Constant-flow systems are generally of two basic types: reciprocating piston and positive displacement (syringe) pumps. The basic advantage of both systems are their ability to repeat elution volume and peak area, regardless of viscosity changes or column blockage, up to the pressure limit of the pump.
Reciprocating piston pump can maintain a liquid flow for indefinitely long time, while
a syringe pump has to be refilled after it displaces the whole syringe volume. On the
other hand, a syringe pump does not have any flow and pressure pulsations compared to the
reciprocating pump. For the micro-HPLC applications a syringe pump allows for the
maintaining of a constant flow at the microliter per minute flow rate range.