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
nd Reed (2005), 2 ed. 192 pages Reed (1993) Goldstein et al, Paper: New:$40 Paper: New:$55 Used:$35? 3rd Edition. 2003 New:$75 Hard:New: $95 Used $80 Hard:New ~$95 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
BSE
Cu grid is pressed into Al
SE BSE contains compositional information
Fe-Mg silicate Beam Penetration
all Ti
Monte Carlo simulation of electron pathways
•Beam penetration decreases with Z •Beam penetration increases with energy 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-rays Produced 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.
2 Ne = 2n
From Reed, 1996 X-ray Lines - K, L, M Kα 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
Edge or Critical ionization energy: Example: Pt (Z=78) minimum energy required to remove X-ray line energies and an electron from a particular shell. associated critical excitation (absorption edge) energies, Also known as critical excitation in keV energy, X-ray absorption energy, or Line Edge absorption edge energy. It is higher Kα1 K-L3 66.83 78.38 than the associated characteristic Lβ3 L1-M3 11.23 13.88 Lβ1 L2-M4 11.07 13.27 (line) X-ray energy; the characteristic Lα1 L3-M5 9.442 11.56 energy is value measured by our X- M1-N3 2.780 3.296 Mζ M2-N4 2.695 3.026 ray detector. Mγ1 M3-N5 2.331 2.645 Mβ1 M4-N6 2.127 2.202 Mα1 M5-N7 2.051 2.133 K-shell gives maximum critical excitation energy
If L => K transition: electron moves Example: Pt (Z=78) from L filling a hole in K X-ray line energies and at the same time it creates a hole in L associated critical excitation (absorption edge) energies, Vacancy filled by M-shell electrons, in keV producing L-series X-rays Line Edge Kα1 K-L3 66.83 78.38 Lβ3 L1-M3 11.23 13.88 Lβ1 L2-M4 11.07 13.27 As long as an atom contains electrons in Lα1 L3-M5 9.442 11.56 M1-N3 2.780 3.296 the various outer shells, if the K-series is Mζ M2-N4 2.695 3.026 excited, then the L and M series will Mγ1 M3-N5 2.331 2.645 Mβ1 M4-N6 2.127 2.202 also be excited! Mα1 M5-N7 2.051 2.133 Overvoltage
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, Line Edge would be (minimally) this accelerating Kα1 K-L3 66.83 78.38 Lβ3 L1-M3 11.23 13.88 voltage Lβ1 L2-M4 11.07 13.27 Lα1 L3-M5 9.442 11.56 • Lα -- 23 kV M1-N3 2.780 3.296 Mζ M2-N4 2.695 3.026 • Mα -- 4 kV Mγ1 M3-N5 2.331 2.645 Mβ1 M4-N6 2.127 2.202 Mα1 M5-N7 2.051 2.133 * recall: E0=gun accelerating voltage; Ec=critical excitation energy 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 • The filament acts as the CATHODE, filament, Wehnelt grid and anode. defined as the “electron emitting” Wehnelt grid is held at a negative electrode in the system potential, only allowing electrons to be emitted from the very tip of the • The ANODE, located below the filament 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 WDS
Electron gun
Column/ Electron optics
WDS spectrometer
video WDS Spectrometers
An 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
unk unk Ii std std Ci ≈ std Ci = KiCi Ii
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 ...close but not exact
unk unk Ii std std Ci ≈ std Ci = KiCi Ii
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. Raw data needs correction Why 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 above the 1:1 line (have apparently higher Fe than they really do), because the Ni atoms present produce X-rays of 7.478 keV, Z el Ka keV K edge keV 24 Cr 5.415 5.989 which is greater than the Fe K 25 Mn 5.899 6.538 26 Fe 6.404 7.111 edge of 7.111 keV. Thus, 27 Ni 7.478 8.332 additional Fe Ka are produced by this secondary fluorescence. • 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
unk unk unk Ii ZAFi std Ci = std std Ci Ii ZAFi 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). 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.