Offset (Interpolated) MAN backgrounds

[Rom]

1. could you explain, "Attention should be given to the "Z fraction" Zbar averaging method when analyzing moderate to high Z materials" means we have to be very careful when try to use MAN backgrounds for high Zbar materials? We shouldn't use some extra tricks - only think and remember about possible physical mistakes.
2. Thank you.

[John Donovan]

I simply mean that at higher Zbars (average atomic number materials), the continuum production is higher (Kramer's Law), and therefore the P/B ratios are lower. That means the background correction is more critical for higher Z materials. 

https://probesoftware.com/smf/index.php?topic=571.0

For common silicate and oxide materials the accuracy of the MAN method is around 200 to 300 PPM. For higher Z materials the accuracy will be lower (larger variance), but can be improved in all cases by utilizing the blank correction with high purity blank standards run as unknowns in Probe for EPMA:

https://probesoftware.com/smf/index.php?topic=307.0

Therefore one should make additional effort to be sure that ones MAN standards properly cover the Zbar range of the standard and unknown materials being investigated.

I also suggest the use of the Z fraction averaging (using Z^0.7 or Z^0.666) method in the MAN curve fits, as the mass bias from using mass fraction Zbar averaging in compounds with elements of different A/Z ratios, will become more visible at higher atomic number.

https://probesoftware.com/smf/index.php?topic=4.msg10036#msg10036

I suggest you read this paper carefully:

https://epmalab.uoregon.edu/publ/A%20new%20EPMA%20method%20for%20fast%20trace%20element%20analysis%20in%20simple%20matrices.pdf

[JonF]

Is it possible to use an interpolated multipoint background to create a MAN curve?

I'm getting an error when trying this. I've collected multipoint off peak backgrounds for carbon on a number of standards.

I enable "Use Off Peak Elements for MAN Fits" and can create a MAN curve that, carbon being carbon, doesn't look great.

I'd like to try using the interpolated background calculated using the multipoint off peaks, so I enable the "Use Interpolated Off Peaks for MAN Fit" in the Analysis Calculations Options window and go back to the "Assign Man Fits" window, but when selecting carbon I get "WARNING - No MAN count data (all marked as missing) was found to calculate the MAN background fit on C ka on spectrometer 3 on crystal LDE2H at 7 keV."   

I can disable the "Use Interpolated Off Peaks for MAN Fit" in the Analysis Calculations Options window and can again see the poor MAN curve.

The "Clear All MAN Assignments (use default)" doesn't make any difference.

Am I doing something daft?

[John Donovan]

Wow, I don't think we've even thought about using multi-point backgrounds for the interpolated MAN, much less tested it!       Always pushing the envelope, hey Jon?      But it should work I would have thought...

[insert rant on impure natural standards]

https://probesoftware.com/smf/index.php?topic=1415.msg10620#msg10620

I'm at M&M at the moment so we are pretty swamped, but please send me a sample MDB file to my probe software email and we'll look at it later this week.

[JonF]

Wow, I don't think we've even thought about using multi-point backgrounds for the interpolated MAN, much less tested it!       Always pushing the envelope, hey Jon?      

I'm finding new and interesting ways to confuse my users.

"What, you don't actually measure the background, and you don't actually measure the background at the energy you don't actually measure it at?"

"err, yeah"

 


Impure standards are one thing, but light elements like carbon are a pain because of the deposition and the million different interferences you have to try and avoid. Interpolated MPB-MAN with APF (if necessary) will hopefully be a nice way to dodge the interferences and the carbon deposition and hopefully give good quant maps for light elements using the LDE/PC crystals.   

Hope M&M is going well.


PS I've emailed you an MDB file, hopefully it'll make it through the email filters.

[John Donovan]

Wow, I don't think we've even thought about using multi-point backgrounds for the interpolated MAN, much less tested it!       Always pushing the envelope, hey Jon?      

I'm finding new and interesting ways to confuse my users.

"What, you don't actually measure the background, and you don't actually measure the background at the energy you don't actually measure it at?"

"err, yeah"

 

It does get rather esoteric, doesn't it...  but hey, it works.

Impure standards are one thing, but light elements like carbon are a pain because of the deposition and the million different interferences you have to try and avoid. Interpolated MPB-MAN with APF (if necessary) will hopefully be a nice way to dodge the interferences and the carbon deposition and hopefully give good quant maps for light elements using the LDE/PC crystals.   

Hope M&M is going well.


PS I've emailed you an MDB file, hopefully it'll make it through the email filters.

That a good point. Because the standard intensities (in Probe for EPMA), are corrected not only for dead time and background, but also for interferences and APF shape/shift effects, this could be quite useful for light elements.  Note that these intensities are also corrected for standard intensity drift (again, in Probe for EPMA).

I got your MDB file and will look at this as soon as I work through the small mountain of notes I have from the meeting.  The meeting went quite well and Tuesday had some great talks on EPMA (if you didn't count mine!).  I ended my presentation talking about the "constant" k-ratio method from John Fournelle and the new dead time correction from Aurelien Moy for WDS as described here:

https://probesoftware.com/smf/index.php?topic=1466.msg11025#msg11025

Actually the conference was great. Really nice to see most of everyone after 3 years.

OK, here's my joke from the conference:  the difference between EDS and WDS is OCD.

 

[John Donovan]

OK, I found that our code was deliberately skipping utilizing multi-point backgrounds (MPBs) for the interpolated MAN method.  Because, you know who would have thought someone would want this?   

Anyway that is fixed now and it seems to work. So just update PFE as usual and you should be good to go.  Oh, and thank-you for sending a small test run demonstrating the issue. Usually we get a massive probe run with 400 samples and 27 elements!

By the way, I assume you've looked over this topic on using MAN for trace carbon:

https://probesoftware.com/smf/index.php?topic=48.0

I found it unnecessary to resort to the (interpolated) off-peak MAN method for trace carbon.  So I suspect that your standards are much more contaminated with carbon that you might have thought.  At least that's what I'm seeing with your test run...

In fact, I found that what was really necessary for trace carbon using the normal MAN was the TDI correction and of course the usual spectral interference corrections.

That said, our instrument has a 100K degrees cryo trap that really pumps the hydrocarbons, so this may not be possible on all instruments.

Finally, I'm assuming that both your standards *and* unknowns are uncoated with carbon and carefully cleaned?  Carbon is a non-trivial exercise on most EPMA instruments for sure.

[JonF]

Brilliant, cheers John. That fix seems to work beautifully. Below is a before/after for the C Ka energy using the on peak C Ka and the MPB interpolated off peak from a larger range of standards.

Before (on peak C Ka)



After (MPB interpolated)


This is going to help a lot with light element mapping! It'll certainly help with oxygen, as that's even more a pain to get around. 

The samples are definitely in need of a clean, but I suspect in this case the majority of the contamination is coming from the samples next to the standards. I was provided with a number of 30mm bakelite blocks (!) that took a couple of turns through the JEOL airlock before the software would open the gate valve to the chamber. TDI and interference correction are definitely needed. I guess as long as the contamination rate is linear, the TDI can take care of it with a good degree of certainty.

Preventing contamination is something that I've spent more time than I care to admit looking in to. I've replaced our liquid nitrogen dewar with an in vacuo peltier cold finger, but I'm very interested in the PCC cooler. Do you notice any vibration from the device? It is referred to as "low vibration" in the product documents, but relative to what?

[John Donovan]

After (MPB interpolated)

That is cool, but the Be and Al intensities bother me.  Continuum intensity should be proportional to average atomic number.  That is, the intensities should be lowest at the lowest atomic numbers.

If this is real then there could be some interesting physics going on (absorption correction?), but it's probably just some sort of artifact.  Please share some plots with us showing intensities with and without the absorption correction.  Or with different absorption corrections.

Preventing contamination is something that I've spent more time than I care to admit looking in to. I've replaced our liquid nitrogen dewar with an in vacuo peltier cold finger, but I'm very interested in the PCC cooler. Do you notice any vibration from the device? It is referred to as "low vibration" in the product documents, but relative to what?

Here's the Polycold system on our Sx100:

https://probesoftware.com/smf/index.php?topic=646.msg3823#msg3823

It just runs and runs (air cooled) and always stays around 100 Kelvin. It cools a baffle over the diffusion pump. It was originally ordered because we couldn't afford Cameca Turbo system and a diffusion pump plus this cryo baffle was about half the price. 

But now that I see how great it works, I would order another on my next instrument.  In fact I would order a turbo pump with this baffle for sure.  Here's a comparison between the "dry" pumped system at NETL and our cryo pumped system:

https://probesoftware.com/smf/index.php?topic=646.msg10157#msg10157

The black line of carbon contamination is our JEOL "dry" system and the invisible carbon line is our Polycold system. No comparison which is better.

As for vibration, it's true it did add a small amount of vibration originally, but then we added a number of wraps of foam lined lead sheet around the head and the vibrations were completely damped. 

I'm sure there are designs that could eliminate vibrations completely.  And one can always turn the head off temporarily for high mag imaging I guess.

[Rom]

Thank you for answering my previous question.
Now could you explain why "correct for continuum absorption" check box gives worse result for my MAN background. What direction should I look when I want to use this option?
Thank you!

[John Donovan]

Thank you for answering my previous question.
Now could you explain why "correct for continuum absorption" check box gives worse result for my MAN background. What direction should I look when I want to use this option?
Thank you!

I assume this is using the interpolated off-peak MAN method?  You should always use the continuum absorption correction with the MAN background method.  It is especially important for low energy emission lines.  The only reason it can be turned off is to demonstrate the absorption effect magnitude in the Assign MAN Fits dialog. That is, the MAN intensities are always absorption corrected during an actual quantitative analysis.

Hard to say what your specific problem is. Did you show your data to your lab manager?  It can be difficult to utilize the MAN correction for oxygen since many factors can influence the results even when using the interpolated off-peak MAN method, e.g, the background fit (interpolation across absorption edges), off-peak interferences, etc.

You may want to use a normal off-peak method for oxygen, and MAN for the other elements.

[Rom]

I assume this is using the interpolated off-peak MAN method?  You should always use the continuum absorption correction with the MAN background method.  It is especially important for low energy emission lines.  The only reason it can be turned off is to demonstrate the absorption effect magnitude in the Assign MAN Fits dialog. That is, the MAN intensities are always absorption corrected during an actual quantitative analysis.

Ok, thank you.

Hard to say what your specific problem is. Did you show your data to your lab manager?  It can be difficult to utilize the MAN correction for oxygen since many factors can influence the results even when using the interpolated off-peak MAN method, e.g, the background fit (interpolation across absorption edges), off-peak interferences, etc.

the background fit (interpolation across absorption edges) - how I can check the issue?
off-peak interferences - checked, there are not
what else?

Edited by John to utilize "quote" feature

[John Donovan]

the background fit (interpolation across absorption edges) - how I can check the issue?
off-peak interferences - checked, there are not
what else?

Please utilize the "quote" feature to make your context clear as I have here and as shown in your previous post.

With the normal (on-peak) MAN method both of these issues disappear, but one requires standards that are high purity. So your best bet is to obtain better (high purity) standards, but for oxygen that is difficult because most materials have at least trace levels of oxygen, which is why I suggest you do not attempt the MAN method for oxygen. At the very least, avoid standards for oxygen background by MAN that contain major oxygen!   

If you must use MAN for oxygen I would start with high sensitivity wavescans to check the background regions and try to avoid absorption edges but it is almost impossible to see these spectral details since the peak region is obscured by the major oxygen peak. Again, I would utilize materials that do not contain oxygen! Then the absorption edges will be more visible in your wavescans.

Think about your strategy when using the MAN method: ideally you want to select standards that do not contain the element of interest, and that also cover the range of average atomic number that your standards and unknowns.

Yes, with the interpolated off-peak MAN method, one can utilize standards that contain (major amounts of) the element of interest.  But that doesn't mean that you should!   

I suggest using a number of materials that do not contain oxygen (e.g., pure metals or sulfides, etc.), and because they will probably still contain some small amounts of oxygen (from oxidation), use the interpolated (off-peak) MAN method as you are doing.

Just get rid of the standards that contain major oxygen (for the oxygen MAN background calibration) and see if that works better!

[Rom]

Thank you, I'll try.

[Rom]

Hi John,
could you explain a huge difference of Continuum Absorption correction on Cr (close to 2), Sb (close to 2) and InAs (close to 10!).
it means that for InAs the measured data is not so important as correction based only on math.
All measurements (Ka for oxygen) done after WS and based on BG approximation.
Thank you very much.