Fluid separation with sampling unit selectively coupling upstream and downstream of separation unit

A method for processing fluid, the method comprising:

• providing a fluid processing apparatus comprising:
• a fluid separation apparatus comprising a fluid pump configured to drive a mobile phase, and a separation unit configured to separate a fluidic sample when comprised within the mobile phase;
• a sampling unit comprising a modulation buffering unit and a modulation drive, wherein the modulation drive comprises an active metering unit and is configured to introduce fluid into the modulation buffering unit;
• and
• a switching unit configured to switch to a first switching state and to a second switching state;
• in the first switching state, introducing fluid into the modulation buffering unit from downstream of the separation unit by operating the modulation drive;
• and
• in a second switching state of the switching unit, introducing fluid buffered in the modulation buffering unit into the mobile phase in a flow path between the fluid pump and the separation unit for separating the introduced fluid by the separation unit.