University

JEOL JXA-8230 Electron Microprobe Lab

To schedule time on the instrument, please contact Brian Joy at joy@geol.queensu.ca

The department's JEOL JXA-8230 electron microprobe was installed in 2011 and is available for general use; the instrument is located in Bruce Wing, room 122. Clients from academia and industry are welcomed. Services offered include quantitative and qualitative chemical analysis, backscattered and secondary electron imaging, and X-ray mapping. The instrument is equipped with five two-crystal wavelength dispersive spectrometers and one energy dispersive spectrometer (SDD). Among the five WD spectrometers, one contains large-area LiF and PET diffracting crystals, and one contains “high-intensity” LiF and PET crystals. Two spectrometers contain TAP crystals and LDE “pseudocrystals,” the latter for detection of elements of atomic numbers five through nine.

Hourly Rate to use the JEOL JXA-8230 Electron Microprobe
  CAD/hr assisted CAD/hr unassisted
Intradepartmental 70 35
Academic 80 45
Industrial 155 120
The set of images of a zoned pyrite grain above was acquired using the WD and ED spectrometers and backscattered electron detector. The As L-alpha image was acquired in differential mode using a TAP diffracting crystal. The Ni K-alpha image was acquired using the “high-intensity” LiF crystal. The S K-alpha image was acquired using EDS (with X-ray limiting aperture in place). Accelerating potential was 20 kV, probe current was 200 nA, and dwell time was 50 ms. The colour intensity scales illustrate X-ray counts per pixel. Within the pyrite, maximum As concentration is ~1200 ppm, and maximum Ni concentration is ~1.7 weight %.
The superiority of WDS (left side) over EDS (right side) in imaging the distribution of elements present in low concentrations is demonstrated clearly in the above Ni K-alpha and As L-alpha images of pyrite, which were collected simultaneously. Not only does the LiFH crystal produce more X-ray counts per pixel at the Ni K-alpha wavelength, but the peak-to-background ratio is much greater, and interferences are less severe. Had the mapping been done using only EDS, As would not have been detected in the pyrite.
The set of images of a cluster of zoned amphibole grains above was acquired using the WD spectrometers and backscattered electron detector. The Cl K-alpha image was acquired using the “high-intensity” PET crystal. Accelerating potential was 15 kV, probe current was 200 nA, and dwell time was 50 ms. The colour intensity scales illustrate weight % oxide (Na2O, Al2O3) or weight % element (Cl). Within the amphibole, weight % Al2O3 ranges between 1.3 and 9.4, weight % Na2O ranges between 0.3 and 1.8, and weight % Cl ranges between 0.2 and 0.5. Note that, despite the strong zoning, the electron backscatter coefficient is nearly constant across the amphibole.
Mosaic imaging allows entire samples to be mapped either as beam scans or stage scans using the secondary electron detector, backscattered electron detector, WDS, and EDS (all elements) simultaneously. The BSE mosaic above consists of 63 panels (9x7), each measuring 1.125x1.125 mm (512x512 pixels); each panel was collected as a beam scan with optical autofocus executed prior to each scan. In the long dimension, the mosaic thus measures 10.125 mm and consists of 4608x3584 pixels. Acquisition time for the entire mosaic was roughly two hours. The panel from the lower left corner is displayed in order to give a sense of the true image resolution.
Qualitative analysis (wavelength scanning) can be used to verify the presence of an element and aid in the assessment of interferences at the peak position or at intended background offset positions. A wavelength scan of scapolite around the Sr L-alpha peak position using large-area PET crystal is shown above in red, and it is clear from the plot that Sr is detectable. However, a satellite of the Si K-beta peak as well as the second-order reflection of K K-alpha complicate selection of background offsets. The latter interference can be minimised by application of pulse amplitude discrimination. Superimposed in blue is a wavelength scan collected simultaneously using TAP (with spectrometer L-values scaled to match those of PETL); dwell time was the same for each scan. Although count rate on TAP is greater, resolution is poorer, and peak-to-background ratio is smaller (thus detection limit is higher).
Although the electron microprobe is used primarily as an analytical tool, with some care one can relatively easily produce images with spatial resolution on the order of tens of nanometers, even with a tungsten hairpin filament. In the BSE image above, a pyrite framboid measuring roughly 3.5 microns across is partially rimmed by sphalerite. Note that the stated magnification (“x 9500”) is not applicable.