Guard Columns/Retention Gaps

Over time, the inlet end of a capillary GC column can become contaminated from the accumulation of non-volatile material. The phase in the front section of the column can also be damaged from the continuous condensation and vaporization of solvent and analytes. Inevitably, active analytes will adsorb to this contaminated/damaged section (the analytes "drag" when passing through the inlet end of the column). Poor peak shape (peak tailing), loss in resolution, and reduced response may be observed. When the chromatographic system degrades to an unacceptable level, performance is restored by clipping the contaminated/damaged section off the inlet end of the column. A decrease in retention times and resolution occurs each time the column is clipped, as theoretical plates are lost. Eventually, the column will be rendered useless.

The use of a guard column/retention gap is an inexpensive technique to extend the lifetime of capillary columns. A guard column/retention gap is a short (1-5 m) piece of uncoated deactivated fused silica tubing which is placed in-line between the GC injection port and the capillary column. The guard column/retention gap is used to take the brunt of the contamination/damage from the solvent and sample. By clipping the guard column/retention gap periodically to restore performance instead of the capillary column, the capillary column remains unaltered. Therefore, chromatography (retention times and resolution) is not affected.

A guard column/retentipon gap consists of two parts:
  • A short length of fused silica tubing
  • A connector