Sample injector with metering device balancing pressure differences in an intermediate valve state

A sample injector for use in a fluid separation system for separating compounds of a fluidic sample in a mobile phase, the sample injector comprising:

• a switchable valve;
• a sample loop in fluid communication with the valve and configured for receiving the fluidic sample;
• a metering device positioned so as to form a part of the sample loop and configured for introducing a metered amount of the fluidic sample into a portion of the sample loop;
• and
• a control unit configured for controlling the valve to switch among a first position, a second position and an intermediate position in order to transfer the sample loop between a low pressure corresponding to the first position of the valve and a high pressure corresponding to the second position of the valve,
• the control unit being further configured for controlling the metering device to vary the pressure in the sample loop, while the valve is in the intermediate position, between the low pressure for loading the fluidic sample and the high pressure for driving the mobile phase,
• wherein the metering device and the sample loop are in fluid communication in each of the first position and the second position of the valve.