The precise concentration of each of the standards was checked periodically by its extinction coefficient. Hundred micro litre of each of the stock standard solutions of a-tocopherol, d-tocopherol, c-tocopherol, cholecalciferol and ergocalciferol was taken and evaporated with N2, and in each case the residue was redissolved with 10 mL of the appropriate solvent for subsequent spectrophotometric measurement after filtration of the solution [30]. The concentration of each standard was calculated by applying Eq. 1.