It is obvious that these pumps deliver a series of "pulses" of the mobile phase. Most detectors, but in particular the refractive index detector, are sensitive to flow "pulses" and for both trace analysis or good quantitation, it may be necessary to eliminate the pulsations. Several methods have been developed to accomplish this. The most simple involves placing a large coil of narrow-bore tubing in the line between the pump and the injector.
As the pump strokes, the coil flexes, absorbing the energy of the pulsations. This type of pulse damper holds a large amount of liquid which must be purged during solvent changes or when performing gradient elution. Another variation of this is a pressure gauge with a large Bourdon tube (usually of the follow-through type, to prevent trapping of liquid) which also flexes with each pulse. The most usual damper type is a membrane one, usually having low volume (less than 0.5 ml). The left part is filled with inert liquid (heptane). Compressibility of this liquid is enough to compensate for the pulsations of the dual piston pump with piston volume around 100 Ál.
Flow and pressure profiles for different types of pumps and cam shape.
The piston movement profile which is used for new pumps is shown above. First stage (1)
has a sharp slope to compensate liquid compressibility. The second stage (2) is the
solvent delivering step and it is the largest in the pumping cycle, the third stage (3) is
the filling procedure; this stage is about 1/50 of total cycle time. This type of cycle is
controlled by a microprocessor and driven by a stepping motor. Pulsation profile produced
by this pump is shown above. A small volume diaphragm damper combined with electronic
pressure gauge is more than enough to compensate for short term pressure peaks.