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Non-Return Valves For efficient function, it is important that while the piston compresses the solvent to express it from the exit port, the flow is completely stopped at the inlet port. Conversely, when the pump draws fresh solvent into the cylinder during refill, the non-return valves must allow solvent to flow through the inlet valve but, flow-back from the exit valve must be completely stopped. This is achieved by the use of efficient non-return valves. Most non-return valves are of

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. 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. The pulses, however, are not completely eliminated and the detector noise resulting from these pulses is probably the most serious disadvantage of the single piston pump. Nevertheless, as a result of its low cost it remains one of the more popular LC

With careful design and exacting construction these types of valve can be extremely efficient. In practice, to ensure the most effective performance, 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

to be filled with solvent. In position (2), the piston advances and when it passes the pumping fluid inlet, it starts compressing the diaphragm expressing solvent to the column. In position (3) the diaphragm has been compressed to its limit and the piston starts to return. In position (4) the piston moves back withdrawing the diaphragm sucking liquid into the pumping cavity ready for the next thrust. The inlet from the solvent supply and the outlet to the column are fitted with non-return valves in the usual manner. Due to the large volume of the pumping cavity, any gradient profile would be seriously distorted so this type of pump is not often used for analytical purposes but is often used in preparative chromatography. The Sample Valve In general, LC sample valves must be able to sustain pressures up to 10,000 p.s.i., although they are most likely to operate on a continuous basis, at pressures of 3,000 p.s.i. or less. The higher the operating pressure the tighter the

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