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PostPosted: 2009.03.28 10:02 
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Does anyone know the exact calculation/formula used by Primus to calculate the Porod volume? Why does it depend on the scattering range? If it uses the Porod Invariant, then its a numerical integration and the Porod volume, Vp is then 2*Pi*I(0)/Q.

Any thoughts?

RPR


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 Post subject: Porod volume calculation
PostPosted: 2011.11.01 15:54 
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rambor wrote:
Does anyone know the exact calculation/formula used by Primus to calculate the Porod volume? ... If it uses the Porod Invariant, then its a numerical integration and the Porod volume, Vp is then 2*Pi*I(0)/Q.

According to the PRIMUS paper (Konarev et al. 2003, J. Appl. Cryst. 36, p. 1279) the Porod volume is calculated as
VP = 2π2I(0)/Q
where
Q = ∫s2[I(s) - K]ds
where K is a constant determined to ensure the asymptotical intensity decay proportional to s-4 at higher angles, see Glatter & Kratky 1982, p. 30.

rambor wrote:
Why does it depend on the scattering range?

I guess the data is extrapolated to I(s=0) using the Guinier approximation which will depend on the chosen nBeg (i.e. smin). The constant K will also depend on the chosen scattering range.
See the AUTOPOROD paper (Petoukhov et al. 2007 Automated SAS data analysis. J. Appl. Cryst. 40, p. s225) for details.


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PostPosted: 2014.02.13 15:34 
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Hi,

related to the porod volume calculation;
due to contribution of internal structures at high q, there might be error in calculation of volume. or may be even the noisy data at high q adds additional error to the volume calculation.
for my data, Porod volume changes significantly by changing the q range. Is there an empirical maximum q range for volume calculation?


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PostPosted: 2014.02.20 10:38 
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saminakbari wrote:
Is there an empirical maximum q range for volume calculation?
Yes, datporod uses qmax = 7.5/Rg. This removes the unwanted scattering contribution from the internal structure. See Petoukhov et al. 2012, J. Appl. Cryst. 45, 344 for details.


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