Dr. Shangshang Mu, Application Scientist, Gatan/EDAX
Over the past year, I’ve rekindled my enjoyment of traveling as I visited customers in the Americas, Asia, and across Europe. During my return journey, I was deeply touched by an airline billboard at the Munich, Germany airport that read, “We all live under one sun. Let’s see it again.” Indeed, it is genuinely nice to see the world once more since reemerging from the pandemic.
While flying over Hudson Bay, an inland marginal sea of the Arctic Ocean, I saw numerous ice caps floating on the water from the aircraft’s belly camera view. To me, these were very reminiscent of the counts per second (CPS) map (Figure 1) in one of the wavelength dispersive spectrometry (WDS) datasets I shared with customers during these trips. Although they were orders of magnitude larger than the micron-scale sample, the resemblance was striking.
Figure 1. Ice caps in Hudson Bay (left) resemble the CPS map of a Si-W-Ta sample (right).
Throughout these journeys, our EDAX Lambda WDS system was one of the hot topics drawing customers’ attention. This parallel beam spectrometer features a compact design compatible with almost every scanning electron microscope (SEM). The improved energy resolution and sensitivity and lower limits of detection make it an excellent supplement to your energy dispersive spectroscopy (EDS) detectors. The CPS map I referred to was captured from a Si-W-Ta sample. The energy peaks of Si K, W M, and Ta M are heavily overlapped, with only approximately 30 eV energy difference between each other. Lambda WDS systems provide up to 15x better energy resolution than typical EDS systems, effectively resolving the ambiguity in analysis.
Figure 2. Overlay of EDS (red outline) and WDS (cyan color) spectra from the central area of the Si-W-Ta sample.
The overlay of EDS/WDS spectra from the central area of the map shows that the Lambda WDS system intrinsically resolves the overlapping EDS peaks (red outline), as depicted by the cyan color WDS spectrum (Figure 2). The shortcoming of EDS in resolving these overlapping peaks results in the distributions of the three elements appearing identical in EDS maps. However, the WDS maps provide clear and distinct visualizations of the individual distributions of the three elements (Figure 3).
Figure 3. EDS (top) and WDS (bottom) maps of the Si-W-Ta sample. The WDS maps resolve the artifacts due to Ta M, Si K, and W M peak overlaps in the EDS maps.
This year’s M&M meeting is just around the corner. If you are traveling to this entirely in-person event, stop by our booth (#504) to check out our integrated EDS-WDS SEM solutions and many other products that will capture your interest.
Dr. Shangshang Mu, Applications Engineer, Gatan/EDAX
The new APEX™ 3.0 is the ultimate materials characterization software, integrating Energy Dispersive Spectroscopy (EDS), Electron Backscatter Diffraction (EBSD), and Wavelength Dispersive Spectrometry (WDS) to deliver previously unattainable solutions. This optimized configuration offers the uncompromised performance of each technique and allows users to combine them for the ultimate materials insight. All three techniques seamlessly operate within the APEX, blending powerful elemental and crystallographic analysis routines through an intuitive interface to deliver outstanding data collection, faster analysis, and flexible reporting for users of all levels.
What does APEX WDS look like?
WDS functionalities are implemented seamlessly with the EDS graphical user interface. The user can quickly adapt to the new functionalities and employ WDS when and where EDS reaches the limit. With one-click from start to finish, Auto WDS allows fully automated WDS scan list generation, optimum sample height determination, and spectrum collection. It simultaneously collects EDS and WDS spectra and displays them side-by-side or overlaid for easy data visualization and interpretation (Figure 1), with no overlapping or overloading of windows.
Figure 1. Simultaneous EDS-WDS spectrum acquisition user interface.
APEX allows you to set an intermediate position for the EDS detector to ensure optimal count rates for both techniques.
Figure 2. Simultaneous EDS-WDS mapping user interface.
Sets of combined EDS-WDS spectrum, linescan, and mapping data at different stage positions can be done via automated batch collection routines (Figure 2) to streamline SEM experiments. EDS and WDS data collection settings are managed in one user-friendly batch scan list (Figure 3).
Figure 3. Combined EDS-WDS batch list.
The quantitative elemental analysis supports individual technique or combined EDS-WDS standards. You can easily switch between EDS and WDS standards for each element by clicking on the icon in front of the element (Figure 4).
Figure 4. Quantitative results with combined EDS-WDS standards.
With the addition of WDS capabilities, APEX 3.0 now includes EDS, EBSD, and WDS. Each characterization tool can operate independently to utilize EDAX’s technological advancements or integrates data to provide solutions that were once unachievable.
Dr. David Stowe, Senior Product Manager – EDS and SEM Products, EDAX/Gatan
My friends and family have always thought that, as a microscopist, I spend my working days in a darkened room staring at dimly lit screens or developing negatives. Yet, the reality of working for a commercial company in the electron microscopy business could hardly be more different—scientific meetings, workshops, and spending time with users have allowed me to travel the world and make friends with some of the most interesting people. It’s always been a source of wonder and amazement for my family that the microscopic world could provide so many opportunities to see our world at large!
Sadly, over the last two years, our daily routines have been aligned much more closely to those visions of darkened rooms and computer screens than we care to remember. However, for many of us, there does appear to be (sun)light at the end of the tunnel. In recent weeks, I attended my first in-person workshop in almost two years. Together with colleagues and more than 60 researchers, I traveled to Munich to attend the Gatan-EDAX Leading Edge Workshop held at the Allianz Arena (home to Fußball-Club Bayern München e. V.—known to many as Bayern Munich). As lovely as it was to visit such a famous sporting stadium, the enjoyment of attending scientific talks and engaging in exciting technical discussions with leading researchers far outweighed the attraction of the venue. The feedback from everyone who participated in the day was incredibly positive. Many of us walked away from the event with new ideas inspired by discussions that day.
One of my fears regarding the impact of COVID-19 on the scientific community is the impact that the lack of these in-person interactions has had on innovation, both in terms of new scientific ideas and technological advancements. While working from home, many of us have missed those stimulating ‘coffee break conversations’ with colleagues outside of our teams. To add, there has been a noticeable drop in interactions on virtual platforms at scientific conferences and commercial webinars, with many preferring to review material offline at a time of their choosing.
Fortunately for EDAX, the opportunity for engagement with others during the pandemic had rarely been as high. Since 2019, we have been joined in the Materials Analysis Division of AMETEK by Gatan; the exchange of ideas between the R&D and Applications teams of the two companies has been significant. Within the last year, we have already seen the first innovations arising from the interactions between the two companies with the announcement of EDAX EDS Powered by Gatan for elemental analysis in the transmission electron microscope, simultaneous EDS and cathodoluminescence spectroscopy, and the development of a workflow solution for lithium analysis in the scanning electron microscope.
Within the last month, our applications teams were able to quantify the lithium content in lithium-ion battery cathode materials for the first time using the lithium-composition by difference method (Li-CDM).
Figure 1. First quantitative analysis of lithium content in cathode materials using the Li-CDM technique.
Their analysis used a range of tools from EDAX and Gatan to prepare, transfer, and analyze the specimen in the scanning electron microscope, highlighting the potential for innovation by Gatan and EDAX working together.
Figure 2. The tools from EDAX and Gatan used for the quantitative analysis of lithium in metal oxide cathode materials.
With the imminent arrival of the latest Microscopy and Microanalysis conference in Portland, OR, I am sure we will learn far more about how the changes to working practices have impacted innovation in our world. I am excited to leave my darkened room and discover your latest works in electron microscopy. I am sure that all participants will enjoy and value the personal interactions that are so important for innovation.
Matt Chipman, Sales Manager – Western U.S., EDAX and Gatan
I recently watched a local news story about inflation in consumer goods. The reporter wanted to know if the dollar store could save you money on groceries. The general answer was perhaps on some items, but it wasn’t significant. However, it was interesting to see how some stores focus on a perceived value instead of a real value to its consumer. First, the dollar store raised its starting price from $1.00 to $1.25. Then they used odd-sized packages that were not equivalent to regular grocery store items, making a direct comparison difficult and offering minimal to no real savings. Finally, the dollar store’s selection was very limited so you may end up back at the regular grocery store for anything other than packaged goods.
So, what does this have to do with the microanalysis business? Well, I believe it’s important to look at the big picture with real, tangible benefits that can impact your research. By offering both EDAX and Gatan products, there are more opportunities to combine different technologies to enable unique analyses that can provide a tremendous value to your material studies.
One great example is the quantification of lithium on a scanning electron microscope. By uniting Gatan’s low-kV OnPoint™ Backscattered Electron Detector with EDAX’s Octane Elite Super EDS Detector, this one-of-a-kind analysis is now possible, surpassing what can be done by either technique alone.
Figure 1. The lithium mapping from joint characterization of the EDAX Octane Elite EDS Detector and Gatan OnPoint BSE Detector.
Not to forget, we’ve also been combining the strengths of the Gatan DigitalMicrograph® Software with the EDAX EDS detector technology for TEMs. I believe we are just beginning to scratch the surface of creative things we can do by joining microanalysis systems and techniques. I love discussing creative ways my customers can coalesce microanalysis techniques to do something new.
Figure 2. Multimodal data acquisition of EELS and EDS data combines the chemical sensitivity of EELS with the broad compositional mapping of EDS. Pictured – STEM EELS/EDS mapping of vertical channel 3D NAND acquired with DigitalMicrograph software.
I hope we can all figure out ways to get a real, noticeable value from the equipment we purchase during this time of inflation. I hope to hear ideas from some of you as you tell me about the needs of your laboratories.
It has been interesting to recently witness EDAX and Gatan working together to combine the technologies in our portfolios. Although technically, Gatan was acquired by AMETEK back in late 2019, it seems like 2021 has been a year where the integration of our two companies has really begun to hit its stride.
For example, I’ve seen how Gatan’s ion polishing instruments can dramatically improve indexing success for EDAX’s Electron Backscatter Diffraction (EBSD) users compared to the conventional methods for sample preparation. And I’ve seen EDAX’s Elite T Energy Dispersive Spectroscopy (EDS) System undergo a tremendous workflow improvement and ease-of-use overhaul with the implementation of Gatan’s Microscopy Suite user interface. It has been great stuff!
However, the most recent integration between our two companies is truly groundbreaking, and I’m thrilled to see what it will do to enhance the research being done in its field.
Hopefully, you’ve already seen the news mentioned on our website. For the first time, we’ve been able to perform quantitative mapping of lithium in the Scanning Electron Microscope (SEM) by combining the power of EDAX and Gatan detectors and software! These breakthrough results will enable a new level of lithium research that was previously never possible with the SEM.
Figure 1. EDAX and Gatan bring you lithium.
So who cares about lithium? Everyone should. Lithium compounds and alloys are very important materials with significant commercial value. The compounds are being implemented into lightweight structural alloys in the aerospace and automotive industries. They’re also utilized in lithium-ion batteries for small electronic devices and vehicles. Many governments worldwide have proposed plans to reduce dependence on legacy energy sources, which makes further development of lithium-based technologies critical to the adoption of these plans. This means significant investments are currently being made in R&D, failure analysis, and quality control of these materials.
Figure 2. (left) Lithium-ion battery cross-section prepared by Ilion II broad beam argon milling system. (right) EBSD IQ + orientation map revealing the microstructure of the heat-affected zone in a lightweight structural alloy.
So what are the issues with lithium? While electron microscopy and EDS are already essential characterization tools in this industry, there is a distinct inability to map lithium distribution in the SEM. This has presented a significant obstacle, holding back research on these tools. EDS is typically a valuable technique for material characterization, with high sensitivity and spatial resolution to allow for quantitative analysis on a wide range of sample types. But it is not possible to identify lithium in commercially important materials by EDS because:
There is no guarantee that lithium X-rays will be produced from the sample. The X-ray energy and the number of photons produced from the specimen depend on the lithium bonding state. So, even if you have lithium in your sample, it does not mean that lithium X-rays will be generated.
Even if a sample does generate lithium X-rays, they are easily absorbed back into the sample itself, contamination or oxidation, or by the EDS detector window before they can even reach the EDS detector.
Indeed, specialized windowless EDS detectors can detect lithium, but these have drawbacks that impede their practicality and largescale adoption. Even on samples that have a high lithium fluorescence, these special detectors have a limit of detection of about 20 wt %. This is equivalent to about half of the atoms in the sample being lithium, which restricts analysis to only metallic lithium or simple lithium compounds that may not be relevant to advanced lithium research or applications.
And having a specialized windowless EDS system potentially introduces a slew of operational issues/limitations with the detector that aren’t present with a “standard” windowed EDS system. It also restricts the detector’s utility on non- lithium -research-based applications in the lab.
So what have EDAX and Gatan done? We have solved these issues by using a patent-pending technique called the Composition by Difference Method. In this method, we quantify the backscattered electron signal to determine the mean atomic mass for all elements in a particular area of a sample. And from the same region, we collect the EDS signal to quantify the non-lithium elements. From that information, we have two data points that tell us the actual mean atomic mass from the region and a calculated value based on the EDS results — when they don’t agree with one another, it tells us we are missing something in the EDS data. That something we’re missing is lithium.
Figure 3. Data from the OnPoint and the Octane Elite Super are combined and analyzed to quantify lithium.
By using this method, and specifically by combining the EDAX Octane Elite Super EDS Detector and the Gatan OnPoint Backscattered Electron Detector to collect these two signals, we can now generate lithium maps quantitatively with single-digit mass percentages of lithium with sub-micron spatial resolution. This accuracy has been verified to ~1 wt. % lithium by an external accredited laboratory using Glow-discharge Optical Emission Spectroscopy (GDOES).
Figure 4. Secondary electron image and elemental metal fraction maps (by wt. %) of the same region of the MgLiAl alloy; white pixels are regions excluded from the analysis due to the influence of topography (identified by arrows in the secondary electron image) shown here for clarity.
This is a cutting-edge capability in the SEM, and it is a huge opportunity for anyone wanting to discover where lithium exists in their specimens. Just to reiterate, this method does not use a specially designed EDS system for lithium detection! It uses EDAX’s standard (windowed) Octane Elite Super and Gatan’s OnPoint BSE detector, along with EDAX and Gatan software. Simply amazing!
Now that EDAX and Gatan have introduced the ability to provide quantitative lithium analysis, that is:
A substantial improvement in limits of lithium detection
Insensitive to the lithium bonding state
More tolerant to contamination and oxidation
Not limited to metallic materials or simple lithium compounds
Free from windowless detector-related limitations on the SEM
It seems that we have helped open an avenue for our customers to expand their lithium research beyond anything previously possible. We are truly beginning a very exciting new stage in lithium analysis, and I can’t wait to see how this new capability is used and what comes next!
You can find more information on this new development in our experiment brief.
Rudolf Krentik, Sales Manager – Central and Eastern Europe, EDAX
It has been some time since I started working at EDAX as an Area Sales Manager for Central and Eastern Europe. When I think about it, Russia is by far the largest region compared to all the others. If sales grew linearly with the size of the area, I would probably be a millionaire. Unfortunately, it is not the case. The primary purpose of my work is to take care of our distributors and business partners in individual countries. I give them support in business cases, provide up-to-date information about our products, and sometimes I am also an intermediary for the serious requirements of our end customers. The work is very interesting, especially because I meet interesting people. EDAX’s customers are primarily scientists and engineers studying materials, solving complex problems, and dealing with development and innovation. Such meetings are often very fun, inspiring, and rewarding.
Figure 1. My new office.
The market situation has changed dramatically since 2015, when I started. COVID-19 has completely changed the way we work. Instead of meeting customers at scientific conferences, we all locked ourselves in our homes for a long time. After three months, I couldn’t stand it and rented a small office so that I wouldn’t go crazy at my home office with my wife and two small children, who were also schooling and working from home. So I was moving from my home office to an actual office, doing just the opposite of what others were doing during the pandemic.
Moving from real life to the online world was probably frustrating for many of us. Still, we had to adapt and start selling and communicating over the phone and especially over the internet. Online presentations and meetings are still the order of the day. This way of communication will be maintained in the future, that is quite certain. Unfortunately, this does not replace personal contact, which is essential for creating a relationship with customers. It can already be seen that interest in virtual conferences is declining. People are inherently interactive and need to share their needs and feelings with each other. This is not possible in the world of the internet. Therefore, we all hope that everything will return to normal soon. Our service technicians have been traveling to places where it is safe for quite a long time, and we salespeople are also starting to plan our first trips abroad. I’m actually partly writing this blog in Turkey on my first trip in 18 months.
Although it does not seem so, COVID has not yet caused significant losses or loss of orders in terms of business results. Our business is still in good condition. One of the factors that affects this is the life cycle of a business case. This can take months or even years. If we do not soon return to the life we are used to; it will have very negative consequences for our field. I mention this because we are currently at the stage where we want to launch several exciting products. You probably know that Gatan also belongs to our AMETEK family. The company is known for its leading technology in detection systems in TEM and SEM and other devices, e.g., for sample preparation. The acquisition of Gatan is a great benefit not only for AMETEK but also for EDAX. The combination of know-how, development, and experience in the electron microscopy field creates space for innovation and synergies that would not be possible.
Several novelties were introduced three weeks ago at M&M 2021. It is worth mentioning the EDAX EDS Powered by Gatan, in which EDAX hardware is now integrated into the software from Gatan. This brings many benefits, such as a unified GUI for all the TEM techniques available from Gatan. EDS analysis with Elite T can now be performed seamlessly with Gatan EELS, 4D STEM (STEMx), or other techniques. This makes it all much easier and faster. And as we know, time is money, and this is doubly true for time spent at the TEM.
Another interesting novelty is the cooperation of EDS and CL detectors. Thanks to an EDS-compatible cathodoluminescence (CL) mirror that enables line of sight from the sample to the EDS detector while still collecting the CL signal, we can obtain information about the material’s structure that was previously difficult to achieve.
When it comes to EBSD, EDAX has been the leading provider of this technique since the 90s. But for reliable analysis, one needs a high-quality sample preparation tool. Again, with the Gatan PECS II, we can offer a complete workflow from getting the sample ready to post-processing of acquired data. The latest news is also the hottest news. With the help of the highly sensitive OnPoint BSE and Octane Elite EDS Detectors, it is possible to detect lithium for the first time and quantify it. Unique technology, the accuracy of which is verified by another method, is now available and we are very anxious to introduce this product to our customers.
That is why we need to get the COVID-19 pandemic under control. Without the opportunity to travel and meet our customers, our work will be inefficient and not as much fun. However, the newly introduced devices and the ongoing development of the EDAX-Gatan collaboration gives us a strong hope that everything is on track and that our efforts are worthwhile.
This last year has been different in many ways, both personally and at work. For me, it meant being in the office or working from home instead of being out and about and meeting customers and performing operator schools in person. This does not exactly mean that things are quieter, though! At home, I got confronted with lots of little maintenance things in and around my house that otherwise somehow manage to escape my attention. At work, lots of things vying for my attention have managed to land on my desk.
The upside is that with almost everything now being done through remote connections. I get to sit more at the microscope in the lab to work on customer samples, collect example datasets, perform system tests, and also practice collecting data on difficult samples so that I can support our customers better. To do that, I have the privilege of being able to choose which EBSD detector I want to mount, from the fast Velocity to the familiar Hikari to the sensitive Clarity Direct Electron System. But how do I decide what samples to use for such practice sessions?
Figure 1. A common garden snail (Cornu aspersum) and an empty shell used for the analysis.
In the past, I wrote about my habit of occasionally going “dumpster diving” to collect interesting materials (well, to be honest, I try to catch the things just before they land in the dumpster). That way, I have built up a nice collection of interesting alloys, rocks, and ceramics to keep me busy. But this time, I wanted to do something different, and an opportunity presented itself when I was working on a fun DIY project, a saddle stool for my daughter. On one of the days that I was shaping wood in my garden for the saddle-support, I noticed some garden snails moving about leisurely. These were the lucky chaps (Figure 1). While we occasionally feel the need to redecorate our walls to get a change of view, the snail’s home remains the same and follows him wherever he goes; sounds great! No need to do any decoration or maintenance, and always happy at home!
But all kidding aside, I have long been interested in the structure of these snail shells and have wanted to do microstructural analysis on one. So, when I found an empty shell nearby belonging to one of its cousins that had perished, I decided to try to do some Scanning Electron Microscope (SEM) imaging and collect Electron Backscatter Diffraction (EBSD) data to figure out how the shell was constructed. The fragility of the shell and especially the presence of organic material in between the carbonate crystals that make up the shell makes them challenging for EBSD, so I decided to mount my Clarity Detector and give it a very gentle try.
The outer layer that contains the shell’s color was already flaking off, so I had nice access to the shell’s outer surface without the need to clean or polish it. And with the Gatan PECS II Ion Mill that I have available, I prepared a cross-section of a small fragment. I was expecting a carbonate structure like you see in seashells and probably all made of calcite, which is the stable crystal form of CaCO3 at ambient temperatures. What I found was quite a bit more exotic and beautiful.
In the cross-section, the shell was made up of multiple layers (Figure 2). First, on the inside, a strong foundation made of diagonally placed crossed bars, then two layers of well-organized small grains, was topped by an organic layer containing the color markings.
Figure 2. A PECS II milled cross-section view of the shell with different layers. The dark skin on the top is the colored outer layer.
At the edge of the PECS II prepared cross-section, a part of the outer shell surface remained standing, providing a plan view of the structure just below the surface looking from the inside-out. In the image (Figure 3), a network of separated flat areas can be recognized with a feather-like structure on the top, which is the colored outer surface of the shell. An EDS map collected at the edge suggests that the smooth areas are made up of Ca-rich grains, which you would expect from a carbonate structure. Still, the deeper “trenches” contain an organic material with a higher C and O content, explaining why the shell is so beam-sensitive.
Figure 3. A plan view SEM image of the structure directly below the colored surface together with EDS maps showing the C (purple), O (green), and Ca (blue) distribution.
The EBSD data was collected from the outer surface, where I could peel off the colored organic layer. This left a clean but rough surface that allowed successful EBSD mapping without further polishing.
My first surprise here was the phase. All the patterns that I saw were not of calcite but aragonite (Figure 4). This form of calcium carbonate is stable at higher temperatures and forms nacre and pearls in shells in marine and freshwater environments. I was not expecting to see that in a land animal.
Figure 4. An aragonite EBSD pattern and orientation determination.
The second surprise was that the smooth areas that you can see in Figure 3 are not large single crystals but consist of a very fine-grained structure with an average grain size of only 700 nm (Figure 5). The organic bands are clearly visible by the absence of diffraction patterns – the irregular outline is caused by projection due to the surface topography.
Figure 5. Image Quality (IQ) and aragonite IPF maps of the outer surface of the shell. The uniform red color and (001) pole figure indicate a very strong preferred crystal orientation.
After this surface map, I wanted to try something more challenging and see if I could get some information on the crossbar area underneath. At the edge of the fractured bit of the shell, I could see the transition between the two layers with the crossbars on the left, which were then covered by the fine-grained outer surface (Figure 6).
Figure 6. An IQ map of the fracture surface. The lower left area shows the crossbar structure, then a thin strip with the fine-grained structure, and at the top right some organic material remains.
Because the fractured sample surface is very rough, EBSD patterns could not be collected everywhere. Nevertheless, a good indication of the microstructure could be obtained. The IPF map (Figure 7) shows the same color as the previous map, with all grains sharing the same crystal direction pointing out of the shell.
Figure 7. An IPF map showing the crystal direction perpendicular to the shell surface. All grains share the same color indicating that the [001] axes are aligned.
But looking at the in-plane directions showed a very different picture (Figure 8). Although the sample normal direction is close to [001] for all grains, the crystals in the crossbar structure are rotated by 90° and share a well-aligned [100] axis with the two main directions rotated by ~30° around it.
Figure 8. An IPF map along Axis 2 showing the in-plane crystal directions with corresponding color-coded pole figures.
Figure 9. Detail of the IPF map of the crossbar area with superimposed crystal orientations.
I often have a pretty good idea of what to expect regarding phases and microstructure in manufactured materials. Still, I am often surprised by the intricate structures in the smallest things in natural materials like these snail shells.
These maps indicate a fantastic level of biogenic crystallographic control in the snail shell formation. First, a well-organized interlocked fibrous layer with a fixed orientation relationship is then covered by a smooth layer of aragonite islands, bound together by an organic structure, and then topped by a flexible, colored protective layer. With such a house, no redecoration is necessary. Home sweet home indeed!
A couple of weeks ago, a question regarding the minimum detection limits (MDL) of our Energy Dispersive Spectroscopy (EDS) quantitative analysis was forwarded to me from a potential customer. This is a frequently asked question I get from customers during EDS training. We understand researchers are looking for a simple answer; however, they don’t get a straightforward answer from us most of the time. This is not because we don’t want to tell the customer the configurations of our systems, but detection limits depend on various factors, including detector window, geometry, detector resolution, collection time, count rate, and sample composition. The detection limit for a given amount of an element in different sample matrixes is not the same. For example, calcium in indium has a much higher detection limit than it has in carbon because calcium energy lines are heavily absorbed by indium, but not by carbon. The limit also changes if you have a bit more of a given compound in the sample. The limits are lower if the data collection time is doubled. So, it is impossible to provide a general MDL for an EDS system or even a given element, but we can calculate the MDL for a given spectrum.
This function is available in APEX™ Software for EDS version 2.0 or later. For each element identified in the spectrum, the MDL is given in the quantification table and flagged if it is below the detection limit (Figure 1). To determine the MDL for a given spectrum, one must look at the statistical significance of the signal above the background. We generally use the single-channel definition for peak and background counts.
Figure 1. Quantification table with MDL.
Figure 2. Illustration of background and peak counts.
For a given element to be above the significance level, it requires that the total number of counts on the peak NP be above background counts NB by a predetermined confidence, see Figure 2. For significance, we use 1.7 standard deviations (SD) in a one-tail test since we are only concerned about having counts above the threshold (Figure 3). A SD of 1.7 corresponds to about 95% confidence for a single-tail.
Figure 3. Single-tail normal distribution. NB is the background mean level.
The significance level can be calculated as:
NS=NB+1.7σB= NB+1.7√(NB)
This means that the requirement for an element to be considered significant is:
NP≥ NB+1.7√(NB)
For the MDL calculation, we are considering the net counts on the peak (NP-NB). Analog to the significance level, it is required that the counts are above the background plus a significance level, but we are now considering net counts instead of gross counts.
NDL=NB+∆(NP-NB)
To calculate the error, we consider the error of the peak and the background. If an element is close to the detection limit, the number of counts are comparable to the background counts, and we can approximate the total error:
∆(NP-NB)=√(NP+NB)≈√(2NB)
Using a 2s/95% confidence level, we can write the count detection limit as:
NDL=NB+2√(2NB)~2.8√(NB)
With the count-based detection limit and assuming the counts are linear with concentration, the concentration MDL can be calculated from the concentration C of a given element in a spectrum:
MDL=2.8√(NB)*C/NP
As I mentioned earlier, the detector window is one of the most important factors determining the MDL. With the introduction of Silicon Drift Detectors (SDD) and the development of fast and low-noise pulse processors, EDS analysis has seen remarkable increases in throughput and reliability in the last decade. But one often overlooked aspect of the detection technology is the detector window. A variety of window technologies are available, including beryllium, polymer films, and the most recent addition by EDAX, silicon nitride. Due to the polymer window’s composition and thickness, a significant part of the low-energy X-rays is absorbed before reaching the X-ray detector. This absorption effect is vastly reduced in the range below 2 keV for the silicon nitride windows, as shown in Figure 4.
Figure 4. Transmission curves for silicon nitride and polymer windows measured using synchrotron radiation.
The MDL for spectra acquired from the same samples with different window configurations can be calculated by employing the derived equation above. This study was led by Dr. Jens Rafaelsen at EDAX using five different standards. To eliminate the detector resolution and response as a variable in the experiment, the window was removed from a standard detector, and exchangeable caps with silicon nitride and polymer windows were mounted in front of the electron trap. Figures 5 and 6 show the relative improvement in MDL for the window-less and silicon nitride window configurations compared to the polymer window. Figure 6 documents the silicon nitride’s superiority over the polymer window in the low energy range with improvements of over 10% for the MDL of oxygen. While Figure 5 shows that further improvements can be gained in the window-less configurations, the silicon nitride window still allows for the use of variable pressure mode and spectrum collection from samples exhibiting cathodoluminescence (CL).
Working as a cathodoluminescence (CL) application scientist at Gatan, I observe a great variety of interesting specimens from semiconductor devices, plastics, and geological samples to novel nanoscale optical devices demonstrating the capabilities of the Monarc® Pro CL detector. In case you don’t know, CL is the visible, ultraviolet, and infrared light emitted by many specimens in the scanning electron microscope (SEM). Recently, I was contacted regarding a meteorite sample and asked what analysis I could demonstrate using CL. As a physicist and amateur astronomer, I was naturally very excited at the rare opportunity to analyze something that literally came from out of this world! You might say I was… over the moon 🌙!
The sample is a thin-section from a meteorite collected from Antarctica – Miller Range 090010, you can read more about the classification here: Meteoritical Bulletin: Entry for Miller Range 090010 (usra.edu). Likely to have been a constituent of the asteroid belt, our specimen had a trajectory that eventually led it to fall to Earth. The study of these meteorites allows us to understand more about the age and history of our solar system. Given the origins and unusual conditions experienced by meteorites, the microstructure can be incredibly complex, but often, chondritic meteorites like this one contain calcium aluminum inclusions (CAIs) and corundum grains which are among the first solids to condense from the solar nebula! Now, before I get wrapped up with the Cosmic Calendar, let’s take a look at our specimen!
Figure 1. Image overlay from a CAI region of meteorite specimen (gray) secondary electron and (green) unfiltered CL.
CL revealed so much new information, and this was an exciting first result! For geological specimens, unfiltered CL images can be very useful to reveal mineral texture, but the real nitty-gritty information is found in the spectrum. So many of the grains showed such strong luminescence that I was eager to learn more.
Our friends at EDAX recently installed an Octane Elite Energy Dispersive Spectroscopy (EDS) Detector on the same SEM as the Monarc. EDS and CL are fantastically complementary techniques for sample analysis. EDS is great for elemental quantification but falls short when trying to identify trace elements, crystallographic phases, or grain boundaries – where CL shines! Equipped with these powerful tools, I collected my first multi-hyperspectral data, capturing CL and EDS signals simultaneously. Take a look at some of the results:
Figure 2. (left) True color representation of the CL spectrum image (color) overlaid with SE image (gray), and (right) extracted CL spectra from points 1 (aqua fill), 2 (red), and 3 (green).
Figure 3. (left) Elemental quantity maps extracted from the EDS spectrum image corresponding to aluminum (blue), calcium (green), and magnesium (red); and (right) extracted EDS spectra from points 1 (aqua fill), 2 (red), and 3 (green). Points 1, 2, and 3 are the same locations as in Figure 2.
Both techniques were very revealing. In addition to Mg, Ca, and Al, the EDS spectrum image (hyperspectral image) detected other elements, some in high abundance like O and Si, and others which were less abundant, including Fe, C, Ti, and Na. We discovered geological materials like hibonite, corundum, and apatite but could not discern which mineral complexes they were involved in. At first glance, the CL and EDS maps looked very similar, but the more I looked, the more I realized there were significant differences, and so I decided to dig a little deeper with the CL spectrum image. The CL spectrum shown in Figure 2 indicates the presence of several trace elements. By looking at the difference of intensities at the smaller sharp peaks in contrast with the surrounding intensities, I was able to differentiate two maps from the CL data, which likely correspond to the presence of trace elements, one with an emission peak at 460 nm (Fe in corundum) and the other at 605 nm (Sm in apatite).
Figure 4. Extraction of CL trace elements (Fe in corundum) found at 460 nm (red) and (Sm in apatite) 605 nm (green).
Figure 5. (left) Bandpass CL image displaying 580 ± 20 nm and (right) colorized EDS map for Al (blue), Ca (green), and Mg (red).
Figure 6. EDS and CL composite image including EDS elemental maps for aluminum(blue) and magnesium (yellow); and trace elements iron in corundum (green) and samarium in apatite (red) as revealed by CL.
The data gathered from this sample may give a glimpse into the history of our solar system’s evolution. It also demonstrates the need for complementary techniques when analyzing complex samples. I want to thank NASA for generously providing the sample used in this study, Gatan and EDAX for providing me the opportunity to work with it, and the nature of the universe for generating this message in a bottle and letting it find its way to our lab!
As I prepared for some analytical work yesterday, I had to repolish a standard block. This made me think about how important these little blocks are and how often they are not cared for properly. With that in mind, I thought it might be useful to pass on some little nuggets of information I have gathered over the years from many sources.
The most important thing about caring for a block is knowing what is in it. Standard blocks can be purchased as a whole or personally made. No matter what, you need to know what you have! To do so, you should keep several copies of the following for every standard you have:
Optical light images of the whole block
SEM Montage image of the whole block (BSE and SE)
Individual image of each standard material
Composition of each standard material with sources
Notes on each standard
Figure 1. Each of our standard blocks has a name and a duplicate document. This packet has optical, BSE, and SE images of the standard. This allows us to quickly find the standard we want while having all the information easily accessible in hand.
Each of these above items is important. You want to keep both a visual record of your standards, a record of what it is and the condition that it is in, to allow you to track any issues that may pop up (Figure 1). Therefore, having a note section is important. You may find that one of the areas of your standard gives anomalous values and should be avoided. You want to make sure this information is easily accessible to everyone that uses the standard. I suggest scanning and keeping electronic copies in a shared folder on your desktop.
Besides the documentation aspect of care, physical care is just as critical. Most commercial standard blocks come pre-polished and carbon-coated. Over time, both of those will degrade and need to be redone. Usually, the carbon coating damages first, but you also need to check for burn marks and other beam damage done to the standard material itself. When repolishing and recoating, I usually do a solid 10 minute repolish with diamond paste. This removes enough material to eliminate the carbon coating and get new clean, undamaged surfaces while not change the physical appearance all that much. I try my best to avoid using an Al-based polishing material, as they tend to stick around too much and can interfere with my analysis on elements I use. With carbon-based polishing material, it is much easier to see the effects of the carbon. In the end, I do not tend to do quant work on carbon that much, while I often try to quantify aluminum. Whatever you do, document what was done. It can help you both head off and understand issues that may present.
While physically handling your sample, it shouldn’t need to be said, but you should never be touching your sample with ungloved hands. Your oils are bad for both the SEM cleanliness and the sample cleanliness. Avoid any sort of colloidal products with standards, as they do tend to flake with age. When not in use, samples should be held in a desiccator with good desiccant (Figure 3).
Figure 3. A good desiccator should have a rubber molding to help it hold a seal at a minimum. You should try to keep it under vacuum for the best results. While taking this picture, I noticed I should dry my desiccant or replace it. I have seen some users keep a small plastic bag of fresh desiccant in the desiccator as a quick visual reference.
There are many other tips I can think of sharing, but to wrap it up, standards are valuable in our industry. A good, well cared for standard will last multiple careers while giving consistent results time after time. Take the time to keep your standards in the best condition, and they will repay your time spent on them tenfold.
![An IPF map showing the crystal direction perpendicular to the shell surface. All grains share the same color indicating that the [001] axes are aligned.](https://i0.wp.com/edaxblog.com/wp-content/uploads/2021/06/figure7.jpg?resize=620%2C396&ssl=1)
