Myth 3: Guard Columns Do Not Affect a Separation False. First of all, having a guard column is a good idea. However, the choice of a guard column can have a great effect on a separation if the wrong configuration or phase is chosen. Remember, the purpose of a guard column is to protect the analytical column against fouling by highly retained sample components, particulates, and other undesirable materials. The guard column is much less expensive than the analytical column that it protects. Thus, it can be replaced more often rather than the more expensive analytical column. Ideally, the guard column chosen should have the exact same stationary phase as used in the analytical column. If the phase is more retentive (for example, has a greater carbon loading or is a mixed-mode phase), then it can affect the separation in a detrimental manner by causing retention time shifts or even selectivity differences. If the phase is less retentive, it might or might not cause a problem unless the phase chosen impacts the overall selectivity. For the guard column to have minimal impact on separation performance, it must be properly plumbed into the flow stream. Obviously, the guard column is inserted between the injector and the analytical column but if too much connecting tubing (either too long or two large of an internal diameter) is used, extracolumn band spreading can occur that can affect the overall separation. Systems employing integrated guard columns where the guard column literally butts up against the analytical column would offer the best solution. However, integrated guard columns usually are associated with cartridge column systems, which seem to have come into disfavor. Whatever the configuration, the guard column should be able to be removed easily and replaced from the HPLC system without affecting its operation. Theoretically, if the guard column is adding additional column length to the analytical column, there should be additional plates generated that should increase the chromatographic resolution. However, due to some of the factors discussed previously, often the addition of a guard column just maintains the separation at best and sometimes detracts from it at worst. The advantage of the guard column is increasing column life and not necessarily increasing the overall efficiency.
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