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