Mike Jercinovic and Mike Williams (U Mass Amherst) recently contacted us with a feature request for the "shared" background method. As many of you know, the "shared" background method, which is a variation of the multi-point background method, was originally suggested by Karsten Goemann (U Tasmania). Let's quickly review these various background methods.
When setting up a quantitative analysis in Probe for EPMA, one can select three different methods for background calibration: traditional off-peak, mean atomic number (MAN), and multiple-point background acquisition methods.
Everyone is familiar with the traditional off-peak method where one acquires a background measurement on each side of the emission peak, and then interpolates the background intensity under the peak (or alternatively one can acquire both background measurements on the same side of the peak and then extrapolate to the background intensity under the peak).
The MAN background method, described in various topics throughout the forum, e.g., here:
https://probesoftware.com/smf/index.php?topic=4.0https://probesoftware.com/smf/index.php?topic=307.0has advantages in saving time and improving precision, by constructing an MAN calibration curve that can be applied to both standards and unknowns of diverse compositions, by making measurements at the *on-peak* position, versus average atomic number (Kramer's Law), in standard materials that *do not* contain the element of interest. A recent variation on the MAN method, originally suggested by Ben Hanson (Corning Glass) and further suggestions by Phil Orlandini (U Texas, Austin), utilizes interpolated *off-peak* measurements on standards to construct an MAN calibration curve for unknowns, where only the on-peak intensities are measured. Thus avoiding on-peak contamination effects (from impure standards), on the on-peak measurements on the standards, and also off-peak interferences on the unknowns. More on this can be found here:
https://probesoftware.com/smf/index.php?topic=987.0Please note that these topics go back a long way, so you might want to skip to the end of the topics, to start with the most recently posted material. A nice summary of the original MAN method can be found in Donovan et al., (2016), Amer. Min.
The multi-point background method (originally suggested by Mike Jercinovic, Julien Allaz (ETH Zurich) and Mike Williams) is similar to the off-peak method, but it utilizes more than one background measurement on each side of the peak. The idea being to characterize the background with greater rigor, particularly in cases where extreme accuracy is required and/or the shape of the background is significantly curved. Here is an example of a typical MPB acquisition for U mb by Mike Jercinovic:
The MPB method is especially useful for complex matrices (e.g., monazites) where the composition can change unexpectedly, thus causing unanticipated off-peak interferences. And because the MPB has multiple background measurements, it can quickly iterate to find the best background interpolation as seen here:
One can also specify these MPB adjustments manually using the so called "manual" over-ride flags if necessary in post processing.
The "shared" background method is a variation of the MPB method, where one acquires traditional off-peak backgrounds on more than one element, which "share" the same spectrometer and Bragg crystal as seen here:
and described in more detail here:
https://probesoftware.com/smf/index.php?topic=9.0and then utilizes the MPB arrays to process these off-peak data *as though* they had been acquired using the MPB acquisition method. The curious aspect to the "shared" bgd method is that because the off-peak measurements are *shared* for all the elements using the same spectrometer and Bragg crystal, the background fits for all these shared elements are exactly the same!
What Mike Jercinovic and Mike Williams wanted us to do was to allow the user to assign the same background fit obtained for one shared element, to all the other shared elements on that same spectrometer and crystal. Which is perfectly reasonable of course, but it turned out to be a bit of a hassle to code for a couple of reasons.
So when we first designed the MPB method we decided to limit the acquisition to 12 off-peak measurements on each side of the emission peak for a total of 24 off-peak measurements. No one would ever need to acquire more than 12 backgrounds on each side of an emission peak, right?
Not a bad assumption when utilizing multi-point backgrounds of course. But when using the shared background method, the first and last element on that spectrometer/crystal can contain 2 * n -1 backgrounds on one side of the peak (where n is the number of elements). This means that one could have up to seven elements per spectrometer/crystal combination and all the background measurements would be available when the limit is 12 backgrounds. But Mike and Mike had a monazite run where they had *nine* elements on the same spectrometer and crystal, not seven!
https://www.youtube.com/watch?v=xwKaXcTPKjESo that means the elements at the top and bottom of the spectrometer range could have up to 17 background on one side of the peak. Oh, god. The problem wasn't so much just increasing the size of the MPB arrays, but finding enough space to fit all the additional manual over-ride flags in the GUI:
So we settled on adding 6 more backgrounds to each side of the peak for a total of 36 off-peak backgrounds per elements. Yes, that's right, thirty-six off-peak measurements.
So, in practical terms what does this mean? Well looking at the above plot, we can adjust our "iterate to" parameters to get the MPB background fit we want, as seen here:
and then we utilize this handy new button seen here, to assign this fit to all the other "shared"background elements as seen here:
The other less visible difficulty was figuring out how to keep track of which backgrounds corresponded to each of the elements on that spectrometer/crystal, as the load order will be different for each element depending on all kinds of details in the acquisition. In the end, we figured out a way to base the background selections on the actual off-peak spectrometer positions, because, after all, they are "shared" backgrounds!
Version 12.8.4 of Probe for EPMA is now available for downloading using the Help menu as usual.
