Two methods are available for rapid qualitative determination of the elements present and estimation of their relative abundances in an unknown sample, WDS scanning and energy dispersive spectrometry (EDS). Both methods are based on analyzing the spectra of characteristic x-rays emitted by the specimen. Applications where these techniques are useful include rapid phase identification, qualitative determination of the presence or absence of major or trace elements, and the determination of interferences and background positions needed for subsequent quantitative analysis by WDS analysis.
WDS Scanning
In this method the crystals in the WDS spectrometers are scanned over a predetermined wavelength range and the intensity of the x-rays detected plotted vs. wavelength (or energy). The elements present are identified by peak matching. WDS analysis is particularly useful for determining the presence of trace elements (< 0.1 wt%) in an unknown sample and also for identifying suitable background positions and potential interferences for WDS analysis. A reasonable full scan for all elements from B to U can be obtained in approximately 10 minutes. Longer dwell times, repeat scanning, and smaller wavelength ranges can be used for better resolution or for elements present in very low concentrations.
WDS scan of PET for La and other rare earth elements in Monazite. Scan time 8 minutes.
EDS (Energy Dispersive Spectrometry)
Our microprobe is equipped with a venerable PGT-5000 EDS detector. The detector simultaneously collects all x-ray wavelengths emitted from the sample and displays an intensity vs. energy (or wavelength) spectra in a matter of a few seconds. The elements present are identified by peak matching. This method is very useful for quickly identifying phases and minerals by pattern matching, rapid determination of the presence or absence of an element, and qualitative analysis of non-flat samples. However, x-ray resolution and peak-background ratios are significantly lower than can be obtained by WDS scanning, and on our system, detection is limited to elements with Z > 9 and present in concentrations >1 wt%.
EDS scan of a chrome-bearing augite. Scan time 5 seconds.