Before the start of the injection sequence, and during an analysis, the injection valve is in the mainpass (main path) position (see Mainpass (main path) Position - standard position during runs and when the sampler is idle). In this position, the mobile phase flows through the autosampler’s metering device, sample loop, and needle. This ensures that all parts in contact with the sample are flushed during the run, thus minimizing carryover.
When the sample sequence begins, the valve unit switches to the bypass position (see Valve in bypass - needle in needle seat, metering device ejects the excess liquid). The solvent from the pump enters the valve unit at port 1 and flows directly to the column through port 6. Then, the metering piston returns to its home position, ejecting the excess liquid into the waste through port 4.
Next, the vial is positioned by the gripper arm below the needle. The needle moves down into the vial, and the metering unit draws the required sample volume into the loop (see Valve in bypass - needle in vial, metering device aspirates sample volume). Then, the needle lifts, and the metering unit draws a 0.7 µL air plug into the needle tip to prevent sample loss during needle washing.
After drawing the sample, the vial is returned to the sample tray. Then, the needle moves into the wash port, where a fresh stream of wash solvent rinses its outer surface (see Outer face of needle getting washed in wash port).
After washing, the needle leaves the wash port and moves into the needle seat. Finally, the injection valve switches back to the mainpass (main path) position, flushing the sample onto the column (see Valve switches to mainpass (main path) - sample is transferred towards the LC column).
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