Author Topic: Questions about MAN background use  (Read 28952 times)

JohnF

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Re: Questions about MAN background use
« Reply #90 on: July 17, 2018, 08:05:22 am »
I am doing some quality control checking of MAN results at minor and trace el levels: Mg, Sr, Mn and Fe in carbonates (using defocused 20 micron beam). I have wavescan data to compare the MAN-predicted bkg counts with. Sr, Mn and Fe MAN numbers match nicely with the wavescan numbers. The particular sample which I am testing has a mean Z of 12.6.

From both wavescans and discrete background measurements on the sample (which has a very small distinct peak; backgrounds are clear), the background should be 12.5.

However, the MAN fitted background, using the continuum absorption correction, gives a background of 17, which doesn't seem correct. If I turn off the cont. abs. correction, there is a wonky curve but the value at a mean Z of 12.6 is closer to the actual measured background level.

(Note: conincidence that the carbonate Z is 12.6, and the actual back ground is 12.5 c/s/20nA)

Again all other 3 elements, Sr, Mn and Fe work great with the con. abs. corrected MAN values being what the wavescans show.

Could there be something weird going on for Mg?

MAN stds were acquired in normal acquisition mode (not TDI). Carbonates later were run in TDI.

Attached are 2 plots

Probeman

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Re: Questions about MAN background use
« Reply #91 on: July 17, 2018, 08:29:47 am »
Hi John,
Both plots are correct.  But it is a little unintuitive, so good question!

The plotted (absorption corrected) MAN intensities in the Assign MAN Fits dialog are the "ideal" background intensities. They cannot be compared to actual measurements.  But by turning off the absorption correction in the MAN dialog, you are seeing the raw measured values, however which cannot be fitted together, because each MAN standard has different matrix physics.   But if the unknown matrix is the same as an individual MAN standard, then you can compare this "raw" intensity to off-peak measurements, as you observed. Please re-read the Donovan, et al. 2016 paper in Amer. Min for a complete explanation:

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

In short, this is because each MAN standard is a different matrix and therefore needs to be corrected for the specific emission line individually before fitting.  Then once we have the equation of fit for these "ideal" intensities for the MAN standards, we can apply that fit to an actual sample after it is "de-corrected" for the actual absorption in the sample.

If you want to compare the calculated MAN background intensity with off-peak measurements, see the procedure here:

http://probesoftware.com/smf/index.php?topic=4.msg189#msg189

Then compare the line in the output labeled "UNBG:". Which are the MAN *or* off-peak intensities for the sample.
john
« Last Edit: July 17, 2018, 08:53:07 am by Probeman »
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Julien

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Re: Questions about MAN background use
« Reply #92 on: January 28, 2019, 09:07:07 am »
Hi John,

It is my understanding that if we do have a MAN correction curve for an element, and if we do have analyzed an element with 2-point background, it should be possible to switch between the 2-point background correction and the MAN. And even if it is not the case, we could use the 2-point background for determining a MAN background correction curve.

My problem is that - for testing purpose - I want to switch between the 2-points background and the MAN background, yet I can NOT change the background type in elements/cations window. The background field are all grayed out, and I cannot change a thing...



I do have checked the option "Use MAN correction for Off-peak measurement..." in menu Analysis, and I do have a correct set of MAN correction (the program indeed asked me to reset the MAN correction, which I did).





Maybe I'm doing something wrong? Note that at first, I was using the “shared background” in this run. To enable the MAN background, I did the following:
  • REMOVE the shared background
  • Activate the “Use MAN correction for off peak elements”
  • Hit the “Clear All MAN assignments”
  • Re-assigned the MAN fits.
Nonetheless, in each case (with shared bkg or with linear 2-pts bkg after resetting the MAN fit), the MAN option in elements/cations remain unavailable… Am I doing something wrong? Any help would be greatly appreciated!

Julien

John Donovan

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Re: Questions about MAN background use
« Reply #93 on: January 28, 2019, 09:57:25 am »
Hi Julien,
I think you are basically doing it correctly.   You don't need to change anything in the Elements/Cations dialog. Those options are for the background *acquisition* type (we'll modify the description to make it more obvious!).

I don't quite understand how you are seeing the MAN curve fits if you acquired off-peak standards and the Use Off-Peak Elements for MAN Fits menu is unchecked.  Maybe you had it checked earlier at some point?

Anyway, see here for an explanation of using standards acquired using off-peak bgds, but for the MAN fit:

https://probesoftware.com/smf/index.php?topic=4.msg189#msg189

The steps to use MAN backgrounds based on your off-peak acquired standards, is to first check the Use Off-Peak Elements for MAN Fits menu and then assign the MAN fits.  This allows the program to use standards acquired with off-peak backgrounds, for the MAN assignments, by "throwing away" the off-peak measurements on those standards.

Next if your samples are acquired using MAN backgrounds you are done. But if your samples are off-peak, then you need to check the next menu down which is the Use MAN Correction for Off-Peak Elements menu. This will force the program to utilize MAN background corrections even if the sample (std or unk) data was acquired using off-peak backgrounds.

To get real fancy you can also check the Use Interpolated Off-Peaks for MAN Fit checkbox in the Analytical | Analysis Options dialog as described here:

https://probesoftware.com/smf/index.php?topic=987.0

With this method, first suggested by Ben Hanson at Corning, the program obtains the MAN background intensities by interpolating the off-peak measurements.  This option can be very useful when you want to use MAN backgrounds for faster acquisition (especially quant x-ray maps), but your standards are contaminated with traces of the element of interest, for example using MAN fits for O ka!

I hope this makes sense, but let me know if you are still having trouble.  Feel free to also send me your MDB file.
john
« Last Edit: January 28, 2019, 11:25:35 am by John Donovan »
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BenjaminWade

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Re: Questions about MAN background use
« Reply #94 on: January 28, 2019, 03:43:02 pm »
Hi John and all
My wish would be to be able to (post acquisition) mix and match elements with MAN and off-peak post acquisition rather than all one or the other... I am sure it probably wouldn't be simple to implement though...

Cheers

John Donovan

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Re: Questions about MAN background use
« Reply #95 on: January 28, 2019, 05:07:36 pm »
Hi John and all
My wish would be to be able to (post acquisition) mix and match elements with MAN and off-peak post acquisition rather than all one or the other... I am sure it probably wouldn't be simple to implement though...

Cheers

Hi Ben,
Yeah it would be more than a little bit of work, I'll have to think how it might be possible.   But you can always do what I do, which is decide which elements should be MAN and which elements should be off-peak *before* you acquire data!   ;)
john
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Julien

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Re: Questions about MAN background use
« Reply #96 on: January 31, 2019, 06:42:00 am »
Thanks John,

I agree with Ben and it might be possible if you would enable the background fields two-point, MAN, MPB... in the Elements/Cations window. Then you could ask your software to check which option is being selected in the Elements/Cations window, and if MAN is selected (and if a MAN correction is available), then it uses the MAN for the background and if the two-point option is selected, then it would use the two-points. This solution could be further extended (with certainly more work...) for the MPB and more specifically shared background. Right now, you have an option to enable or remove the shared background, but if you do have a shared background, and if the MPB option in Elements/Cations is set to MPB, then it uses the shared background. Otherwise it would use the classical two-point background or the MAN background. Of course, if you do so, the software should be able also to determine if (a) MAN background is available, and (b) if a set of MPB or shared background is available.

This solution might also add some more clarity: the user can simply check in the Elements/Cations to see what background correction is being used for that specific element, whereas the current solution (your answer to my question) is done by clicking an option in a menu.

Julien

JakubHaifler

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Re: Questions about MAN background use
« Reply #97 on: June 07, 2019, 01:40:40 am »
Hi,
I have been trying to use some advanced measurement (or rather processing) techniques, i.e. multi-point background fitting, interference corrections and (now) mean atomic number background calculation WITHOUT Probe for EPMA (Punk’s not dead!). Although it is quite demanding, I have been quite successful in some example tasks (in all these approaches) having only measured intensities, CalcZAF and MATLAB.

Measurement of Pb in my complex material (REE-, actinide-, Zr-, Ti-, Nb-rich minerals) seems to be very complicated. Recently, I have been trying MAN method. So, here I share my experience with fitting the Z vs background curve without PfE, but I also have a question.

I carefully selected some standards for background measurement at PbMb position (c. 58010 on LPET of our Cameca SX100). The intensities were measured at 15kV, 180 nA for 240-300 s.   

The approach of how to calculate the continuum absorption is mentioned in Ware and Reed (1973), which is referred in Donovan et al. (2016). However, for better understanding, I recommend a book “Scanning Electron Microscopy and X-ray Microanalysis” by Goldstein and co-workers and its electronic supplement, where the method of Philibert, Duncumb and Heinrich is explained in detail. At first, I tried to calculate their example on NiKa absorption in Ni-Fe alloy in Excel, then I got the same results in CalcZAF. Continuum absorption is calculated similar to the absorption of a characteristic line with the exception of zero concentration of the emitter in the former case. The values of f(chi), which is the fraction of the generated x-rays, which were not absorbed, can be simply and quickly calculated in CalcZAF. First of all I selected Philibert/Duncumb-Reed matrix correction and MAC table (Ware and Reed refer to Heinrich, 1966; however MAC’s for PbMb are not published in this set – see below). Then, I loaded a standard from our dataset, e.g. GdPO4. Subsequently, I added PbMb line to the calculation with Pb concentration set to zero. After the calculation, the value of f(chi) for PbMb can be found in results. This approach seems to work well.



Here is my average Z vs corrected background intensity diagram:



Somewhat enigmatic for me are the deviations of the two Zr-bearing standards, ZrO2 and pure Zr. I have studied a WDS scan of a wide region around PbMb performed on a pure Zr, but there should be absolutely no noticeable zirconium (or some contaminant) peak at that position – according to the scan as well as according to the x-ray spectral database. Any idea for this behaviour?



According to calculated absorption corrections, Zr is a somewhat stronger absorber (f(chi)=0.55) than most other standards, at least those with similar Z. Surprisingly, the raw background intensities before the correction seem to be much closer to the general trend of the most other standards (except for TiO, which shows much higher f(chi)=0.85 = much weaker absorber that the others, which usually have c. 0.6-0.7 or lower).



One observation I find interesting or surprising: I tried to explain the deviation of the Zr-bearing standards in my diagram. So, I decided to compare the values of all the MAC’s of PbMb in Zr (originally, I selected Heinrich, 1966) among various datasets. Then, I found out, that there are no values for absorption of PbMb in the dataset of Heinrich, 1966 and CalcZAF automatically took MAC’s from a different dataset (McMaster, I think). The MAC's for PbMb in Zr from different datasets, however, are relatively similar, so the backgrounds of Zr-bearing still deviate. 

One more question, on PfE – is the continuum absorption correction done using the approach by Philibert (1963) as referred in Ware and Reed (1973) in all cases or can it be switched to a different approach?

Best regards, Jakub Haifler   
« Last Edit: June 07, 2019, 08:50:19 am by John Donovan »
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Re: Questions about MAN background use
« Reply #98 on: June 07, 2019, 03:11:18 pm »
Hi Jakub,
Wow.  Very impressive work.  I must say, I think you would really have a blast with PFE on your instrument, maybe someday?

As to why the Zr containing materials are behaving like this, there are a lot of parameters involved here so it will be interesting to figure this out.  As you point out, the mass absorption coefficient values being the most dominant for this correction. I will have to try some MAN measurements myself, using the Pb Mb emission line, and see what I can see, but in the meantime I will try to answer a few of your questions as much as possible.

But first I ran a *simulation* of these MAN curves in Probe for EPMA (in demo mode) and got this MAN curve for your standard calibration materials:



Turning off the continuum absorption correction I got this:



Note that stds 40 (Zr oxide) and 540 (Zr metal) both drop below the curve without the absorption correction, so that is consistent with your measurements.  The default MAC table in Probe for EPMA (and CalcZAF) is Henke (for emission lines less than 10 keV). Looking in CalcZAF we can see the MACs for this emission line in Zr:

MAC value for Pb Mb in Zr =    2580.40  (LINEMU   Henke (LBL, 1985) < 10KeV / CITZMU > 10KeV)
MAC value for Pb Mb in Zr =        .00  (CITZMU   Heinrich (1966) and Henke and Ebisu (1974))
MAC value for Pb Mb in Zr =    2750.53  (MCMASTER McMaster (LLL, 1969) (modified by Rivers))
MAC value for Pb Mb in Zr =    2620.36  (MAC30    Heinrich (Fit to Goldstein tables, 1987))
MAC value for Pb Mb in Zr =        .00  (MACJTA   Armstrong (FRAME equations, 1992))
MAC value for Pb Mb in Zr =    2528.02  (FFAST    Chantler (NIST v 2.1, 2005))
MAC value for Pb Mb in Zr =    2620.00  (USERMAC  User Defined MAC Table)

The Henke MAC is quite a bit smaller than the MAC30 table value which you used. So this might cause the over correction you are seeing for the Zr containing materials.

As to what continuum absorption correction is being utilized in Probe for EPMA for the MAN curve fit, it is whatever absorption correction is currently selected in the ZAF/Phi-Rho-Z method.  Originally we tried several absorption corrections specifically intended for continuum emissions, but we got worse results than just using the absorption corrections from the currently selected matrix correction.  So now the MAN method in PFE just uses the absorption correction from whatever matrix correction has been selected.

By the way, you might want to contact Julien Allaz, Mike Jercinovic and Karsten Goemann as both have worked on using the MAN method with complex high Z materials. All three will be at the QMA 2019 conference in Minnesota later this month and all will be giving presentations on various background correction methods including MAN, off-peak and MPB (multi-point backgrounds) and Karsten will present the MPB method with "shared" backgrounds (that is utilizing normal off-peak measurements from other emitters measured on the same spectrometer).

Also see these topics:

https://probesoftware.com/smf/index.php?topic=4.msg17#msg17

https://probesoftware.com/smf/index.php?topic=9.msg7764#msg7764

Or maybe your materials are slightly contaminated in Pb? I can assure you that some of the Smithsonian REE standard are contaminated with Pb!  Note how my MAN curves excluded these materials automatically because it knows from the standard composition database that these materials contain Pb!
john
« Last Edit: June 08, 2019, 08:57:15 am by Probeman »
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JakubHaifler

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Re: Questions about MAN background use
« Reply #99 on: June 12, 2019, 03:40:24 am »
Hi John,

thank you very much for your answer. As you wrote, PfE would be a great help, particularly for such complicated measurements I am doing at the moment. On the other hand, these are extraordinary and are only for my research, I do not perform such calculations for routine measurements. So, the increased effort is acceptable in this case. Unfortunately, I am not powerful enough to affect what software and hardware equipment we have in our laboratory. But as you wrote – maybe sometime. I hope, my little DIY is not going to break your business with PfE  :), but may allow some people, particularly students, to try how the method works.

As for my data: I did some additional measurements. Something is clearer now, something not. I did two 5-point background measurements on Zr (btw, we have two pure Zr standards – intensities in both somewhat differ, but neither is compatible with my MAN curve). I observe that intensities at PbMb position are c. 7% higher than what I would expect from the exponential fit through the other four points. So, some contamination seems to be highly probable (may be Pb, Mo, Bi, Nb... at this position). On the contrary, the theoretical background at PbMb calculated from the fit remains incompatible with the MAN curve.



So, after additional measurements, following 12 standards are compatible within a single fit: YAG, Fe2O3, YPO4, LaPO4, GdPO4, GdAlO3, MnTaO4, CrTa2O6, CaWO4, Hf, Ta, W. I excluded TiO and Ti, as TiKb(II) shows overlap with PbMb. Similar to Zr, Sn also have PbMb-intensity higher than a fit through 4 other points nearby, so the contamination may also occur. Pure elemental standards Cr, Fe are not compatible with the MAN curve (they give higher PbMb-intensities), but I do not have additional data for resolving the reason.



Very mysterious for me is the behaviour of cheralite, CaTh(PO4)2, intensity of which shows very deep “negative” anomaly. It has avg. Z of 51.158, but PbMb-intensity of only 1.24 cps/nA (based on 3 measurements). CaWO4 and MnTaO4 with very similar avg. Z give c. 1.53-1.54, which is the theoretical, MAN-curve-compatible, background value at that Z.

I read some older comments on the MAN method (from about 2013). According to them, the method works well for low-avg Z matrices, while not so well for high-avg Z, which is not a very good message for me. However, I have not found much information on that. I think, my MAN curve looks relatively nice (of course, I have not explained incompatibility of Fe, Cr and particularly cheralite yet). Moreover, the used standards are relatively simple compounds (pure elements, binary or ternary compounds). So, I guess, the complex natural material is going to be much more tricky.
I am somewhat confused of a more general question of whether there are some serious limits and irregularities in this method. Or what should be the main pitfall for the high-Z matrices?
One example: from the many posts in this thread we can see that the MAN curve is usually fitted by a linear/exponential/2nd order polynomial, fluently increasing, function. This seems to be a rule: the higher Z, the higher the (corrected) background intensities. But for example in this post, John's data show a concave curve (which John explains there). 
https://probesoftware.com/smf/index.php?topic=4.msg1070#msg1070   

Best regards, Jakub     
« Last Edit: June 12, 2019, 08:12:59 am by JakubHaifler »
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Re: Questions about MAN background use
« Reply #100 on: June 12, 2019, 11:16:58 am »
Hi Jakub,
Please understand that there is nothing extraordinary about using the MAN background correction.  My guess is that, at least for geologists who utilize the software, the MAN correction is used almost all the time, at least for the major and minor elements (note that one can utilize a mix of MAN and off-peak elements for any given sample setup). The reason for it being utilized so often, is that the MAN correction saves so much time time (and improves precision) on major, and even minor elements, where very small accuracy errors in the MAN calibration curve, are generally smaller than the precision of the peak intensity measurement. 

For typical geological samples with K emission lines and low to moderate average Z materials, this is usually the case.  In my experience with typical silicates and oxides, the MAN background correction is accurate down to 200-300 PPM, even without a blank correction.  The use of the MAN correction with higher Z matrices and high Z emission lines is still an area of active investigation.   But maybe a little "MAN" history would be appropriate here...

The very first rationale for the MAN considerably preceded my own initiation to EPMA, and was invented to deal with EPMA instruments which utilized fixed monochromaters for commonly measured elements. For example, on my first EPMA instrument, which I inherited at UC Berkeley in the 1980s, it had 4 fixed monochromaters (for Si, Fe, Ca and Al), and 4 tunable spectrometers for another 4 elements. The reason of course being that fixed monochromaters *cannot* be de-tuned for the off-peak measurement!  So people came up with the idea of utilizing a calibration curve of standards which did not contain the elements of interest, for the background correction for these "fixed" spectrometers. Which meant of course that using the MAN correction, we could measure 8 elements in 10 seconds!  This was very useful as you might imagine for beam sensitive materials.

Since then the MAN background correction has been significantly improved and as my Amer. Min. 2016 paper showed, the MAN background correction can even be utilized for improving sensitivity for trace elements.  Basically this results in a 40% or more improvement in detection limits, in 1/2 the acquisition time, because only the on-peak intensity is measured.  And of course, of interest to you, one automatically avoids off-peak interferences, because there are no off-peak measurements when using the MAN correction!  In addition, interpolation issues with non-linear backgrounds and/or absorption edges simply disappear with the MAN method because, again, only the on-peak position is utilized.

But even though the MAN method can result in better accuracy than off-peak methods when such off-peak issues are present, the main caveat for the MAN method remains accuracy (at least at trace levels), because we are not performing a direct measurement of the continuum on the sample.  Therefore for ultimate accuracy, the MAN method is commonly combined with the so-called "blank" correction. The blank correction works well for relatively simple matrices where suitable (usually synthetic) blank samples can be obtained, e.g., quartz, zircon, pyrite, TiO2, Fe2O3, Fe3O4, etc.  Of course for complex materials, zero blanks may be difficult to obtain. But it is also of interest to note that the blank correction as implemented in Probe for EPMA, can be applied to non-zero blank samples, so there is that option.

Now having said all that, the remaining practical difficulties of the MAN method is obtaining pure standards that do not contain the element of interest.  As you pointed out in the linked post, many standards have not been properly characterized for trace elements. For example, I have two synthetic zircons, one appears to have a couple hundred PPM more phosphorus than the other. I have heard from many colleagues that utill they started utilizing the MAN method, they had not realized how contaminated some of their standard materials were.  On the other hand, the MAN method can teach us quite a lot that we didn't already know about our standards!

Just as a small aside, Ben Hanson at Dow Corning came up with a really crazy idea for the MAN method, which is to utilize the *off-peak* interpolated intensities for the MAN calibration curve, or "offset MAN" as he called it, as described here:

https://probesoftware.com/smf/index.php?topic=987.msg6447#msg6447

"Crazy as a fox" as they say!  The reason he requested this because he found that many of his standards weren't as pure as he thought they were. I thought it was a good idea for another reason: sometimes I've wanted to utilize the MAN method for oxygen analysis, and it's difficult to find standards that don't contain even trace oxygen (since we live in an ocean of oxygen)!  Whatever.

It boils down to this: almost without exception, the lowest intensity one can measure (by definition!) is the background.  Yes, there are possible sample and/or detector absorption edges and "holes" in the background continuum due to secondary Bragg diffraction (e.g, the hole in the continuum seen in FeS2 near the Au Ma position and in some PET crystals at the Ti Ka position, though there is still some debate about this), but these are fairly rare. So in general, any standards that fall above the general trend of the MAN calibration curve are either from contamination (from the element being present), or an interference from another analytical peak.

The general strategy at the user level is to simply remove those (higher intensity) offending standards from fit of the MAN curve. Now because you are conducting a bit of a research project here with your high Z materials, you are interested in exactly what is causing these outliers in the MAN calibration curve.  And that is great and you should pursue this line of inquiry because it is interesting and helpful for greater understanding of the details.

As to particular difficulties with high Z matrices, the main difficulty is due to the fact that the P/B ratios are worse for high Z matrices since from Kramers Law we know that we produce more continuum in higher Z materials. Second, when analyzing high Z emission lines, we are generally restricted to L or even M emissions and these are also typically lower P/B than K emissions. So combined this means that the calibration of the MAN curve is even more critical for high accuracy work in high Z materials, especially at low concentrations.

That said, there are advantages to using the MAN method even in complex high Z materials, as has been pointed out (no off-peak interferences and no interpolation). But I think other approaches such as the MPB (multi-point background) and even "shared" bgds can be useful for such matrices. I believe Karsten Goeman and Julien Allaz have published these methods and also have examples of these alternative off-peak methods posted elsewhere on the forum.
« Last Edit: June 12, 2019, 09:45:15 pm by Probeman »
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