Liquid Chromatography - The Twin-Headed Pump 1
The pump consists of two cylinders and a single common piston. The expression of the solvent to the column is shown in the upper part of figure 8. As the piston progresses to the right, solvent is displaced to the column system and, simultaneously, fresh solvent is withdrawn from the solvent reservoir into the right hand chamber. When the piston arrives at the extent of its travel, a step in the driving cam is reached and the piston is very rapidly reversed. As a result the contents of the chamber on the right-hand side are displaced into the left-hand chamber. This situation is shown in the lower part of figure 8. The transfer rate of the solvent to the left-hand chamber is 100 times faster than the delivery rate to the column and consequently reduces the refill-pulse very significantly. In addition, if a solvent gradient is being used and the right-hand chamber is being filled with a solvent mixture, excellent mixing is achieved during the refill of the left-hand chamber. It is clear, however, that there will not be a smooth transition from one solvent concentration to the next but will be a step-wise change.
An alternative approach to the elimination or reduction of pump pulses and one which is probably the more successful (though more expensive) is the use of twin pump heads. During the operation of a two-headed pump, one cylinder is filled while the other is delivering solvent to the column.
The Twin-Headed Pump.
The cylinders and pistons of a two-headed pump are constructed in the same manner to the single piston pump with sapphire pistons and stainless steel cylinders fitted with non-return valves to both the inlet and outlet. The driving cams of both pistons are carefully cut to provide an increase in flow from one pump while the other pump is being filled. This compensate for the loss of delivery during the refill process and the consequent fall in pressure. A diagram of a twin-headed pump is shown in figure 9. It is seen that there is a common supply of mobile phase from the solvent reservoir or solvent programmer to both pumps and the output of each pump joins and the solvent then passes to the sample valve and then to the column. In the diagram, a single cam drives both pistons, but in practice, to minimize pressure pulses, each pump usually has its own cam drive from the motor.