The sample preparation device includes a vessel, such as a sample tube, having a bottom end with a bottom opening. A valve for control of the flow of any liquid outwardly of the vessel through the bottom opening is located at the bottom end of the vessel. The valve includes a deflectable membrane positioned against the bottom opening to close the bottom opening when the valve is in its normally closed condition. The membrane is deflectable away from the bottom opening when the valve is in an open condition to permit any liquid in the vessel to flow through the bottom opening to an outlet port of the valve for dispensation into a suitable collection vessel.