How is membrane selectivity calculated for gas separation?

Selectivity (alpha) for gas separation is the ratio of permeabilities: alpha_A/B = P_A/P_B, where permeability P = D x S (diffusivity x solubility). For polymeric membranes, selectivity arises from differences in both molecular size (affecting diffusivity) and condensability (affecting solubility). There is typically a trade-off between selectivity and permeability - highly selective membranes often have lower flux. Selectivity determines product purity and recovery achievable.