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

V

_{P} = 2π

^{2}I(0)/Q

where

Q = ∫s

^{2}[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. s

_{min}). 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.