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The Diaphragm Pump The unique property of the reciprocating diaphragm pump is that the actuating piston does not come into direct contact with the mobile phase and thus, the demands on the piston-cylinder seal are not so great. The diaphragm has a relatively high surface area and thus, the movement of the diaphragm is relatively small and consequently the pump can be operated at a fairly high frequency. High frequency pumping results in a very significant reduction in pulse amplitude and, in addition, high frequency pulses are more readily damped by the column system. Nevertheless, it must be emphasized that diaphragm pumps are not pulseless. A diagram of a diaphragm pump, showing its mode of action is depicted in figure 10   Figure 12. The Action of a Diaphragm Pump

The wheel driving the crank rotates in an anti-clockwise direction and in position (1) the diaphragm has been withdrawn causing the pumping cavity behind the diaphragm to be filled with solvent. In position (2), the piston advances and when it passes the pumping fluid inlet, it starts compressing the diaphragm expressing solvent to the column. In position (3) the diaphragm has been compressed to its limit and the piston starts to return. In position (4) the piston moves back withdrawing the diaphragm sucking liquid into the pumping cavity ready for the next thrust. The inlet from the solvent supply and the outlet to the column are fitted with non-return valves in the usual manner. Due to the large volume of the pumping cavity, any gradient profile would be seriously distorted so this type of pump is not often used for analytical purposes but is often