Transcript Electron Probe Microanalysis

Electron Probe Microanalysis
Electron Probe Microanalysis
A technique to quantitatively analyze samples for their chemical composition on a micro-scale (~1μm)
Instrument:
Known as probe, microprobe or electron microprobe – all the same
Main components
Electron Gun
Beam Column
Sample Holder/Air Lock
X-ray Spectrometers – Wavelength or Energy Dispersive
Vacuum System
Illumination System
Brief Description of Main Components
Electron Gun – Source of electrons generated by thermionic emission of Tungsten filament
Grid cap (Wehnelt cylinder) negatively biased to constrict electron beam
(From Reference 1, page 30)
Beam Column
Serves to de-magnify image of filament to achieve ~ 1m beam size
Contains a condenser lens and an objective lens
(From reference 2, page 45)
Sample holder/air lock
Provides a means of holding a sample for examination – typically a polished thin section or mineral grain mount
Air lock provides a means to exchange samples – be careful here!
X-ray spectrometers
Two kinds of spectrometer found on probes – wavelength (WDS) and energy dispersive (EDS) – WDS 10X sensitivity of EDS
Measure the same thing – characteristic spectra of an element present in the sample
E = hc/λ = 12.396/ λ, E = energy in Kev, h = Planck’s constant, c = velocity of light, λ = wavelength in angstroms
This relationship shows energy and wavelength are inversely related
Method utilizes the characteristic radiation from an excited atom to quantify the amount of an element in a sample –
Peak height proportional to amount of element present, after background correction
Example: a K shell electron is ejected and an L electron drops to fill vacancy – leads to a Kα line (this radiation is specific to
the atom excited)
(From reference 3, page 7)
Origin of X-ray Lines for K and L Transitions
Quantitative analysis
Purpose: to quantify the amount of an element in a sample – must compare signal from sample to that of a known standard
To first order, counts from sample and counts from standard are directly related to concentration
k ratio = Isample/Istd = Csample
In practice, k ratio must be corrected for sample effects
k ratio = Isample/Istd = Csample Z A F
Corrections: Commonly referred to as “ZAF” corrections
ZAF means we have to make three types of corrections to our sample data
Z is the so called atomic number correction – is made up of stopping power and backscatter terms
A is the absorption correction – takes into account that some of the X-rays produced in sample volume don’t make it out of
the sample
F is the fluorescence correction – corrects for X-ray induced excitation in the sample
There is an alternate correction process utilized by many of the modern probes – it is the “Phi–rho-z” method, it basically
combines the Z and A effects into one method, you still need to do the fluorescence correction as well
Software packages included with all modern probes make the corrections for you (Buyer beware!)
Sample - Electron Beam Interaction:
(From reference 1, page 69)
Vacuum System
Required to sustain tungsten filament and stop residual gas X-ray absorption
Illumination System
Two modes usually provided
1. Optical illuminator – reflected light, sometimes transmitted light, for visual identification of sample features
2. SEM mode – the probe can function as an SEM (scanning electron microscope) with both secondary and backscatter images
available – extremely useful method for selecting spots to probe
Helpful Hints
Beam energy must be 1 to 2 times energy required to excite desired X-ray line
Beam current should be as low as possible for minimum spot size
Spot size can be varied if sample is reactive under the electron beam (be sure to use same spot size for standards)
Our Instrument:
CAMECA SX-100
(note: only one other manufacturer of probes, JEOL)
R100191 Roselite
Oxides
MgO
CaO
ZnO
MnO
As2O5
CoO
NiO
Total
Date
x
y
z
1.638543
24.06168
0.000012
0.286896
49.26317
12.80333
0.102457
87.26375
10/6/2010 13:16
-14599
646
835
1.59662
24.12704
0.000012
0.160339
49.08891
13.1324
0.006092
86.1962
10/6/2010 13:20
-14600
680
835
1.071248
24.23088
0.05624
0.158607
49.61437
13.21772
0.138841
88.03637
10/6/2010 13:23
-14606
755
835
1.072605
24.14428
0.06135
0.289297
50.12297
13.49821
0.141022
87.80859
10/6/2010 13:27
-14619
751
835
1.353601
24.1974
0.000012
0.197133
49.81471
13.44682
0.081916
87.91537
10/6/2010 13:35
-14787
751
835
1.211323
24.17074
0.000012
0.167884
49.4537
13.39434
0.000013
88.0152
10/6/2010 13:38
-14787
751
835
1.198884
23.93616
0.000012
0.147767
50.02366
13.34733
0.083508
86.75258
10/6/2010 13:41
-14770
751
835
1.293255
24.01254
0.043498
0.192441
49.24385
13.52351
0.03376
87.50651
10/6/2010 13:44
-14782
783
835
1.709265
24.21972
0.127935
0.189742
49.79366
13.01528
0.026582
88.88528
10/6/2010 13:47
-14782
816
835
1.707457
24.39864
0.128095
0.222202
49.00893
12.99749
0.128013
85.96765
10/6/2010 13:50
-14782
816
835
1.787547
23.60133
0.000012
0.908881
50.02984
10.78678
0.041535
88.81791
10/6/2010 13:54
-14598
667
835
2.446573
24.23414
0.000012
0.992909
50.55445
10.81757
0.062686
86.68926
10/6/2010 13:58
-14602
666
835
1.928083
24.24879
0.132762
0.942763
50.85159
11.42332
0.000013
87.06113
10/6/2010 14:01
-14529
614
835
2.040862
24.05773
0.000012
0.88676
50.49967
11.50833
0.035461
86.67274
10/6/2010 14:06
-14474
416
835
1.978735
24.20343
0.000012
1.008217
50.83835
11.15598
0.000013
87.03284
10/6/2010 14:09
-14478
28
835
Ave.:
1.602306733
24.12296667
0.036666
0.450122533
49.880122
12.537894
0.058794133
Std. Dev.:
0.387013667
0.176478249
0.051026
0.355207273
0.590885254
1.022139584
0.049448279
Wt % Oxide
Oxide MW
Mol #
Atom Prop.
Anion Prop.
# Ions/formula
MnO
Oxide
0.45
70.94
0.006343
0.006343389
0.029335609
0.029
ZnO
0.04
81.38
0.000492
0.000491521
0.002273087
0.002
CoO
12.54
74.93
0.167352
0.167351732
0.773934109
0.774
NiO
0.06
74.93
0.000801
0.000800747
0.003703133
0.004
MgO
1.6
40.31
0.039691
0.039691006
0.183554855
0.184
CaO
24.12
56.08
0.4301
0.430099857
1.98903797
1.989
As2O5
49.88
229.84
0.217021
1.08510268
5.018161237
2.007
Total:
88.69
1.729880933
Enter Oxygens in formula:
8
Oxygen Factor Calculation:
F=
4.624596
F is factor for anion proportion calculation
Ideal Chemistry:
Ca2(Co,Mg)(AsO4)2·2H2O
Measured Chemistry:
Ca1.989(Co0.774Mg0.184Mn0.029Ni0.004Zn0.002)Σ=0.993(As1.004O4)2·2H2O
References
1. Scanning Electron Microscopy and X-ray Microanalysis, 3rd ed., J. Goldstein and others,
Springer Verlag, 2003
2. Electron beam X-Ray Microanalysis, K. F. J. Heinrich, Van Nostrand Reinhold Co., 1981
3. Elements of X-Ray Diffraction, B. D. Cullity, 2nd ed., Addison Wesley Pub. Co., 1978
Other useful references:
Principles and Practice of X-ray Spectrometric Analysis, 2nd Ed., E. P. Bertin, Plenum Press, 1975
(a comprehensive treatment of X-ray spectrometry)
Principles of Modern Physics, R. B. Leighton, McGraw-Hill, 1959 (extensive treatment of origin of
X-ray spectra)
Quantum Physics, R. Eisberg and R. Resnick, J. Wiley and Sons, 1974 ( Less rigorous treatment of
modern physics than Leighton)
Electron Probe Microanalysis, L. S. Birks, Wiley-Interscience, 1971 (classic reference with a
worked example of the ZAF correction)
Electron Microprobe Analysis and Scanning Electron Microscopy in Geology, S. J. B. Reed,
Cambridge, 2005 (current brief reference to both probe and SEM for geologists)