Liquid Chromatography - The Sample Valve 1

The wheel driving the crank rotates in an anti-clockwise direction and in position (1) the diaphragm has been withdrawn causing the pumping cavity behind the diaphragm to be filled with solvent.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 valve seating surfaces must be forced together to eliminate any leak. It follows that any abrasive material, however fine, that passes into the valve can cause the valve seating to become scored each time it is rotated which will ultimately lead to leaks. This will cause the sample size to vary between samples and eventually affect the accuracy of the analysis. It follows that any solid material must be carefully removed from any sample before filling the valve.

The two basic types of LC sample valve have been discussed in Principles and Practice of Chromatography. In LC however, there is a third type of valve which is similar to the external loop valve but contains an extra loading port and behaves like an internal loop valve. a diagram of which is shown in figure 13. The basic difference between this type of valve and the normal external loop sample valve is the introduction of an extra port at the front of the valve. This port allows the injection of a sample by a syringe directly into the front of the sample loop. Position (A) shows the inject position. Injection in the front port causes the sample to flow into the sample loop. The tip of the needle passes through the rotor seal and, on injection, is in direct contact with the ceramic stator face. Note the needle is chosen so that it's diameter is too great to enter the hole.