The D4 Endeavor fits in a unique niche in the XRD world. It’s basically the same thing as a Bruker D8, but built into a very compact cabinet with a large autosampler on top. These machines see heavy use in the cement, pharmaceuticals, and aluminum industries among others. Today we have one headed out to a new home. It started life in the pharmaceutical world, but had light use so it was a great candidate for refurbishment.
Blog
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Hybrid sample holders for Bruker FlipStick Autosamplers
There’s a simple and inverse relationship between the average atomic number of a given matrix and the efficiency with which it scatters X-rays. The effect isn’t all that much different from shining a flashlight on a white piece of paper. While it’s nothing like a mirror, you’ll definitely get light scattering off of it to some degree. This effect has strong effects on most XRD and XRF applications in that the additional photons must be dealt with somehow. This post addresses plastic sample holders and their effects on XRD data specifically.
Click on the scan images for full size versions:
This odd hump is the result of scatter off the sample holder body. 
Running 1mm anti-scatter and divergence slits solves the problem as long as we’re able to fill at 25mm sample well. Any smaller and we can’t avoid irradiating the sample holder. The hybrid holder performs nearly identically to the PMMA holder. Smaller anti-scatter and divergence slits work well as you can see in these scans, however, the intensity we lose throughout the remainder of the scan makes this an undesirable approach.
The scatter is gone, but we’ve lost 50% of our intensities along with it. Automatic anti-scatter and divergence slits allow the irradiated area to be held constant throughout the scan which gives us the best of both worlds. These are not terribly common though and the data must be corrected as the intensities will vary dramatically from theoretical and historic data.
Perhaps the easiest way to deal with this effect is to simply change the sample holder material. While we make a large number of standard PMMA sample holders for Bruker XRD systems, the scatter from the plastic material is an undeniable problem. For that reason, we have also made Aluminum bodied holders for quite a while. Even steel holders are used, but usually not with Cu incident x-rays as Fe fluoresces strongly under Cu excitation.
While this works for most users, those using a FlipStick autosampler are in a difficult position. Due to the way this autosampler positions and rotates the sample holder during data collection, the only viable options for the body have been PMMA plastic or steel. Both of which handle the pressure well, but between the scatter off the plastic and the Fe fluorescence from the steel, we’re left without an ideal solution.
We make some very large zero background sample holders which are really the perfect solution in that there is near zero scatter or any effect from the sample holder itself. It’s like the sample material is floating in space. These are incredibly powerful, but not inexpensive so their use is usually limited to cases where they are required.
Recently, we decided to try an experiment with an outer edge of PMMA plastic surrounding an inner body and sample well made of Aluminum. Al will still scatter a bit, but it’s much better than plastic. We’re hopeful that this will be a useful solution for many XRD users who struggle with scatter in the 5-13 degree 2Theta range. Contact us if you’re interested in a solution like this.
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NIST 1976c custom mount
It’s easy to forget how much of our scientific work hinges upon comparative data. The entire field of metrology is concerned with the verification and maintenance of “standard reference materials” (SRMs). Creating a perfect reference standard essentially involves proving a negative. In the XRD world, we need to prove that there are no impurities, no crystalline defects, no unaccounted for thermal variations, no stress/strain effects present, and above all, that the first unit produced is effectively identical to the last and all between.
There really isn’t any one material that checks all the boxes, but the NIST gets very close thanks to the efforts of Mr. Jim Cline and his associates. The hardware they’re using is completely custom to the point that it bares no resemblance to the instrumentation we normally work with. There’s a great page on their divergent-beam lab here.
The NIST 1976 material has been a mainstay in regular monitoring and certification of XRD system performance for many years and is now on its third (c) generation. With this most recent revision the shape of the standard has been changed from a flat plate to a round disk. This allows for much greater compatibility across the range of sample holders in the market.
This week we made a custom mount for a company using a Siemens D5005 with a 40-position autosampler. It’s common to mount these, but this is the first time I can recall doing so for this particular autosampler so I thought it was worth sharing a little about the material and a picture of the finished product.
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XRD tube bisection
XRD tubes rely on the ability to precisely regulate the flow of electrons from the filament to the anode in order to create x-ray emissions. That requires a completely evacuated envelope to avoid having the high-voltage short to ground. The resulting symptom is that the high voltage generator will shut down almost instantly when the high-voltage potential is applied.
This particular tube looked fine on the outside and didn’t even look all that old so it was a little surprising to find it behaving like the vacuum in the envelope had been compromised. However, after two tests, it was definitively bad so it was set aside for disposal. The first step is simply to remove the head of the tube which is little more than a mounting flange and cooling water distribution device, but when it was removed, the back side of the anode is cooled directly with water and showed some of the most extreme pitting we’d ever seen!
The next step is to break the ceramic envelope off the metal body of the tube. It was full of water which was obviously the cause of the problem. The pitting must have broken through to the vacuum envelope, which then sucked in water until it was 100% full. Hence the lack of any “sloshing” while we handled the tube.
Cross sectioning the tube anode showed just ow extensive the pitting was. The actual hole was so small that I could only guess where it had been, but it was definitely there.
We disassemble the x-ray tube on XRD systems during every preventative maintenance procedure and definitely would have caught this before it failed completely. It was most likely caused by either very low quality tap water being used in the recirculating chiller or some additive designed to prevent scale or algae.
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Mystery crystals
Curiosity may have killed the cat, but it’s the lifeblood of a well-functioning analytical lab. A few days ago, Todd was preparing a water chiller for shipping and washed out some corrosion products with acetic acid. The resulting solution was left in a beaker over the weekend and when we returned, he noticed that it had formed rather large crystals. So he did what any curious person with a lab full of XRD instrumentation would do. He ground it up into a fine powder and ran it through a D4 Endeavor. The resulting pattern was definitive and made complete sense given the brass metals involved in the corrosion product and the acetic acid solution.
The data was imported into MDI Jade Pro for phase identification. Jade makes it very easy to take the analysis all the way through whole pattern fitting (WPF/Rietveld) when paired with the COD or ICDD PDF-4 databases. I generated a full report even though the data was not nearly the quality we would usually require for this type of analysis for a client.
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PMMA vs Aluminum sample holders
Plastic sample holders have been the default option XRD for decades. They’re inexpensive to make, good enough for most purposes, and very resistant to a wide range of chemicals. Seems like a “win” all around right? As long as they’re made correctly and from the proper materials, these work just fine. Spoiler alert: 3D printed thermoplastics have a distinct structure so if you try making your own, be sure they’re well out of the irradiated area.
Comparison of the same powder in an aluminum holder vs a standard PMMA holder. Click the image for a full view of the scan. We’ve seen a big move toward more zero-background holders in the last few years and they’re definitely incredible, but many XRD users aren’t concerned with running extremely small volumes as much as they are eliminating or minimizing the scattering of x-ray by the plastic at low angles. As a general rule for polycrystalline materials, the lower the average atomic number of the material, the more efficiently it scatters x-rays. We’re not talking about Bragg diffraction. More like shining a flashlight on a white sheet of paper. Compton and Raleigh scattering are the two effects at work here.
We use this effect to great advantage in the XRF world when we want to check out the emission spectrum of the x-ray tube. Running a piece of graphite as the sample allows us to collect a scan which shows all the different energies in the emission spectra. Unfortunately, scatter is highly problematic in XRD.
This is why we supply aluminum holders with our refurbished systems and why we use mostly aluminum holders at Texray. Different plastics can perform differently, but the problem is characterized by a wide, low hump centered around 10 degrees 2Theta and with a width of about 10 degrees. This is a key area in many materials so anything we can do to clean up the background is usually beneficial.
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What’s really coming out of my XRD tube?
We’ve been experimenting with better ways to quantify the quality of XRD tubes in the shop. We use these tests on new and used tubes to monitor performance in two key areas. 1) Intensity 2) Spectral purity.
What we’ve settled on is a test that involves a wavelength-dispersive approach which gives us a lot of intensity to work with while eliminating background scatter and fluorescence effects. Basically, we’re able to extract more information from the data because the “noise” is almost zero.
We used Jade Pro to evaluate the scans, but they’re not D-spacing vs intensity as one would normally expect. This scan represents Wavelength vs intensity more like one would see in a WDXRF spectrometer. Cu KA1 and 2 are obvious, as is Cu KB1. Many of the current generation of XRD users have never seen a W LA1 peak in their data, but it’s clearly visible here as this is an older tube. What I’ve never been able to see before is the W La2 peak in the green scan. You’re looking at a peak that is ~62eV separated from W La1. No XRD detector on the market has energy resolution like that so these would always be lumped together so you’d see a series of additional peaks from every d-spacing in the sample in the diffractogram. Only a handful of detectors (our SDD-150 for example) could even separate the W La from the Cu K lines. That’s the power of wavelength-dispersive techniques. Incidentally, the most common device for cleaning up superfluous energy emissions in XRD data is a diffracted-beam monochromator and they eliminate all the W La through a secondary diffraction event much like what we’re doing.
Characterizing emissions is nothing new. In fact, I started wanting to improve this after listening to a talk at DXC about Jim Cline’s famous XRD system which is used at NIST to perform the primary data collection on the CRMs we all use. To paraphrase Sir Arthur Conan Doyle, “When you explain every extraneous data point, the remaining information is the pure truth of the sample”.
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Bruker D2 Phaser Sample Holders
KS Analytical Systems now offers custom sample holders for the Bruker D2 Phaser 6-position autosampler. These can be finished to order with any depth and diameter of well or a zero-background plate (ZBH) with or without a well ground in the surface.
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Fully digital autosampler install
Another fully-rebuilt, digital autosampler out in the wild. This one is on a system that already has one of our Si-Drift Detectors and an awesome ICDD Jade Pro/PDF-4+ software package. We’ve got all the fancy new hardware at our in-house lab, but when we need the absolute best data, this is our goto configuration.
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Sample holders for Rigaku Miniflex systems
KS Analytical Systems now offers custom sample holders for Rigaku Miniflex systems. The 6-position autosamplers and rotation stages (and even some fixed stages) make use of the magnetic disk design for holding powder without taking up much extra room in the diminutive benchtop.
- Top-loading
- Rear-loading
- Zero-background
