A guard column acts as a trap for many particulates and strongly retained components arising from samples and your HPLC system itself, and is installed immediately before the analytical column. The use of a guard column is a cost-effective way to increase substantially the lifetime of your analytical column (sometimes by up to 3-fold). Guard columns are recommended especially when biological or environmental samples are analyzed, and can also be very beneficial when analyzing pharmaceutical and agrochemical active ingredients, in-process samples, and formulated samples.
In order to get the maximum benefit from the guard column, it is important to change it before its capacity has been exceeded and contaminants have reached the analytical column. Some guidance on how often to change guard columns is also available.
A guard column, however, is not a substitute for a precolumn filter, which is always strongly recommended to protect the column(s) from particulates from samples, pump seals, and injector valve wear. A precolumn in-line filter should be placed immediately before the guard column (or analytical column, if no guard column is used). Agilent sells a very low dead volume in-line filter, which can even be used before Rapid Resolution HT 1.8 micron columns, with very little loss in performance.