CALCULATION OF THE NECESSARY LENGTH OF A RETENTION GAP
In the injector of a gas chromatograph samples are evaporated at certain temperatures. Because the main component of a sample is the sample solvent the following considerations are pointing at the solvent only.
The ideal gas law, p · V = n ·R · T,, gives a correlation between amount of a component and its the volume at a certain temperature and pressure. The vapour volume depends also on the chemical structure/molecular weight of the component. The smaller a molecule is, the more vapour for a given liquid volume is produced. Higher pressure decrease this volume and higher temperatures will increase it.
In a split injector one part is injected onto the column and another part is vented out of the system, the ratio between these two streams is the split ratio. Direct or splitless injections do not have a secondary vent flow, they should use retention gaps for better focussing effects and then the injected volume should not exceed the retention gap volume. The right choice of a retention gap deactivation is dependent on the polarity of the sample solvent.
The total vapour volume has to fit into the liner to get sharp injection bands and good resolutions in the separation process and the column's maximum injection volume should not be exceeded by the partial volume injected to the column.