文档介绍：Version: November 16, 2009
EPMA of Particles
Electron probe microanalysis EPMA
What’s the point?
EPMA is traditionally done for bulk material. What are the issues for particles?
How precise/accurate are such analyses?
Bulk vs particle
Normal EPMA assumes that the electron beam is exciting a homogeneous volume, . there is no difference either laterally or vertically. Thus, the matrix correction is being applied in a uniform matter, and there is one applicable f(rz) profile for each element.
However, particles create
many difficulties, including:
Their size (small)
Their venue (location, surroundings)
Their surface (not flat nor smooth)
The Good, the Bad, the Ugly
Most operators of electron microprobes, using WDS, understand the difficulties of trying to do EPMA on non-flat surfaces (particles).
However, the real problem is that there are probably 100 EDS systems on SEMs for every 1 WDS on an electron microprobe. And with SEMs and EDS systems so easy to operate, a large number of users make major errors without knowing it, assuming whatever the software spits out as position is really what it is.
After these slides, you should know the difference between the GOOD, the BAD, and the UGLY… particle analyses.
Size: “Mass Effect”
Goldstein et al, 1992, p. 479, 481
Consider a 2 um diameter sphere (say of NIST glass K412).
Mass effect/error: electrons escape from sides of small particles if the E0 is great enough so that electrons scatter out of the body before using up all their energy.
Size: Absorption Effect
Absorption effect of non-flat upper surface: there is a different path length from the normal flat geometry. In this case, the emergent x-rays will travel ~50% shorter distance in the material, and thus have ~50% of the absorption, and all else being equal, will yield a K-ratio twice as large as it would be if it were larger and had a flat polished surface.
Goldstein et al, 1992, p. 479, 481
Size - Detector-sample Geometry
EDS (shown to right