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

XRD work is categorized into two major groups. Single crystal and powder analysis. While single crystal work is usually highly customized to particular applications and involves a largely unique hardware set, powder (PXRD) work covers a broad range of applications. Many of which can be performed without any special hardware at all. Perhaps it would be more accurate to call it “Randomly oriented small particle” diffraction. Somehow I think “ROSPXRD” would be slow to catch on. At the risk of oversimplifying the options, I’d like to take a few posts to showcase some of the more common analyses which can be performed with a basic PXRD system and perhaps a few that require minimal additional attachments.

This is an example of a Bruker D8 Advance configured in its most basic PXRD state with only a scintillation counter, sample stage and source.

This is an example of a Bruker D8 Advance configured in its most basic PXRD state with only a scintillation counter, sample stage and source.

This is the same D8 base instrument configured for single crystal XRD. Note the Chi, phi, XYZ stage, area detector (2D) and Goebel focusing mirrors.

This is the same D8 base instrument configured for single crystal XRD. Note the Chi, phi, XYZ stage, area detector (2D) and Goebel focusing mirrors.

 

 

 

 

 

 

 

 

 

 

 

My last post involved a basic phase identification and this seemed like a great place to start. Most PXRD users are asked to identify some unknown bit of corrosion, rock or contaminant at some point. I once took a shot at something which later turned out to be sewage sludge ash. I have no idea what they hoped to find in that. Exotic, mundane or distasteful, the most basic XRD can collect the necessary data to perform this analysis. Phase ID is usually the first step most users take toward more advanced software. In addition to the simple pattern analysis features that usually come standard, you’ll need an engine designed to search one of the many commercial or open-source databases available. The ICDD, NIST and AMCSD are probably the most popular with several others on the fringe. There are even user-developed databases which are usually compiled in a particular lab to cover the range of phases they expect to see based on their product or application.

Limiting the search to categories of phases which are likely to be present greatly improves the relevance of the results list. There’s obviously no reason to search through a huge list of minerals when trying to identify a metallic oxide coating. Hit lists can also be refined based on data from other sources such as qualitative elemental analysis. We use our WDXRF systems and the built in elemental filter in Jade to trim the options substantially.

Any good search/Match engine will have support not only for multiple databases, but also offer the option to limit your search to certain subfiles which are group my material categories.

Any good search/Match engine will have support not only for multiple databases, but also offer the option to limit your search to certain subfiles which are group my material categories.

Semi-quantitative or simple qualitative elemental data can be used to eliminate a large percentage of erroneous hits so the analyst can focus on only pertinent options. We prefer to bundle an XRF scan with any Phase ID project.

Semi-quantitative or simple qualitative elemental data can be used to eliminate a large percentage of erroneous hits so the analyst can focus on only pertinent options. We prefer to bundle an XRF scan with any Phase ID project.

 

 

 

 

 

 

 

 

 

 

Isolating the valid hits from erroneous is where experience comes into play. Non-ideal particle size, preferred orientation and crystallographic imperfections can make the process quite difficult. Relative peak intensity ratios, peak width and sometimes even the complete absence of a particular peak which would theoretically be present all present opportunities to gain additional insight. Sometimes this is relatively easy as in the case I presented in the previous post, but other situations are not so simple. These difficulties are amplified in the case of low concentrations and complex mixtures.

This is a great example of Phase ID the way we all wish it came out. The peaks are sharp, intense and located right on their theoretical angle.

This is a great example of Phase ID the way we all wish it came out. The peaks are sharp, intense and located right on their theoretical angle.

This is an example of something a little harder to nail down. Overlapping peaks, several additional phases and a highly imperfect sample. Refining the options based on external measurements and in depth sample prep make the difference between success and failure in cases like this.

This is an example of something a little harder to nail down. Overlapping peaks, several additional phases and a highly imperfect sample. Refining the options based on external measurements and in depth sample prep make the difference between success and failure in cases like this.

 

 

 

 

 

 

 

 

 

 

XRD pattern analysis has come along way in the last 40 years and most of the major improvements have come on the heels of increased computing capability which enables us to perform exhaustive iterative calculations on complex patterns quickly and at comparatively low cost. However, there is nothing on the market as of now which has made an experienced analyst obsolete.

 

I stumbled into Dondero’s Rock Shop a few weeks ago and struck up a conversation with the owner. He had been interested in geology all his life and was now operating a very nice shop in North Conway, NH with just about every type of mineral one could imagine on display. It was a great opportunity to have an expert identify a few specimens my boys had collected the previous day and he was more than happy to help. These were very large single crystals of relatively common minerals, but it was obvious that experience makes all the difference when one is trying to identify them by sight. I offered to return the favor by collecting XRD data on anything that ever managed to stump his well trained eye and he immediately brought out an interesting sedimentary formation which he’d sliced into cross to sections. He had been very curious about its composition and I brought home a sample. My technical expertise is primarily in the hardware we use at Texray while the real science is handled by other, more highly skilled hands, but this seemed like a fun little project and good practice if nothing else.

Geological samples are particularly difficult to analyze by XRD as they contain various defects which are difficult if not impossible to model based on theoretical data. Our precious Rietveld refinements roll off of this type of data like water off a ducks back all too often and we’re left wondering how on earth this mud could be mistaken for moon rocks. As wonderful as Rietveld is in well-trained hands, we tend to rely much more on comparative data when we’re working with this type of sample. We can thank Dennis Eberl of USGS in Boulder, CO for bringing RockJock into the world to solve exactly these types of problems. RockJock is relies on what’s called RIR. That is Relative Intensity Ratio analysis to provide both qualitative and quantitative results. The  algorithm has been massaged into a number of commercial products in an effort to improve the user interface and add additional functionality, but the core of all that is still readily available on the internet for anyone interested to download. If you’re interested in something a little more user friendly, we offer ClaySim from MDI.

To the left you can see the data I collected after mild grinding. It’s not uncommon to spend several hours collecting data before it’s adequate for quantification or other advance analysis, but as we’re only interested in qualitative phase ID, this will more than suffice. I was quite surprised to find only two major phases present since the sample clearly shows four distinct layers with completely different coloration. The scan actually ran all the way to 120°2Θ, but the “action” is mostly concentrated at the lower angles. Hardcore geologist actually push the lower limit all the way down to 2.5°2Θ in an effort to catch a few illusive peaks. The analysis program you see here is MDI Jade 2010. It’s their flagship product and for good reason. Almost all of our users are running some form of Jade for their analysis and all have had nothing but glowing praise for it.

So it appears that the mystery rock was actually little more than Quartz and Dickite. It’s possible that there’s a bit of Kaolinite mixed in there as well, particularly because Dickite and Kaolinite share a chemical composition. The real fun started when I let Jade loose using a feature called “One Click Analysis”. This is as close to a “black box” as XRD analysis will ever get. With good data collected on a solid, well-aligned XRD, this little button can provide impressive results with no user input at all. It’s not the magic bullet for every situation, but in this case, it recommended yet another phase with the same chemical composition as Kaolinite and Dickite. Nacrite. Adding this into our phase list improved the difference pattern and allowed Jade to model nearly every bump in the pattern.

As  the owner of KS Analytical Systems, I’ve seen XRD and XRF instrumentation used throughout industry and academia. Over the years we’ve expanded from a simple, on-site service company to a much more comprehensive organization offering complete systems, software and hardware upgrades and even sample preparation equipment. As our demo laboratory has grown to include more and more systems of increasing complexity, we started looking for an opportunity to put our in-house systems to good use. Texray Laboratory Services was founded to serve existing KSA customers who needed specialty work, method generation and training services as well as the greater industrial and scientific community with routine qualitative and quantitative analysis. We’ll be using this blog to showcase special applications and interesting projects so come back often.