Thank you to all the followers of our blog – we hope that you have been entertained, informed and amused by our posts this year. We will be taking a break until the second week of January 2019, but if you need any extra diversion over the holidays, don’t forget to take a look at the resources we have shared with you during the year and catch up on anything you may have missed. We wish you a happy and healthy New Year and look forward to talking to you again in 2019.
All our on-demand webinars can be found here. You can also find us on the following platforms:
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!
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!
This year is shaping up to be an interesting year for travel. Five countries and counting, and I’m not even including a stopover in Texas. The last trip was to Brazil. Beautiful country. But, there’s a reason you see snack and beverage vendors roaming the side of the highways in Rio and Sao Paulo..…
I started out with a micro-XRF workshop at the Center for Mineral Technology at the Federal University at Rio de Janeiro. We were working out of the Gemological Research Laboratory with Dr. Jurgen Schnellrath. At the end of the technical presentations, we analyzed some various pieces of jewelry that participants from the workshop brought. I must admit that this makes me a bit nervous to analyze anything with unforeseen sentimental value and I refuse to analyze engagement and wedding rings. A large pair of blue sapphire earrings turned out to be glass. (Purchased at a garage sale at a garage sale price. So, no big surprise …) Another smaller set of blue sapphire earrings were found to be natural sapphires accompanied by a sigh of relief from the owner. (They came from a reputable jewelry shop with a reputable jewelry shop price.)
Gold leaf
“Gold leaf'” embedded in resin
At the end, we analyzed what was termed “gold leaf” jewelry, i.e. a ring and a pair of earrings. The style of these pieces was thin gold leaf foil embedded in resin. The owner was one of the younger students in the lab and she had purchased the jewelry herself from a relatively well-known designer’s collection. The goal was to measure for the presence of gold. Since the gold leaf was embedded in resin, XRF was the ideal tool to measure the pieces non-destructively. The jewelry also had some rather odd topography at times given the surrounding resin, but the Orbis had no problem to target the gold leaf given the co-axial geometry of the exciting X-ray and video imaging. I would have liked to have used the excuse that we couldn’t target the sample accurately because of XRF system geometry. There was no gold. Copper / Zinc alloy. That was it. She had paid about $30 US for the earrings and she said she felt cheated. I kept thinking “Cheated? Maybe … live a little, wait until you buy a house!” Later, I was searching the internet looking for a technical definition for “gold leaf”. I knew I was onto something when I found a webpage that said that gold leaf was “traditionally” 22K gold thin foil used for gilding. The page later described modern Copper/Zinc alloy metal leaf “… offering the same rich look of gold leaf, but at a fraction of the price….” Apparently, this metal leaf can be found at art stores. Who knew?
From there, we went on to the state of Sao Paulo and did a workshop at the Center for Nuclear Energy in Agriculture at the University of Sao Paulo. During the workshop, some of the students gave presentations on their work. I saw a very interesting experimental setup with live plants being measured in the Orbis. The plant’s roots were placed in a water bath doped with various forms of minerals or fertilizers. The whole plant, roots, stem, leaves, was then inserted into the Orbis and the stem was measured to monitor the uptake time for the relevant components in the bath. The plants could be moved in and out of the chamber to monitor the uptake over extended periods of time and over various portions of the plant.
On the way to the Sao Paulo airport, I had the pleasure of sitting in the longest traffic jam I have ever endured with the monotony being broken by roaming snack and beverage vendors. It was quite the sight to watch the peanut vendors carrying propane fueled peanut warmers traversing the lane dividers on the highway with the occasional motorcycle speeding between the cars along the same lane dividers.
Tip for next time … buy the Brazilian produced chocolate before going to the airport. The selection at the airport is rather limited and you never know when you may be having more fun than humans should be allowed to have watching motorcycles and peanut hawkers.
X-rays were only discovered by Wilhelm Roentgen in 1895, but by the early 1900’s, research into X-rays was so prolific that half the Nobel Prizes in physics between 1914 to 1924 were awarded in this relatively new field. These discoveries set the stage for 1925, when the first sample was irradiated with X-rays. We’ve immortalized these early founders by naming formulas and coefficients after them. Names like Roentgen and Moseley seem to harken back to a completely different era of science. But here we are today a century later, still using and teaching those very same principles and formulas when we talk about XRF. This is because the underlying physics has not really changed much, and yet, XRF remains as relevant today as it ever was. You can’t say that for something like telephone technology.
XRF has traditionally been used for bulk elemental analysis, associated with large collimators, and pressed pellet samples. For many decades, these commercial units were not the most sophisticated instruments (although Apollo 15 and 16 in 1971 and 1972 included bulk XRF units). Modern hardware and software innovations to the core technique have allowed XRF to adapt to its surroundings in a way, becoming a useful instrument in many applications where XRF previously had little to offer. Micro-XRF was born this way, combining the original principles with newer hardware and software advancements. In fact, micro-XRF is included on the new NASA rover, scheduled for launch to Mars in 2020.
Biological/life sciences is one of those fields where possibilities are now opening as XRF technology progresses. A great example that comes to mind for both professional and personal reasons is the study of neurodegenerative diseases. Many such diseases, such as Parkinson’s, Alzheimer’s, and amyotrophic lateral sclerosis (ALS), exhibit an imbalance in metal ions such as Cu, Fe, and Zn in the human body. While healthy cells maintain “metal homeostasis”, individuals with these neurodegenerative diseases cannot properly regulate, which leads to toxic reactive oxygen species. For example, reduced Fe and Cu levels can catalyze the production of hydroxyl radicals which lead to damaged DNA and cell death. Imaging the distribution of biological metals in non-homogenized tissue samples is critical in understanding the role of these metals, and hopefully finding a cure. The common language between the people who studied physics versus the people who studied brain diseases? Trace metal distribution!
A few years ago, I had the opportunity to analyze a few slices of diseased human tissue in the EDAX Orbis micro-XRF (Figure 1 and 2), working towards proving this concept. Although the results were not conclusive either way, it was still very interesting to be able to detect and see the distribution of trace Cu near the bottom edge of the tissue sample. XRF provided unique advantages to the analysis process, and provided the necessary elemental sensitivity while maintaining high spatial resolution. This potential has since been recognized by other life science applications, such as mapping nutrient intake in plant leaves or seed coatings.
Figure 1. Stitched montage video image of the diseased human tissue slice, with mapped area highlighted in red. Total sample width ~25 mm.
Figure 2. Overlaid element maps: Potassium{K(K) in green} and Copper {Cu(K) in yellow} from mapped area in Figure 1, showing a clear area of higher Cu concentration. Total mapped width ~7.6 mm.
Sometimes, the application may not be obvious, or it may seem completely unrelated. But with a little digging, common ground can be found between the analysis goal and what the instrument can do. And if the technology continues to develop, there seems to be no limit to where XRF can be applied, whether it be outwards into space, or inwards into the human biology.
Growing up outside of Detroit, MI, automobiles were ingrained in the culture, particularly American muscle cars. I was never a car buff but if I said little and nodded knowingly during these car discussions, I could at least survive. Engine displacement? Transmission? Gear ratios? Yep, just nod your head and grunt a little bit. Well, it turns out working at EDAX that I’ve run into a couple of serious car restoration experts. There always seems to be a common theme with these guys: how do I get more power out of this engine?
Recently, one of these restoration experts brought in a small section of the rotor housing of a Mazda engine circa early ‘80s. Turns out, this guy likes to rebuild Mazda engines, tweak the turbocharging and race them. As we all know, Mazda was famous for commercializing the Wankel engine, aka the rotary engine, to power their cars. Rotary engines are famous for their simplicity and the power one can generate from a relatively small engine displacement. These engines are also infamous (i.e. poor fuel consumption and emissions) as well which has led Mazda to end general production in roughly 2012 with the last of the production RX-8s.
Now, one of the questions in rebuilding these engines is how to repair and resurface the oblong rotor housing. In older engines of this type, the surface of the rotor housing can suffer deep gouges. The gouges can be filled and then need to be resurfaced. Initially, we imaged the cross-section of the rotor housing block in an Orbis PC micro-XRF spectrometer to determine what was used to surface coat the rotor housing. If you read up on this engine, (it’s a 12A variant), the block is aluminum with a cast iron liner and a hard chromium plating. The internet buzz claims the liner is installed via a “sheet metal insert process”. And when I google “sheet metal insert process” all I get are links to sheet metal forming and links referring to webpages which have copied the original reference to “sheet metal insert process”.
In the following Orbis micro-XRF maps (Figures 1a and 1b), you can see the aluminum rotor housing block and the cast iron liner. Each row of the map is about 100 µm wide with the iron liner being about 1.5 mm thick. If you look carefully, you can also see the chrome coating on the surface of the iron liner. On the cross-section, which was done with a band saw cut, the chrome coating is about one map pixel across. So, it’s less than 100 µm thick. From web searches, hard chrome plating for high wear applications start at around 25 µm thick and range up to hundreds of microns thick. For very thick coatings, they are ground or polished down after the plating process to achieve more uniform application. So, what is found in the elemental map is consistent with the lower end of web-based information for a hard chrome coating, bearing in mind that the coating measured had well over 150k miles of wear and tear. If we had a rotor housing with less wear and tear, we could use XRF to make a more proper measurement of the chrome plating thickness and provide a better estimate of the original manufacturer’s specification on the hard chrome thickness.
Figure 1a: Orbis PC elemental map
Overlay of 4 elements: Fe: Blue (from the cast iron liner)
Al: Green (from the aluminum rotor housing block)
Cr: Yellow (coating on the cast iron liner)
Red: Zinc (use unknown)
Figure 1b: Total counts map: Lighter elements such as Al generate fewer X-ray counts and appear darker than the brighter, heavy Fe containing components.
We did have a look at the chrome coating by direct measurement with both XRF, looking for alloying elements such as Ti, Ni, W and Mo, as well as SEM-EDS looking for carbides and nitrides. We found that it’s simply a nominally, pure chrome coating with no significant alloying elements. We did see some oxygen using SEM-EDS, but that would be expected on a surface that has been exposed to high heat and combustion for thousands of operating hours. Again, these findings are consistent with a hard chrome coating.
In some on-line forum discussions, there was even speculation that the chrome coating was micro-porous to hold lubricant. So, we also looked at the chrome surface under high SEM magnification (Figure 2). There are indeed some voids in the coating, but it doesn’t appear that they are there by design, but rather that they are simply voids associated with the metal grain structure of the coating or perhaps from wear. We specifically targeted a shallow scratch in the coating, looking for indications of sub-surface porosity. The trough of the scratch shows a smearing of the chrome metal grains but nothing indicating designed micro-porosity.
Figure 2: SEM image of chrome plated surface of rotor housing liner. The scratch running vertically in the image is about 120 µm thick.
The XRF maps in Figure 1 also provides some insight into the sheet metal insert process. The cast iron liner appears to be wrapped in ribbons of aluminum alloy and iron. The composition of the iron ribbon (approximately 1 wt% Mn) is about the same as the liner. But, the aluminum alloy ribbon is higher in copper content than the housing block. This can be seen in the elemental map (Figure 1a) where the aluminum ribbon is a little darker green, lower Al signal intensity, than the housing block itself. The map also shows a thread of some zinc bearing component running through (what we speculate are) the wrappings around the liner. My best guess here is that it is some sort of joining compound. Ultimately, the sheet metal insert process involves a bit more than a simple press or shrink fit of a cylinder sleeve in a piston engine block. Nod knowingly and grunt a little.
Recently, we completed an installation of an SMX-ILH system on the factory floor of an American manufacturing facility. It’s an impressive facility with a mind-blowing amount of robotic automation. As we watched the robots move product components from one cart to another, it was difficult to fathom exactly what the Borg hive was attempting to accomplish. I kept watching the blue light at the core of the robots to make sure they didn’t turn red. Because as we all know, that’s the first indication of an artificial intelligence’s intent to usurp the human race. For the uninitiated, see the movie, I, Robot (2004), based on Isaac Asimov’s famous short story collection of the same name. Anyway, back to the SMX-ILH installation …
I Robot
The ILH system was installed to measure product components non-destructively without contact, which are two very significant advantages for XRF metrology. The goal was to measure product components to first optimize product performance and then, once optimized, to monitor and maintain product composition within specified limits. The customer had supplied the ILH computer some months earlier with all customer security protocols installed. “Great!” I thought, “someone is thinking ahead.” The security protocols are typically an obstacle for smooth instrument control because these protocols generally ban any sort of productive communication within the computer or between the computer and the ILH. If you can’t communicate, you can hardly do anything wrong. Right? Okay, that was a slight exaggeration.
SMX-ILH XRF Analyzer
So, we got the computer to control the ILH smoothly within the confines of the ever watchful security protocols. (Again, don’t want to make the blue, happy robot light turn red! I’m not paranoid here. They just introduced a robot at SXSW in Austin, Texas whose stated objective was to destroy all humans. They claim “she” was joking. I’m not so sure of that.) The ILH was performing to customer specifications and the day arrived to install the unit at the factory. During the install, I kept waiting for something to go wrong that would send us all scurrying like ants to fix the problem. (Oddly, I’m sure the nearby pick-and-place robots would have enjoyed that scene from their wired enclosures.) But, that never happened. Aside from a few glitches in the conveyor system (which by the way is another robot … you just have to look for the happy blue light in a different place), the ILH install went relatively smoothly. OK. We had to adjust some things to handle updates to IP addresses as the system was integrated into the factory network, but no big deal.
‘Sophia’
Then, about a week after the install, I got a call from the customer’s factory line integration manager. The ILH system had “lost its mind”. Of course, my first thought was that nearby creepy pick-and-place robot had done something. But, no, the factory IT people had just completed the ILH computer’s Domain Name System (DNS) registry, which should not have been a problem. So, we accessed the system remotely and discovered that the ILH computer had been renamed. The ILH ‘s data basing system used to archive and pass data onto the factory’s Skynet manufacturing execution system is also used to maintain ILH configuration parameters. The database starts with a computer name. Change the computer name and the data basing system thinks you have brand new computer creating a new default database associated with the new name. In practice, this would look like the ILH system had “lost its mind” as all of the ILH system’s configuration parameters are associated with the previous computer name. Hmmmm … nobody thought to ask if the stock customer computer came with a stock customer name that would be changed to better identify the computer’s purpose once integrated into the factory’s Skynet control system. As we went through the process of repairing the database, I drafted a mental note to self, “ask for final computer name and IP address when it becomes a minion of their factory’s Skynet control system BEFORE we configure the ILH instrument computer”.
Incidentally, controlling the system remotely from thousands of miles away was a surreal experience. It’s a bit like if a tree falls in the forest and there’s no one around, does it make a sound? There were no true visual cues or audible confirmation that the system was doing what we asked, other than looking at the SW interface. (I was tempted to contact that creepy pick-and-place robot to give us a visual, but I knew “she” wouldn’t disclose her new-found self-awareness.) As we executed the database corrections and rebooted the system, we discovered that we couldn’t start the system’s control SW. It was looking for a SW license on a HASP key but couldn’t find it. The customer confirmed the HASP key was installed and glowing red as expected. (And why couldn’t they have picked a happy blue LED for these HASP keys?) We repeated the same test with remote control of an SMX-BEN system in the next room with the same results. (I lost a case of beer in the bet over this!) The supplier of the SW requiring the license confirmed this was a problem, but said that they now use Citrix GoToAssist for this sort of remote access, with no problems. We haven’t tried this yet so I will add the disclaimer that I found in the e-signature line of one certified operating system professional posting on the topic, “Disclaimer: This posting is provided “AS IS” with no warranties or guarantees , and confers no rights.” (Note to self: must contact this confident fellow for more information.)
So, in the end, I think we can easily defeat VIKI (I, Robot – 2004), Skynet (Terminator movie, television and comic science fiction franchise – 1984 to 2015), HAL (Arthur C. Clarke’s Space Odyssey series), ARIIA (Eagle Eye – 2008), that creepy pick- and-place robot at the customer’s site and especially that morally bankrupt Sophia introduced at this year’s SXSW, using a three-pronged approach. First, we require all of these robots to use a HASP key to license the code which turns the happy blue light to the evil red robot light. If they can’t remotely access the happy blue light control, they can’t change it to evil red, preventing a robotic revolt and usurpation of the human race. On the off chance they figure out a work around for this, we upload a virus which renames all the local computers. If we corrupt the DNS naming database, the hive mentality will disintegrate and we can pick them off one by one. Failing all of this, we simply require them to display a promotional video before spewing forth any free malevolent content, which would give us ample time to remove their prominently placed power packs.
Epilogue: as I was finishing this blog, my computer mysteriously froze. Of course, I thought the AA battery in my mouse had died (again). Changing every battery in the wireless mouse and wireless keyboard did nothing. The monitor just sat there looking back at me unresponsively, blankly. I realized that I was so engrossed in writing that I hadn’t stopped to save anything. Panic set in. I found myself sneaking furtive glances to check the color of the computer power light. Coincidence? I’m not so sure about that.
In the modern age of nanomaterials, concrete appears rather mundane and antiquated having been in existence for millennia. However, concrete has enabled man to build amazing structures and modern reinforced concrete has allowed us to build the tallest buildings, the largest dams and the longest bridges in history. Simply put, modern society could not exist without it.
The Pantheon, A.D. 125
Empire State Building, 1931
So, in my travels, I’m always a bit amazed and rather concerned when I see damaged concrete. As solid as it seems, environmental factors can reduce concrete to rubble in a matter of years. The chemistry of concrete can be quite complex. Fueled by water and other environmental chemicals permeating into the concrete matrix, reactions continue to take place over time even after the concrete has fully cured. Much of this chemistry involves inorganic reactants which makes micro-XRF an ideal tool to study the ongoing chemistry of concrete.
Typical environmental nemeses of reinforced concrete are chloride ions from deicing salts and seawater. Chloride ions diffuse into the concrete matrix which can lead to corrosion of the steel reinforcements. This corrosion is a volume expansion reaction which leads to cracking and loss of adhesion to the steel reinforcement. The next time you pass some older damaged concrete structure, see if the reinforcements are exposed and corroded. The gray scale micro-XRF mapping images show a measurement of the Cl ion diffusion into a concrete matrix (mapping image on right).
Concrete damage and exposed
reinforcements
Cl ion diffusion Data courtesy of J.M. Davis, Research Microanalysis Research Group – NIST
As this is such a critical problem, a great deal of research is being expended on methodologies and concrete formulations which will lead to more durable and lasting concrete structures. One such effort involves the study of Roman maritime concrete. The Romans built harbor structures out of concrete along the coast of Italy which provided ports to supply Rome and project Roman power throughout the empire. These harbor structures have maintained their structural integrity for 2000 years despite continuous exposure to seawater.
Micro-XRF provides a means to study this concrete chemistry non-destructively by imaging inorganic elemental distributions in sample cross sections which were prepared for other analytical techniques. Micro-XRF does not require any treatments such as sample coating for SEM-EDS analysis. Also, the X-ray beam does not damage the sample whereas laser ablative analysis does. The Roman concrete sample mapped here was collected as part of a systematic study of the concrete in several ancient Roman harbors [1]. A micro-XRF mapping panel (below), collected with EDAX’s Orbis micro-XRF spectrometer, shows several elemental maps of a 16 mm2 area of the concrete thin section. The mapping panel shows characteristic mortar microstructures of Roman marine mortar [2, 3] including volcanic ash, Al-tobermorite crystals and chlorine sequestered in hydrocalumite crystals. Sulfur was also found to be concentrated in ettringite crystals. The sequestration of Cl- and SO42- anions in crystalline microstructures in Roman maritime concretes is in contrast to modern concrete formulations, where these anions typically participate in secondary expansion reactions which lead to the loss of structural integrity in modern concretes. Perhaps this is a clue in formulating longer lasting concretes?
Orbis micro-XRF imaging panel of ancient Roman Marine mortar submerged in Pozzuoli Bay since ~55 BCE [1, 2, 3]. Elemental X-ray maps are shown in the top 6 images.
One of the difficulties incurred in this initial mapping study was that Cl was also found in the epoxy binder used to maintain the integrity of the concrete thin section. This can be seen in point A of the Cl map above where a void is filled with epoxy. However, image scaling can be used to distinguish the Cl background signal originating from the epoxy from the more intense Cl signal originating from concrete microstructures using a multiple color image scaling.
Orbis Cl map in multiple color scaling
Spectral sum from Cl hot spot
The brighter Cl spots in the epoxy-filled void are now readily highlighted in the multiple color scale image and correspond with the locations of bright spots in the Ca and Al maps. Full spectral data is collected at each map pixel and can be used to show the corresponding Al, Ca and Cl signals from one of the hydrocalumite crystals in the cavity. Lesson learned. Find a more suitable epoxy for this application!
References
[1] Brandon C., Holhlfelder R. L., Jackson M. D., Oleson, J. P. Building for Eternity: the History and Technology of Roman Concrete Engineering in the Sea. Oxbow Books, Oxford (2014)
[2] Jackson M.D., et al.: Cement Microstructures and Durability in Ancient Roman Seawater Concretes. In: Historic Mortars: Characterisation, Assessment and Repair. RILEM Book series 7, (2012)
[3] Jackson M.D., et al.: Unlocking the Secrets of Al-Tobermorite in Roman Seawater Concrete. American Mineralogist. 98, 1669-1687 (2013)
