Ge 116 Module 2: Electron Probe. Part 1: instrument basics, WDS X-ray analysis and standardization. Electron Probe: Parts. (5 of these). Electron Microprobe Instrumentation. What Makes a Microprobe? High Stability Electron Source Focussing WDS X-ray Optics High Precision Stage
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Part 1: instrument basics, WDS X-ray analysis and standardization
(5 of these)
This is a Cameca spectrometer, but JEOL design is pretty similar
Crystals (2 pairs)
Proportional Counting Tube (note tubing for gas)
Goldstein et al, p. 280
WDS provides roughly an order of magnitude higher spectral resolution (sharper peaks) compared with EDS. Plotted here are resolutions of the 3 commonly used crystals, with the x-axis being the characteristic energy of detectable elements.
Note that for elements that are detectable by two spectrometers (e.g., Y La by TAP and PET, V Ka by PET and LIF), one of the two crystals will have superior resolution. When there is an interfering peak and you want to try to minimize it, this knowledge comes in very handy.
Reed, 1995, Fig 13.11, in Williams, Goldstein and Newbury (Fiori volume)
This plot of Fe Ka X-ray intensity data demonstrates why we must correct for matrix effects. Here 3 Fe alloys show distinct variations.
The Fe-Ni alloys plot above the 1:1 line (have apparently higher Fe), because Ni atoms present produce 7.278 keV X-rays, above Fe K edge of 7.111 keV.Thus, additional Fe K are produced by this secondary fluorescence.
In addition to absorption (A) and fluorescence (F), there are two other matrix corrections based upon the atomic number (Z) of the material: one dealing with electron backscattering, the other with electron penetration (or stopping). These deal with corrections to the generation of X-rays. C is composition as wt% element (or elemental fraction).
The matrix corrections assume that all elements present (and interacting with the X-rays) will be included. There are situations, however, where either an element cannot be measured, or not easily, and thus the analyst must make explicit in the quantitative setup the presence of unanalyzed element/s -- and how they are to be input into the correction.
Typically oxygen (in silicates) is calculated “by stoichometry” (which requires valence of cations). Elements can also be defined in set amounts, or relative proportions, or “by difference” – although this later method is somewhat dangerous as it assumes that there are no other elements present.
EPMA’s claim to fame as a microanalytical tool rests upon (1) faith in a correct matrix correction and (2) use of “good”, “correct”, “true” standards.
How do you know whether to trust a standard?
* This is based upon experience, be it from prior probe usage, from a more experienced user, from a book or article, or trial and error (experience comes from making mistakes!) It is commonly a multiple iteration, hopefully not more than 2-3 efforts.
On occasion, probe labs will cooperate in “round robin” exchanges of probe standards, where one physical block of materials will be examined by several labs independently, using their own standards (usually there will be some common set of operating conditions specified). The goal is to see if there is agreement as to the compositions of the materials.
T = max temp rise
E = accelerating potential in keV
I = beam current in A
k = thermal conductivity in W/mK
d = beam diameter in m
3. Detection Limit