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Consensus K-Ratio Measurements

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No one is posting consensus k-ratios here. I'm just sharing my surprise and pleasure with some quant results.  Related to this I'm also collecting k-ratios at different beam currents for the possible purpose of a better dead time calibration procedure.  More on this soon.

In the meantime the FIGMAS group is working with the initial participants and is collating results returned to them for the specific materials currently being distributed.  More materials are "in the pipeline".   I'm as impatient as you are...   :D

Indeed there are many synthetic oxides "out there", but we want to utilize materials that we know are currently available in bulk quantities, so we are starting afresh working with various suppliers.

The characterization of high purity synthetic materials is indeed a separate process from the k-ratio data collection. Your suggestions for bulk purity characterization are helpful. Thanks.

John Donovan:
We added a new feature to Probe for EPMA to assist in the accurate acquisition of consensus k-ratios.

If you're like some people and utilized element setups from the SETUP.MDB element database or loaded a sample setup from a previous Probe for EPMA run, you probably loaded the element setups, then peaked the spectrometers, checked the PHA settings and maybe even re-ran a wavescan sample to check on your off-peaks, but that might not be enough.

Recently Probeman contacted us to let us know that although he did all of the above, when loading the element setups from previous runs, he neglected to note that the dead time constants for his WDS spectrometers had since been re-calibrated. Therefore that the stored elemental setups still referenced the old dead time constants, from prior to the dead time re-calibration.  And they should of course since they are a record of that calibration!  ::)

But to prevent this from happening again, in the latest version of Probe for EPMA, if one has updated the dead time constants in the SCALERS.DAT file since those element setups were saved, Probe for EPMA will now check for this and provide the following user dialog, if it finds that the dead time constants have been updated since then:

Therefore this new PFE update will allow one to utilize older element setups but with the new dead time calibrations from the SCALERS.DAT file.

Of course one can always manually update these dead time constants "after the fact" using the Analytical | Update Dead Time Constants menu dialog as described here:

John Donovan:
I wanted to mention that the new consensus k-ratio export/import format agreed by Nicholas Ritchie and myself is now documented in the Probe for EPMA User Reference manual and is described here in case anyone else wants to write a script for those using JEOL or Cameca OEM software.

This format (which could change I guess) currently consists of 12 header lines which are pretty self explanatory:

"Instrument Type (make and model)
"Data File Name"
"User Name"
"File Title"
"File Description"
"Nominal Beam: "
"Database Created:
"Last Updated: "
"Last Modified: "
"Current Date and Time: "

These are followed by the data lines which contain 39 column labels as shown here:

"Sample Name"
"Line Number"
"Spectro Num"
"Spectro Orientation"
"Beam Size"
"Beam Current"
"Background Type"
"Off Peak Bgd Type"
"On-Peak Pos"
"Hi-Peak Pos"
"Lo-Peak Pos"
"Baseline (volts)"
"Window (volts)"
"Bias (volts)"
"Detector Type"
"Dead Time (usec)"
"On-Peak Time (sec)"
"Hi-Peak Time (sec)"
"Lo-Peak Time (sec)"
"Unknown Counts (cps/nA)"
"Standard Name"
"Standard Counts (cps/nA)"
"Raw On Counts"
"Raw Hi Count"
"Raw Lo Count"
"Raw Off Count"

All these data columns are tab delimited for easy import into Excel. Most of these are pretty self explanatory, but a couple of notes. First the "Spectro Orientation" column is referenced to *North* being zero degrees and proceeds clock-wise around the instrument looking down from the top. 

EDS detectors should be given the spectrometer number zero (even if you have more than one), and the spectrometer orientations will differentiate them.  The column "Crystal" is just a text string so for EDS detectors it could be SDD (10 sq) or SDD (30 sq) or whatever to further facilitate differentiating them.

The on and off-peak positions for EDS systems could simply refer to the emission energy, and the off-peaks could document the range of the ROI fit, or just leave blank for filter methods.

Here is an image loaded into Excel:

This picture was taken pre-Covid but here is a snapshot (by who?) of Nicholas and myself which showed up in my Google "OneDrive" feed this morning:

John Donovan:
Of course I should hasten to add that if you are using Probe for EPMA to acquire your consensus k-ratios, you can simply use the string selection tool in the Analyze! window to select the samples to output (typically the secondary standards), and then right click the sample list to output those selected samples to the consensus k-ratio output format that Nicholas Ritchie and I have come up with:

Or you can output all sample to the consensus k-ratio format using the Output | Save Custom Analysis Output menu...

This is the FIGMAS export/import format that we should be sending our k-ratio data to Will Nachlas and Aurelien Moy.

Of course if you think of anything in this format that you think should be modified, please chime in here!

While at M&M last week we (of course) discussed issues of accuracy in EPMA.

One metaphor I utilized was what I called the "holy trinity of microanalysis".  That is standards, k-ratios and physics.

Standards because of course if our standard compositions are not what we think they are, we will introduce inaccuracy in our analytical results.  This is true whether we utilize high purity synthetic standards or natural matrix matched standards:

K-ratios because that is what we measure, at least on (WDS) EPMA/SEM instruments. And to obtain accurate k-ratios we need to correct not only for beam current and count time, but also dead time, background, standard intensity drift, spectral interferences, time dependent intensity effects (TDI) and even sometimes peak shift/shape effects (APFs).

It is worth mentioning that in Probe for EPMA the reported k-ratios (KRAW) are corrected for all the above items.  This comes in very handy when performing further evaluation and analysis using k-ratios exported from PFE. But as we have been discussing recently we need to make sure that our k-ratios agree not only with all the spectrometers on our own instrument (simultaneous k-ratios), but also with k-ratios measured on other instruments (consensus k-ratios).

And finally physics, because we need to deal with matrix effects of absorption, energy loss, backscatter and fluorescence.  Today we still have some issues with matrix corrections, e.g., backscatter corrections for high Z compounds containing elements disparate A/Z ratios, but matrix corrections have come a long way over the last 30 years.

Now it is my working hypothesis that we have, as a community, tended to drift towards (or remain fixated on) matrix matched standards for several reasons:

Historically our matrix corrections did not model the physics of electron-solid interactions very well, but this has dramatically improved in recent decades. If you are utilizing a modern phi-rho-z method today (with accurate mass absorption coefficients), you are probably in very good hands for most (not all!) matrix situations.

Second, our instruments are not as well calibrated as they ought to be, For many reasons, but particularly the reasons mentioned above: dead time and effective takeoff angles (which by the way, includes sample tilt!). The good news is that using measurements such as the constant k-ratio method we can perform these instrument calibrations and improve our accuracy.

And of course we need to properly correct our k-ratios for background, spectral interferences, TDI effects, etc. using software tools that can handle these issues when they arise.

So, let's start making consensus/constant k-ratio measurements on our instruments and see how well they actually perform.  I'll end with noting that I've attached my M&M presentation of quant mapping below (login to see attachments), because it ends with a few slides regarding the points above.


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