Qualitative X-ray Analysis and Imaging

Abstract

It is a waste of time to proceed with quantitative analysis of your XEDS spectrum or image without first carrying out qualitative analysis. Qualitative analysis requires that every peak in the spectrum be identified unambiguously, with statistical certainty, otherwise it should be ignored for both subsequent quantitative analysis and imaging. We emphasize this point because of the many opportunities for the misidentification of small peaks in the spectrum. In this chapter, we'll deal initially with acquisition and identification of the elemental information in spectra and images. First, we will show you how to choose the best operating conditions for your particular AEM and XEDS system. Then we'll explain the best way to obtain a spectrum for qualitative analysis. You have to acquire a spectrum with sufficient X-ray counts to allow you to draw the right conclusions with a given degree of confidence. There are a few simple rules to follow which allow you to do this. Two advantages are gained from rigorous qualitative analysis. First, you may be able to solve the analytical problem at hand without needing to perform full quantification. Second, when quantification is carried out (see the next chapter), you will not spend an inordinate amount of time analyzing an element that isn't there, and you can be confident that your results are valid. We'll go over the many ways to misidentify peaks in your spectra, particularly small ones, which may, in fact, arise from important trace elements but, which might be artifacts, could be peaks from another element, or are possibly statistically insignificant. Commercial peak-identification software, while improving all the time, is not error-free. We'll end with a few words about qualitative X-ray imaging. 34.1 MICROSCOPE AND SPECIMEN VARIABLES When you first acquire a spectrum, the operating conditions should maximize the X-ray count rate to give you sufficient intensity in the characteristic peaks in your spectrum, in the shortest time, with the minimum number of artifacts. You need sufficient counts so you can detect, unambiguously, the presence of all the elements in your specimen (within the limitations of your XEDS detector) with statistical certainty. As we'll explain, the best conditions for such qualitative analyses require that you obtain the spectrum from a reasonably thick, large area of your specimen, using a large probe and a large aperture to give the most current, but in doing this you'll compromise other desirable analysis qualities, particularly high spatial resolution. So right up front you need to know two key points & There are only three requirements for good qualitative analyses; counts, counts and more counts & The conditions for the best qualitative (and quantitative) analysis (which are also those that give the best analytical sensitivity) are precisely the worst for obtaining the best spatial resolution. A more complicated factor in getting the most X-ray counts in your spectrum is choosing the right operating voltage. Remember, back in Figure 33.10.C, we showed that you get a higher detection and collection efficiency if you decrease the kV because the scattering cross section (s) increases when the kV decreases; that was a specimen effect. Now we are talking about a gun effect; A QUALITATIVE MUST! Although such an approach may seem time-consuming and unnecessarily tedious, the need for initial qualitative analysis of the spectrum cannot be stressed too strongly.