Pre-Columns - the forgotten art of using retention gaps

Pre-columns, guard columns, retention gaps, DuraGuards, EZ-Guards - these are just a few of the names used in capillary GC to indicate a certain type of inlet section on the capillary column. From a technical point of view they are all the same. They consist of a small piece of fused silica, usually 1 - 5 meter in length, installed in front of the analytical column. The coupling with the analytical column is done using various types of connectors. In case of EZ-Guard columns the pre-column is an integrated part of the analytical column without the need for an extra coupling. All of these pre-columns are made of uncoated but deactivated fused silica tubing. More precisely they are almost uncoated. They are coated and deactivated using a thin layer of stationary phase. The film thickness of this layer is only a fraction of that to be found on the analytical column even when compared to the thinner film columns of 0.10 µm.

The difference between pre-columns

The main difference is only their naming, which is directly linked to their usage.

Retention gaps are used to introduce a liquid sample into the analytical column via (cold) on-column or splitless injection techniques. Retention gaps are essential to obtain small injection bands, narrow sample peaks and maintain column efficiency if these injection techniques are applied.

Pre-columns are referred to as guard columns when their main purpose is to protect the analytical column from contamination and it allows the injection of relatively „dirty samples“ on the column. Obviously pre-columns are often used both as retention gap and as guard column at the same time e.g. in case of on-column injections of dirty samples. Also the DuraGuard/EZ-Guard columns can be used either way.

Which pre-column polarity do I need?

As mentioned, the pre-column is coated and deactivated with a thin layer of stationary phase with a certain polarity. A coating with a 100 % methylpolysiloxane phase will result in a non-polar pre-column; polyethylene glycol phases in polar ones. The stationary phase used for deactivation of the guard column piece for EZ-guard columns is always identical to the stationary phase of the analytical column. In case of separately installed pre-columns the polarity of the analytical column can be different from the polarity of the pre-column. In practice this will rarely happen. It is the choice of sample solvent, which will dictate the choice of pre-column polarity.

The table above indicates pre-column polarity and solvent compatibility. In general non-polar pre-columns should be combined with apolar solvents, polar solvents with polar pre-columns. There is however some overlap possible in the intermediate polar solvents. A fairly polar solvent like e.g. Isopropanol can be combined with an either apolar, polar or medium polar pre-columns. However, it is important to note that especially in cases where the pre-column is used as retention gap for on-column injections this compatibility issue is crucial. The liquid sample solvent should be evenly spread on the entire phase layer of the pre-column in order to obtain good sample focusing and prevent peak splitting. This can only occur when there is a good polarity match between solvent and pre-column. The nature of the analytes as such plays a minor role in the pre-column polarity choice.

Which pre-column length do I need?

The determination of the length of the pre-column depends on application, injection technique and some other factors. For those applications where it is used as guard column the length is not very critical. In most cases the interfering contaminants will be trapped in the first meter of the guard column. One meter is often enough. However, it is more practical to work with a longer length guard column, as is the case for EZ-guards columns. The front-end of the EZ-Guard can be cut off when it becomes too contaminated. Monitoring of the column efficiency and peak symmetry will indicate the moment to remove the first section of the guard column. Cutting off one meter will be sufficient in most cases.

Retention gap lengths are determined differently. During a splitless injection the retention gap usually functions as an extension of the injector liner. It prevents the injector from getting overloaded. As a result the volume of the retention gap for splitless is linked directly to the total vapor volume of the sample solvent. The analytes are „cold trapped“ in the stationary phase at the front of the analytical column. The length of the retention gap needed depends on injected volume, type of solvent, injector temperature, column head pressure and internal diameter of the retention gap. The table below shows some retention gaps lengths for 1 µl of solvents at 250 °C and 100 kPa (14.5 psi). It is clear that the 530 µm retention gaps are the most practical because of their shorter length. Water in particular has a large vapour volume and that has its effect on the retention gap length. In practice one installs 25 % extra retention gap length just to be on the safe side.

Our GC Calculator can help calculating those volumes.