A container assembly for use with a high-pressure liquid chromatography (HPLC) instrument is disclosed, in which the container assembly, when coupled to a source of pressurized gas, provides fluid medium to the HPLC instrument at positive pressure. The container assembly has an external exterior container shell, an internal fluid container for holding fluid medium, an interstitial volume between the external exterior container shell and the internal fluid container, a port for fluidly connecting the volume to a pressurized gas source, and a port for fluidly connecting the internal fluid container to the HPLC instrument. As a pressurized gas in the interstitial volume increases, fluid medium flows out of the port connected to the internal fluid bag and container assembly at a positive pressure. A system incorporating the container assembly, and method of use of the same, are also disclosed.