The trapping pumps remove gas molecules by sorption or condensation on its internal surfaces (see Figure 6). If the gas molecules chemically react with the internal material of the pump, the new material created by the reaction of gas and inner material molecules will be deposited as a thin film. This is called sorption of the gas molecules. Furthermore, if the gas molecules come in touch with the refrigerated surfaces of the pump, gas will be condensed and removed as a liquid.