EOM fits to a slightly different data profile

Linear (OLIGOMER), and non-linear (MIXTURE) analysis, singular value decomposition (SVDPLOT), addition of missing fragments (BUNCH, CORAL), analysis of flexible systems (EOM/RANCH & GAJOE), flexible refinement of high-resolution models (SREFLEX)
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papatrobonis
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EOM fits to a slightly different data profile

#1 Post by papatrobonis » 2017.10.31 18:32

Hello
Thank you so much for all your amazing job concerning SAXS. So great for the community.
I have a question...
The output of EOM fit (I used EOM online for that) gives me a data profile slightly different from the original data file input to the EOM interface.

I show the differece In the figure :
BLUE spectrum : EOM data profile output
GREEN spectrum : the original data profile
RED: EOM fit !!! very good indeed but I have to understand what is happening
Sélection_130.png
Sélection_130.png (23.67 KiB) Viewed 3836 times
My question is WHY ? What does exactly EOM to the data ?

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AL
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Constant subtraction

#2 Post by AL » 2017.11.02 12:41

EOM (well, actually CRYSOL) subtracts a small constant to compensate imperfect background subtraction. In your case the constant was negative.
Make sure next time you remove the very low angles that don't follow the Guinier law from your data.

papatrobonis
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Re: EOM fits to a slightly different data profile

#3 Post by papatrobonis » 2017.11.04 01:28

Thank You very much for your answer
However my question remains...if the substracted background is constant then why the original data are superimposed within the low resolution region and then mismatch in the upper resolution region. It seems like a linear function y=a+bx is substracted ruther than a constant y=a

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AL
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log plot vs. linear plot

#4 Post by AL » 2017.11.06 11:46

papatrobonis wrote:...if the substracted background is constant then why the original data are superimposed within the low resolution region and then mismatch in the upper resolution region.
Because you are looking at it on a logarithmic scale. Plot it on a linear scale and it will be obvious that it is just a constant.

papatrobonis
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Re: EOM fits to a slightly different data profile

#5 Post by papatrobonis » 2017.11.07 01:29

thank you so much...Obviously, I have the same problems my students have even after 27 years on pure experimental physics (i am not biologist) and physics teacher too ! I just forgot that I look the lnI !!!
and THANKS for ATSAS online !!!! Your servise is really great

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