Gas Chromatography - Gas Supplies > Flow Programmers > Page 10

 

(3)

It is seen that at high values of (g), the retention time approaches a constant value.

The relationship between and (g) is depicted in figure 4.

Figure 4 Graph of against (g)

Figure 4 shows that there is little advantage in employing inlet/outlet pressure ratios much above 5 as values in excess of this do not reduce elution time significantly. If the column is very long, and consequently has a high flow impedance, higher inlet pressures may be necessary to obtain the optimum flow rate but this may not significantly reduce the elution time.

In figure 5, the log of the retention time is plotted against (g) for both compressible and incompressible mobile phases. It is seen that for a compressible mobile phase the retention time falls to a constant level when (g) is about 5 or 6. In contrast, for an incompressible mobile phase (i.e. in liquid chromatography), the retention time is continuously reduced as (g) is increased. The advantages of flow programming with a compressible mobile phases are much less than for incompressible mobile phases.