(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!
Electron Probe Microanalysis
Electron Probe Microanalysis
A technique to quantitatively analyze samples for their chemical composition on atrument: 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
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)
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 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!)