When drying solids in the desiccator, often the apparatus is evacuated. This procedure is useful to favor evaporation of volatile substances like water by lowering the pressure inside the desiccator. For the evacuation, a membrane pump should be used. A water aspirator as vacuum ejector will evacuate the vessel only to the vapor pressure of water at the given temperature and fill the atmosphere inside the desiccator with water vapor, which has to be removed while drying too. The use of oil-based pumps, like a rotary vane pump, will lead to an oil vapor inside the desiccator consisting of many volatile organic compounds (VOC). Some desiccants release water on standing in a low-pressure atmosphere, so they are not suitable when evacuating the desiccator.