Electron Probe Microanalysis

1. Electron Probe Microanalysis

2. Electron Probe Microanalysis A technique to quantitatively analyze samples for their chemical composition on a micro-scale (~1μm) Instrument: Known as probe, microprobe or electron microprobe – all the same Main components Electron Gun Beam Column Sample Holder/Air Lock X-ray Spectrometers – Wavelength or Energy Dispersive Vacuum System Illumination System Brief Description of Main Components Electron Gun – Source of electrons generated by thermionic emission of Tungsten filament Grid cap (Wehnelt cylinder) negatively biased to constrict electron beam (From Reference 1, page 30)

3. Beam Column • Serves to de-magnify image of filament to achieve ~ 1m beam size • Contains a condenser lens and an objective lens (From reference 2, page 45)

4. Sample holder/air lock • Provides a means of holding a sample for examination – typically a polished thin section or mineral grain mount • Air lock provides a means to exchange samples – be careful here! • X-ray spectrometers • Two kinds of spectrometer found on probes – wavelength (WDS) and energy dispersive (EDS) – WDS 10X sensitivity of EDS • Measure the same thing – characteristic spectra of an element present in the sample • E = hc/λ = 12.396/ λ, E = energy in Kev, h = Planck’s constant, c = velocity of light, λ = wavelength in angstroms • This relationship shows energy and wavelength are inversely related • Method utilizes the characteristic radiation from an excited atom to quantify the amount of an element in a sample – Peak height proportional to amount of element present, after background correction • Example: a K shell electron is ejected and an L electron drops to fill vacancy – leads to a Kα line (this radiation is specific to the atom excited) (From reference 3, page 7)

5. Origin of X-ray Lines for K and L Transitions

6. Quantitative analysis • Purpose: to quantify the amount of an element in a sample – must compare signal from sample to that of a known standard • To first order, counts from sample and counts from standard are directly related to concentration • k ratio = Isample/Istd = Csample • In practice, k ratio must be corrected for sample effects • k ratio = Isample/Istd = Csample Z A F • Corrections: Commonly referred to as “ZAF” corrections • ZAF means we have to make three types of corrections to our sample data • Z is the so called atomic number correction – is made up of stopping power and backscatter terms • A is the absorption correction – takes into account that some of the X-rays produced in sample volume don’t make it out of the sample • F is the fluorescence correction – corrects for X-ray induced excitation in the sample • There is an alternate correction process utilized by many of the modern probes – it is the “Phi–rho-z” method, it basically combines the Z and A effects into one method, you still need to do the fluorescence correction as well • Software packages included with all modern probes make the corrections for you (Buyer beware!)

7. Sample - Electron Beam Interaction: (From reference 1, page 69)

8. Vacuum System • Required to sustain tungsten filament and stop residual gas X-ray absorption • Illumination System • Two modes usually provided • 1. Optical illuminator – reflected light, sometimes transmitted light, for visual identification of sample features • 2. SEM mode – the probe can function as an SEM (scanning electron microscope) with both secondary and backscatter images available – extremely useful method for selecting spots to probe • Helpful Hints • Beam energy must be 1 to 2 times energy required to excite desired X-ray line • Beam current should be as low as possible for minimum spot size • Spot size can be varied if sample is reactive under the electron beam (be sure to use same spot size for standards) • Our Instrument: • CAMECA SX-100 • (note: only one other manufacturer of probes, JEOL)

11. References • Scanning Electron Microscopy and X-ray Microanalysis, 3rd ed., J. Goldstein and others, Springer Verlag, 2003 • Electron beam X-Ray Microanalysis, K. F. J. Heinrich, Van Nostrand Reinhold Co., 1981 • Elements of X-Ray Diffraction, B. D. Cullity, 2nd ed., Addison Wesley Pub. Co., 1978 Other useful references: Principles and Practice of X-ray Spectrometric Analysis, 2nd Ed., E. P. Bertin, Plenum Press, 1975 (a comprehensive treatment of X-ray spectrometry) Principles of Modern Physics, R. B. Leighton, McGraw-Hill, 1959 (extensive treatment of origin of X-ray spectra) Quantum Physics, R. Eisberg and R. Resnick, J. Wiley and Sons, 1974 ( Less rigorous treatment of modern physics than Leighton) Electron Probe Microanalysis, L. S. Birks, Wiley-Interscience, 1971 (classic reference with a worked example of the ZAF correction) Electron Microprobe Analysis and Scanning Electron Microscopy in Geology, S. J. B. Reed, Cambridge, 2005 (current brief reference to both probe and SEM for geologists)