Energy Dispersive Spectrometer (EDS)

diagram illustrating how x-rays form during electron-matter interactions

Conceptual model for explaining how x-rays form under an electron beam (4.9 Mb original image)

How electron beams cause x-rays to glow from sample

The diagram above is conceptual, of course.  Electrons don't orbit atomic nuclei like planets, nor are beam electrons exactly like cue balls in billiards.  The reality of what happens is incompressible, which is true of most of the world, but this simplified model is useful and predicts observations well.

When beam electrons strike an inner-shell electron in an atom in the sample, 

the inner-shell electron can be ejected, creating an empty "vacancy" in that inner shell 

the beam electron continues after being deflected becoming an "inelasically-scattered" electron that goes off to do more damage elsewhere

An electron from a higher-energy outer shell drops down into the lower-energy vacancy in the inner shell.  

Because energy is conserved, the change in energy levels (ΔE) releases an x-ray photon of exactly that amount of energy.

Like the rainbow of colors of visible light, x-rays also come in a spectrum of "colors" that are invisible to us, but differ just as red from blue for our eyes.  The EDS (Energy Dispersive Spectrometer) detects two things:

1. which energy wavelengths ("colors") of x-ray are glowing from the sample, and

2. how much of each wavelength of x-ray glows from the sample.

The "which energy wavelengths" data tells us which chemical elements are present in the sample because each element on the periodic table glows a characteristic color of x-ray when beamed with energy (either in the form of an electron beam for EDS or an x-ray beam for XRF).

The "how much" data tells us how much of each chemical element is present at each point in the sample because if the sample has lots of an element, then proportionately lots of x-rays of that element's characteristic color will glow.

The EDS measures the chemical composition at each pixel in the image.  This is only a 2k image, so 2048 × 1557 pixels = 3,188,736 individual chemical analyses.  The SEM/EDS can do those 3 million analyses in less than 10 minutes.  The EDS reports those data as maps of individual elements (see below) or the microscopist can overlay selected maps to highlight different crystals in the sample.

(4.9 Mb original image)

Composition maps - Colorado andesite

The EDS x-ray analyzer can map out the chemical concentration of each element on the sample individually to highlight the distribution of different elements among different crystals.

All of the images in this collage are of the same spot on a sample of volcanic rock.  Each single-color image shows the distribution of one element.  The multi-color image is what we see when we stack all of the single-color images on top of one another into one picture.  Please click here to see more detail.

(5 Mb original image)

chemical composition maps of volcanic rock

Individual and composite layered chemical composition maps of a sample of andesite from Colorado (Please click here for more detail)

Composition maps - phenocryst in basalt

Although backscatter images are a great first view of different crystals in a sample, composition maps help us distinguish between crystals with the same backscatter brightness.

This image compares the chemical compositions of the different kinds of crystals in a sample of basalt.

Please click here to see a more complete explanation with bigger images.

(2.3 Mb original image)

Individual and composite layered chemical composition maps of a sample of phenocryst in basalt  (Please click here for more detail)

Point analyses - granite

In addition to mapping out the chemical variations across a sample surface, the EDS x-ray analyzer can very precisely determine the exact chemical composition of any single point.  The spikey graphs report the types of x-rays and amounts of each type of x-ray that glow from the sample at each spot analyzed.  From this, we can calculate the concentrations of each element and identify the type of mineral.

Please click here to see more detail.

(2.7 Mb original image)

Individual point analyses of spots on a sample of granite and the corresponding x-ray spectra that tells us the exact concentrations of elements at each individual point (Please click here for more detail)