Trace elements detection limits using QEMSCAN

In geochemistry, a trace element is a chemical element whose concentration is less than 1,000 ppm or 0.1% of a rock's composition. This definition however does not take into account the type of distribution of the element; is the element a minor and/or optional component of complex minerals (REE), and is it likely to occur as discrete mineral phases. Distribution plays a major part in how well automated mineralogy analysis can detect trace elements.

There are two different methods for QEMSCAN to detect trace elements: the first is through phase identification and calculation; the second is through direct identification in the measured energy-dispersive X-ray (EDX) spectra.

1) Trace element detection through phase identification

There are a number of requirements for trace elements to become accurately identified by mineral/phase identification:

• The phase with the trace element is intercepted in the sample surface and measured by the electron beam. Depending on the homogeneity and concentration of the phase, for this to occur will require a certain statistically defined minimum area of measurements and therefore possibly multiple cuts through the sample.
• Presence of an adequate SIP entry that correctly identifies the phase. Otherwise, it will remain 'Unclassified' (see Fig. 1).
• Presence of a phase pure enough not to result in a mixed EDX spectrum. This generally requires phases to be large enough >3-5 cubic um, depending on the accelerating e-beam voltage (15-25 kV).
• Accurate calculation of the trace element also requires that it is accounted for in the nominal or known composition of the phase, i.e. that it is a priori assigned in the Primary Mineral List and used for the elemental assay report.

2) Direct trace element detection from EDX spectrum

Similarly, for successful trace element identification from the EDX spectrum in QEMSCAN elemental maps, a number of requirements need to be fulfilled:

• The trace element must be detectable by QEMSCAN, i.e. is one of the 72 elements currently supported by the SAE (see Fig. 2).
• The element must be activated in the SIP. SIP development using the ‘element concentration method’ introduced in iDiscover 5.1 will generally try to limit the number of elements to those required to identify all typically present phases in a particular applications (e.g. 16 in the current FEI O&G SIP). The reason is that additional non-essential elements increase the background noise and thereby reduce the overall quality of mineral identification.
• The X-ray count must be adequate to discriminate the elemental energy peaks in the spectra in the elemental spectral match step of the spectral analysis. Automated mineralogy is working with low-count spectra, and depending on the application QEMSCAN is typically set to 1-5,000 X-rays per acquisition points.
• Adequate energy peak discrimination is further a function of the accelerating e-beam voltage, which, depending on the application, is set to 15-25kV. The higher the voltage, the better the resolution of the energy lines of heavier elements. However, the increase in voltage comes with an increase in the size of the excitation volume which results in more mixed spectra.
• Finally, the elemental concentration must be within the detection limits. Limited empirical tests suggest that the detection limit for lighter elements such as Mg is within 1.5-3%, REE >20%, and those in-between around 7-8%.

In conclusion, a lot depends on a priori knowledge of the sample, and the quality and relevancy of the SIP being used to measure the sample. The very fact that elemental assays are calculated in iDiscover, using assumptions on the phase composition, and that phases need to be intercepted by the measurement, arguably make X-ray_fluorescence (XRF) the preferred choice for basic reports on the chemical  bulk composition of rock samples. However, XRF cannot provide context, such as elemental deportment and textural phase association. Automated mineralogy has clear advantages over X-ray crystallography (XRD) when it comes to accurately identifying trace elements and phases - as long as they are intercepted.

Figure 1. Trace element Ti here identified in Rutile (TiO2 in red - black circles), but unidentified in Ilmenite (FeTiO3 in black - red circle)

Figure 2.  All 72 elements currently supported by elemental spectra in the new Spectral Analysis Engine (SAE) in iDiscover 5.1 (QEMSCAN)