Microprobing with electrons
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Transcript of Microprobing with electrons
Electron MicroProbe Analysis
Characterizing Mineral ChemistryJon Price, Rensselaer Polytechnic Institute
EMPA is a powerful tool for compositional analysis at the micrometer scale
Virtues
•Non destructive
•Highly accurate (up to 10 ppm)
•Fast
•Appropriate for chemical scale
•Somewhat inexpensive
W A R N I N G
This is complicated technology - avoid “black box” syndrome
Most minerals are sized between 0.1 - 100’s of mm.The rather ordinary rock slab on the left is composed of small (1-5mm) grains of quartz and feldspar.
The feldspar below is large (15 mm) but is concentrically zoned.
Feldspars are solid-solutions and exhibit a range of compositions.
How might we determine the composition of the minerals in our rocks?
What is unique about each element?
MM
TT
TT
Ephoton = EH - EL = h f = h c / λ
1. To obtain composition, we need a measurable characteristic for each element.
Electron structure is element specific. In other words, Ephoton is the result of a specific jump in a specific element.
Fluorescence: electromagnetic radiation results from moving electrons closer to the nucleus
Examples of transition levels in Barium
K 37.44 keV
LI 5.99 keV
LII 5.63 keV
LIII 5.25 keV
So LII to K (K α1) is…
31.81 keV
Heavier atoms have many energy levels
So LIIto K is 31.81 keV or 31,810 eV
The wavelength of the photon produced by this jump is
λ = h c / E
h = 6.626 × 10-34 m2 kg/s
c = 3 × 108 m/s
E = 31,810 eV × 1.602 × 10-19 J/ eV = 5.096 × 10-15 J
So λ = 3.900 × 10-11 m
2. To get analysis at micron scale, we need high energies (keV) focused on small area
Electrons are charged particles that can be focused and redirected using a magnets
Lower energy example: the CRT
Raymond Castaing formulated the technique for microanalysis and built the first working unit by 1951.
3. Fluoresced x-rays need to be collected and counted.
Recall that crystalline structure diffracts x-rays(XRD)
Bragg equation: λ = 2d sin θ
Crystal with unknown d spacing
X-ray source with known λ
Castaing’s machine: focused electron beam that produces x-rays in an unknown, that may be counted at known diffraction angles.
Wavelength dispersive spectrometry (WDS)
Bragg equation: λ = 2d sin θ
The intensity of x-rays is much smaller relative to those generated from a tube (as in XRD)
The EMP wavelength spectrometer uses crystals with curved lattices and ground curvature to reduce lost x-rays
The Rowland Circle
Crystal
Detector
InboundX-rays
Example of a modern EM probe
Locate the following:Cathode and
anodeBeamMagnetsSampleCrystalDetector
The Cameca SX100• Five spectrometers• Each with 2-4 crystals
The new RPI facility
Cameca SX 100 EMPRontec EDS detectionGatan mono CL
Electron forces jumpChar. photon producedGlancing background phn
Produced photon adsorbed - may produce Auger e-
Electron bounces off atom (high E): backscattered
Electron knocks out another e- (low E): secondary
More on electron-sample interactions
EMPA does not analyze surfaces (thin film), but penetrates a small volume of the sample.
The collectable products of electron collision origin originate from specific volumes under the surface.
Secondary electrons emitted from the first 50 nmImages surface topography
Backscattered electron intensity are a function of atomic densityImages relative composition
Other electron-sample interactions are useful
Ti
Characteristic x-ray emission
The x-ray volume changes as a function of a number variables.
A sample with higher average atomic density will have a shallower but wider volume than one with a lower density.
A beam with higher energy (keV) will produce a larger volume than one with a lower E0.
Nonunique nature of emission volume
From the excitation volume behavior, it is clear atomic density (Z) makes a difference in the emitted intensities.
Some of the x-rays are absorbed into atoms within and adjacent to the excitation volume.
Some of the x-rays promote electron jumps in atoms within and adjacent to the excitation volume.
Z
A
F
Raw data are corrected for ZAF influences. The total correction produces a rather long equation that may be satisfied only through iteration.
The microprobe advanced as a tool because of the microprocessor
Sample effects
The number of x-rays counted at the appropriate diffraction angle is proportional to the concentration of the fluorescing element. But the excitation volume is not unique.
Quantification requires comparison to a well-characterized standard.
Standard analyzed by other means
Your sample with unknown composition
Castaing’s micro WDS machine was a breakthrough. By 1960, advances in semiconduction permitted the construction of a new detector that could collect all of the emitted x-ray energies (pulses and background) within a few seconds.
Energy Dispersive Spectrometry (EDS)
•Measures charges in semiconductor [Si(Li)]
•Makes histogram of measured charges
•Extremely fast
•Very inexpensive
•Lower accuracy relative to WDS
EDS spectrum for a 15kV beam on a gemmy crystal from the Adirondacks (M. Lupulescu, NYSM).
Al Kα & β
Si Kα & β
K Kα
K Kβ
EM P A t r a v e r s e s o f s p in e l u s in g WD S
Formula for the spinel
Nom: Mg Al2O4
Act: Mg1-3x Al2+2x O4
EMPA is a powerful tool for compositional analysis at the micrometer scale
High voltage electron beam can be focused on one micrometer area
Composition is determined by characteristic x-rays from excited atoms
WDS
•Characteristic x-rays are focused through diffraction
•Permits better resolution
EDS
•All x-rays are counted simultaneously
•Permits faster analysis / identification
Limitations
•Good standards are essential
•Quantification is dependant on accurate correction for ZAF effects
•User needs to be aware of excitation volume
Results
•Accurate assessment of mineral stoichiometry
•WDS provides trace element compositions
•May assess inhomogeneity at small scales