Liquid Chromatography - Column Switching
The valve usually consists of five parts. Firstly there is the control knob or handle that allows the valve selector to be rotated and thus determines the load and sample positions. Secondly, a connecting device that communicates the rotary movement to the rotor. Thirdly the valve body that contains the different ports necessary to provide connections to the mobile phase supply, the column, the sample loop if one is available, the sample injection port and finally a port to waste. Then there is the rotor that actually selects the mode of operation of the valve and contains slots that can connect the alternate ports in the valve body to provide loading and sampling functions. Finally there is a pre-load assembly that furnishes an adequate pressure between the faces of the rotor and the valve body to ensure a leak tight seal.
The technique of column switching can increase the versatility of the liquid chromatograph significantly. An example of a six port valve arranged for column switching is given in figure 15. The arrangement utilizes the same valve as that used for the external loop sampling system. It is seen that column (1) is connected between ports (5) and (6) and column (2) is connected between ports (2) and (3). Mobile phase from a sample valve, or more usually from another column, enters port (1) and the detector is connected to port (4). In the initial position of the rotor shown in the diagram on the left hand side, the rotor slots connect ports (1) and (6), (2) and (3) and (4) and (5).
Courtesy of Valco Instruments Inc.
Figure 16. Valve Arrangement for Column Switching