Abstract: Disclosed is a septum-mounting and shielding assembly for a sample injector of the type utilized with gas chromatographs. The arrangement comprises an adapter flange designed for mounting on the inlet end of the sample injector block and containing a small diameter aperture through which a sample injection probe such as a hypodermic needle can be inserted. A septum holder is mounted for rotation relative to the adapter flange member about an axis eccentric to the aperture in the adapter flange. The septum holder contains a small diameter aperture which is eccentric relative to the axis of rotation, the degree of eccentricity being the same as that between the aperture and the adapter flange and the axis of rotation. Consequently, the two apertures are in registration in only one relative angular position of the septum holder. A silicone rubber septum is mounted in the septum holder coaxially with the aperture therein.