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Analytical chromatography requires a pulse free flow of the solvent under high pressure. To achieve pressures up to 500 bar requires piston pumps and special Teflon or graphite based seals where the pressurised liquid improves the sealing power.
Make sure that pistons of modern LC pumps are made of synthetic sapphire to reduce wear and to extend the working life of the pump. The cylinder is usually made of stainless steel and is attached to two non-return valves in line with the inlet and outlet connections to the pump. The piston is driven by a stainless steel cam which forces the piston into the cylinder expressing the solvent through the exit non-return valve. After reaching the maximum movement, the piston follows the cam and returns as a result of the pressure exerted by the return spring. During this movement the cylinder is loaded with more solvent through the inlet non-return valve. For pulsation free operation two or three head are required. The shape of the cam is cut to provide a linear movement of the piston during expression of the solvent but a sudden return movement on the refill stroke. In this way the pulse effect that results from the refill action is reduced.
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