Liquid Chromatography - The Rapid Refill Pump
Most pistons of modern LC pumps are made of synthetic sapphire to reduce wear and extend the working life of the pump. The cylinder is usually made of stainless steel and is attached to two non-return valves in line with the inlet and outlet connections to the pump. The piston is driven by a stainless steel cam which forces the piston into the cylinder expressing the solvent through the exit non-return valve. After reaching the maximum movement, the piston follows the cam and returns as a result of the pressure exerted by the return spring. During this movement the cylinder is loaded with more solvent through the inlet non-return valve. The shape of the cam is cut to provide a linear movement of the piston during expression of the solvent but a sudden return movement on the refill stroke. In this way the pulse effect that results from the refill action is reduced. The pulses, however, are not completely eliminated and the detector noise resulting from these pulses is probably the most serious disadvantage of the single piston pump. Nevertheless, as a result of its low cost it remains one of the more popular LC pumps.
The Rapid Refill Pump
In order to avoid the refill pulses resulting from a single piston pump, a number of rapid refill systems have been developed. The designs have ranged from cleverly designed actuating cams to drive the piston rapidly in the refill mode to electronically operated piston movements. One successful approach to this problem is exemplified by the pump design shown in figure 8.
Courtesy of Perkin Elmer Inc.
Figure 8. The Rapid Refill Pump