Spectrum library matching

Saying What You Mean and Meaning What You Say!

Shawn Wallace, Applications Engineer, EDAX

A recent conversation on a list serv discussed sloppiness in the use of words and how it can cause confusion. This made me consider that in the world of microanalysis, we are not immune. We are probably sloppiest with two particular words. They are resolution and phase.

Let us start with how we use the word phase and how phases are commonly defined in microanalysis. In Energy Dispersive Spectroscopy (EDS), we use phase for everything, for example, phase mapping, phase library. In Electron Backscatter Diffraction (EBSD), the usage is a little more straightforward.

So, what is a phase? Well to me, a geologist, a phase has both a distinct chemistry and a distinct crystal structure. Why does this matter to a geologist? Two different minerals with the same chemistry, but with different structures, can behave in very different ways and this gives me useful information about each of them.
The classic example for geologists is the Al2SIO5 system (figure 1). It has three members, Kyanite, Sillimanite, and Andalusite. They each have the same chemistry but different structures. The structure of each is controlled by the pressure and temperature at which the mineral equilibrated. Simple chemistry tells me nothing. I need the structure to tease out that information.

Figure 1. Phase Diagram of the Al2SiO5 system in geological conditions. Different minerals form at different pressures and temperatures, letting geologists know how deep and/or the temperature at which the parent rock formed.**

EDS users use the term phase much more loosely. A phase is something that is chemically distinct. Our phase maps look at a spectrum pixel by pixel and see how they compare. In the end, the software goes through the entire map and groups each pixel with like pixels. The phase library does chi squared fits to compare the spectrum to the library (figure 2).

Figure 2. Our Spectrum Library Match uses as Chi-squared fit to determine the best possible matches. This phase is based on compositional data, not compositional and structural data.

While the definition of phase is relatively straight forward, the meaning of resolution gets a little murkier. If you asked someone what the EDS resolution is, you may get different answers depending on who you ask. The main way we use the term resolution when talking about EDS is spectral resolution. This defines how tight the peaks in a spectrum are (figure 3).

Figure 3. Comparison of EDS vs. WDS spectral resolution. WDS has much higher resolution (tighter peaks) than EDS, but fewer counts and more set-up are required.

The other main use of resolution, in EDS is the spatial resolution of the EDS signal itself (figure 4). There are many factors which determine this, but the main ones are the accelerating voltage and sample characteristics. This resolution can go from nanometers to microns.

Figure 4. Distribution of the electron energy deposited in an aluminum sample (top row) and a gold sample (bottom row) at 15 kV (left column) and 5 kV (right column). Note the dramatic difference in penetration given by the right hand side scale bar.

The final use of resolution for EDS is mapping resolution. This is by far the easiest to understand. It is just the step size of the beam while you are mapping.

Luckily for us, the easiest way to find out what people mean when they use the terms resolution or phase, is just to ask. Of course, the way to avoid any confusion is to be as precise as possible with your choice of words. I resolve to do my part and communicate as clearly as I can!

** Source: Wikipedia

A New Year and a New Market!

Tara Nylese, Global Applications Manager, EDAX

The landscape of the microscopy and microanalysis field has been changing for years. In the EDAX Application Labs we see this change throughout most of our activities such as system demos, training courses and customer support. The trend shows that many users are 1) looking for ways to increase their productivity due to increased workload demand and 2) using multiple tools in combination to solve their increasingly diverse, yet still deeply scientific, applications. The result is the growing need for Quick & Easy solutions in microanalysis – which also happens to be the topic of my recent webinar!

The latest addition to our Mahwah applications lab is a tabletop SEM, which we added at the end of last year. This offers simplicity of use without a compromise in the quality of the analysis, which is remarkable enough for both this blog and the related webinar.

Raw materials verification is a common materials application in a variety of industrial settings. Typically the analysis requires image collection, spectral collection and analysis, quantification and then data interpretation. Experienced analysts can spend time reviewing the results to determine what the material is, and whether the concentrations are within acceptable limits. But with the new tools available for these new market environments, there’s a better way.

Figure 1

Figure 1.

Figure 1 shows an image of a block of raw steel. In order to quickly and conclusively determine the specific type of steel, nowadays all the analyst needs to do is collect a quick spectrum and click the “Match” button. The software then compares the unknown material to a user’s previously collected alloy library.  The routine matches the spectral fingerprint and doesn’t even require a peak identification or quantitative analysis.

Figure 2.

Figure 2.

The results in Figure 2 show the red unknown spectrum matched to the three closest results in the alloy library. Values show the percentage of the match, giving an even better metric to report, and in this case the material is identified as SS 316.

Figure 3.

Figure 3.

The quality of the solution can be further increased by performing a peak ID and using the match routine on the concentration results. In this case Figure 3 shows a % match of concentration to SS316. An added benefit of this option is that the sample spectra collection conditions (kV, system configuration etc.) can be different from  the library conditions, such as 15 kV sample collection conditions compared to a 20 kV library, (see Figure 4). This increases analytical flexibility while still maintaining quality results.

Figure 4.

Figure 4.

The results show that a confident and conclusive raw materials verification analysis can now be accomplished in just a few mouse clicks without the need for time consuming interpretation by an analyst. So when you want quality results quickly, check out the new software available for your best solutions!