Author Topic: Quantitative Spectral Interference Corrections  (Read 5325 times)

John Donovan

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Quantitative Spectral Interference Corrections
« on: October 05, 2013, 07:21:00 pm »
One very nice feature of Probe for EPMA is the ability to toggle on and off (globally for a run), a large number of various analytical corrections, without the need to un-assign and re-assign each correction to every sample in a run. For example, from the Analytical | Analysis Options menu dialog we can turn off the quantitative interface correction by simply unchecking the box shown here:



This of course results in a high total since the many interferences that are specified for this sample are ignored when the Use Assigned Interference Corrections On Standards and Unknowns checkbox is unchecked as seen here: (it is automatically selected for you, whenever you assign a new spectral interference).



Though we could simply return to full interference corrections by simply checking the above checkbox, let us instead, add a new interference assignment which, as noted above, will then select the Use Assigned Interference Corrections... checkbox, automatically.

While the quantitative interference corrections in Probe for EPMA are extremely accurate and easy to use there are some pitfalls awaiting the unwary user (including "yours truly" on more than one occasion!).  Let's start by assigning a well known interference usually observed in monazite compositions that is critical for U-Th-Pb chemical age calculations: Y LG3 on Pb Ma. We begin by opening the Standard Assignments dialog from the Analyze! window as seen here:



Note however, that the All Samples option is selected in the Analyze! window (and then the Select All button clicked to select all samples in the run).  This ensures that the program automatically assigns the interference correction to all standards and unknowns in the run.

If we had previously observed a non-zero, but still significant concentration of Pb that we did not expect in a sample containing a major amount of Y, say a Yttrium Aluminum Garent (YAG) standard, we might suspect that this is indeed an interference as opposed to a contaminant, but how can we confirm this? One way is to click the Pb element row since that is the element that is suspected to being interfered with, and then click the Calculate Interferences button as seen here to confirm if the interference is even possible. This calculation merely assumes Gaussian peak shapes and calculates the nominal spectral overlap one might expect to observe. If the suspected interference does not show up in the list of possible interferences, we might then consider contamination rather than spectral interference as the culprit.



But as can see in the above screen capture, the Y LG3 line is indeed listed, so we now know that the interference is most likely actually present. So normally, we would just assign the YAG standard as the standard for the interference, but here is where it gets a bit interesting. In Probe for EPMA, we only need to note: the element being interfered with, the element causing the interference, the order of the x-ray line that is causing the interference and a standard to be used for the interference that should ideally contain a major amount of the element causing the interference and none of the interfered element- and (this is important), no other elements causing an interference on the interfered element.

Therefore, although a YAG standard would normally suffice as an interference standard for Y on Pb because Probe for EPMA automatically accounts for the difference in the matrix between the unknown and the standard used for the interference (see Donovan, et al. attachment below), it does *not* account for peak shifts due to chemical states in the interference correction (at least not yet!). And as noted by Mike Jercinovic a few years ago (see Jercinovic and Williams attachment below), the peak position of the Y LG3 line is sensitive to chemical states and therefore one must choose another standard for the interference correction. Fortunately we can assign a YPO4 standard because it does not contain any Pb as shown here:



However, the clever observer will also note the interference of La La1 (II means 2nd order) on Pb listed just below the Y LG3 interference which is even slightly larger. Unfortunately on this occasion, the LaPO4 standard *does* contain a small amount of Pb and therefore is unsuitable as an interference standard for La on Pb, although a Pb free LaPO4, LaB6 or LaF3 standard could be utilized.

The end result of all the interference corrections assigned is shown here, which results in a much better total than previously:



Finally, if we like to note the magnitude of the interferences on each element, we can see them listed in the log window as shown here:



Thank-you to Julien Allaz who made me aware of the Y LG3 peak shift issue between YAG and YPO4.
« Last Edit: September 30, 2016, 11:53:26 am by John Donovan »
John J. Donovan, Pres. 
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Re: Quantitative Spectral Interference Corrections
« Reply #1 on: April 29, 2014, 01:48:32 pm »
Here is the critical page from the paper showing how the  fully quantitative interference correction is derived:

« Last Edit: April 29, 2014, 04:10:56 pm by Probeman »
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JohnF

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Re: Quantitative Spectral Interference Corrections
« Reply #2 on: November 12, 2017, 04:54:19 pm »
Yesterday I had a frustrated morning as I couldn't get PfE to correct an overlap.
Let me explain: We are studying Fe-Ni compounds using low (7) kV, and experimenting with the use of the La and Ll lines, and testing different crystals of different spectrometers. This complication was the source of my confusion.
Ni La on LTAP
Ni La on PC1
Ni La on PC0
Ni Ll on LTAP
Ni Ll on PC1
Ni Ll on PC0
Fe La on LTAP
Fe La on PC1
Fe La on PC0
Fe Ll on LTAP
Fe Ll on PC1
Fe Ll on PC0

Got that? All being run on each standard and unknown.

Eventually based upon the wavescans, the decision came down to using the Ll lines on PC0.

However, on the PC0 the Fe La (actually Lb, as they are merged together on the PC0) has a major overlap upon the Ni Ll.

Now in the interference correction in PfE, John Donovan doesn't only show the element interfering with the studied element, he lists the x-ray line. That is what confused me.

Add: to run the quant, I had to D! (disable quant) on the 4 "other" lines.

Since the PC0 Fe La (short for La/Lb) overlapped the PC0 Ni Ll, I selected PC0 Fe La as the interfering line....but Pf'E refused to run the quant. After some consultation with John, and more scratching my head, I did something that seemed illogical, but worked....I put the interfering line as the PC0 Fe L1...and the quant worked! Then I realized that I was focusing on the wrong thing, the _line_ rather than the _element_.


John Donovan

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Re: Quantitative Spectral Interference Corrections
« Reply #3 on: November 12, 2017, 05:38:22 pm »
Since the PC0 Fe La (short for La/Lb) overlapped the PC0 Ni Ll, I selected PC0 Fe La as the interfering line....but Pf'E refused to run the quant. After some consultation with John, and more scratching my head, I did something that seemed illogical, but worked....I put the interfering line as the PC0 Fe L1...and the quant worked! Then I realized that I was focusing on the wrong thing, the _line_ rather than the _element_.

Hi John,
Complex example, but very cool!

Sorry for the confusion but I add the x-ray line to the Elements/Cations interfering element listbox, when an element is duplicated.  It really just serves as an identifier for the element channel in the software.  I could just as well say Fe (1), Fe (2), Fe(3), etc.  But more often people (like you) are comparing the performance of different lines for the same element, and in these cases the x-ray line string helps to identify the quantitative element channel.

And because the interference correction in PFE is completely quantitative, it doesn't matter what emission line we are utilizing for the interfering element, it only matters what the *concentration* of the interfering element is.   So depending on the accuracy of the interfering element quant, one should be able to obtain an accurate interference correction by simply assigning an interfering element channel that isn't disabled for quant.

It gets complicated doing what you do!  You should plot an example of this nasty but intertesting interference correction.  Very cool stuff you and Moy are doing.
john
« Last Edit: November 12, 2017, 05:50:59 pm by John Donovan »
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Re: Quantitative Spectral Interference Corrections
« Reply #4 on: November 14, 2017, 11:38:23 am »
Hi John,
I really haven't looked into the Fe and Ni La vs. Ll situation as you have, but does it make any sense to try acquiring the La emission lines using the integrated intensity acquisition option where the software performs a scan over the peak to get the intensity for quantification?

I know the integrated intensity scan method works for chemical shift issues with sulfur, but of course the La region is more complex.

http://probesoftware.com/smf/index.php?topic=733.msg4660#msg4660

john
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