Applications

Want a Free Set of Microanalysis Standards?

Dr. Shangshang Mu, Applications Engineer, EDAX

Modern EDS systems are capable of quantitative analysis with or without standards. Unlike standard-less analysis, the k-ratio is either calculated in the software or based on internal standards. For analysis with standards, it is measured from a reference sample with known composition under the same conditions as the unknown sample. As an applications engineer, sometimes users ask me where to order these standards. Usually, I point them to the vendors that manufacture and distribute reference standards where you can order either off-the-shelf or customized standard blocks. In addition to these commercial mounts, I always tell them that they can request a set of mineral, glass, and rare earth element phosphate standards from the National Museum of Natural History free of charge! These are very useful standards that I’ve seen widely used in not only the geoscience world but also in various manufacturing industries. These free standards are also great for those graduate students with limited budgets and ideal for practicing sample preparation (yes, I was one of them).

This set of standards is officially called the Smithsonian Microbeam Standards and includes 29 minerals, 12 types of glass, and 16 REE phosphates. You can find out more information about these standards and submit a request form by clicking on the link below:
https://naturalhistory.si.edu/research/mineral-sciences/collections-overview/reference-materials/smithsonian-microbeam-standards

I mentioned sample preparation earlier. Yes, you read that right. These standards come in pill capsules containing from many tiny grains to a few larger ones and you need to mount them on your own (Figure 1).

Grains in a pill capsule.

Figure 1. Grains in a pill capsule.

Since you can get the information such as the composition, locality, and references for each standard from the website, what I want to discuss in this blog post is how to prepare them properly for X-ray analysis. The first tricky thing is to get them out of the capsules. The grains in Figure 1 are almost the largest in this set and you won’t get too many of this size. Some of the grains are even too tiny to be seen at first glance. For the majority that are really tiny, you need to tap the capsule a couple of times to release the grains that get stick to the capsule wall, then you can open the capsule very carefully and let the grains slide out with a little tapping.

For mounting, the easiest way is to mount the standards in epoxy using a mounting cup and let it cure. I did this in a fancy way to make it look like a commercial mount (Figure 2). I ordered a 30 mm diameter circular retainer with 37 holes used by commercial mount manufacturers (Figure 3) and filled the holes with standards on my own. I must admit that the retainer is not cheap, but you can machine the mount by yourself or have a machine shop do it for you. In addition to looking pretty, the retainer ensures a good layout so you can quickly locate the standards you need during microanalysis, and you can mount the same type of standards on one block and get rid of the hassle of frequently venting and pumping the SEM chamber to switch standard blocks.

Examples of commercial mounts.

Figure 2. Examples of commercial mounts.

 

30 mm diameter circular retainer with 37 holes.

Figure 3. 30 mm diameter circular retainer with 37 holes.

To prevent the tiny grains from moving and floating up when pouring the epoxy mix, I placed the retainer upside down and pressed it onto a piece of sticky tape (Figure 4a) and positioned the grains on the sticky surface of the tape within the holes. When tapping the capsule to let the grains slide out and fall into the hole, the other holes were covered to prevent contamination (Figure 4b). These holes are small in diameter and pouring the epoxy mix directly will trap air bubbles in the hole to separate the grains from the epoxy mix. To overcome this problem, I filled up the hole by letting the epoxy mix drip down very slowly along the inner surface of the hole.

Positioning grains within the holes of the retainer.

Figure 4. Positioning grains within the holes of the retainer.

For general grinding, I start with wet 240 grit SiC sandpaper with subsequent use of 320, 400, 600, 800, and 1,200 grit wet SiC sandpapers. But coarser grits can grind off tiny grains in this case, so I would recommend starting with a relatively fine grit based on the sizes of the grains you receive and always use a light microscope or magnifier to check the grinding. For polishing abrasive, I used 1 micron and 0.3 micron alumina suspensions on a polishing cloth. For the grains used as standards or quantification in general, the surface needs to be perfectly flat. However, the napped polishing cloth tends to abrade epoxy and the grains at different rates, creating surface relief and edge rounding, especially on tiny grains. To mitigate this effect, the polishing should be checked under a light microscope constantly and stopped as soon as the scratches are removed. A vibratory final polishing with colloidal silica is optional. Followed by ultrasonic cleaning and carbon coating, the standard mount is ready to use.

Note that commercial mount manufacturers may prepare standards individually (especially for metal standards) and insert them into the holes from the back of the retainer and fasten them with retaining rings (Figure 5a). A benefit of this approach is that the standards on the mount are changeable, so you can load all the standards you need on one mount before microanalysis. I used to make several individual mounted standards that can fit into the retainer (Figure 5b) but this process is very time consuming and much trickier to keep the small surface flat during grinding and polishing.

a) The back of a commercial metal standard mount. b) A tiny cylindrical mount that can fit into the retainer holes.

Figure 5. a) The back of a commercial metal standard mount. b) A tiny cylindrical mount that can fit into the retainer holes.

This is definitely a good set of standards to keep in your lab. With EDAX EDS software, in addition to quantification with these standards, you can also use them to create a library and explore the Spectrum Matching feature. The next time you want to quickly determine the specific type of a mineral, you can simply collect a quick spectrum and click the “Match” button, and the software will compare the unknowns to the library you just created.

Between the Lines

Dr. René de Kloe, Applications Specialist, EDAX

While I am testing new hardware and software versions, I use it as an opportunity to collect some data on unique materials. Testing detector speed or general software functionality is easiest on a simple material like an undeformed Ni or Fe alloy. But, I think it is a shame to perform longer duration tests on materials I have already seen many times before. For such occasions, I look through my collection of materials for something nice to map. During testing of the upcoming APEX™ 2.0 EBSD software, I collected a few larger scans on fossils that I had found during geological fieldwork and family holidays. This included large single-field scans and a Montage map, where we combine beam scans with stage movements for a large mosaic map.

Cross-section through a fossil crinoid stem and IPF on PRIAS™ center map of the fossil crinoid stem sample collected from the indicated area.

Figure 1. a) Cross-section through a fossil crinoid stem. b) IPF on PRIAS™ center map of the fossil crinoid stem sample collected from the indicated area.

For example, Figure 1a shows a cross-section through a fossil crinoid stem. At the edge, the lighter areas represent the structure of the organism, while the darker areas are later sedimentary infill.

This is beautifully visible in the 2.1 x 1.7 mm IPF on PRIAS™ center map, where the biomineral structure appears smooth and fine-grained. In contrast, the infill is more equiaxed and shows topography due to compositional differences (Figure 1b).

Another beautiful scan was collected while I was trying out the new APEX™ 2.0 EBSD Montage map wizard. This wizard allows easy pre-imaging of the entire scan field to set the actual scan area. With the wizard, setting up such a large, 18 million point, 30-field Montage map over a 1.3 x 7 mm area can be done in a few minutes.

Calcite rock sample with fossils and EBSD Montage map of one of the nummulite fossils.

Figure 2. a) Calcite rock sample with fossils. b) EBSD Montage map of one of the nummulite fossils.

We collected these two scans on calcite rocks for which you can simply load the appropriate crystal structure. But collecting data is not always that easy, especially if you are not sure what phase(s) you have in your sample. And ultimately, EBSD data collection is based on pattern analysis and then matching the detected bands against a lookup table. In most cases, you can just search the included EDAX structure file database that contains close to 500 phases and covers most commonly studied materials, such as the calcite used for the scans above.

But where do these files come from? Partly, they are a result of our combined legacy. Over the years, we have seen many materials and often painstakingly identified which bands to select to get reliable indexing results. Nowadays, you can create phase files directly using atomic and crystallographic information. However, you can continue to extract the majority of “new” phase files from XRD databases, such as the AMCS, ICSD, or ICDD PDF databases. These databases contain 10’s to sometimes 100’s of thousands of phase descriptions that are based on XRD measurements. The XRD data shows which lattice planes are effective X-ray diffractors, and are also useful to construct a structure file for electron diffraction patterns.

Indexed olivine EBSD pattern.

Figure 3. Indexed olivine EBSD pattern.

And there the fun starts. Often there are multiple possibilities for phases or minerals (e.g., solid solution series) available in the database. Which one to select? And in many cases, there is no single-phase file that matches the pattern exactly. There are always bands that do not get labeled or are shown in the overlay that are not visible in the real pattern. This is due to the differences between X-ray and electron diffraction intensities or simply incomplete database entries. Time for some human intervention. The APEX™ EBSD software contains advanced tools to modify and optimize the reflector tables of imported or calculated structure files. First, the color-coding itself. All bands are labeled with a color that corresponds to the IPF color triangle, so equivalent lattice planes get identical colors. This allows a visual inspection if bands that are designated with the same color also appear identical.

IPF color triangle.

Figure 4. IPF color triangle.

Then there is a band ID tool to help identify non-labeled bands in the diffraction patterns. When a pattern appears correctly indexed, but a number of bands are not labeled, the user can draw a line on the missing band. The software then shows which lattice plane corresponds to that band and also indicates all crystallographic equivalent planes. When it is still difficult to identify the correct indexing solution, it can be beneficial to bypass the Hough band detection and instead manually draw the bands for indexing. A good trick for low symmetry crystals is only to select the thinnest bands. These correspond to the lattice planes with the largest d-spacings and should be the important low-index crystallographic planes. By excluding the (often) large number of bands with similar bandwidths, you reduce the number of options and more quickly land at the best matching orientation or phase.

Manual Band Selection tool.

Figure 5. Manual Band Selection tool.

When a solution is found that matches the thin bands, you can start drawing in the other ones. When drawing a band, the software automatically shows where all the crystallographic equivalent planes should be. If a line is drawn where no band is present, you have the wrong candidate, and you need to look further. If all the indicated bands match in appearance and width, you can enable the reflector. This way, it is easy to interactively generate a matching phase file. Just keep in mind that when you have optimized a structure file to a pattern, it is a good idea to find some more patterns from that phase (if necessary, just rotate the sample to get a different orientation) and verify that all the bands in the other patterns are also properly identified. This is especially important for low symmetry materials where few lattice planes are equivalent.

Band optimization sequence on an EBSD pattern from W2C. The initial reflector table (a) misses a number of strong bands. Manually selecting a band (b) shows which reflector this is and where the crystallographic equivalent bands should be. This can be repeated (c) until all clear bands have been labeled.

Figure 6. Band optimization sequence on an EBSD pattern from W2C. The initial reflector table (a) misses a number of strong bands. Manually selecting a band (b) shows which reflector this is and where the crystallographic equivalent bands should be. This can be repeated (c) until all clear bands have been labeled.

Although it can be rewarding to identify a new phase and optimize the structure file to allow for EBSD mapping of a new and interesting material, I would like to end with a word of warning. When you are working with a good pattern and successfully identify the phase and orientation, it is very tempting to keep looking for bands and completely fill the pattern with everything you can see. But that is often a bad idea, as the weaker bands will typically not get selected by the Hough transformation on the poorer patterns that are used during indexing. Enjoy playing with the materials and structure files, but don’t overdo it.

Diffraction pattern with all visible bands enabled for indexing.

Figure 7. Diffraction pattern with all visible bands enabled for indexing.

How to Get a Good Answer in a Timely Manner

Shawn Wallace, Applications Engineer, EDAX

One of the joys of my job is troubleshooting issues and ensuring you acquire the best results to advance your research. Sometimes, it requires additional education to help users understand a concept. Other times, it requires an exchange of numerous emails. At the end of the day, our goal is not just to help you, but to ensure you get the right information in a timely manner.

For any sort of EDS related question, we almost always want to look at a spectrum file. Why? There is so much information hidden in the spectrum that we can quickly point out any possible issues. With a single spectrum, we can quickly see if something was charging, tilted, or shadowed (Figure 1). We can even see weird things like beam deceleration caused by a certain imaging mode (Figure 2). With most of these kinds of issues, it is common to run into major quant related problems. Any quant problems should always start with a spectrum.

Figure 1. The teal spectrum shows a strange background versus what a normal spectrum (red) should look like for a material.

Figure 1. The teal spectrum shows a strange background versus what a normal spectrum (red) should look like for a material.

This background information tells us that the sample was most likely shadowed and that rotating the sample to face towards the detector may give better results.

Figure 2. Many microscopes can decelerate the beam to help with imaging. This deceleration is great for imaging but can cause EDS quant issues. Therefore, we recommend reviewing the spectrum up front to reduce the number of emails to troubleshoot this issue.

Figure 2. Many microscopes can decelerate the beam to help with imaging. This deceleration is great for imaging but can cause EDS quant issues. Therefore, we recommend reviewing the spectrum up front to reduce the number of emails to troubleshoot this issue.

To save the spectrum, right-click in the spectrum window, then click on Save (Figure 3). From there, save the file with a descriptive name, and send it off to the applications group. These spectrum files also include other metadata, such as amp time, working distance, and parameters that give us so many clues to get to the bottom of possible issues.

Figure 3. Saving a spectrum in APEX™ is intuitive. Right-click in the area and a pop-up menu will allow you to save the spectrum wherever you want quickly.

Figure 3. Saving a spectrum in APEX™ is intuitive. Right-click in the area and a pop-up menu will allow you to save the spectrum wherever you want quickly.

For information on EDS backgrounds and the information they hold, I suggest watching Dr. Jens Rafaelsen’s Background Modeling and Non-Ideal Sample Analysis webinar.

The actual image file can also help us confirm most of the above.

Troubleshooting EBSD can be tricky since the issue could be from sample prep, indexing, or other issues. To begin, it’s important to rule out any variances associated with sample preparation. Useful information to share includes a description of the sample, as well as the step-by-step instructions used to prepare the sample. This includes things like the length of time, pressure, cloth material, polishing compound material, and even the direction of travel. The more details, the better!

Now, how do I know it is a sample prep problem? If the pattern quality is low at long exposure times (Figure 4) or the sample looks very rough, it is probably related to sample preparation (Figure 4). That being said, there could be non-sample prep related issues too.

Figure 4. This pattern is probably not indexable on its own. Better preparation of the sample surface is necessary to index and map this sample correctly.

Figure 4. This pattern is probably not indexable on its own. Better preparation of the sample surface is necessary to index and map this sample correctly.

For general sample prep guidelines, I would highly suggest Matt Nowell’s Learn How I Prepare Samples for EBSD Analysis webinar.

Indexing problems can be challenging to troubleshoot without a full data set. How do I know my main issues could be related to indexing? If indexing is the source, a map often appears to be very speckled or just black due to no indexing results. For this kind of issue, full data sets are the way to go. By full, I mean patterns and OSC files. These files can be exported out of TEAM™/APEX™. They are often quite large, but there are ways available to move the data quickly.

For the basics of indexing knowledge, I suggest checking out my latest webinar, Understanding and Troubleshooting the EDAX Indexing Routine and the Hough Parameters. During this webinar, we highlight attributes that indicate there is an issue with the data set, then dive into the best practices for troubleshooting them.

As for camera set up, this is a dance between the microscope settings, operator’s requirements, and the camera settings. In general, more electrons (higher current) allow the experiment to go faster and cover more area. With older CCD based cameras, understanding this interaction was key to good results. With the newer Velocity™ cameras based on CMOS technology, the dance is much simpler. If you are having difficulty while trying to optimize an older camera, the Understanding and Optimizing EBSD Camera Settings webinar can help.

So how do you get your questions answered fast? Bury us with information. More information lets us dive deeper into the data to find the root cause in the first email, and avoids a lengthy back and forth exchange of emails. If possible, educate yourself using the resources we have made available, be it webinars or training courses. And always, feel free to reach out to my colleagues and me at edax.applications@ametek.com!

Is It Worth The Salt?

Felix Reinauer, Applications Specialist, EDAX

When you are in Sweden at Scandem 2019 it is the perfect time to order SOS as an appetizer or for dinner. It is made of smör, ost and sill (butter, cheese and herring) served together with potatoes. Sometimes the potatoes need a little bit of improvement in taste. It is very easy to take the salt mostly located on all tables and salt them. Doing that I thought about how easy it is to do this today and what am I really pouring on my potatoes?

Salt was very important in the past. In ancient times salt was so important that the government of Egypt and other countries setup salt taxes. Around 4000 years ago in China and during the Bronze age in Europe, people started to preserve food using brine. The Romains had soldiers guarding and securing the transportation of salt. Salt was as expensive as gold. Sal is the Latin word for salt and the soldiers used to get their salare. Today you still get a salary. Later ‘Streets of Salt’ were settled to guarantee safe transportation all over the country. As a result, cities along these roads got wealthy. Even cities, like Munich, were founded to make money with the salt tax. Salt even destroyed empires and caused big crises. Venice fought with Genoa over spices in the middle ages. In the 19th century soldiers were sent out to conquer a big mountain of salt of an Inconceivable value, lying along the Missouri River. We all know the history of India´s independence. Mohandas Gandhi organized a salt protest to demonstrate against the British salt tax. The importance of the word salt is also implemented in our languages, “Worth the salt”, “Salz in der Suppe” or “Mettre son grain de sel”.

The two principle ways of getting salt are from underground belts and from the sea. It can be extracted from underground either by mining or by using solution mining. Sea salt is produced in small pools which were filled up during high tide and water evaporates under sunny weather conditions. Two kinds of salt mining are done. Directly digging the salt out of the mountain, then dissolving it to clean it. Or hot water is directly used to dissolve the salt and then the brine is pumped up.

Buying salt today is no longer that expensive, dangerous or difficult. But now a new problem arises. I´m talking about salt for consumption, which usually means NaCl in nice white crystals. So, are there any advantages to using different kind of salts? If we believe advertisements or gourmets, it is important, where the salt we use came from and how it was produced. Today the most time-consuming issue is the selection of the kind of salt you want in the supermarket!

For my analysis I chose three kinds of salts from three different areas. The first question was, are the differences big enough to detect them using EDS or will the differences be related to minor trace elements which can only be seen in WDS. It was a surprise for me that the differences are that huge. I had a look at several crystals from one sample. Shown as examples are the typical analysis of the different compounds and elements for that provenance.

First looking at the mined salt. I selected a kind of salt from the oldest salt company in Germany established over 400 years ago. One kind from Switzerland manufactured in the middle of the Alpes and one from the Kalahari, to be as far away as possible from the others. The salt from Switzerland is the purest salt only containing NaCl with some minor traces. The German salt contains a bigger amount of potassium and the Kalahari salt a bigger amount of sulfur and oxygen (Figure 2.).

Figure 2.

Secondly, I was interested in the salt coming from the sea. I selected two types of salt from French coasts one from the Atlantic Ocean in Brittany and another one from the Mediterranean Sea. The third one came from the German coast at the Baltic Sea. The first interesting impression is that all the sea salt contains many more elements. The Mediterranean salt contains the smallest amount of trace elements. The salt from the Atlantic Ocean and the Baltic sea contains, besides the main NaCl, phases containing Ca, K, S, Mg and O. A difference in the two is the amount of Ca containing compounds (Figure 3.).

Figure 3.

Finally, I was interested in some uncommon types of salt. In magazines and television, experts often publish recipes with special types supposedly offering a special taste, or advertising offers remarkable new kinds of healthy salt. So, I was looking for three kinds which seem to be unusable. I found two, a red and a black colored, Hawaiian salt. The spectrum of the red salt shows nicely that Fe containing minerals cause the red color. Even titanium can be found and a bigger amount of Al, Si and O. The black salt contains mainly the same elements. Instead of Fe the high amount of C causes the black color. A designer salt is the Pyramid finger salt, which is placed on top of the meat to make it look nicer. Beside the shape, the only specialty is the higher amount of Ca, S and O (Figure 4).

Figure 4.

It was really interesting that salt is not even salt. As the shape of the crystals varies, so they differ in composition. In principle it is NaCl but contain more or less different kinds of compounds or even coal to color it. There are elements found in different amounts related to the type of salt and area it came from. These different salts are located in a few very small areas in and on the crystals.
And finally, I pour salt onto my potatoes and think, ok it is NaCl.

 

Hats Off/On to Dictionary Indexing

Dr. Stuart Wright, Senior Scientist EBSD, EDAX

Recently I gave a webinar on dynamic pattern simulation. The use of a dynamic diffraction model [1, 2] allows EBSD patterns to be simulated quite well. One topic I introduced in that presentation was that of dictionary indexing [3]. You may have seen presentations on this indexing approach at some of the microscopy and/or materials science conferences. In this approach, patterns are simulated for a set of orientations covering all of orientation space. Then, an experimental pattern is tested against all of the simulated patterns to find the one that provides the best match with the experimental pattern. This approach does particularly well for noisy patterns.

I’ve been working on implementing some of these ideas into OIM Analysis™ to make dictionary indexing more streamlined for datasets collected using EDAX data collection software – i.e. OIM DC or TEAM™. It has been a learning experience and there is still more to learn.

As I dug into dictionary indexing, I recalled our first efforts to automate EBSD indexing. Our first attempt was a template matching approach [4]. The first step in this approach was to use a “Mexican Hat” filter. This was done to emphasize the zone axes in the patterns. This processed pattern was then compared against a dictionary of “simulated” patterns. The simulated patterns were simple – a white pixel (or set of pixels) for the major zone axes in the pattern and everything else was colored black. In this procedure the orientation sampling for the dictionary was done in Euler space.
It seemed natural to go this route at the time, because we were using David Dingley’s manual on-line indexing software which focused on the zone axes. In David’s software, an operator clicked on a zone axis and identified the <uvw> associated with the zone axis. Two zone axes needed to be identified and then the user had to choose between a set of possible solutions. (Note – it was a long time ago and I think I remember the process correctly. The EBSD system was installed on an SEM located in the botany department at BYU. Our time slot for using the instrument was between 2:00-4:00am so my memory is understandably fuzzy!)

One interesting thing of note in those early dictionary indexing experiments was that the maximum step size in the sampling grid of Euler space that would result in successful indexing was found to be 2.5°, quite similar to the maximum target misorientation for modern dictionary indexing. Of course, this crude sampling approach may have led to the lack of robustness in this early attempt at dictionary indexing. The paper proposed that the technique could be improved by weighting the zone axes by the sum of the structure factors of the bands intersecting at the zone axes.
However, we never followed up on this idea as we abandoned the template matching approach and moved to the Burn’s algorithm coupled with the triplet voting scheme [5] which produced more reliable results. Using this approach, we were able to get our first set of fully automated scans. We presented the results at an MS&T symposium (Microscale Texture of Materials Symposium, Cincinnati, Ohio, October 1991) where Niels Krieger-Lassen also presented his work on band detection using the Hough transform [6]. After the conference, we hurried back to the lab to try out Niels’ approach for the band detection part of the indexing process [7].
Modern dictionary indexing applies an adaptive histogram filter to the experimental patterns (at left in the figure below) and the dictionary patterns (at right) prior to performing the normalized inner dot-product used to compare patterns. The filtered patterns are nearly binary and seeing these triggered my memory of our early dictionary work as they reminded me of the nearly binary “Sombrero” filtered patterns– Olé!
We may not have come back full circle but progress clearly goes in steps and some bear an uncanny resemblance to previous ones. I doff my hat to the great work that has gone into the development of dynamic pattern simulation and its applications.

[1] A. Winkelmann, C. Trager-Cowan, F. Sweeney, A. P. Day, P. Parbrook (2007) “Many-Beam Dynamical Simulation of Electron Backscatter Diffraction Patterns” Ultramicroscopy 107: 414-421.
[2] P. G. Callahan, M. De Graef (2013) “Dynamical Electron Backscatter Diffraction Patterns. Part I: Pattern Simulations” Microscopy and Microanalysis 19: 1255-1265.
[3] S.I. Wright, B. L. Adams, J.-Z. Zhao (1991). “Automated determination of lattice orientation from electron backscattered Kikuchi diffraction patterns” Textures and Microstructures 13: 2-3.
[4] Y.H. Chen, S. U. Park, D. Wei, G. Newstadt, M.A. Jackson, J.P. Simmons, M. De Graef, A.O. Hero (2015) “A dictionary approach to electron backscatter diffraction indexing” Microscopy and Microanalysis 21: 739-752.
[5] S.I. Wright, B. L. Adams (1992) “Automatic-analysis of electron backscatter diffraction patterns” Metallurgical Transactions A 23: 759-767.
[6] N.C. Krieger Lassen, D. Juul Jensen, K. Conradsen (1992) “Image processing procedures for analysis of electron back scattering patterns” Scanning Microscopy 6: 115-121.
[7] K. Kunze, S. I. Wright, B. L. Adams, D. J. Dingley (1993) “Advances in Automatic EBSP Single Orientation Measurements.” Textures and Microstructures 20: 41-54.

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 Light on Leonardo

Sue Arnell, Marcom Manager, EDAX

I recently spent 10 days’ vacation back in the UK, but my visit “home” turned into somewhat of a busman’s holiday when I visited the current exhibition at the Queen’s Gallery in London: LEONARDO DA VINCI: A LIFE IN DRAWING. While all the drawings were very interesting, one particular poster particularly caught my eye.

Figure 1: Poster showing the use of X-ray Fluorescence (XRF) analysis on one of the drawings in the exhibition.

It may be hard to see in this small image, but the drawing in the bottom left corner of the poster showed two horses’ heads, while the rest of the sheet showed very indistinct lines. When viewed under ultraviolet light, however, it is clear that there were an additional two horses depicted on the same page.

Figure 2: Drawing of horses seen under ultraviolet light

A video on the exhibit site shows a similar result with a second page:

Figure 3: Hand study seen in daylight

Figure 4: Hand study seen under ultraviolet light

According to the poster, researchers* at the Diamond Light Source at Harwell in Oxfordshire used X-ray fluorescence, which is non-destructive and would not therefore harm the priceless drawing, to explain the phenomenon in the first drawing of the horses. Scanning a small part of the drawing to analyze individual metalpoint lines, they were able to extract the spectrum in Figure 5.

Figure 5: the results of XRF analysis on the drawing showing the presence of copper (Cu) and Zinc (Zn) in the almost invisible lines and almost no silver (Ag).

The conclusion was that Leonardo must have used a metalpoint based on a Cu/Zn alloy and that these metals have reacted over time to produce salts and render the lines almost invisible in daylight. However, under ultraviolet light, the full impact of the original drawings is still visible.

When I shared this analysis back in the EDAX office in Mahwah, NJ, Dr. Patrick Camus (Director of Engineering) had a few additional (more scientific) observations.

  • XRF may be useful in determining the fading mechanism by looking for elements associated with environmental factors such as Cl, (from possible contact with human fingertips), or S in the atmosphere from burning coal over the centuries. It may be related to exposure to sunlight as well.
  • The use of ultraviolet light as an incoming beam has a similar reaction but slightly different with the material as the x-rays producing emissions at much smaller energy level. This process is called photoluminescence. The incoming beam excites valence electrons across an energy gap in the material to a higher energy level which during relaxation to the base energy releases a photon. The energy of these photons is typically 1-10 eV or much less than x-ray detectors can sense. Interestingly, this excitation does not occur in conductors/metals, thus proving more evidence of the picture material being a band-gap or insulating material like a salt.
  • This example shows that a single technique does not always provide a complete picture of the structure or composition of a sample, but the use of multiple techniques can provide information greater than the sum of the individual contributions.

From my point of view, I have been trying to explain, promote and market the EDAX products and analysis techniques for over eight years now, so it was very interesting to see the value of some of ‘our’ applications in a real-world situation.

* Dr. Konstantin Ignatyev, Dr. Giannantonio, Dr. Stephen Parry