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(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 profound as its effect in GC separations, but can be critical when closely similar substances are being separated. In LC a change in temperature will change the free energy of the solute in both phases, (generally in a commensurate manner) and so the net  change in the free energy difference with temperature, which

. A G-PN column was used which was 30 m long and 0.25 mm I.D. and operated at 130˚C employing helium as the carrier gas. The basic materials are patented and the technique of bonding and coating the material onto the column is extremely difficult and involves much proprietary art. The Column Oven and Temperature Programmer The column oven should operate over a fairly wide temperature range (e.g. from 5˚C to 400˚C). In practice, however, the maximum oven temperature needed is usually less than 250˚C, particularly when synthetic stationary phases are being used, as many of them tend to be unstable and either decompose or volatilize 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

contains the column, the essential device that actually achieves the necessary separation, and an oven to control the column temperature. It is interesting to note that despite the complexity of the apparatus, and its impressive appearance, the actual separation is achieved either in a relatively short length of packed tube or a simple wall-coated open tube. The rest of the apparatus is merely there to support this relatively trivial, but critical device. The oven also will contain a temperature sensor and if necessary an appropriate temperature programmer. As the mobile phase is a gas, there are virtually no interactions between the sample components and the mobile phase and thus the elution time can not be controlled by techniques such as solvent programming or gradient elution. The counterpart to gradient elution in gas chromatography is temperature programming. The column temperature is raised continuously during development to elute the more retained peaks in a

that forces a backward flow of carrier gas through the column and the strongly retained solutes are eluted to waste. As suggested above, to accelerate the purging process, the column temperature can be raised. When back flushing is complete, the valve is returned to the sampling position and the column temperature brought back to the initial conditions for analysis. The sampling stage of the back flushing technique shown in figure 20 and depicts the sample passing through the valve to the column and from the column back to the valve and through the detector. The normal split injection system places the sample on the column.   Figure 21. Removal of Highly Retained Solutes in the Back Flushing Procedure After the solutes of interest have been eluted, the valve is rotated, and the connection of the column inlet to the sampling system is now directed to waste. At the same time the column exit is disconnected from the detector and connected to a separate carrier gas supply that

, thus, using equation (74) and values for (b) of 0, 0.5, 0.1, 0.15, 0.17, and 0.2, the temperature curves for the (n) th plate of a column having 2500 plates can be calculated. The results obtained are shown in Figure 22. It is seen that the expected S-shaped curve is produced. As the solute dissolves in the stationary phase, and the heat of solution is evolved, the temperature rises. After the peak maximum is reached, the solute desorbs from the plate, the heat of solution is absorbed, and thetemperature falls below that of its surroundings. This effect can be simply demonstrated by inserting a thermocouple into a column and monitoring the temperature as the solute band passes. An example of a set of such curves (27) is given in Figure 23. It is seen that the expected temperature profiles are realized. It is also interesting to note that the front of the peak is eluted at a higher temperature than the back of the peak throughout the whole length of the column.  This, as is

the majority of GC analyses are carried out between temperatures of 75˚C an 200˚C. In contrast, LC column ovens cover a more limited range of temperatures viz., 0˚C to 120˚C. Temperature programming is an essential feature of all GC column ovens and is necessary to handle a sufficiently wide molecular and polarity range of samples. Linear programming is the most common although other functions of time are often available. LC column ovens are rarely provided with temperature programming facilities as the technique appears to be far less effective compared with GC gradient elution being a far more ffective alternative. The thermostatting medium of GC ovens is almost exclusively 'forced air' as the heat capacity of the GC mobile phase (i.e., a gas) is relative small. Consequently, air has sufficient heat capacity to change the column temperature rapidly without significant cooling from the carrier gas. Air ovens are also employed in LC column ovens