 small particles in liquid suspension, the only contrast leading to scattering in the typical range of resolution of the SAS is simply Δρ, the difference in "average" scattering length density between the particle and the surrounding liquid, because variations in ρ due to the atomic structure only become visible at higher angles.This means that the total integrated intensity of the SAS pattern (in 3D) is an invariant quantity proportional to the square Δρ2.In 1-dimensional projection, as usually recorded for an isotropic pattern this invariant quantity becomes formula_4, where the integral runs from q=0 to wherever the SAS pattern is assumed to end and the diffraction pattern starts.