I agree with Ed's suggestion - the folks on that forum are very helpful and
very insightful with regards to small-angle scattering.
Some thoughts to offer:
Mark - it's hard to evaluate from the Primus screen shots, simply because
Primus is not rendering the experimental noise. I'd suggesting plotting
your data out on a log-log plot in Origin or via the Igor Pro macros if this
data is from an American beamline (NIST, ORNL) and using those renderings to
evaluate. Rendering the experimental noise is important, as in those middle
D2O concentrations you're going to have a considerable amount of incoherent
scatter contributing to your profiles.
In your first 30% data shown, the discrepancy is concerning. Check the
reduction parameters and make sure the correct correction files were used
for that particular sample to detector distance. The rendering of the
experimental noise would be helpful in evaluating.
In your 50% and 70% data, the experimental noise would be helpful to
evaluate, but on the whole they seem pretty good. The discrepancy seen at
middle Q in this profile (http://postimg.org/image/m358pazb7/) is a little
concerning though.
I'm concerned with the first set of 90% data you show, assuming the
incoherent scatter is very low at that concentration of D20 and
signal-to-noise is high. There seems to be a discrepancy in the middle Q
regime (~0.15 Q) Again, might be worth double-checking the correction files
used for reduction.
HTH,
Kushol
Kushol Gupta, Ph.D.
Research Associate - Van Duyne Laboratory
Perelman School of Medicine
University of Pennsylvania
BLOCKED::mailto:kgu...@stwing.upenn.edu kgu...@mail.med.upenn.edu
215-573-7260 / 267-259-0082
From: CCP4 bulletin board [mailto:CCP4BB@JISCMAIL.AC.UK] On Behalf Of Ed
Pozharski
Sent: Wednesday, August 07, 2013 11:54 AM
To: CCP4BB@JISCMAIL.AC.UK
Subject: Re: [ccp4bb] Problems with SANS data analysis
This question may be better suited for more small-angle-oriented forum, e.g.
http://www.saxier.org/forum/
On 08/07/2013 11:22 AM, Remec, Mark wrote:
Dear CCP4bb,
I have a few questions concerning SANS data recently collected that I'm
having trouble analyzing. The data was collected at 2 different detector
distances (4m, 2.5m) to achieve higher q-range, but I worry that the curves
don't overlap enough at intermediate q, which might indicate a problem with
the data. The links below are pictures of the corresponding datasets, before
truncating the 4m high-q data and merging them into one. Is there a problem
evident with the data, or am I imagining a problem?
http://postimg.org/image/qb00y20qr/
http://postimg.org/image/8trbp7akj/
http://postimg.org/image/hni86axj7/
http://postimg.org/image/3sjxnu343/
http://postimg.org/image/4ysj0dgsj/
http://postimg.org/image/9ypz8bmf7/
http://postimg.org/image/m358pazb7/
http://postimg.org/image/jzuthmzib/
My second question concerns the values obtained in the analysis of the final
scattering curves. The second sample in my experiment shows serious
deviation in the values obtained for I(0) and Rg by Guinier analysis
compared to the values obtained by the P(r) analysis. In other words, either
the P(r) values match the Guinier and the P(r) fit is terrible, or else the
P(r) fit is good but doesn't match the Guinier at all (5-10 difference in
Rg, 2x difference in I(0)). I've checked to make sure the buffer subtraction
algorithm was OK, and I'm pretty certain that the buffers were exact
matches, so I don't know how to explain this variation. There's no evidence
of aggregation or polydispersity to throw off the values, either. Does
anyone know how this can happen?
--
Oh, suddenly throwing a giraffe into a volcano to make water is crazy?
Julian, King of Lemurs
