Enhance internal standard and add external standard quantification #12
Comments
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Note relationship to #11 |
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With the external standard, I don't know if the K factor should only appear in the diffraction pattern of the standard, and you get the value by following the links, or if it should be in both. When I set up refinements in TOPAS, I calc it once in the std, and reference that value in all the unknowns; I don't copy/past the value. |
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Maybe it could be considered to be a _pd_instr.-type value? |
Internal standardNew data names:
Ammended descriptions of existing data names:
External standardNew data names:
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This was the sort of thing that I was reluctant to put in without a formal definition, since I am certain that the value is not transferable between codes. I guess if the software package and version is defined elsewhere then this becomes qualified, but I’d reference those data items in the definition.
Note that it is a histogram property. Definitely not tied to phase. Phase fractions are a property of phase and histogram, like some of the other things we have been discussing such as texture and microstrain.
On Oct 12, 2022, at 10:21 AM, Matthew Rowles ***@***.******@***.***>> wrote:
I would also like to be able to record the Rietveld scale factor (or any other scaling factor used to make a phase fit some data)
for a phase.
Does _pd_phase.scale_factor sound OK?
Is PD_PHASE the place to put that? given that it is a joint property of the phase and the diffractogram? But then again, so is _pd_phase.mass_percent - you can have different weight fractions of the same phase in different diffractograms.
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Yes, you're right. And potentially even not transferable between different versions of different input files, as it all depends on how you scale data. eg do you include the constant in the denominator of Lp? scratch that thought for the moment. |
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I'm confused by the description of
Is this refering to a block containing a histogram or a phase? |
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I would have envisioned a standard as being a single phase material so this would point to a block that could contain a dataset with phase as refinement information. I guess for something more complex, the block could summarize the calibration process where there could be links to other blocks. |
Hi all
Some ideas on reporting quantitative phases analysis from the internal and external standard approaches.
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For quantitative phase analysis (QPA), there are two predominant whole-pattern approaches utilising crystal structures: internal and external standard.
Other methods include RIR, DDM, and single peak methods, but these won't be considered further.
Current data names for QPA consist of
_pd_phase.mass_percent,_pd_calib.std_internal_mass_percent, and_pd_calib.std_internal_name. Other data names currently being added to the dictionary which are of relevance are_pd_char.mass_atten_coef_mu_calcand_pd_char.mass_atten_coef_mu_obsInternal standard
The most applied quantification methodology uses the Rietveld approach and Hill & Howard algorithm.
In this method, the relative mass fraction of a phase is given as
where$s$ is the Rietveld scale factor, $M$ is the mass of the unit cell, and $V$ is the volume of the unit cell. The sum in the denominator is made over all phases present in the specimen, including $a$ .
This approach gives relative mass fractions, as the values are normalised to 100%. If the absolute mass fractions are needed, then an internal standard can be used.
A known amount of a crystalline phase, which doesn't exist in the sample, is added and mixed well. In the analysis procedure, the absolute mass fractions are given as
where the relative mass fractions are corrected by the ratio of the known addition of internal standard, and the calculated relative mass fraction of the internal standard.
The data names, and some definitions, currently in the powder dictionary are just not enough to capture all of the required information. Two additional data names are proposed:
_pd_calib.std_internal_block_id: to link to the block id of the structure used to model the internal std._pd_phase.scale_factor: to record the Rietveld scale factor of that phase used in modelling a diffraction pattern.The definition of
_pd_phase.mass_percentshould be expanded to add:The definition of
_pd_calib.std_internal_mass_percentis incorrect, and also needs expansion:eg add 1 g std to 2 g sample = 33% std in the final specimen. It doesn't matter if the standard is only 90% crystalline. The answer (in this case) is always 33%.
The enumeration of the difference between the total mass added of internal standard and the crystalline mass of internal std added is necessary for the calculation of mass fractions in the original sample, before the std was added.
An example CIF file would look like this:
External standard
Just after the publication of the Hill & Howard algorithm, O'Connor and Raven published their external standard approach to quantification. This method is superior, as it inherently gives absolute quantification, rather than relative.
In this method, the mass fraction of a given phases is given by
$$ w^\mathrm{abs}_a = \frac{(sMV)a \mu{\mathrm{specimen}}^* } {K} $$
where$\mu_{\mathrm{specimen}}^*$ is the mass absorption coefficient of the entire specimen, and $K$ is a diffractometer constant. $\mu_{\mathrm{specimen}}^*$ can be measured, calculated from elemental analysis, or calculated from a Rietveld analysis (which assumes that the amorphous and crystalline fractions are of the same composition, and that the composition of the crystal structures used match those of the actual crystals in the specimen). $K$ must be measured from a well-characterised standard material using exactly the same instrument conditions as the unknown specimen. It is particular concern that beam intensity is consistent.
Two new data names are proposed:
_pd_calib_std.external_k_factor: this is used to record the K factor used in the analysis of a diffraction pattern._pd_calib_std.k_factor: this is used to report the K factor calculated from a standard pattern.An example CIF file would look like this:
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