Hi Andrew,
I again return to this plot of analytical totals versus Ca concentration you posted earlier. With some additional annotations.
I'm probably missing something, so please correct me if you see something, but it seems to me that when making a measurement of CaCO3, why would one expect to obtain the exactly correct stoichiometry for CO2, when the Ca measurement is not exactly correct?
We already know the code calculates the correct CO2 when the correct Ca is provided (relative to the standard of course). So if the measured Ca concentration is different than the expected Ca, the matrix correction will be different and the concentration of stoichiometric carbon, based on the calculated oxygen, which is itself based on the measured Ca content, will also be different.
Am I making any sense?
In fact I think the answer is laying right in front of our noses. In Reply#86 look at the ZCOR values for Ca Ka for the standard CaCO3, the slightly high total CaCO3 and the slightly low total CaCO3. I summarize them here:
ZCOR: 1.0567 --- --- <--- close to 100% total
ZCOR: 1.0568 --- --- <--- slightly high total
ZCOR: 1.0566 --- --- <--- slightly low total
In short, different Ca concentrations are calculated for each different case, meaning that different amounts of stoichiometric oxygen are calculated for each composition, and hence different amounts of carbon are calculated for each composition. Only when the correct Ca is measured, does the correct stoichiometric oxygen get calculated, and subsequently only then does the correct carbon by stoichiometry to oxygen get calculated.
What do you think?
Hi John,
Let us look at how Probe-for-EPMA handles a binary oxide, MgO, in comparison to a simple carbonate, siderite (Fe,Mn)CO3.
In the case of MgO, I measured the Mg K-alpha intensity for 20 points, and for the usual reasons, we have uncertainty in the results. The Mg concentrations range from 60.05 to 60.65 wt%, and the analytical totals from about 99.6 to 100.6 wt%.
However, regardless of this analytical uncertainty, for every point, Probe-for-EPMA reports oxygen-by-stoichiometry
in the ideal ratio of 1:1 (with rounding in the fifth decimal place):
In the case of siderite, I measured the Fe K-alpha and Mn K-alpha intensities for 40 points, and for the usual reasons, we have uncertainty in the results (but beam damage was not an issue). The (Fe+Mn) concentrations range from 47.9 to 48.6 wt%, and the analytical totals from Probe-for-EPMA are reported between 99.7 and 100.4 wt%.
However, Probe-for-EPMA reports CO2-by-stoichiometry
as a function of concentration!Only when the concentration is extremely close to 100.00 wt% is the correct, stoichiometric, amount of CO2 reported.
So, in the case of MgO, regardless of analytical uncertainty, the stoichiometric ratio of 1:1 Mg:O is maintained.
But in the case of a carbonate, such as calcite CaCO3 or siderite (Fe,Mn)CO3, stoichiometry is
not maintained.
Rather, the proportion of CO2 reported by Probe-for-EPMA is a function of concentration.
To follow up on your above post:
"In short, different Ca concentrations are calculated for each different case, meaning that different amounts of stoichiometric oxygen are calculated for each composition, and hence different amounts of carbon are calculated for each composition. Only when the correct Ca is measured, does the correct stoichiometric oxygen get calculated, and subsequently only then does the correct carbon by stoichiometry to oxygen get calculated." Yes, in Probe-for-EPMA, different amounts of Ca result in different amounts of C and O being calculated.
Unfortunately, they are
not calculated in the stoichiometric ratio for CaCO3 where Ca : C : O should be 1:1:3.
The question is why do we see this behaviour?
We need to look into the matrix correction code more closely.
Best regards,
Andrew