Liquid Chromatography - Column Ovens

This results in mobile phase passing from port (1) to port (6), through column (1) to port (5), from port (5) to port (4) and out to the detector. Thus, the separation will take place in column (1). The ports connected to column (2) are themselves connected by the third slot and thus isolated.

When the valve is rotated, the situation is depicted on the right hand side of figure (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 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 controls the magnitude of the absolute retention, can be relatively small. Its effect on relative retention, however, can be very significant and, in fact, be the determining factor in achieving a satisfactory resolution. (5-7) The effect of temperature on diffusivity will be similar in both GC and LC. An increase in temperature will increase the diffusivity of the solute in both phases and thus increase the dispersion due to longitudinal diffusion and decrease dispersion due to resistance to mass transfer. As a result, at the optimum velocity, the efficiency of both the LC and GC column will be largely independent of temperature, however, the optimum velocity will be higher at higher temperatures and provide the potential for faster analyses. Due to the lesser effect of temperature on solute retention in LC (compared to that in GC), temperature is not nearly so critical in governing absolute retention time but is often essential in achieving adequate resolution, particularly between closely eluting solutes such as isomers. In contrast to the GC column, the thermal capacity of an LC column is much higher as the specific heats of liquids are much greater than those of a gas. As a consequence, a high heat capacity thermostatting fluid is necessary and if retention measurements need to be precise, air ovens would not ideal for thermostatting LC columns. On the other hand, liquid thermostatting media are rather messy to use and tend to make column changing difficult and lengthy. However, if accurate data is required, good temperature control may be essential. If precise retention measurements are not required, an air thermostatting oven might be a reasonable compromise.