Electron probe microanalysis - Electron microprobe analysis EPMA (EMPA) What s EPMA all about? What can you learn?

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1 Electron probe microanalysis - Electron microprobe analysis EPMA (EMPA) What s EPMA all about? What can you learn?

2 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

3 In Library Reed (2005), 2 nd 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 New:$75

4 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

5 e.g. Cu-Al grid SE for morphology, BSE for composition Cu has higher Z, i.e. bright BSE Cu grid is pressed into Al SE

6 BSE contains compositional information Fe-Mg silicate

7 Beam Penetration all Ti Monte Carlo simulation of electron pathways Beam penetration decreases with Z Beam penetration increases with energy

8 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

9 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. N e = 2n 2 From Reed, 1996

10 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.

11 From Reed, 1996

12 Absorption Edge Energy 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 higher than the associated characteristic (line) X-ray energy; the characteristic energy is value measured by our X- ray detector. Example: Pt (Z=78) X-ray line energies and associated critical excitation (absorption edge) energies, in kev Line Edge Kα1 K-L Lβ3 L1-M Lβ1 L2-M Lα1 L3-M M1-N Mζ M2-N Mγ1 M3-N Mβ1 M4-N Mα1 M5-N

13 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! Example: Pt (Z=78) X-ray line energies and associated critical excitation (absorption edge) energies, in kev Line Edge Kα1 K-L Lβ3 L1-M Lβ1 L2-M Lα1 L3-M M1-N Mζ M2-N Mγ1 M3-N Mβ1 M4-N Mα1 M5-N

14 Line Edge Kα1 K-L Lβ3 L1-M Lβ1 L2-M Lα1 L3-M M1-N Mζ M2-N Mγ1 M3-N Mβ1 M4-N Overvoltage Overvoltage is the ratio of accelerating (gun) voltage to critical excitation energy for particular line*. U = E 0 /E c 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 kv Mα -- 4 kv Mα1 M5-N * recall: E 0 =gun accelerating voltage; E c =critical excitation energy

15 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

16 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

17 Field-emission vs normal electron Gun (2) The beam diameter on the sample surface is smallest for the FE gun at a given current.

18 Penetration increases with increasing HV

19 Low HV: small interaction volume High X-ray spatial res. Quant analysis <300nm High Z: L-lines

20 Generic EMPA WDS Electron gun Column/ Electron optics WDS spectrometer video

21 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

22 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

23 Each crystal its λ interval

24 Diffracting Crystals Element Ranges H-crystals: smaller radius => limited range, but higher countrate

25 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 Why flat crystals are not used

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

27 WDS detector (proportional counter) P10 gas (90% Ar - 10% CH 4 ) 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!

28 Castaing s First Approximation C unk I unk i i I std C std = KC std i i i i 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

29 ...close but not exact C unk I unk i i I std C std = KC std i i i i 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.

30 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. Raw data needs correction

31 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 kev, which is greater than the Fe K edge of kev. Thus, additional Fe Ka are produced by this secondary fluorescence. Z el Ka kev K edge kev 24 Cr Mn Fe Ni 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 kev, which is greater than the Cr K edge of kev. Thus, Cr Ka is increased, with Fe Ka are absorbed ( used up ) in this secondary fluorescence process.

32 Z A F C unk i = I unk i std I i ZAF i unk ZAF i std C i std 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).

33 Precision and Accuracy Low Precision High Precision Low Accuracy High Accuracy

34 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?

35 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 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. Fe 3 O 4 ), 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.