Electron probe microanalysis -Electron microprobe analysis

EPMA (EMPA)

What’s EPMA all about?What can you learn?

EPMA - what is it?Precise and accurate quantitative chemical analyses of micron-size domains, mainly major elements

High energy electrons interact with the atoms in the sample, yielding X-rays (and other signals)

Quantify X-ray intensity and compare with counts from standards

Nominally non-destructive

In Library

Reed (2005), 2nd ed. 192 pages

Paper: New:$40

Hard:New: $95 Used $80

Reed (1993)

Paper: New:$55 Used:$35?

Hard:New ~$95

Goldstein et al, 3rd Edition. 2003 New:$75

EPMA - is it for me?This technique has its own characteristics, advantages, weaknesses.

Is it the best technique to get the information you need?

1) It is a micro-technique, and for multiphase samples provides discrete compositions, not the bulk composition.

2) It samples volumes (depths) on the order of ~1 µm, limiting its usefulness for small inclusions or films.

3) It provides major and minor element quantification, and has limited capacity for trace element analysis.

4) Despite being non-destructive, samples need to be mounted and polished; they can be reanalyzed many times.

5) Relatively inexpensive and accessible

GG621 – Electron Microprobe Analysis: how to use our probe…

SE for morphology, BSE for composition

e.g. Cu-Al grid

Cu has higher Z, i.e. bright

Cu grid is pressed into Al

BSE

SE

BSE contains compositional information

Fe-Mg silicate

Beam Penetration

•Beam penetration decreases with Z •Beam penetration increases with energy

Monte Carlo simulationof electron pathways

all Ti

Cathodoluminescence (CL)

In some types of sample (non-metallic, also often Fe-free), electron bombardment stimulates light emission

Excitation: e from valence band to conduction band.

Return by low-E photon emission (visible)

Typical minerals: diamond, qtz, ap, zrc (fsp)...

Exact cause ambiguous

Requires additional detector

Characteristic X-raysProduced by electronic transitions within inner electron shells. Can be explained by examining the Rutherford-Bohr model of the atom, in which electrons orbit in a number of shells around a nucleus.

From Reed, 1996

Ne = 2n2

X-ray Lines - K, L, MKα X-ray is produced due to removal of K shell electron, with L shell electron taking its place. Kβ occurs in the case where K shell electron is replaced by electron from the M shell.

Lα X-ray is produced due to removal of L shell electron, replaced by M shell electron.

Mα X-ray is produced due to removal of M shell electron, replaced by N shell electron.

From Reed, 1996

Absorption Edge Energy

Example: Pt (Z=78) X-ray line energies and

associated critical excitation (absorption edge) energies,

in keV

Edge or Critical ionization energy: minimum energy required to remove an electron from a particular shell. Also known as critical excitation energy, X-ray absorption energy, or absorption edge energy. It is higherthan the associated characteristic (line) X-ray energy; the characteristic energy is value measured by our X-ray detector.

Line EdgeKα1 K-L3 66.83 78.38Lβ3 L1-M3 11.23 13.88Lβ1 L2-M4 11.07 13.27Lα1 L3-M5 9.442 11.56

M1-N3 2.780 3.296Mζ M2-N4 2.695 3.026Mγ1 M3-N5 2.331 2.645Mβ1 M4-N6 2.127 2.202Mα1 M5-N7 2.051 2.133

K-shell gives maximum critical excitation energy

If L => K transition: electron moves from L filling a hole in K

at the same time it creates a hole in L

Vacancy filled by M-shell electrons, producing L-series X-rays

As long as an atom contains electrons in the various outer shells, if the K-series is excited, then the L and M series will also be excited!

Line EdgeKα1 K-L3 66.83 78.38Lβ3 L1-M3 11.23 13.88Lβ1 L2-M4 11.07 13.27Lα1 L3-M5 9.442 11.56

M1-N3 2.780 3.296Mζ M2-N4 2.695 3.026Mγ1 M3-N5 2.331 2.645Mβ1 M4-N6 2.127 2.202Mα1 M5-N7 2.051 2.133

Example: Pt (Z=78) X-ray line energies and

associated critical excitation (absorption edge) energies,

in keV

Overvoltage is the ratio of accelerating (gun) voltage to critical excitation energy for particular line*. U = E0/Ec Maximum efficiency is at 2-3x critical excitation energy.

Example of Overvoltage for Pt: for efficient excitation of this line, would be (minimally) this accelerating voltage

• Lα -- 23 kV

• Mα -- 4 kV

Line EdgeKα1 K-L3 66.83 78.38Lβ3 L1-M3 11.23 13.88Lβ1 L2-M4 11.07 13.27Lα1 L3-M5 9.442 11.56

M1-N3 2.780 3.296Mζ M2-N4 2.695 3.026Mγ1 M3-N5 2.331 2.645Mβ1 M4-N6 2.127 2.202Mα1 M5-N7 2.051 2.133 * recall: E0=gun accelerating voltage; Ec=critical excitation energy

Overvoltage

Components of the EMP

• Electron Gun– Produces electron

• Focusing lenses– Permit focussed beam to hit sample

• Sample Stage– Allow precise positioning of sample

under beam

• Optical System– Allows visual positioning of sample

and selection of sample sites

• Spectrometers– Allow collection of X-rays emitted

from the sample

Field-emission vs ‘normal’ electron Gun (1)

• “Triod” is formed between the filament, Wehnelt grid and anode. Wehnelt grid is held at a negative potential, only allowing electrons to be emitted from the very tip of the filament

• The filament acts as the CATHODE, defined as the “electron emitting” electrode in the system

• The ANODE, located below the filament, acts as the “electrode collecting” electrode in the system

Field-emission vs ‘normal’ electron Gun (2)

• The beam diameter on the sample surface is smallest for the FE gun at a given current.

Penetration increases with increasing HV

Low HV: small interaction volume

• High X-ray spatial res.

• Quant analysis <300nm

• High Z: L-lines

Generic EMPA WDSElectron gun

Column/ Electron optics

WDS spectrometer

video

WDS SpectrometersAn electron microprobe generally has 3-5 spectrometers, with 1-4 crystals in each. Here, Spectro #1 with its cover off.

Crystals (2)

Proportional Counting Tube

(note tubing for gas)

PreAmp

Key points• X-rays are dispersed by crystal with only one wavelength (nλ) reflected (=diffracted), with only one wavelength (nλ) passed to the detector

• Detector is a gas-filled (sealed or flow-through) tube where gas is ionized by X-rays, yielding a massive multiplication factor (‘proportional counter’)

• X-ray focusing assumes geometry known as the Rowland Circle

• Key features of WDS are high spectral resolution and low detection limits

Each crystal its λ interval

Diffracting Crystals Element Ranges

H-crystals: smaller radius => limited range, but higher countrate

Why flat crystals are not used

The point source of X-rays in the EMP is not optimally diffracted by a flat crystal, where only a small region is “in focus” for the one wavelength of interest

Curved crystals focus

X-ray intensities are improved by adding curvature to the crystals:

WDS detector (proportional counter)P10 gas (90% Ar - 10% CH4) is commonly used as an ionization medium. The X-ray enters through the thin window.

X-rays interact with Ar and produce ion pairs (Ar+ + e), with n of pairs proportional to the X-ray energy.

Electron hits the wire, generating an electric pulse => 1 count!

Castaing’s First Approximation

Castaing’s “first approximation” follows this approach. The composition C of element i of the unknown is the K-ratio times the composition of the standard. In the simple case where the standard is the pure element, then, the fraction K is roughly equal to the fraction of the element in the unknown.

=> „Raw“ k-ratio

Ciunk ≈

Iiunk

Iistd Ci

std = KiCistd

...close but not exact

However, it was immediately obvious to Castaing that the raw data had to be corrected in order to achieve the full potential of this new approach to quantitative microanalysis.

The next two slides give a graphic demonstration of the need for development of a correction procedure.

Ciunk ≈

Iiunk

Iistd Ci

std = KiCistd

Raw data needs correctionWhy correct for matrix effects?

3 Fe alloys: e.g. at 40% Fe.

X-ray intensity of the Fe-Ni alloy is ~5% higher than for the Fe-Mn

Fe-Cr is ~5% lower than the Fe-Mn.

Thus, we cannot use the raw X-ray intensity to determine the compositions of the Fe-Ni and Fe-Cr alloys.

Absorption and Fluorescence• Note that the Fe-Mn alloys plot along a 1:1 line, and so is a good reference.

• The Fe-Ni alloys plot abovethe 1:1 line (have apparently higher Fe than they really do), because the Ni atoms present produce X-rays of 7.478 keV, which is greater than the Fe K edge of 7.111 keV. Thus, additional Fe Ka are produced by this secondary fluorescence.

Z el Ka keV K edge keV24 Cr 5.415 5.98925 Mn 5.899 6.53826 Fe 6.404 7.11127 Ni 7.478 8.332

• The Fe-Cr alloys plot below the 1:1 line (have apparently lower Fe than they really do), because the Fe atoms present produce X-rays of 6.404 keV, which is greater than the Cr K edge of 5.989 keV. Thus, Cr Ka is increased, with Fe Ka are absorbed (“used up”) in this secondary fluorescence process.

Z A F

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).

Ciunk =

Iiunk

Iistd

ZAFiunk

ZAFistd Ci

std

Precision and Accuracy

Low Precision High Precision

Low Accuracy

High Accuracy

Standards: how “good” are they? well characterized? homogeneous?

Instrumental conditions: beam stability; spectrometer reproducibility; thermal stability; detector pulse height stability/adjustment; reflected light optics (stage Z)

Matrix correction: any issues (eg MACs for light elements)? Wide range in Z for binary (eg PbO)

Sample and standard conditions: rough surface? polish? etched? tilt? Sensitive to beam? C coat thickness if used

Counting statistics: enough counting time?

Spectral issues: peak and background overlaps?

Sample size vs interaction volume: homogeneous? small particles? secondary fluorescence?

Summary: How to know if the EPMA results are “good”?

There are only 2 tests to prove your results are “good” – actually, it is more correct to say that if your results can pass the test(s), then you know they are not necessarily bad analyses:

• 100 wt% totals (NOT 100 atomic % totals). The fact that the total is near 100 wt%. Typically, a range from 98.5 - 100.5 wt% for silicates, glasses and other compounds is considered “good”. It extends on the low side a little to accomodate a small amount of trace elements that are realistically present in most natural (earth) materials. These analyses typically “do oxygen by stoichiometry” which can introduce some undercounting where the Fe:O ratio has been set to a default of 1:1, and some the iron is ferric (Fe:O 2:3). So for spinels (e.g. Fe3O4), a perfectly good total could be 93wt%.

• Stoichiometry, if such a test is valid (e.g. the material is a line compound, or a mineral of a set stoichiometry.