Metering device with defined enabled flow direction

A method of operating a sample injector to introduce a sample fluid into a mobile phase, wherein the mobile phase is to be driven by a mobile phase drive through a separation unit for separating compounds of the sample fluid in the mobile phase, wherein the method comprises:

• switching an injector valve into a sample intake mode in which a metering device for displacing fluid is operated to intake a metered amount of a sample fluid from a sample container into the sample injector;
• subsequently switching the injector valve into a separation mode in which the intaken sample fluid is driven by the mobile phase drive for separating the compounds by the separation unit;
• at least during the sample intake mode, providing a flow of fluid displaced by the metering device along a defined flow direction and disabling a flow of fluid in an opposite flow direction, wherein
• the injector valve switches to operate the sample injector selectively in:
• a first mode, in which the first end of the sample flow line is coupled to the inlet of the metering device, the second end of the sample flow line is coupled to the outlet of the metering device, and the mobile phase drive is coupled to the separation unit;
• and
• a second mode, in which the first end of the sample flow line is coupled to the mobile phase drive and the second end of the sample flow line is coupled to the separation unit.