Guard and scavenger columns

The small particle size of the packing makes 3x3 columns susceptible to plugging problems. Samples and mobile phases should be filtered before use, and the column should be protected with guard and scavenger columns (see Section 5.3.2). 

A microparticulate hplc column is a very efficient filter, and if the mobile phase contains any particulate matter, or acquires it from the pump and/or the injection valve due to wear, it will collect at the top of the column. If this happens, the pressure drop across the column for a given flow will gradually increase, and the column may eventually become completely blocked. To prevent this happening, the mobile phase should always be filtered before use, preferably through a 0.5 p,m porosity filter, and guard and scavenger columns should be used as a matter of routine (see Section 5.3.2). 

The useful life of hplc columns can be extended by proper treatment, in particular by the use of guard and scavenger columns. Pretreatment of samples is often necessary, eg very dilute samples may require concentration, or complex samples may need to be cleaned up. Some of the more important techniques are considered. 

For use with 25 cm x 4.6 mm analytical columns, guard columns and scavenger columns are often 4.6 mm internal diameter and 3-10 cm in length. They can be packed with microparticulate stationary phases or with porous layer beads. Porous layer beads are cheaper than microparticulates and are easier to pack, but they have lower capacities and will require changing more often. It is usually difficult to know how long a pre-column will last before it requires changing. In routine work, precolumns are usually repacked or replaced to a fixed schedule. 

Because scavenger columns are located upstream of the injection valve, they do not add to the dispersion of the chromatogram, and their size is not critical in this respect. Guard columns, on the other hand, do cause a slight loss of efficiency, and so need to have a relatively small volume. Reducing the volume, of course, reduces the life of the guard column. 

Why use fast LC Besides the obvious advantage of faster analyses, there is 50 to 80% less solvent use (and less waste generated). Sensitivity is increased 3 to 5 times because there is less dilution of the analyte peaks with the smaller column volumes as a result of tighter packing. These densely packed columns are more prone to clogging. A scavenger column or filter is placed before the injector, in addition to the guard column placed before the column. 

---