How to Solve the Problems Associated With Elemental Maps in Analyzing Minerals
10.22.2013 In our last article, we learned how rapid identification of mineral phases of metamorphic rock can be achieved with quantitative wavelength dispersive spectroscopy (WDS). This article, however, will outline some of the Problems with Elemental Maps in analyzing minerals in metamorphic rocks and what can be done to solve them. In scanning electron microscopy (SEM)-based X-ray microanalysis, samples are mapped to provide basic information about how metamorphic rocks have formed. There are, however, some potential problems with elemental mapping that must be considered to correctly understand the sample. The biggest problem with traditional mapping is that you need to know all the elements of interest that exist in a sample before the data collection is started. If an element is not expected but is present, a “black hole” may appear in the maps at its location or the element may go totally unnoticed. The use of spectral imaging data acquisition reduces this problem by collecting complete spectrum at each location. This permits elemental map extraction of the unexpected element days or even years after the data was collected. However, it is still the responsibility of the user to identify and choose the element(s) for mapping. An additional problem with maps is the high background signal, usually for low energy X-rays, arising from the Bremsstrahlung (electromagnetic radiation), which can lead to poor visibility, bad interpretation of the data, and faulty conclusions. The final problem can arise from pathological peak overlaps. If there is a need to map multiple elements which have important peaks that are closely spaced in energy, improper contributions of each element into the map of the other will occur. Using EDS and Quantitative WDS for Complete Analysis In traditional X-ray microanalysis systems, managing the optimal acquisition condition is a trial-and-error process because each microscopist chooses the elements for analysis, which can lead to different results from the same sample, depending on who performed the analysis. Improvements in WDS acquisition times and automated crystal selections enable WDS data to be acquired concurrently with an Energy dispersive spectroscopy (EDS) acquisition. WDS offers more accurate quantitative analyses, particularly for light elements, and better resolution of overlapping X-rays peaks for improved element identification and quantification. EDS plays the role of quickly and automatically identifying elements, while WDS facilitates richer analysis of the sample. This emerging technique offers a comprehensive solution for understanding the chemical compositions of the minerals in metamorphic rocks, and determining the types of minerals and their textural evolution. And, whereas such analyses required many hours of WDS acquisition time, a complete WDS analysis of geological samples is now achievable in a fraction of the time. Read the application note on avoiding the problems associated with elemental maps and simplifying the interpretation of the Spectral Imaging data set.
Leave a Reply