The short-stroke piston pump

Practical High-Performance J.iquid Chromatography, Fifth edition Veronika R. Meyer 

The pumping action comes from a stepper motor which drives a rotating disk or cam. The cam then guides the piston to its forward and backward direction. With each stroke, the piston conveys a small amount of liquid, usually in the 100 pi range. The check valves are built in such an asymmetric way that they close when pressure comes from the top and open when the pressure at their bottom is higher than at the top. The 

With a circle-shaped cam and a uniformly working stepper motor the outlet of the pump would be discontinuous and not smooth  

In order to obtain as smooth a flow as possible, the cam has an irregular shape and is rotated according to a well designed time program depending on its actual position. During the delivery stroke of the piston the cam turns slowly its shape guarantees a 

The cam can be replaced by an electronically controlled linear spindle. Optimum flow constancy is then obtained by a perfectly matching software. Such systems, however, are more expensive than conventional ones. 

Practical High-Performance liquid Chmmalography, Fourth edition Veronika R. Meyer 2004 John Wiley Sons, Ltd ISBN 0-470-09377-3 (Hardback) 0-470-09378-1 (Paperback) 

The flow can be smoothed by using two pistons which work parallel but in opposite directions. Another design is the use of tandem pistons arranged in series, whereby the first one conveys slightly more than the second. This allows to compensate for mobile phase compressibility and helps to suppress cavitation, i.e. the formation of vapour bubbles during the filling stroke. Some pumps use double-ball check valves with a second seat and ball on top of the first one. The electronics of the pumps are also designed to compensate for the compressibility of the eluent (which fortunately is low, e.g. 1% per 100 bar). 

The valve balls are from ruby, the seats from sapphire. In its seat, the ball seals only through the thin ring where the two parts are in contact. If a particle of dust or a buffer crystal is trapped there, the valve will be leaky. This is one of the reasons why mobile phases must rigorously be kept particle-free. 

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