GC Columns

When selecting the right GC column, four factors are particularly important:

• stationary phase
• inner diameter
• length
• film thickness

The parameters are described in more detail below and should help to make it easier to select the right column.

The simplest method for selecting the stationary phase is to search the manufacturer's databases or the literature. Even if no exactly suitable application is found, this can at least narrow down the choice of stationary phase. Do you need help finding suitable applications? Get in touch with us! We will be happy to support you in your search for suitable columns.

If no applications can be found, the selectivity and polarity of a GC column can help. Selectivity describes the ability of the stationary phase to separate two analytes from each other. This is due to different interactions between the stationary phase and the analytes. The most important interactions are dispersion, dipole-dipole interaction and hydrogen bonding.

The polarity in turn is determined by the modification of the stationary phase and also determines the type of interaction. Almost all manufacturers offer a sequence of non-polar to polar GC columns. Polar phases offer more polar interactions and therefore polar analytes are also retarded for longer. The choice of polarity of a column therefore directly influences the selectivity of the stationary phase and vice versa.

Sort the table according to increasing polarity

Based on this overview, a type of stationary phase can be selected depending on the analyte. If the analytes differ in their dipole moments, a column should be selected that also has dipole properties. If the analytes have different strengths for the formation of hydrogen bonds, then a column should be selected that enables hydrogen bonds. The principle of "like dissolves like" applies here.

If the retention is to be increased or the selectivity of a functional group is to be changed, a stationary phase with more of this functional group can be used. For example, a 50% phenyl phase can be used instead of a 5% phenyl phase. This increases the interactions and changes the selectivity.

However, it is important to note that the phase with the lowest possible polarity should always be used, as these phases are less prone to phase bleeding than more polar phases.

PLOT columns (Porous Layer Open Tubular) and packed columns are used for permanent gases and very volatile substances. PLOT columns are capillary columns which, similar to standard columns, have a high separation efficiency and narrow peaks. However, the loading capacity of such columns is low. Various materials such as aluminium oxide, molecular sieves or carbon materials are used to coat the capillary.

Packed columns have a larger diameter and are filled with the stationary phase (similar to HPLC). Due to the shorter columns with a larger inner diameter, the separation performance is usually poorer, but large sample quantities can be injected.

Depending on the application of the column, either a PLOT or a packed column can be useful. PLOT columns can be used in trace analysis or with MS, for example. While packed columns are often used for process control or routine analysis.

The smaller the inside diameter, the higher the number of trays per metre. However, the required inlet pressure also increases significantly with a smaller inner diameter. Due to the high upstream pressure with small inner diameters, standard analyses are usually carried out with 0.18 mm or larger inner diameters. In GC-MS applications, the vacuum lowers the required inlet pressure so that smaller inner diameters can also be used.

If a high flow rate (carrier gas flow rate) is required, e.g. for headspace applications, columns with a larger inner diameter (0.45mm or 0.53mm) are used to minimise the required inlet pressure.

Another point to consider when selecting the appropriate inner diameter is the loadability of the column. The larger the internal diameter of a column, the higher the loadability.

Consequently, a general recommendation can be summarised: For the greatest possible efficiency (separation performance), the smallest possible inner diameter (0.18mm or 0.25mm) should be used. An inner diameter of 0.18mm or smaller is suitable for GC-MS. If larger sample volumes are to be analysed, a 0.32mm column can be used. If high flow rates are required, columns with an inner diameter of 0.45mm or 0.53mm are used.

As the length of a GC column increases, the bottom number (efficiency) of the column also increases. However, a longer column also leads to longer retention and a higher back pressure. Furthermore, a longer column also shows greater column bleed, as more stationary phase is present. However, this is often only observed to a lesser extent.

The standard length of a GC column is 25-30 metres and can also be used for unknown samples. Short columns (10-15m) can be used for minor separation problems or very few compounds. Short columns are also often used in conjunction with small internal diameters in order to reduce the inlet pressure. Long columns (50-60m) should only be used if an increase in resolution cannot be achieved by other means or if there are very complex samples containing many substances.

The disadvantage of a long column is always the longer analysis time and the higher purchase costs.

A greater film thickness means that more stationary phase is present, which leads to longer retention. Columns with a greater film thickness are therefore used in particular for analytes that elute very quickly or are highly volatile. These include solvents or certain gases. Another effect is that the temperature at which elution is observed also increases. A thicker film provides the same or longer retention at a higher temperature. Thinner films, on the other hand, reduce retention and are therefore used for strongly retarding analytes. In addition, the temperature required for elution is reduced. As a result, the same or earlier elution of the analytes can be observed at lower temperatures. Due to these effects, columns with a thick film are used for very volatile compounds and gases. Columns with a thin film are used for high-boiling analytes.

However, a thicker film is associated with greater column bleed. As a result, late eluting peaks can be overlaid by column bleed. Therefore, the temperature limit is also lower with thicker films compared to thinner films.

The capillary walls are better shielded by a thicker film, which makes a column more inert. For this reason, peaktailing can often be reduced or avoided with a thicker film.

A thicker film also shows a higher capacity compared to thinner films. When analytes are present in large quantities in a sample, a thin film often shows very broad peaks. As a result, these peaks can overlap others. Here, the use of a thicker film can lead to narrower peaks and better separation.

As the film thickness naturally also depends on the internal diameter of the column, there are a few recommendations. For columns with an inner diameter of 0.18-0.32mm, 0.18-0.25µm thick films are used as standard. Columns with an inner diameter of 0.45-0.53mm are often used with film thicknesses of 0.8-1.5µm. Thicker films are used for volatile compounds, are more inert and have larger sample capacities. However, column bleed is greater and the temperature limits are lower. Thinner films are used for high-boiling analytes, are less inert and have a lower sample capacity. However, the temperature limit is generally higher and only minimal column bleed can be observed.