AN ANALYTICAL XRAY SERVICES LABORATORY
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I spend a great deal of time meeting with XRD and XRF users throughout the year, but usually in the context of some problem or time-sensitive project. Luckily I’ve been able to attend the Denver X-ray Conference fairly consistently over the last few years. It’s a great time to catch up with other users who are as deeply invested in X-ray spectroscopy and crystallographic analysis as we are. The vendors always put on a great show in the exhibit hall and poster sessions.

The first three days of the week are filled with technical workshops focused on an array of topics. There are always some introductory classes for both XRD and XRF for new users to attend and then there will be additional topics which are usually more advanced. The educational opportunities alone are well worth the attendance fee. Each session is run by an expert in the field and questions, even from industrial users, are welcomed. The sessions are strictly non-sales oriented as well which lends the event a very egalitarian feeling. See the full program here.

Plenary sessions and more sales-oriented meetings occur later in the week and are a great way to get a feel for the cutting edge technology being released by the various vendors. The exhibit hall opens a few days into the conference so everyone has a few days to see all the different booths. We always spend a great deal of time at the Materials Data, Inc and Bruker-AXS booths in particular.

The conference moves between Westminster, CO just North of Denver, Chicago, IL and Big Sky, MO. I’ve never made the trek up to Big Sky, but I hear it’s beautiful. Some attendees only come when it’s up there.

I’d love to connect with as many of our readers as possible so contact us if you’ll be there and I’ll be sure to see you while I’m at DXC-Big Sky!

The dreaded “amorphous” hump created by x-rays scattering off plastic sample holders has plagued XRD users for decades. It’s a serious enough problem that we make a good volume of these holders from Aluminum which works very well for loose powders. The plastic scatters xrays at around 13 degrees 2Theta (Cu anode tube) which make a real mess of most geological patterns and isn’t fun to model out for Rietveld refinement. Zero background holders like our ZBH-32 work wonderfully in standard sample stages designed for a single sample at a time, but the large plate isn’t compatible with the autosampler.

I recently had a request for a hybrid holder which would allow for analysis of very small volumes of materials while retaining compatibility with the autosampler. This is almost identical to our standard powder holders, but with a well designed specifically for our small ZBH plate.

Key features include:

  • 6061-T6 Al material (anodized or as-machined)
  • Si(510) plate
  • Raised sample well minimizes the area of the sample holder in the plane of diffraction. (Original Siemens design)
  • Beveled well walls minimize the area of Al in the plane of diffraction
  • Other small modifications are made to improve reliability of these holders in the autosampler

One of the fundamental facts of lab-based X-ray production is that our x-ray tubes emit much more than the pure KA1 lines we rely on for material characterization and quantification. Most XRD users are familiar with techniques and hardware for the reduction or elimination of KB1, W LA1 and Bremsstrahlung, but take for granted the inseparable pair of KA1 and KA2 (referred to as the “doublet”). Luckily for us, these energies are present in strict proportion such that we can factor their paired presence into most XRD analysis to the point that one might barely notice their effect. However, the fact remains that we will see peak broadening at lower angles and completely independent additional peaks at higher angles due to this superfluous discrete emission.

Separating the doublet cannot be accomplished electronically or through absorption/attenuation such as might be effective for KB1 energies. It must be done in the primary-beam with an additional diffraction event. Primary-beam monochromators are generally classified by the number of diffraction events required for a photon to pass completely through the device. Single-bounce, 2-bounce and 4-bounce geometries are common with the latter providing the best energy resolution allbeit the lowest intensity (photon flux). My limited experience suggests that while the single-bounce models retain enough intensity to have some application in powder XRD, the others are relegated to HR-XRD applications such as XRR.

The alignment for any of this hardware is not for the faint of heart as it begins with coarse adjustments using fluorescent screens in the beam path. This was essential for us given how dramatically misaligned the monochromator had become after so many attempts to bring it back into operation. We actually needed our SDD system to verify that we were tuning for Cu KA1 energy rather than the KB1 emissions because some of the most basic aspects of the alignment had pushed way beyond their intended position.

Along the way we built ourselves a motorized remote adjustment tool which we’ll return to the user as small adjustments are required on a regular basis with this kind of monochromator to retain maximum intensity. It’s quite useful and even versatile enough to allow for the adjustment of multiple control knobs.

One final note regarding intensity. It’s easy to get excited about energy resolution like this, but bear in mind that we’re looking at ~20x reduction in intensity due to the inherent losses involved in the primary diffraction event. This data was collected at 10x the normal speed and at half the normal 2Theta step increment so it looks very good, but one would need a compelling reason to slow their data collection this much.

Another side effect of performing your energy discrimination in the primary beampath is that other issues such as fluorescence effects (incident x-rays exciting elements in the sample causing high background intensities) are harder to avoid than they would be with a diffracted-beam monochromator. The 4x reduction in intensity inherent in the diffracted-beam monochromatization makes it a poor choice to eliminate these effects when the incident intensities are already so low. We recommend energy-dispersive detectors such as our SDD-150 to eliminate extraneous energies without sacrificing net intensity. We’ve also worked with the Bruker LynxEye XE-T detector which has a very high energy resolution compared to other position sensitive detectors (PSD). Contact KS Analytical Systems for more information on these options.

The majority of the samples we receive come in volumes high enough to completely fill the well in any of our standard sample holders. Some are too large or oddly shaped which calls for a special holding solution like those listed here, but many are simply very small quantities of powder. Placing these in a standard holder would leave them well outside the plane of diffraction and provide terrible data, not to mention substantial scatter
or diffracted background from whatever the powder is placed on. The answer is a zero background sample holder (ZBH). Most our users at KS Analytical Systems run the original Siemens/Bruker plates, but others are using Si(100) and even glass substrates. We’re very happy to say that
we’re able to offer a direct replacement for these with our new ZBH-32 holders. These fit most Siemens XRD systems and can be customized for use in most any other system. Contact us for more information on this. The scan below shows the data collected from a single mg of Silicon 640B standard powder spread across a ZBH.

Off Planar Quartz ZBH w-1mg 640B

Full scan of 1mg Silicon 640B standard spread across a ZBH

ZBH-32

ZBH-32 sample holders mounted for Siemens and Bruker single sample stages.

 

Some users report acceptable results using simple glass plates. While there are serious caveats here, it may be a reasonable solution for some users. The issue with amorphous glass is not diffracted peaks in the background, but rather, scatter off the surface. X-ray scattering off a surface is inversely proportional to the average atomic number of that material. That is to say, the lighter the matrix, the more efficiently it will scatter X-rays. This is why we use a pure Graphite sample to characterize the emission spectra of our XRF instrumentation. The glass sample shows the expected scatter “hump” starting at a very low angle and it doesn’t flatten until nearly 100°2Θ. While some of this can be modeled and subtracted with good profile fitting software like Jade 2010, it can be challenging to match the data quality of a good ZBH. We’re working on a series of videos to guide new users through some of these features, but on-site training classes are also available.

 

Glass plate

Amorphous glass empty

Glass-Qtz-Si510 overlay

Glass, ZBH-32 and off-planar quartz scans overlayed for comparison

 

 

 

 

 

 

 

 

Several of our customers in the geological industry use standard Si(100) wafers. These can be a great solution, but again have serious drawbacks for some applications. The Si(100) material creates diffracted peaks which are very sharp and therefore easier to model out sometimes, but also very high as the material is monocrystalline. The scan below shows what happens when one tries to run a normal scan across a bare plate. The largest peaks are actually only one or two which have over loaded the detector and caused it to drop out. All of these scans were collected with our SDD-150 which can handle up to 1×10^6 cps, but for the sake of good comparison, we left it tuned as it would be for a standard pattern. The monocrystalline nature of this material causes big problems, but it also allows for a creative solution. See the second scan for the results of the same measurement with the plate angled 1 degree off of theoretical. With this geometry, it’s unlikely this would affect the data quality dramatically, but the offending peaks are drastically diminished.

 

Si-100 wafer

Si-100 empty

Si-100 locked vs unlocked

Si-100 standard vs skewed scan

 

 

 

 

 

 

 

 

 

 

Off-planar Quartz holders have been the industry standard for decades. Historically, these have been made from solid, monocrystalline quartz material cut at a specific angle (6° off the C axis if I’m not mistaken). While these work well, they can be inconsistent. Even some of the OEM holders we’ve tested have shown some peaks which we can’t explain. Talking to some very experienced crystallographers, we find that they’ve had similar experiences.

 

 

Off Planar Quartz ZBH

Off-planar Quartz empty

ZBH-32 empty

ZBH-32 empty

 

 

 

 

 

 

 

 

We’ve been looking for a better answer for several years, but there are few off-the-shelf materials which work as well as off-planar quartz. The ideal answer was to cut solid Si(100) oriented billets such that the face presented to the diffractometer had no d-spacings which would diffract in the normal range of these machines. This is not unlike the off-planar Quartz method, but the starting material is much more consistent and durable. Si(510) offers very low background as well as the consistency of a manufactured product. The new ZBH-32 sample holders from KSA come in two versions, ZBH-25 and ZBH-32 with the latter being ideally suited for rotating stages and low angle work.

 

 

 

 

Some months ago we had a pair of scientists visit our space to look over a refurbished Siemens D5000. Interestingly enough, they’d planned to use the system for some basic XRD, but mainly for the development of polycapillary optics. This is a fascinating new technology that’s gaining steam out in the R&D centers around the world and merits a full post dedicated to it ASAP. This is the type of “game changing” innovation which hasn’t come along in XRD in decades. They took delivery of their machine a few weeks ago, but while on-site they asked about another project. All that’s really needed is simple phase analysis by XRD with one complication, the material reacts violently when exposed to air.

We roughed out a basic design the same day, but that was only the beginning. Dealing with materials like this necessitates careful consideration of all material and procedures involved in getting it from the lab where it’s synthesized to ours and back. The final design involved billet Aluminum, BUNA rubber o-rings and polyimide (Kapton) film and took quite a while to flesh out. The end result is a complete system which allows the customer to load the samples into individual cells inside their own glove box. The individual cells can be loaded into the case inside the glove box as well. The outer o-rings on the cells seal against chamfered edges on the pockets of the case creating a second sealed area above and below the sample. It’s unlikely that the inner cell would ever rupture, but this protects the polyimide film and adds a substantial extra level of safety.

DSC_0348

This is the complete case as it will ship. The flat-head bolts hold everything together and provide the necessary force to seal the chambers.

DSC_0336

With the case open, you can see the individual cells along with the top and bottom of the case. The labeling shouldn’t be necessary since there are strict protocols in place for transportation of this material, but it never hurts to be cautious.

DSC_0331

The original design called for a slightly more secure seal, but when we started working up the procedure to assemble them, it became clear that something simpler was needed, particularly considering that these will be loaded in a glove-box. An 11th hour rehash necessitated new o-rings, modification of the outer cell rings and complete redesign and fabrication of the cell center sections. I think this was well worth the effort though as they’re much easier to assemble now. There’s even a nice little tool to make it easier to open them back up for disposal or reloading. The original design would have been essential disposable.

DSC_0331 (2)

 

 

 

 

 

 

 

 

XRD is the preferred analytical method of testing free crystalline silica in workplace atmospheres.   At Texray, we offer respirable silica testing using the NIOSH 7500 method, so we thought those of you in industries such as concrete, construction, glass, milling, mining, hydraulic fracking and sandblasting will find it important to know that OSHA has proposed an update to the current rules.  The most notable change is a decrease in OSHA’s Permissible Exposure Limits (PEL) from 100 mg/m3 of air for general industries and 250 mg/m3 of air for construction industries to 50 mg/m3 of air for all industries.  OSHA, along with other independent occupational health institutions, feel the current PELs are outdated and inadequate, and rightfully so, considering limits were last set in 1971 and based on 1960s research.  However, most of these industries may not feel an impact from such as change, as many employers already use preventative measures to reduce dust exposure.

Would this rule change affect the analytical testing procedure?

Photo Courtesy of New Jersey Department of Health

Photo Courtesy of New Jersey Department of Health

No, XRD limits of detection (LOD) for NIOSH 7500 are 0.005 mg/m3 for an 800 L air sample.  This rule change would not affect the testing method, since the LODs for silica in the form of quartz and cristobalite are well below the newly proposed PEL.  However, the rule change may increase the number of industrial sites needing to test their work environments for respirable silica.

According to OSHA, “The proposed rule is expected to prevent thousands of deaths from silicosis, lung cancer, other respiratory diseases, and kidney disease. OSHA estimates that the proposed rule will save nearly 700 lives and prevent 1,600 new cases of silicosis per year once the full effects of the rule are realized.”  The rule change was proposed last September and the hearing was recently concluded at the beginning of April.  Final decision will be coming soon. We will keep you posted.

In my last post, I hinted towards the importance of literature searches before attempting to analyze a new material, and then someone in one of my discussion groups posted a link to the 30 Most Downloaded Advanced X-ray Analysis Articles from the ICDD. I thought some of you might find this helpful, so I am reposting the link.

30 Most Downloaded AXA Publications

The articles are free to download and written by well-known, respected professionals that have been in the business of X-ray analysis for many years. The topics range from sample preparation to applications to instrumentation hardware. Enjoy!

Over the past 15 years, Barnett Shale has become a major resource for natural gas in Texas.  Being located in the North Texas region, it is easy to see the boom of drilling rigs and wells popping up in the suburban and rural areas between Ft. Worth and Denton.  Collaborations between Geologists at universities and major oil companies have put a large amount of research into characterizing shale.  In 2001, Środoń et al. published a journal article in Clays and Clay Minerals that discussed the importance of sample preparation for sediments, such as shale, to be analyzed using X-ray diffraction.

Powder X-ray diffraction is the preferred and best technique to identify and quantify mineral compositions in geological materials such as rocks, sediments, and soils.  Sample preparation and loading are two important factors for accurate quantitative XRD analysis using Rietveld refinement.  Proper sample grinding and using a side-loader or backside loader are common practices to avoid preferred orientation.  At Texray, we have a variety of sample holders for different applications, and we can even custom build holders for those random parts.  However, in this study we wanted to see for ourselves the effect of sample grinding and particle size, and also we wanted to test out our new McCrone Micronizing Mill.  We already knew what the results would be from experience and previous work by Środoń et al., 2001 and Klug and Alexander, 1974, but this was a fun experiment to try with shale.

Shale rock from the North Texas region

Shale Rock from the North Texas region

The rocks (pictured above) were broken up into smaller pieces using a mortar and pestle, and then half was transferred to the McCrone mill for wet grinding and the other half we continued to grind manually using the mortar and pestle.  By the way if you are running out of bench space in the laboratory and are looking for a mill, I highly recommend the McCrone Micronizing Mill because it takes up very the little space and it’s capable of grinding below 10 μm in less than 10 minutes.  After grinding, we loaded the powder samples into a backside loader and analyzed them using a Bruker D5000 X-ray Diffractometer.

Shale XRD Pattern

XRD pattern of Mortar & Pestle Ground Shale (blue) vs McCrone Mill Ground Shale (red)

In the XRD pattern shown above the main differences you will notice between the two grinding methods are peak intensities and a small 2-theta peak shift.  Both of these differences are effects related to particle size distribution and sample loading.  Wet grinding the shale in a McCrone Mill creates smaller uniform particles (~5μm), therefore when loading the sample into holders the powders pack easier and tighter creating a denser layer of material for the X-rays to penetrate, hence higher peak intensities compared to manual grinding.  Sample preparation is one of the most important aspects to quantitative XRD because of preferred orientation and sample displacement.  In order to reduce user error such as, induced preferred orientation, it is essential we learn from previous research and take the proper steps to prepare samples.  The ICDD is a great source for free literature on applications involving XRD and XRF.  We will be posting more discussions on sample preparation and applications in the future.