Sigma

In equilibrium sedimentation experiments, the reduced apparent molecular weight. This is the term which is directly measured by experiment; often it is fit by nonlinear least square curve fitting (NONLIN computer program or equivalent). In samples not consisting of a single monomer of one solute, the σ's of the various components may be fit from the single concentration gradient. To find the molecular weight from σ, it is necessary to know the density of the solution, the partial specific volume of the solvent, and the conditions of the experiment (temperature and rotor speed).