A retention gap can be applied before between or after a column. Its purpose is exactly as its name, a gap or delay of retention by a deliberate introduction of 'dead space' or a void in the flow path of the analysis.
A guard column is a column which is usually similar in composition to a column after it but is placed before this column in order to receive and retain components of the sample that would ordinarily contaminate or destroy the analytical column. Thus it may be replaced more often at a cheaper cost to the analyst than replacement of the analytical column.
For on-column injections it usually is a good idea to use a guard column or a column that is empty, without packing, that is, a retention gap column, so that the sample may be injected directly into it instead of the analytical column. Since on-column injections are usually performed COLD, the sample is often a liquid, and liquids can damage the phase of the analytical column, especially if the phase is not bonded to the column wall.
Yes the guard column does act as a retention gap but not exactly as an empty column, and the guard column may actually be a retention gap.
The naming of an empty tube, and the naming of a packed tube can be confusing and redunant.
I hope I have not confused you.
best wishes,
Rod
