Electron Probe Microanalysis 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) Beam Column Serves to de-magnify image of filament to achieve ~ 1m beam size Contains a condenser lens and an objective lens (From reference 2, page 45) 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) Origin of X-ray Lines for K and L Transitions 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!) Sample - Electron Beam Interaction: (From reference 1, page 69) 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) R100191 Roselite Oxides MgO CaO ZnO MnO As2O5 CoO NiO Total Date x y z 1.638543 24.06168 0.000012 0.286896 49.26317 12.80333 0.102457 87.26375 10/6/2010 13:16 -14599 646 835 1.59662 24.12704 0.000012 0.160339 49.08891 13.1324 0.006092 86.1962 10/6/2010 13:20 -14600 680 835 1.071248 24.23088 0.05624 0.158607 49.61437 13.21772 0.138841 88.03637 10/6/2010 13:23 -14606 755 835 1.072605 24.14428 0.06135 0.289297 50.12297 13.49821 0.141022 87.80859 10/6/2010 13:27 -14619 751 835 1.353601 24.1974 0.000012 0.197133 49.81471 13.44682 0.081916 87.91537 10/6/2010 13:35 -14787 751 835 1.211323 24.17074 0.000012 0.167884 49.4537 13.39434 0.000013 88.0152 10/6/2010 13:38 -14787 751 835 1.198884 23.93616 0.000012 0.147767 50.02366 13.34733 0.083508 86.75258 10/6/2010 13:41 -14770 751 835 1.293255 24.01254 0.043498 0.192441 49.24385 13.52351 0.03376 87.50651 10/6/2010 13:44 -14782 783 835 1.709265 24.21972 0.127935 0.189742 49.79366 13.01528 0.026582 88.88528 10/6/2010 13:47 -14782 816 835 1.707457 24.39864 0.128095 0.222202 49.00893 12.99749 0.128013 85.96765 10/6/2010 13:50 -14782 816 835 1.787547 23.60133 0.000012 0.908881 50.02984 10.78678 0.041535 88.81791 10/6/2010 13:54 -14598 667 835 2.446573 24.23414 0.000012 0.992909 50.55445 10.81757 0.062686 86.68926 10/6/2010 13:58 -14602 666 835 1.928083 24.24879 0.132762 0.942763 50.85159 11.42332 0.000013 87.06113 10/6/2010 14:01 -14529 614 835 2.040862 24.05773 0.000012 0.88676 50.49967 11.50833 0.035461 86.67274 10/6/2010 14:06 -14474 416 835 1.978735 24.20343 0.000012 1.008217 50.83835 11.15598 0.000013 87.03284 10/6/2010 14:09 -14478 28 835 Ave.: 1.602306733 24.12296667 0.036666 0.450122533 49.880122 12.537894 0.058794133 Std. Dev.: 0.387013667 0.176478249 0.051026 0.355207273 0.590885254 1.022139584 0.049448279 Wt % Oxide Oxide MW Mol # Atom Prop. Anion Prop. # Ions/formula MnO Oxide 0.45 70.94 0.006343 0.006343389 0.029335609 0.029 ZnO 0.04 81.38 0.000492 0.000491521 0.002273087 0.002 CoO 12.54 74.93 0.167352 0.167351732 0.773934109 0.774 NiO 0.06 74.93 0.000801 0.000800747 0.003703133 0.004 MgO 1.6 40.31 0.039691 0.039691006 0.183554855 0.184 CaO 24.12 56.08 0.4301 0.430099857 1.98903797 1.989 As2O5 49.88 229.84 0.217021 1.08510268 5.018161237 2.007 Total: 88.69 1.729880933 Enter Oxygens in formula: 8 Oxygen Factor Calculation: F= 4.624596 F is factor for anion proportion calculation Ideal Chemistry: Ca2(Co,Mg)(AsO4)2·2H2O Measured Chemistry: Ca1.989(Co0.774Mg0.184Mn0.029Ni0.004Zn0.002)Σ=0.993(As1.004O4)2·2H2O References 1. Scanning Electron Microscopy and X-ray Microanalysis, 3rd ed., J. Goldstein and others, Springer Verlag, 2003 2. Electron beam X-Ray Microanalysis, K. F. J. Heinrich, Van Nostrand Reinhold Co., 1981 3. 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)