Author Topic: Using CalcZAF To Teach EPMA  (Read 11738 times)

Probeman

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Re: Using CalcZAF To Teach EPMA
« Reply #15 on: July 17, 2015, 08:01:08 AM »
Of course it would only be a problem if you are analyzing all trace elements, since if a major element is present in the iteration that will dominate the matrix correction loop...
John, does it mean that iteration routine does not wait until every element is converged, but the first one to reach the difference limit? 
Sergei

In my code the iteration waits until all elements have converged:

' Normalize and check for convergence
If zaf.ksum! < ZAFMinTotal! Then GoTo ZAFSmpInsufficientTotal
For i% = 1 To zaf.in0%
r1!(i%) = r1!(i%) / zaf.ksum!
ZAFDiff!(i%) = Abs(zaf.conc!(i%) - r1!(i%))
If zaf.conc!(i%) > ZAFMinToler! And ZAFDiff!(i%) > zaf.conc!(i%) / 1000# Then r0% = 1 ' not converged yet
zaf.conc!(i%) = r1!(i%)
Next i%


My previous point was simply that if the major elements are fixed (as is often the case for trace element analysis, e.g., Ti in quartz), the matrix correction is essentially constant and converges quickly.
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Gseward

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Re: Using CalcZAF To Teach EPMA
« Reply #16 on: July 30, 2015, 02:36:11 PM »
this is a mundane point, and probably irrelevant in this case, but:

always make sure the standard and unknown coating parameters are correctly defined!

When the analysed composition of a Standard acquired as an Unknown is not as expected (especially when standardised against itself!), this is my first sanity check.

Gareth

Probeman

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Re: Using CalcZAF To Teach EPMA
« Reply #17 on: September 24, 2020, 09:23:41 AM »
I thought I would post a short primer on the matrix correction equations utilized in EPMA.

We begin with the so called "raw" k-ratio which is the unknown intensity divided by the standard intensity. Normally, both intensities are corrected for all measurement effects such as time, beam current, background, etc:



The above image is from the glossary in the Probe for EPMA help file (which is also distributed with the CalcZAF EPMA utility). To download CalcZAF, see the first post in this topic. This ratio should be reproducible when two materials (unknown and standard) are measured on any instrument with the same electron beam energy and detector takeoff angle (hence the need for globally agreed upon standard compositions!).

Next we examine the standard k-factor term as shown here:



This is to deal with standards that are *not* a pure element. In the case of a pure element standard both the concentration and the matrix correction term [ZAF] are equal to 1 and hence have no effect on the raw k-ratio.

Note also that the term [ZAF], refers to *any* matrix correction scheme including pr(z) methods. The difference in pr(z) methods is that the absorption (A) and stopping power and backscatter terms (Z), are treated in a single analytical expression. The fluorescence factor (F) is treated separately. Alpha factor methods would utilize the so called "beta" term.  But we simply use the [ZAF] term to describe them all.

When the raw k-ratio and the standard k-factor are combined with the unknown matrix correction term, we have the full expression as shown here:



which is iteratively calculated until everything converges.

In the CalcZAF application, the following output shows the raw k-ratio (K-RAW) and the elemental k-ratio (K_VALUE), the latter which is the k-ratio if a pure element standard were utilized:

« Last Edit: September 24, 2020, 12:52:55 PM by Probeman »
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