How Does the Pump Work?

In the quaternary pump, the liquid runs from the solvent reservoir through the degasser to the MCGV/4CGV and from there to the inlet valve.

For the isocratic pump, the solvent bottle is directly connected to the inlet valve.

The pump assembly comprises two substantially identical piston/chamber units. Both piston/chamber units comprise a ball-screw drive and a pump head with one sapphire piston for reciprocating movement in it.

A servo-controlled variable reluctance motor drives the two ball screw drives in opposite directions. The gears for the ball-screw drives have different circumferences (ratio 2:1) allowing the first piston to move at twice the speed of the second piston. The solvent enters the pump head close to the bottom limit and leaves it at its top. The outer diameter of the piston is smaller than the inner diameter of the pump head chamber allowing the solvent to fill the gap in between. The first piston has a stroke volume in the range of 20 – 100 µL depending on the flow rate. The microprocessor controls all flow rates in a range of 1 µL/min – 10 mL/min. The inlet of the first pumping unit is connected to the inlet valve.

The outlet of the first piston/chamber unit is connected through the outlet valve and the damping unit to the inlet of the second piston/chamber unit. The outlet of the purge valve assembly is then connected to the following chromatographic system.

When turned on, the pump runs through an initialization procedure to determine the upper dead position of the first piston. The first piston moves slowly upwards into the mechanical stop of the pump chamber and from there it moves back for a defined distance. The controller stores this piston position in memory. After this initialization the pump starts operation with the set parameters. The inlet valve opens and the down-moving piston draws solvent into the first pump chamber. At the same time, the second piston moves upwards delivering to the system. After a controller-defined stroke length that depends on the flow rate the drive motor is stopped and the inlet valve closes. The motor direction is reversed and moves the first piston up until it reaches the stored upper limit and at the same time the second piston moves downwards. Then the sequence starts again moving the pistons up and down between the two limits. During the up movement of the first piston, the solvent in the pump chamber is pressed through the outlet valve into the second pump chamber. The second piston draws in half of the volume displaced by the first piston and the remaining half volume is directly delivered to the system. During the drawing stroke of the first piston, the second piston delivers the drawn volume to the system.

Quaternary pumps: For solvent compositions from the solvent bottles A, B, C, D the controller divides the length of the intake stroke in certain fractions in which the gradient valve connects the specified solvent channel to the pump input.

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