One Analysis Technique – So Many Options!

Roger Kerstin, North America Sales Manager, EDAX

X-ray Fluorescence (XRF) solutions – which type of XRF instrument should I choose?

Most of the XRF systems out there are very versatile and can be used in many different applications, but they are typically suited for a specific type of analysis. Since the discovery of XRF many decades ago there have been new developments and new instruments just about every year. The term Florescence is applied to phenomena in which the absorption of radiation of a specific energy results in the re-emission of radiation of a different energy. There are two different types of detectors for XRF systems: Wavelength Dispersive (WDS) and Energy Dispersive (EDS).

In energy dispersive analysis, the fluorescent X-rays emitted by the material sample are directed into a solid-state detector which produces a “continuous” distribution of pulses, the voltages of which are proportional to the incoming photon energies. This signal is processed by a multichannel analyzer (MCA) which produces an accumulating energy spectrum that can be processed to obtain analytical data.

In wavelength dispersive analysis, the fluorescent X-rays emitted by the material sample are directed into a diffraction grating monochromator. The diffraction grating used is usually a single crystal. By varying the angle of incidence and take-off on the crystal, a single X-ray wavelength can be selected. The wavelength, and therefore the energy, obtained is given by Bragg’s law:

nλ = 2d Sinθ

In the XRF world there are many different types of instruments to choose from: large systems to small systems; high powered systems to low powered systems, floor standing systems to benchtop to portable systems.

What do I choose, where do I start?

The answer to these questions is that it really depends on the samples you are trying to measure and the performance you are trying to achieve. I really classify these instruments in 3 different categories: bulk, portable, and small spot.

Bulk XRF: This typically means that you have samples that are either powders, liquids or even solids that you need to analyze quickly. Bulk instruments have a large x-ray spot size to excite a lot of the elements fast and get a quick answer. They can be EDS or WDS instruments, benchtop or floor standing, and low or high power. The kind of analyzer will determine what you can or cannot measure. The higher the power, the lighter the elements and the lower the concentrations. The benchtops typically are lower power (50kv and lower) and are usually decent for go/no go type analysis and even everyday type of analysis when super low LOD’s are not needed, or light elements (below Na) are not of a concern. If you need lighter elements or lower LOD’s then typically you would go with a high power WDS system and these typically can go up to 4kw of power and have a vacuum chamber or He environment .

Portable XRF: This is just what is says – portable. These analyzers are typically used for sorting metals, in the geological field, or anything that you can’t just bring to the lab. The performance of these have come a long way and they are a critical tool for many industries. They tend to have a larger spot size but since they are portable they must be light to carry around all day. They are typically lower power and lower current, which does not allow them to have the same type of performance as the lab type instruments but usually they are good for sorting and identifying samples. They are also very good for ancient artifacts or paintings that can’t be brought to a lab.

μXRF (Micro spot XRF): These are the instruments that have a small spot size compared to all other XRF systems and they are used in smaller sample identification or mapping of a sample. There are several different types of μXRF analyzers. Some use collimators to focus the beam (this typically loses intensity) for applications like coating thickness testing or alloy id. These are usually designed to be inexpensive and benchtop for quality control applications. They are versatile but also limited to the elements they can measure. Most of these only analyze down to Potassium as they usually do the analysis in an air environment. Then there are μXRF systems that use optics to focus the x-ray to smaller spot sizes. These are used for more in-depth analysis, and are equipped with a vacuum chamber, mapping and low LODs.

Before buying an XRF system many factors must be taken into consideration and you need to ask yourself some of the following questions to really determine the best fit for your applications.

• How big is my sample?
• Can I destroy my sample?
• What levels of detection do I need to measure?
• How many samples per day will I measure?
• Can I pull a vacuum with my sample?
• What elements do I need to measure?
• What type of flexibility do I need for multiple sample types?
• What size features or samples do I need to measure?
• How much money do I have?

As you can see there are many questions to answer and many options for XRF instruments. The more you know about what you want to measure, the better you can narrow down your search for the proper instrument.

XRF is a very powerful technique but you do need to get the proper tool for the job.
Happy hunting and good luck!

Orbis XRF Analysis of Ceramic Monoliths

Dr. Bruce Scruggs, Product Manager XRF, EDAX

Over the last several months, I’ve had a couple of opportunities to analyze a ceramic monolith. For me, this was interesting because I’ve never analyzed one of these and they have been around for a long time. Ceramic monoliths have been used for decades to support metal catalysts, providing a large surface area for reactants to interact with the catalyst. One of the most common uses is found in the automotive catalytic converter. The car’s engine exhaust passes through the catalytic converter changing environmentally polluting gases (e.g. NOx, CO and residual hydrocarbons) into more innocuous ones. (Well, they used to be more innocuous anyway until some clever person decided that CO2 emissions were problematic as well. But, I digress.) Some quick literature reading suggests there is a renewed interest in these for other areas of application besides automotive emission control.

Ceramic monolith with hexagonal channels.

Ceramic monoliths can be made from a variety of ceramics or minerals depending on the application. While it’s true in some cases that the ceramic material is inactive, there are reactions where the ceramic substrate influences the catalytic reaction. Hence, material selection is important. Application of the catalytic metals onto the monolith is another critical step which influences the overall performance of the catalyst. In one typical application process, the untreated monolith is dipped into a liquid slurry of catalytic precursors and then calcined to activate the catalyst.

Ceramic monolith with square channels assembled in an external housing.

The initial goal for Orbis micro-XRF analysis was to analyze the metal distribution within the channels of the monolith. The monoliths were cross-sectioned to expose the interior of a plane of channels and the starting question was to look at the distribution of applied metals along the length of the channels. This is easy enough to do and we can clearly see distributions as we measure from the channel entrance to the center of the channel. It’s what you would expect when dipping a narrow tube in a slurry. But, we could also see distributions across the width of the channel as well. It’s not something I immediately thought about, but it makes sense as the slurry pools in the corner of the channels where two channel walls meet. As we discussed the results we had so far, the question of quantification came up. (Questions about quantification always come up!) As we discussed quantification methodologies, I was measuring at different points within a single channel and noticed that light element signals from the substrate (e.g. MgK or AlK) were sometimes present in the spectrum and sometimes not. This was a surprising result as the belief was that the catalytic wash coat was thick enough to completely absorb these signals. So, we also learned that mass coverage of the catalyst treatment was not as heavy as expected and this also provided some valuable insight into how to go about quantifying the catalytic distributions within the monolith.

If the Orbis micro-XRF analysis can provide data on how well the catalyst is distributed throughout the monolith channel, then this could potentially enable improvements in application techniques, which in turn may lead to dramatic improvements in catalyst efficiency. Overall, I thought that wasn’t bad for a couple of hours of instrument time!