This is going to sound strange, but let's discuss how the blank correction feature in Probe for EPMA *might* be utilized in certain cases to improve the accuracy of major elements. Yes, the blank correction was originally introduced to correct for continuum artifacts in order to improve trace element accuracy, both for off-peak measurements as discussed here:
https://probesoftware.com/smf/index.php?topic=29.msg387#msg387Full paper here:
https://epmalab.uoregon.edu/pdfs/3631Donovan.pdfand also for MAN background acquisitions as described here:
https://probesoftware.com/smf/index.php?topic=307.msg5427#msg5427Full paper here:
https://epmalab.uoregon.edu/publ/A%20new%20EPMA%20method%20for%20fast%20trace%20element%20analysis%20in%20simple%20matrices.pdfI first had the idea of using the blank correction for major elements when analyzing oxygen to determine water in hydrous glasses (from Withers) using the method first proposed by Barbara Nash where one quantitatively measures oxygen and also all the cations, then one calculates the oxygen expected from these cations and then subtracts them from the measured oxygen, to obtain the actual excess or deficit oxygen.
The difficulty in this case being the accuracy of the oxygen measurement. To test this method I used MgO as my oxygen standard, and then applied Area Peak Factors (APFs) from Bastin, empirically determined mass absorption corrections and also TDI curves. Read the white paper here for details:
https://pages.uoregon.edu/epmalab/reports/Withers%20hydrous%20glass.pdfBut the bottom line was when testing the oxygen accuracy on the NIST mineral glasses, I found that I was still off by some 1% absolute or about 2% relative, which gave somewhat reasonable results on the Wither's glasses, but just wasn't good enough. Hence the application of the blank correction to improve the accuracy of a major element, in this case oxygen, by applying the NIST glass as a blank sample which also brings me to the strange subject of this topic: using the blank correction for major elements (yes, one could have simply utilized the NIST mineral glass a the primary standard for oxygen (and TDI curves), but what would be the fun in that?).
It turns out that the blank correction equation as shown here:
is designed to handle not only blank samples that are "true" blanks, that is they contain a zero concentration of the element in question, but also blank samples that contain non-zero concentrations of the element of interest. E.g., my synthetic SiO2 which contains 1.4 PPM Ti.
So in principle the blank sample could be any concentration of the element of interest, so long as it is an *accurate* concentration (note the blank sample will usually be a standard, but it must be measured as an unknown to be applied as a blank, but that's another discussion).
So let's look at another example I recently ran into in the lab. In this case we were analyzing a Mg-Li alloy, where Li (of course) had to be measured by difference. The samples were beautifully polished even though they were very soft (approaching pure Mg), but I cannot say the same for our standard block which contained pure Mg metal, but even after being freshly polished, it had a bit of an "orange-peel" surface. Most likely because it was polished along with all the other standard materials causing a bit of cross contamination (e.g., the infamous NIST Au-Cu-Ag sample polishing issues). The standard block was also carbon coated, while the unknown sample was not carbon coated. However we did specify a carbon coat correction for the standards, but not for the unknowns, as described here:
https://probesoftware.com/smf/index.php?topic=23.0Anyway, after all that we obtained the following analysis for the Mg-Li alloy as seen here:
Element Li is Calculated by Difference from 100%
No Sample Coating and/or No Sample Coating Correction
Un 2 Mg-Li, 630C, Results in Elemental Weight Percents
ELEM: Mg Mn O Mo Si Li
TYPE: ANAL ANAL ANAL ANAL ANAL DIFF
BGDS: EXP LIN EXP LIN LIN
TIME: 25.00 25.00 25.00 25.00 25.00 ---
BEAM: 27.54 27.54 27.54 27.54 27.54 ---
ELEM: Mg Mn O Mo Si Li SUM
...
340 99.375 .023 .435 -.007 .160 .014 100.000
341 99.542 .013 .515 .000 .102 -.173 100.000
342 99.758 .021 .499 .009 .147 -.433 100.000
343 99.582 .017 .495 -.009 .106 -.192 100.000
344 99.825 .014 .513 .009 .138 -.499 100.000
345 99.800 .020 .536 -.042 .149 -.463 100.000
346 99.934 .018 .491 -.003 .084 -.524 100.000
347 99.682 .016 .362 .009 .049 -.118 100.000
348 99.413 .028 .610 .015 .166 -.230 100.000
349 99.907 .043 .536 .012 .117 -.615 100.000
350 99.849 .028 .594 -.023 .191 -.640 100.000
351 99.532 .015 .580 -.003 .146 -.269 100.000
352 99.641 .023 .545 -.042 .183 -.350 100.000
AVER: 98.054 .018 .634 -.003 .142 1.154 100.000
SDEV: 1.945 .010 .346 .016 .054 1.724 .000
SERR: .106 .001 .019 .001 .003 .094
%RSD: 1.98 53.41 54.50 -611.61 37.77 149.42
STDS: 512 525 913 542 514 ---
STKF: .9984 1.0000 .2509 .9910 1.0000 ---
STCT: 44026.0 20271.5 8110.8 8847.3 4129.0 ---
UNKF: .9776 .0002 .0029 .0000 .0008 ---
UNCT: 44134.6 3.2 103.1 -.2 3.3 ---
UNBG: 35.7 17.9 56.5 5.3 .8 ---
ZCOR: 1.0030 1.1951 2.1997 1.4395 1.8420 ---
KRAW: 1.0024 .0002 .0127 .0000 .0008 ---
PKBG: 1242.53 1.18 2.80 1.00 6.17 ---
INT%: ---- ---- -.01 ---- ---- ---
The averages don't look right because I deleted most of the (300+) analysis lines to save space.
It's not an awful analysis, but the Li by difference numbers do go slightly negative, and I suspected that this might be due to the Mg standard being surface contaminated during polishing by the other materials in the standard mount. Let's continue in a bit in the next post.
