Why should I use a guard column with my analytical or preparative column?
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A guard column is recommended to protect the analytical/preparative column from contamination from particulates from the injection, debris from worn pump seals/injector rotor seals or unfiltered mobile phases.
Filtration through a 0.22um to 0.45 um should be done in order to remove particles and help degas the mobile phase at the same time.
Solid Phase Extraction or Liquid Liquid Extraction also help produce a cleaner sample for direct injection.
Failure to use a guard column directly exposes the analytical or preparative column to contamination and therefore reduces its practical lifetime.
Care should be taken to use, whenever possible, the same material in the guard column as in the analytical/preparative column especially when doing method development.
Typical analytical guard columns are 1 or 2 cm long with either 2.0 (2.1) or 4.0 (4.6) mm depending on the column dimensions and 1cm long for 10mm, 21.2mm & 30mm column id.
Whether you opt for 1 or 2 cm long guard columns is tied to how harsh your mobile phase is and how messy your sample is. Ideally, in order to keep your chromatography and to avoid increasing the system pressure, you should use the shortest guard column available and use the same id as the column whenever available. Otherwise you should choose the closest smaller id guard column available.