EDXRF

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By Marky

EDXRF- also known as Energy Dispersive X-Ray Fluorescence.  It’s a method for detecting or quantitating elements based on the number and energy of x-rays given off.  You may also see it abbreviated at XRF.  And there is a closely related method used with Scanning Electron Microscopes called EDS.

Why are we discussing such an esoteric method here?  It is not as esoteric as you might think.  If you’ve had a commercial company check your house for lead paint, you probably used EDXRF. And the instruments don’t stop at just detecting lead.  There are a wide range of elements EDXRF can detect.

Yes, you can spend hundreds of thousands of dollars on an instrument (you can also spend a lot less).

But handheld versions of these instruments can be rented (the rental is relatively expensive, but affordable if shared between many).  An internet search gives a quick list of companies and the cost.

You’re used to visible spectrum.  The light is separated into individual components and then sent to a detector.  In the x-ray world, this techniques is wavelength dispersive x-ray fluorescence (WDXRF).  It works, but has disadvantages.  Here is the worst problem.  WDXRF requires high levels of x-ray power (kilowatts), leading to safety issues.

Compare this to EDXRF.  A sample is exposed to high energy radiation (say an x-ray beam).  The high energy radiation causes an electron close to the nucleus to be kicked out of the atom, leaving a charged  atom (hence the term “ionizing radiation“). Other electrons fall into the orbit close to the nucleus and emit characteristic x-rays (fluorescing).   These fluoresced x-rays are then detected.  While EDXRF requires x rays, the radiation level is much lower (under 50 watts).  And EDXRF testing is generally nondestructive.

With visible spectrometry, the spectrum produced are usually very broad.   With EDXRF, electrons are jumping between very well defined “shells” and limited x-rays are produced.  Spectral interpretation and element identification is simplified.

Other kinds of radiation measures the intensity of light produced.  So does EDXRF.  But the concept used is “Photon counting“.

Each x-ray photon hits the detector.  Think of it as a fancy solar cell.  When the photon hits, it creates a burst of electrons.  This burst of electrons is then sent to an A/D card and with the help of some math and a computer, the energy of the x-ray (in kev)  can be calculated.  The computer creates a graph of number of x-ray photons versus photon energy.  This is the display you see on the front.

There are disadvantages to this approach.  Let’s say two x-rays hit the detector at once (a 5 kev x-ray and a 3 kev x-ray).  The detector can’t tell the difference between this and one 8 kev x-ray (5 kev + 3 kev = 8 kev).

Most detectors are silicon based.  When an x-ray enters the detector, the silicon can fluoresce, resulting in a lower energy x-ray.  Silicon has a characteristic x-ray energy of 1.74 kev.  Again, if a 5 kev x-ray enters the detector, the instrument sees a 3.26 kev x-ray (5 kev – 1.74 kev= 3.26 kev).

Most instrument’s software eliminates these peaks.

The next question asked is “What can you do with this instrument?”

First, understand that EDXRF identifies elements, not molecules.  For example, EDXRF might tell you that zinc and sulfur were present in your sample.  But, is it zinc sulfide or a mixture of metallic zinc and sulfur powder?  EDXRF can’t tell.

Here’s another example.  Many companies test for hexavalent chrome (Cr+6) as part of a European requirement.  EDXRF can’t tell the valence of chrome ions present.  However, if the total chrome (the sum of all forms of chrome present) is under the limit, the hexavalent chrome will be, too.  EDXRF can detect total chrome.

Next, EDXRF isn’t equally sensitive to all elements.  Take a look at the periodic table below.

In the upper left hand corner of each element box, you will see a number.  Carbon, for example, is 6.  This  represents the atomic number of the element.   Many manufacturers claim elements 9 (fluorine) through  92 (uranium) are detectable.  Don’t believe it. Low atomic number elements produce weak x-rays.  So weak that air attenuates them.  The x-rays produced by elements like magnesium (number 12) and aluminum (number 13) are so weak  they can’t be detected unless present in large amounts.  On the other hand, high atomic number elements, such as lead  (number 82) and bismuth (number 83), are easily detected.  There are special techniques to detect low atomic number elements (pulling a vacuum, for example).  But, you can’t use them on a hand held  device.  Bottom line, these instruments are useful for detecting heavy elements.

A few tips before you rent the instruments.  Read as much about the instrument as possible before you rent it.  This should include the operating manual.  And, if you are going to transfer information to a computer, make sure you have the necessary hardware.

Another  note- EDXRF will work well on solids, but not on liquid samples.   Water absorbs x-rays.

Next, understand that you are using ionizing radiation, either in the form of x-rays or weak radioactivity (similar to a smoke detector).  Do not point the instrument at any human flesh.  Do not look into the instrument.  Keep the instrument away from children.

Always check the safety requirements in your area.  Some states require radiation exposure measurement while using these instruments.  This is usually in the form of a film badge worn as a ring on a finger.

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