Inert Surfaces and Connections

Pumps are basically devices for pulling in solvent, pressurizing it, and driving it out through the injector, column, and detector. As described earlier, it does this with a plunger driven through a Teflon seal into a pumping chamber (Fig. 9.2). 

Inlet and outlet check valves ensure a one-way solvent flow. Problems arise when the plunger must be pulled back to refill the chamber to ready itself for the next stroke (Fig. 9.3). The pressure drops until the plunger starts forward again. This results in pulsation, which causes variations in solvent delivery flow and, more importantly, variation in pressure to the column. The column acts as a pressure dampener, which can easily seen by watching baseline fluctuation of a system with and without a column in place. 

Pump manufacturers continue to work to reduce pulsing. The first attempt was the two-headed pump, which is connected through an output manifold or pressure transducer (Fig. 9.4). One head delivers solvent while the second refills. This improvement led to the first high-performance FIPLC pumps, but 

The next step was the electronically compensated pump. All pumps speed the motor as resistance increases to maintain a constant solvent slow. These pumps also add a major plunger speed-up during refill and repressurization. With this modification, a pump with a single pump head and a pulse dampener could give 90% of the performance of a two-headed pump for 50% of the cost. An overall dramatic price reduction for the dual-pump HPLC system resulted. 

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