We have devised an apparatus useful for and a method of removing impurities from vaporous precursor compositions used to generate reactive precursor vapors from which thin films/layers are formed under sub-atmospheric conditions. The method is particularly useful when the layer deposition apparatus provides for precise addition of quantities of different combinations of reactants during a single step or when there are a number of different individual steps in the layer formation process, where the presence of impurities has a significant affect on both the quantity of reactants being charged and the overall composition of the reactant mixture from which the layer is deposited. The method is particularly useful when the vapor pressure of a liquid reactive precursor is less than about 250 Torr at atmospheric pressure.