Liquid Chromatography - The LC Pump 2

It is seen in figure 4 that the total air pressure on the piston, diameter (y), is transferred to a piston controlling the liquid pressure, of diameter (x). Because the radii of the pistons differ, there will be a net pressure amplification of . For example, if the upper piston is 5 cm in diameter and the lower piston 1 cm in diameter, then the amplification factor will be . It is seen that the system can provide very high pressures in a relatively simple manner.

The pump operates in the following manner. Air enters port (1) and applies a pressure to the upper piston that is directly transmitted to the lower piston. If connected to a liquid chromatograph, or column packing manifold, liquid will flow out of the left hand side non-return valve as shown in the diagram which continues until the maximum movement of the piston is reached. At the extreme of movement a micro switch is activated and the air pressure transferred from port (1) to port (2).

The piston now moves in the opposite direction and draws mobile phase through the right hand non-return valve refilling the cylinder with solvent. Again on reaching the limit of the piston movement a second micro switches the air supply from port (2) back to port (1) and the process repeated. The refilling process is extremely fast and, if an appropriate pulse dampener is used, the outlet pressure remains reasonably constant during the refill cycle.