Dionex AXP/AXP-MS Manual
4 THEORY OF OPERATION
4.1 Mechanical Operation
4.1.1 Liquid System Flow Path
The flow path of the Dionex AXP/AXP-MS pump starts at the inlet
reservoir filter, passes through the inlet check valve, then through the
pump head, and finally exits through the outlet check valve.
4.1.2 Pump Cycle
The pump cycle consists of two phases: the pumping phase, when
fluid is metered out of the pump at high pressure, and the refill phase,
when fluid is rapidly drawn into the pump.
During the pumping phase, the pump piston moves forward at a
programmed speed; this results in a stable flow from the pump. The
piston is driven by an eccentric bearing which is directly driven by the
motor.
At the end of the pumping phase, the pump enters the refill phase. The
piston quickly retracts, refilling the pump head with solvent, and the
piston begins to move forward again as the pumping phase begins. The
motor speed is increased during refill to reduce refill time and to pre-
compress the solvent at the beginning of the pumping phase.
For optimal operation of the check valves, a backpressure of at least 25
psi is required. Operating at lower pressures can lead to improper
seating of the valves and cause inaccurate flow rates.
4.1.3 Pulse Damping
The diaphragm-type pulse damper consists of a compressible fluid
(isopropanol) held in an isolated cavity by an inert but flexible
diaphragm. During the pumping phase of the pump cycle, the fluid
pressure of the mobile phase displaces the diaphragm, compressing the
fluid in the cavity and storing energy. During the pump refill phase,
the pressure on the diaphragm is reduced and the compressed fluid
expands, releasing the energy it has stored. This helps to stabilize flow
rate and pressure. The amount of mobile phase in contact with the
pulse damper is small, only 0.9 mL at 2500 psi, and the geometry used
ensures that the flow path is completely swept, so solvent “memory
effects” are virtually eliminated.
14
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