Abstract: An on-line gas chromatographic liquid inject valve which comprises (a) a sampling shaft having a channel defined therein which is reciprocal between a sampling position wherein a liquid stream flows across the shaft and through the channel and an inject position wherein the channel traverses a length of a seal and delivers a liquid sample carried from the liquid stream in the channel to a gas chromatograph for analysis, (b) a fluid-driven piston- or plunger-type arrangement at an outer end of the shaft for causing the sampling shaft to reciprocate between the sampling position and the inject position, (c) a seal around the sampling shaft and between the liquid stream and the gas chromatograph in use of the valve, and (d) a fluid-driven piston having a central opening defined therein in which the sampling shaft reciprocates, and which bears against the seal for placing and maintaining a constant pressure against the seal as the seal wears from frictional contact between the sampling shaft and the seal.