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
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.
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.