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. Column Switching 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

(7); port (1) is connected to port (2), port (3) connected to port (4) and port (5) connected to port (6). This results in the mobile phase from either a sample valve or another column entering port (1) passing to port (2) through column (2) to port (3), then to port (4) and then to the detector. The ports (5) and (6) are connected, this time isolating column (1). This arrangement allows either one of two columns to be selected for an analysis or part of the eluent from another column pass to column (1) for separation and the rest passed to column (2). This system, although increasing the complexity of the column system renders the chromatographic process far more versatile. The number applications that require such a complex chromatographic arrangement are relatively small, nevertheless, when required, column switching can provide a simple solution to certain difficult separation problems.  Column Ovens The effect of temperature on LC separations is often not nearly so

at higher temperatures. Similarly, initial temperatures below 50˚C are also rarely needed. The oven usually has air circulation driven by a powerful fan to ensure an even temperature throughout the oven. The temperature in any part of the oven should be stable to ± 0.5 ˚C and when operating isothermally the column temperature should be constant to ± 0.2 ˚C. The oven should have a capacity of 1-2 cu. ft. and is supplied with fittings to accept more than one column and some switching valves if so desired. Such equipment is needed for multidimensional chromatography. The temperature programmer (hardware and software) usually has a range of linear gradients from 0.5˚C/min. to about 20˚C/min. Some programmers include nonlinear programs such as logarithmic and exponential, but most GC analyses can be effectively accomplished using linear programs only. The program rate can be changed at any time in the chromatographic development or

The mobile phase passes through several stationary columns which contain the stationary phase which may be silica gel, or a bonded phase, or, in GC a suitable coated support. There are, also, a number of different ports, one for the mobile phase and one for the return of the mobile phase. In addition there is a central feed port and two take-off ports. These ports can, by appropriate valve programming (usually by means of a computer) connect, sequentially, each column to its neighbor. In modern simulated moving bed systems, disc valves are no longer employed and have been replaced by sets of individual valves. It is clear that the apparent counter current movement of the stationary phase, relative to the mobile phase, is achieved by appropriate valve switching which simulates the rotation of the ports on the disc valve between each column. Referring to figure 31, it is seen that part of the feed moves with the mobile phase and is collected by a

Column Switching Techniques   The use of column switching techniques is not exclusive to capillary column systems, but the procedures lend themselves more readily to open tubular columns than packed columns, and, consequently, are extensively employed for certain types of analyses. Switching procedures are particularly useful in 'back flushing' techniques and in 'heart cutting' specific samples from a partially resolved mixture.   Figure 20. Sample Injection in the Back Flushing Procedure