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) 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

. In the early years of the LC renaissance, there were two types of pump in common use; they were the pneumatic pump, where the necessary high pressures were achieved by pneumatic amplification, and the syringe pump, which was simply a large, strongly constructed syringe with a plunger that was driven by a motor. Today the majority of modern chromatographs are fitted with reciprocating pumps fitted with either pistons or diaphragms. For more information on HPLC pump requirements see the pump section in the HPLC supplement. The Pneumatic Pump The pneumatic pump has a much larger flow capacity than the piston type pumps but, nowadays, is largely used for column packing and not for general analysis. The pneumatic pump can provide extremely high pressures and is relatively inexpensive, but the high pressure models are a little cumbersome and, at high flow rates, can consume considerable quantities of compressed air. A diagram of a pneumatic pump is shown in

, a single non-return valve assembly usually contains two non-return ball valves connected in series as shown in figure 5. The Syringe Pump The syringe pump is a large, electrically operated simulation of a hypodermic syringe. Although used in the early days of LC renaissance, it is rarely used today as, due to its design, it can provide only a limited pressure and the volume of mobile phase available for use is restricted to the pump volume. Unless the separation is stopped while the pump is refilled and the development subsequently continued, the pump can only elute solutes that have retention volumes equal or less than the pump capacity. A diagram of a syringe pump is shown in figure 6. Courtesy of the Perkin Elmer Corporation Figure 6. The Syringe Pump

of the piston and between the piston and the outlet there is a coil that acts as a dampener. This type of pump is still occasionally used for the mobile phase supply to microbore columns that require small volumes of mobile phase to develop the separation. It is also sometimes used for reagent delivery in post column derivatization as it can be made to deliver a very constant reagent supply at very low flow rates. The Single Piston Reciprocating Pump The single piston reciprocating pump was the first of its type to be used with high efficiency LC columns (columns packed with small particles) and is still very popular today. It is simple in design and relatively inexpensive. A diagram of the single piston pump is shown in figure 7. Figure 7. A Single Piston Reciprocating Pump

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

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