Analysis of Minerals and Metals Using XRF Mapping

Wavelength dispersive XRF spectrometers are normally used for quantitative elemental analysis of several material types such as petrochemical and geochemical materials, metals, glass and ceramics, mining and cement. These samples are presented to the XRF instrument as uniform and homogenously prepared samples. The Thermo Scientific ARL PERFORM’X spectrometer offers a new dimension of analysis: sample surface mapping. The mapping capability enables taking into account heterogeneity, contamination, gradient, segregation and inclusion determination. In addition, the ARL PERFORM’X mapping option allows to construct detailed composite maps of elemental distribution within the sample. The fine step resolution of 0.1 mm provides excellent analysis for process improvement and problem solving applications. This ability bridges the gap between conventional bulk analysis and standard micro-analysis using microscopic techniques such as scanning electron microscopy (SEM).

Instrument

The ARL PERFORM’X spectrometer used for these tests was a 4200 W system (Fig. 1). This system has a standard configuration with 6 primary beam filters, 4 collimators, up to nine crystals, two detectors, and 5 GN Rh X-ray tube for best performances, from ultra light to heavy elements. The ARL PERFORM’X analyzer features mapping and small spot analysis (1.5 mm and 0.5 mm areas).

Figure 1. Thermo Scientific ARL PERFORM’X Spectrometer Series.

Mapping of Geological Samples

In the following example, the ARL PERFORM’X mapping of the examined geological sample shows that it is possible to define and locate grain boundaries and crystalline structures that are not visible to the naked eye (Fig. 2). The formation of pseudotachylite is usually studied without reference to its chemistry, but in this example we can see a wealth of chemical information that bears upon the genesis of the material. Al, Si, K, Ca and Fe show large gradients while Na, Mg, Ti and Sr are more evenly distributed (Fig. 3).

Figure 2. Pseudotachylite sample with analyzed area.

Figure 3. Distribution of nine elements in the sample.

Another example is shown in figures 4 and 5 with a feldspar sample displaying an inhomogeneous structure. The ARL PERFORM’X determines the element constituents in the selected section of the sample. The interesting aspect of this analysis is in the differences of chemistry of the two feldspars and how the plagioclase comes to mantle the earlier alkali feldspar. The chemistry offers a lot of fundamental information regarding this process.

Figure 4. Heterogeneous feldspar sample seen in the 30 mm aperture sample holder.

Figure 5. Distribution of eight elements in the sample.

Mapping of Metallic Samples

Mapping can be utilised for the identification and quantification of contamination or metallic inclusions as shown in figures 6 to 9.

Figure 6. Selected area for mapping in green.

Figure 7. Distribution of Cr.

Figure 8. Distribution of Cu.

Figure 9. Distribution of Fe.

Standard-less Analysis for Mapping

The most useful development of XRF analytical programs has been the availability of “standard-less” Thermo Fisher Scientific (Ecublens) SARL, Switzerland (ISO certified).

These packages enable quantitative data to be obtained for completely unknown samples. As in many real life situations, obtaining any or enough standards to create a calibration is not always possible. This is certainly the case during analysis of defects or unknown contamination. In such situations, the company offers the most comprehensive standard-less software on the market: Thermo Scientific UniQuant package.

The software includes a factory calibration based on 64 pure element standards that helps determining the concentration of unknown samples in any matrix by using complex mathematical algorithms for up to 79 elements. These algorithms correct for matrix effects and inter-elemental effects to offer an accurate quantitative result.

Conclusion

The elemental mapping capability is an excellent tool for all applications dealing with heterogeneity or concentration gradients in samples and also when impurities or inclusions are found in given specimens. It is seen that analysis using mapping can easily be performed with the ARL PERFORM’X sequential XRF spectrometer. The operations are easy thanks to the sophisticated Thermo Scientific OXSAS software which is able to operate with the latest Microsoft Windows 7 packages.

This information has been sourced, reviewed and adapted from materials provided by Thermo Fisher Scientific - Elemental Analyzers.

For more information on this source, please visit Thermo Fisher Scientific - Elemental Analyzers.