Abstract: A funnel-shaped monolithic low-density polyethylene sample-vial septum comprises a flange, a capture wall, a diffusion-barrier wall, and a membrane. The capture wall is conical, extending from its truncated apex at the diffusion barrier wall to its mouth about which the flange is disposed. The membrane is disposed at the end of the diffusion-barrier wall away from the capture wall. This structure defines a septum aperture, including a conical capture section, defined by the capture wall, and a cylindrical diffusion-barrier section, defined by the diffusion-barrier wall. The 7.0 mm length of the diffusion-barrier section is 12/mm times the square of its diameter 0.76 mm. The diameter of the diffusion-barrier section which is selected to be 0.05 mm greater than the diameter of the largest needle to be used with the septum, i.e., a 22 gauge needle typically used for liquid chromatography. The minimum thickness of the membrane is 0.