A guard column protects the analytical column from damage caused by particulate matter and strongly adsorbed material. Ideally, the packing of the guard column should be the same as the analytical column so that the chromatography of the analytical column is not altered. If the identical material is not available, then similar or less retentive packing may be used for the guard column. To maintain an adequate capacity for sample impurities without introducing excessive peak dispersion, choose a guard column with an internal diameter close to or the same as the analytical column and with a similar or slightly larger particle size. The guard column should be no more than 20% of the length of the analytical column.