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 Post subject: Porod Volume and CalculationPosted: 2009.03.28 10:02
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Joined: 2008.02.29 03:29
Posts: 27
Location: USA
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 calculationPosted: 2011.11.01 15:54

Joined: 2007.08.03 18:55
Posts: 535
Location: EMBL Hamburg, Germany
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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 Post subject: Re: Porod Volume and CalculationPosted: 2014.02.13 15:34
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Joined: 2014.01.23 18:04
Posts: 5
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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 Post subject: Re: Porod Volume and CalculationPosted: 2014.02.20 10:38

Joined: 2007.08.03 18:55
Posts: 535
Location: EMBL Hamburg, Germany
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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