Offset (Interpolated) MAN backgrounds

[John Donovan]

could you explain a huge difference of Continuum Absorption correction on Cr (close to 2), Sb (close to 2) and InAs (close to 10!).

You do understand x-ray absorption, don't you?   

Is this in reference to measuring O ka as an emitter?  Please take a look at the mass absorption coefficients for oxygen absorbed by other elements.  CalcZAF has a very nice output for this:



Notice how the MACs increase until just after In and Sn as absorbers.  Then it drops off. That is called an absorption edge. If you would like to learn more I suggest Goldstein et al., 1992.

it means that for InAs the measured data is not so important as correction based only on math.

Was that a question?  And why do you say "only based on math"?  At least say "only based on physics"!   

I assume you meant to say that because there is a large absorption correction (due to a nearby absorption edge for O ka by In), the results are highly dependent on the absorption correction model *and* the MAC values selected for O ka.

One way to evaluate the matrix corrections is to utilize the Use All Matrix Corrections checkbox in the Analyze! window in Probe for EPMA.

One can select the various MAC tables from the Analytical | ZAF, Phi-Rho-Z, Alpha Factor, Calibration Curve Selections menu as seen here:



One can also try various MAC tabulations from the literature as described here:

https://probesoftware.com/smf/index.php?topic=1340.msg9631#msg9631

[Rom]

I assume you meant to say that because there is a large absorption correction (due to a nearby absorption edge for O ka by In), the results are highly dependent on the absorption correction model *and* the MAC values selected for O ka.

Yes, you are right.
Does it mean it will be better to use (if it is possible) the standards for MAN BG with small MACs?

[John Donovan]

I assume you meant to say that because there is a large absorption correction (due to a nearby absorption edge for O ka by In), the results are highly dependent on the absorption correction model *and* the MAC values selected for O ka.

Yes, you are right.
Does it mean it will be better to use (if it is possible) the standards for MAN BG with small MACs?

Well it depends on what the composition of your unknowns are.

Normally the only thing that matters for the MAN background correction is the average atomic number of the unknown.

But if the unknowns contain elements that yield an very large absorption correction for a particular emission line (e.g., O ka), you are probably better off using MAN standards that contain those elements, just to obtain a self consistent MAN correction for your unknowns.

[Rom]

Ok, thank you very much.
Can I estimate MACs for my standard sample (contains more then 1 element) from "Table of MACs for elements" of PFE soft?

[John Donovan]

Can I estimate MACs for my standard sample (contains more then 1 element) from "Table of MACs for elements" of PFE soft?

You could, by using those mass absorption coefficients with Beer's Law:

https://en.wikipedia.org/wiki/Beer%E2%80%93Lambert_law

Because the mass absorption coefficients are already normalized to mass, you can simply sum the mass fractions of each element's MAC to obtain the average MAC (actually we should be using attenuation coefficients and molar concentrations as is done in PENEPMA!).

But you can do this easily in the CalcZAF application by simply entering the compound (by formula or weight %) with a small amount of the emitting element, and calculating the k-ratios.

Here is an example of InAs with a small amount of oxygen (100 atoms of In, 100 atoms of As and 1 atom of oxygen):



After clicking the Calculate button we obtain this output:



But again, in cases of extreme absorption, we should not only be using MAN standards that cover the range of average atomic number for our unknowns (and standards), but we may also want to consider elements that are representative of these extremely absorbing matrices (so long as they do not contain major concentrations of the emitting element in order to avoid extrapolating across absorption edges). 

Also remember the rule of thumb: for major elements, the standards and matrix corrections are most important. For trace elements, the background corrections and interference corrections are most important. For minor elements, it's a little of all these concerns.

Performing background corrections for oxygen using the MAN method is especially tricky because it is difficult to obtain materials that do not contain oxygen and that are not oxidized (which is why I assume you are using the interpolated MAN method). However, you might be better off using MAN on your other elements and off-peak methods for oxygen.

Can you share with us what exactly you are trying to characterize?   In other words what major/minor/trace elements are you trying to measure, and in what materials? 

[Rom]

Sorry for delay with answer, I had a long trip.
1. About CalcZAF application - very cunning, thank you. I'll try.
2. Could you explain your phrase

..., but we may also want to consider elements that are representative of these extremely absorbing matrices (so long as they do not contain major concentrations of the emitting element in order to avoid extrapolating across absorption edges).

3. We are working with many different systems. But with simplified for this topic we try to understand Fe+2/Fe+3 in Fe-Cu-Si-O solid solution or Sn+2/Sn+4 in Sn-Cu-Si-O or Cu+1/Cu+2 in Ni-Cu-Si-O etc (all elements - major).

[Ben Buse]

I am not a coder, but from a user perspective it seems like maybe a stern pop-up warning when un-checking the 'use interpolated off peak' might be enough to get the risk across? If that were ignored or didn't exist, as you say as soon as one looked at their 'Assign MAN Fits' hopefully it would be pretty obvious that something was deeply awry. And even if you ignored that, your results should really make introspection unavoidable?

Hi Phil,

OK, I think have something that will work.  What we did was add a new checkbox to the Analysis Options dialog as seen here along with the original Use Interpolated Off-Peaks For MAN Fit checkbox:



This new checkbox is called Use Interpolated Off-Peak Intensities Intensities for MAN Even If Element Is Present.  That's a mouthful but I think it does what it says.  What this new flag does is basically just ignore the MAN minimum intensity level for MAN standard assignments that is set at 100 PPM. Now the cool thing is that this new MAN fit option actually reveals several interesting problems with using off-peak intensities for determining backgrounds (off-peak or MAN!).  Three separate issues actually I think.  These issues have been described in detail by many of our colleagues, Mike Jercinovic and others, Julien Allaz, Karsten Goemann among them I'm pretty sure.  You've probably seen them yourself as well.

This should be also very useful when you want to map for trace elements in a phase that's constant for major elements, allowing a single measured background to be used

[Probeman]

I am not a coder, but from a user perspective it seems like maybe a stern pop-up warning when un-checking the 'use interpolated off peak' might be enough to get the risk across? If that were ignored or didn't exist, as you say as soon as one looked at their 'Assign MAN Fits' hopefully it would be pretty obvious that something was deeply awry. And even if you ignored that, your results should really make introspection unavoidable?

Hi Phil,

OK, I think have something that will work.  What we did was add a new checkbox to the Analysis Options dialog as seen here along with the original Use Interpolated Off-Peaks For MAN Fit checkbox:



This new checkbox is called Use Interpolated Off-Peak Intensities Intensities for MAN Even If Element Is Present.  That's a mouthful but I think it does what it says.  What this new flag does is basically just ignore the MAN minimum intensity level for MAN standard assignments that is set at 100 PPM. Now the cool thing is that this new MAN fit option actually reveals several interesting problems with using off-peak intensities for determining backgrounds (off-peak or MAN!).  Three separate issues actually I think.  These issues have been described in detail by many of our colleagues, Mike Jercinovic and others, Julien Allaz, Karsten Goemann among them I'm pretty sure.  You've probably seen them yourself as well.

This should be also very useful when you want to map for trace elements in a phase that's constant for major elements, allowing a single measured background to be used

But why use a only single MAN background?     

Yes, if the phase is exactly constant for major elements, a single MAN standard would work, but why not measure two MAN standards with average Z values just on either side of your unknown phase and at least you'd have a slope in case the unknown average Z changes even a tiny bit...

Of course you already know about utilizing the blank correction to improve zero accuracy (for all background methods, including off peak and MAN) but here are some links that others might find useful:

https://probesoftware.com/smf/index.php?topic=454.msg6694#msg6694

https://probesoftware.com/smf/index.php?topic=1378.msg9999#msg9999

https://probesoftware.com/smf/index.php?topic=307.msg4901#msg4901