Pneumatic pumps are simple, inexpensive and pulse free. They consist of a collapsible solvent container housed in a vessel that can be pressurized by a compressed gas. This
pump has limited capacity and pressure output and is not adaptable to gradient elution. The pumping rate depends on solvent viscosity. Screw-driven syringe pumps consist of a large syringe in which the piston is moved by a motor-driven screw. They are pulse free and the rate of delivery is easily varied. They suffer from lack of capacity and are inconvenient when solvents must be changed. Reciprocating pumps are versatile and widely used. They consist of a small cylindrical chamber that is filled and then emptied by the back-and-forth motion of a piston. Advantages include small internal volume, high output pressures, adaptability to gradient elution, and constant flow rates that are independent of viscosity and back pressure. The pulsed output must be damped.