Hi Jose
Errors in the region of 1wt% aren't unheard of in complex mixtures (and the
liklihood of the second decimal place being meaningful is very slim). However
there are other possible explanations and one in particular comes to mind.
Did you micronize your sample? If yes then feel free to ignore the following!
If not then it might be worth thinking about.
We did a systematic study on the effect of particle statistics on the
repeatability of quantitative analysis on one of the NIST cements last year
(presented at the ICCC in Montreal - should really go somewhere more
accessible). We compared different divergence slits, micronized and
unmicronized samples, spun and static, and remounted/repeated the experiments 3
times each for real statistics as opposed to esds.
Funnily enough the phase that showed the biggest anomaly in one of the unground
sample was the periclase - obviously a big (by XRD standards) lump fell into
the sample from the kiln that made up the NIST batch and made it through the
mill at the plant. Without grinding it we got 0.1, 0.1 and 2wt% one series of
3, versus 0.6, 0.6 and 0.6 wt% (this is from memory so don't don't quote me!)
for the micronized sample.
This was the result that we should have got theoretically but it was nice to
see it work in practice, and the periclase did such a nice job for us by
sticking out like a sore thumb even without a Rietveld analysis! BTW there was
no statistically significant difference (95% limits) in the results of the C3S
and C2S phases between ground and unground samples - mostly because the errors
in the unground samples were so large! The minor phases were another matter.
In the micronized samples the choice of divergence slit size made little
difference, but with the unground samples they made a significant difference.
To cut a long story short it all comes down to particle statistics - with an
ideal powder the choice of fixed versus variable slits (or even different fixed
slits) should make no difference (assuming that the Rietveld software can
handle the difference in profiles). Moral of the story - sample prep, sample
prep and more sample prep!
Pam
-----Original Message-----
From: José Carlos Cordeiro [mailto:[EMAIL PROTECTED]
Sent: May 29, 2008 3:53 PM
To: rietveld_l@ill.fr
Subject: ADS
I have a Programmable Divergence Slit (incident beam) in my
difratctometer, that can work in automatic mode (irradiated length) or fixed
mode (0.5 degree or 1.0 degree etc). What is the best collect mode for Rietveld
Refinament??
I did refinement two difractograms collected in booth configuration
(fixed 0.5 degree and automatic mode), the results in the fase "Periclase"
(cement fase) changed, see below:
Periclase with 0.5 degree = 6.08%
Periclase wich automatic mode = 7.14%
Note.: The refinement of the difractograms collected with automatic
mode went converted to fixed mode by software.
What do you think about this?
regards
===========================================
José Carlos Cordeiro
Diretoria Técnica - Laboratório Central
Centro Técnico - Curitiba
Rodovia PR-092, 1303, Abranches, 82130-570 Curitiba-PR
Votorantim Cimentos
[EMAIL PROTECTED] <mailto:[EMAIL PROTECTED]>
www.votorantimcimentos.com.br
Fone: + 55 041 3355-1380
Fax: + 55 041 3355-1358
===========================================
