Thanks for checking in again! Welcome back.
After talking about XRF and OES analyzers last time, I thought I had better talk about some of the challenges that users might face with these technologies. I don't want folks to think there are no limits to their special, magical powers! Hopefully I can offer a few tips to help improve the results and experiences people have using them. I'll start with handheld XRF for this episode.
Before I get into any trouble here and have metal samples crashing through my windows, I want to say that I am not claiming to be an expert or have all of the correct theoretical answers. What I will be sharing here will be (largely) based on my experience, but will be flawed and imperfect, just like me. What I do bring to the conversation is some years of experience, both in manufacturing and sales, of handheld XRF analyzers.
Anyway, handheld XRF is a powerful technology and a great tool to have in your arsenal. These fully featured spectrometers can help you prevent mix-ups, as I highlighted in my previous issue (man was that guy grumpy after that demo!) they can also help you sort scrap or recycled materials to feed new processes. They can be used to screen out inappropriate materials as with lead in toys. They can even be used in mining applications (mining applications are fraught with challenges, not the least of which is having no real "dirt" standards, but I'll talk about that another time) to help determine ore quality. But, as with all things, they have limitations and weaknesses.
The most frequently cited limitation I hear of XRF is... Drum-roll please!
Carbon
Yep. Carbon.
XRF cannot now and likely will never be able to see carbon and there's no use whining about it! Quit asking when it will be available too! (I'm talking to you David!) It's just not in the cards for a handheld device, and the reason is physics.
XRF analysis is all about the x-ray energy that atoms can absorb, which pushes electrons out of their orbits, and the energy atoms emit when electrons fall into the holes left by those pushed-out electrons. Carbon is a very light element. It doesn't have many electrons that are in tune with x-rays and can be pushed out by that energy. Furthermore, when an electron does get pushed out and one falls in the hole it left behind, the little photon it emits is a weak one and can't travel far. It gets swallowed up by pretty much anything it runs into, even air.
It isn't until the sodium-magnesium range of materials do you start getting x-rays that can actually get into a detector IF they can make it through all the air, window materials and other stuff waiting to suck them up! And then it's only about 6 or 7% of them that get to the detector to form a signal. Almost sad...
So you see, carbon analysis ain't gonna happen with handheld XRF anytime soon! Fortunately there are methods for measuring carbon in metals, OES being one of them, though admittedly, there are more precise methods than OES but they tend to be a little more pricey to operate, with more chemistry, additives and consumables involved. With proper sample prep and data averaging, OES can do a respectable job...
But I digress, dang it, we're talking about XRF here so let's get back to it.
This same issue arises when you are analyzing aluminum with XRF. Aluminum and many of the alloying elements used in it are light elements and provide wimpy x-rays. The vast majority of XRF analyzers out there aren't even able to measure the x-rays that aluminum, magnesium and silicon emit. To get around this, many analyzers will instead look for the copper and iron and other heavier elements that are alloyed with aluminum to make identification guesses.
About 5 years ago, SDD hit the market and aluminum analysis became much better. Silicon Drift Detectors offer greatly improved count-rates (how many x-ray photons per second can be measured) over standard SiPIN detectors, sometimes by a factor of 10 or more. Sensitivity improvements inherent in SDD structure also mean better signals from these weak photons.
But since such a low percentage of the photons make it into the detector, aluminum analysis can be improved by using longer analysis times, and I recommend 20-40 seconds at least, sometimes more for 3000 and 5000 series alloys
Well, I better wrap it up; it's a blog not a novel! Until next time!
Walter
Verichek Instruments
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