AN ANALYTICAL XRAY SERVICES LABORATORY
Feel free to call us: 940-784-3002

 

XRD sample prep is like a box of chocolates. You never know what you’re going to get… So many materials are fluffy or sticky that even after fine grinding, it’s common to have some clumps that just don’t want to break up. This became a problem for one of our clients using our side-loading tool so they added a piece of mesh to the mouth of the funnel. Their next order included a request for some type of removable solution for this so we mounted some coarse mesh in an acrylic frame that sits nicely on top of the funnel and makes it very easy to sift through sample material as it’s being loaded. We love these so they’ll be an option on all future orders!

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

It’s relatively common for us to receive very small volumes of material for analysis. Often this is the total amount available so getting the right answers is extremely important. When these come in as powders, the answer is always to run them on a zero background plate, but sometimes that’s not the case. Luckily, there are other options for analysis of very small quantities.

The most common application for filter-membrane sample holders has always been respirable silica quantification. This is mandated by OSHA and is an extremely common industrial hygiene test. Ambient air is sampled with a fixed or mobile suction system and particles are deposited onto a PVC membrane inside a sealed cartridge. Testing procedures are defined by NIOSH7500 and since this is a total quantification method (not a relative method), it’s critical that the entire sample is measured. Unfortunately, the measurement cannot be completed on the PVC membrane as received. Transferring the sample powder to an Ag membrane is accomplished by dissolving or ashing the PVC away, diluting the remainder in a solvent and depositing it onto the Ag membrane by vacuum filtration. The end result is an extremely low loss of analyte even for very small volumes of material.

This preparation method is also very useful for other types of samples which might have crystalline particulate suspended in a solution. Drying samples can be time-consuming, heating them to boil off liquid can cause phase transitions in the crystalline analyte, and handling dry powder in very small quantities is a very good way to lose material. Vacuum filtration solves all these problems.

 

Our most popular custom sample holder is the SC40F25 which is designed to hold the common 25mm Ag membrane filters used for this type of mounting. The anodized Al body is a time-tested design that works very well and causes almost no interference with the data, unlike the original injection-molded plastic parts. However, the most common method for retaining the membrane has always been to drop a metal support disk behind it and use an ID snap ring to retain both the disk and membrane. This can be a frustrating operation even for experienced hands. Snap rings are hard to control and the high spring tension gouges the inner diameter of the Aluminum body to the point that the holders must be replaced periodically.

After watching so many clients struggling with this system, we thought we could find a better option. The first step was a simple, laser cut acrylic backer instead of the metal disk. The acrylic was thicker which limited the depth to which the snap ring needed to be set. This was an improvement but still required the snap ring.

The next step was 3D printed plugs which could be pushed into the well. These supported the membrane and held it in the plane of diffraction at the same time. A standard pair of pliers was all the was needed to grab the plug and gently rotated it to release the membrane. This seemed like the ideal solution, but we heard from one user who claimed that the plug was causing an interfering peak in his measurements. We’ve been around the block with 3D printed sample holders in general and it’s definitely true that the common thermoplastics used will crystallize when cooled rapidly. This causes lots of problems for routine analysis of powders, but this was the first we’d heard of a peak being visible through an Ag membrane. Perhaps this user had a particularly thin membrane, but regardless, we needed a new solution, both for their lab and our own.

 

Our current solution is a laser cut “spring” backer which again combines the function of retainer and support in one part. The spring is easy to install by hand and can even be removed by hand, but forceps or needle-nose pliers make this easier. These have been working very well so we’re hopeful that this is going to be a long-term solution that we can share with our clients.

 

 

 

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

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.

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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.

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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.

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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.

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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.