Friday, 28 October 2011

QEMSCAN® elemental mapping

Colour-coded mineral and elemental QEMSCAN® maps of single dust grain collected during the 'Red Dawn' dust storm.

With iDiscover™ version 5, the new QEMSCAN® Spectral Analysis Engine (SAE) translates low-count energy-dispersive x-ray (EDX) spectra into up to 72 elemental concentrations for each measurement point. The QEMSCAN® Species Identification Protocol (SIP) assigns phase and mineral names based on elemental ranges/ratios, and optional backscatter electron (BSE) brightness thresholds, x-ray count rates, and confidence levels. An exciting new capability has been added to iDiscover version 5.2, which will officially be released in the coming week: elemental mapping.

There are likely to be many applications where elemental mapping will improve mineral identification. Here is one example, which Alan Butcher and I have developed ahead of our INQUA presentation on “SEM-EDS based particle-by-particle characterisation of a large Australian dust storm”. We presented QEMSCAN data from the massive dust storm event that swept across eastern Australia on 23 September 2009, which has been nicknamed the 'red dawn event'. As we were processing the data we asked ourselves the obvious question why don’t we see anything “red”, where are the iron oxides?

The red colour of dust is linked to sub-micron coatings of iron oxides (hematite) on mineral grains. With an excitation volume of 2-5 microns at 20 keV accelerating e-beam voltage, these coatings are too thin to be measured directly. However, they will contribute to mixed spectra. Mineral definitions generally allow for up to 5% of “other elements” to deal with matrix interference effects. Clay mineral definitions often allow for even higher iron concentrations, to account for cation exchanges. As a result, the Fe-oxide coatings did not show up in the standard SIP definitions (figure on the left). However, the Fe-oxide coatings were clearly highlighted in the Fe elemental map (centre figure). This prompted us to duplicate the quartz definition, exclude iron in the standard definition, and add an iron-rich quartz definition below. The result is shown in the figure to the right.

This simple example demonstrates three things: 1) the ability of the new QEMSCAN SAE to decompose low-count EDX spectra into elemental concentrations; 2) the ability of elemental maps to highlight the mineral context in which elements of interest occur, even in mixed spectra; 3) the beauty of position dependency of the SIP, with the first-match approach allowing to create “elemental discriminator” phase definitions.

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