QPA precision with TOPAS

nprietveld

Dear All,
I am planning to perform some QPA Rietveld studies. To evaluate issues and optimize the study I decided to start making up a mixture of three NIST SRMs, namely 674a zincite (32.24 % weight), 674a rutile (35.80%), 676 corundum (31.96%). With a TOPAS refinement I obtain: zincite 30.9%, rutile 36.5%, corundum 35.5% (Rwp 2.7%). So, accuracy of these estimates seems me acceptable. I am a bit confused about the esds obtained (do_errors) with TOPAS on these quantities: zincite weight_percent 30.945`_20.431, rutile weight_percent 36.537`_22.142, corundum 32.518`_20.963, an esd of the same order of magnitude of the measured abundance seem me unexpectedly high. Does anyone have an explanation or suggestions on this? The system I plan to study shall include a relevant amorphous content and zeolites as major and minor phases. How to manage to get in this case a realistic
estimate of the precision of my weight % measurements?
Any reference/suggestion is warmy welcome!

alancoelho

Hi Natale

I didn't previously notice the large error in your e-mail; errors for QUANT in a sample you have described shoudl be n the order of 1 to 2%. If you were to send me your INP and data file then I will have a look at what the problem is.
cheers
alan

alancoelho

The large errors seen is due to the use of PVII peak types and low exponents.

PVII peak types can have very long tails with a large amount of intensity distributed in the tails when the exponent is small (ie. 0.7).

nprietveld

Thank you Alan for your reply! As I told in my preceeding message, the system I plan to study include a relevant amorphous content and zeolites as major and minor phases. Anybody willing to share their experience on QPA with TOPAS? Which are literature studies you consider relevant on this topic?
Natale

johnsoevans

The round robin quant data from Nikki and Ian are in one of the tutorials on my webpage. The simple analysis there gives much lower uncertainties. I think there are links to the papers.

rowlesmr

Yep, no issues at all doing quant.

I second the notion of having a go at Ian's and Nikki's round robin data. THey've gat data for simple ternary mixtures, amorphous content, preferred orientation, and microabsorption.

On the software side, one nice thing you can do is use elemental_composition. This will give you an overall elemental composition of your specimen based on your quant results and the composition of the structures you used to model them.

You can use spike phases to determine amorphous content, and the external standard approach is easy to implement too.

It's the duck's nuts.

nprietveld

Matthew, I appreciate your optimism, but for me it is not always the same. Some time ago, I have been trying with a granite, made up by Na-feldspar, K-feldspar, quartz, white and black mica. I measured the bulk XRF composition, and the composition of each phase by SEM/EDS, so I easily got the phase abundance in the rock. As it was time consuming, I tried with Rietveld, but notwithstanding a nice fit I was not able to reproduce the above results. I always obtained an overestimation of quartz. I guess mainly because I was not able to manually grind all the componenents to the same grain size. And I think it is almost impossible, given the physical features of the involved phases. What would you do in this case?

rowlesmr

You're now talking about accuracy; that's a whole other ball game.

I don't know if you can make it to the next TOPAS user meeting, but Ian Madsen and Nikki Scarlett are on the speaker list, talking about microabsoprtion and data collection strategies for QPA.

You might be running into issues around microabsorption. What are the LACs of your various phases? You could be having issues of contrast between the various phases. There is no real fix for this, apart from trying different x-ray energies to minimise contrast, or to use neutrons.

You mention grinding; How are you grinding? By hand? With a microniser? Are your intensities repeatable? Try running a specimen, then tip out the powder from the holder, give it a light grind by hand to break up any agglomerates, reload the same powder, and run it again. If you do this, and your intensities aren't repeatable, you probably have a particle statistics issue, and need to grind better, or otherwise get more particles in the beam.

Matthew