As many of you know, Probe for EPMA has a very realistic demo or simulation mode which can be utilized for educational purposes.
Probe for EPMA utilizes spectral intensities generated from the Penepma Monte Carlo software in two different ways. First, for WDS simulation it utilizes previously calculated pure element spectra from Penepma calculated using 1 eV resolution in 5 keV steps from 5 to 25 keV. For compounds it simply sums the spectra based on the weight fraction of the element (Yes, we should also be performing an absorption correction on each energy bin, but it's on the to-do list. In the meantime it's good enough...) to produce the final WDS spectrum.
Of course the spectrum is subsequently modified for higher Bragg orders, WDS exponential effects and absorption edges. See the folder C:\UserData\Penepma12\Penepma\pure for these pure element intensity files. If you don't have these files, or you don't have a complete set of these files, you should update your Penepma data files using the Help | Update menu with the Update Penepma Monte Carlo Files Only checkbox checked.
For EDS, Probe for EPMA utilizes the Penepma in "real time", that is the EDS spectra is generated as the Penepma Monte Carlo program runs in the background and displayed in Probe for EPMA. The photon rate depends on a number of factors but is roughly equivalent to having a beam current of a few nA or so.
In a recent post Donovan showed an example demonstrating some new flags for EDS quantification and data was shown where the Fe concentration was not very accurate compared to the other elements as seen here:
ELEM: Si Ca Fe Mg Al Mn O
TYPE: ANAL ANAL ANAL ANAL ANAL SPEC SPEC
BGDS: LIN LIN EDS EDS EDS
TIME: 20.00 20.00 24.00 24.00 24.00 --- ---
BEAM: 30.02 30.02 30.02 30.02 30.02 --- ---
ELEM: Si Ca Fe Mg Al Mn O SUM
5 21.416 11.062 6.624 11.571 4.886 .077 43.597 99.234
AVER: 21.416 11.062 6.624 11.571 4.886 .077 43.597 99.234
SDEV: .000 .000 .000 .000 .000 .000 .000 .000
SERR: .000 .000 .000 .000 .000 .000 .000
%RSD: .00 .00 .00 .00 .00 .00 .00
PUBL: 21.199 10.899 7.742 11.657 4.906 .077 43.597 100.077
%VAR: 1.02 1.50 -14.44 (-.73) (-.41) .00 .00
DIFF: .217 .163 -1.118 (-.09) (-.02) .000 .000
STDS: 162 162 162 160 160 --- ---
This is primarily because of the poor counting statistics resulting from a relatively high energy emission line at 15 keV, only running the Penepma simulation for 12 seconds (it assumed an EDS count time of 12 seconds because of the assumed 50% EDS deadtime), and only a single "measurement". Here one can see the low count rate from the Fe Ka emission line from Penepma:
On-Peak (off-peak corrected) or EDS (bgd corrected) or MAN On-Peak X-ray Counts (cps/1nA) (and Faraday/Absorbed Currents):
ELEM: si ka ca ka fe ka mg ka al ka BEAM1 BEAM2
BGD: OFF OFF EDS EDS EDS
SPEC: 1 2 0 0 0
CRYST: PET LPET EDS EDS EDS
ORDER: 1 1 1 1 1
5G 56.93 33.28 2.82 22.62 9.75 30.016 29.974
AVER: 56.93 33.28 2.82 22.62 9.75 30.016 29.974
SDEV: .00 .00 .00 .00 .00 .000 .000
1SIG: .31 .24 .06 .18 .12
SIGR: .00 .00 .00 .00 .00
SERR: .00 .00 .00 .00 .00
%RSD: .00 .00 .00 .00 .00
However, if we increase the WDS counting time to 64 seconds we see a much better result as seen here:
ELEM: Si Ca Fe Mg Al Mn O
TYPE: ANAL ANAL ANAL ANAL ANAL SPEC SPEC
BGDS: LIN LIN EDS EDS EDS
TIME: 24.00 24.00 64.00 64.00 64.00 --- ---
BEAM: 30.03 30.03 30.03 30.03 30.03 --- ---
ELEM: Si Ca Fe Mg Al Mn O SUM
37 21.533 10.589 7.691 11.640 4.903 .077 43.597 100.030
AVER: 21.533 10.589 7.691 11.640 4.903 .077 43.597 100.030
SDEV: .000 .000 .000 .000 .000 .000 .000 .000
SERR: .000 .000 .000 .000 .000 .000 .000
%RSD: .00 .00 .00 .00 .00 .00 .00
PUBL: 21.199 10.899 7.742 11.657 4.906 .077 43.597 100.077
%VAR: 1.57 -2.85 -.66 (-.15) (-.05) .00 .00
DIFF: .334 -.310 -.051 (-.02) (.00) .000 .000
STDS: 162 162 162 160 160 --- ---
In fact, here we got lucky! Spurious accuracy as they say!
However, instead of increasing the EDS count time, we could simply acquire more data points per standard. Note that because PFE "re-seeds" the random number generator in Penepma for each spectrum "acquisition", the average will usually be more accurate than a single point. Of course, even better is to acquire multiple data points and a longer counting time as seen here:
ELEM: Si Ca Fe Mg Al Mn O
TYPE: ANAL ANAL ANAL ANAL ANAL SPEC SPEC
BGDS: LIN LIN EDS EDS EDS
TIME: 24.00 24.00 64.00 64.00 64.00 --- ---
BEAM: 30.01 30.01 30.01 30.01 30.01 --- ---
ELEM: Si Ca Fe Mg Al Mn O SUM
37 21.540 10.588 7.696 11.396 5.202 .077 43.597 100.098
38 21.430 10.739 7.906 11.129 4.779 .077 43.597 99.657
39 21.481 10.701 6.680 11.716 5.343 .077 43.597 99.595
40 21.296 10.845 7.757 11.773 4.451 .077 43.597 99.796
41 21.675 10.843 7.908 11.969 4.561 .077 43.597 100.630
AVER: 21.485 10.743 7.589 11.597 4.867 .077 43.597 99.955
SDEV: .140 .107 .517 .333 .392 .000 .000 .424
SERR: .062 .048 .231 .149 .175 .000 .000
%RSD: .65 1.00 6.81 2.87 8.05 .00 .00
PUBL: 21.199 10.899 7.742 11.657 4.906 .077 43.597 100.077
%VAR: 1.35 -1.43 -1.97 (-.52) (-.79) .00 .00
DIFF: .286 -.156 -.153 (-.06) (-.04) .000 .000
STDS: 162 162 162 160 160 --- ---
So the Fe is off about 2% which is typical EPMA accuracy. The value here for education is that the students can actually see the effects of increasing count time and/or replicate measurements on their own laptop computers. With a bit of "luck" of course!
