Optimising GC Column Choice
GC Column Type
GC columns can generally be split into two categories: packed columns and capillary columns. Depending on your application, this will influence the type of GC column you need and the specifications of the column in that category. Packed columns are usually made from metal such as stainless steel or glass and have been used since the dawn of gas chromatography. Capillary columns are typically fused silica coated in a protective polymer called polyimide and are currently the most commonly used GC column type.
Packed columns usually range from 2-6 m in length and have an internal diameter of 2-4 mm. Packed columns are filled with particulate solid support available in a range of diameters and are beneficial for large sample capacities and are specified for certain analytical methods but can produce broad peak shapes and give poor separation depending on the application.
Capillary column typically range in length from 15 to 100 m with an internal diameter of 0.18 to 0.53 mm. They provide much better resolution and sharp peaks but have a much lower volume sample capacity. There are different types of capillary columns: porous-layer open tubular (PLOT), wall-coated open tubular (WCOT) and support-coated open tubular (SCOT). For most current day applications, typically a WCOT capillary column will be the best choice which is a fused silica tube coated in a liquid stationary phase.
Stationary Phase
For your application there may be a specified column type or a USP phase classification for the stationary phase to be used for the method i.e. G43 is the USP phase classification for a 624 stationary phase column. The stationary phase of a column is classified due to stationary phase composition which gives the column a certain polarity. The polarity of your column is influenced by the chemical composition and functional groups present in the stationary phase. Choosing the correct stationary phase is key to separating the compounds you wish to analyse as compounds may co-elute on one type of stationary phase but separate on another due to the different chemistry of the phases and the analyte-phase interactions.
When choosing a stationary phase the basic chemistry principle of “like dissolves like” must be observed e.g. choose a non-polar stationary phase if analysing non-polar compounds. The type of interactions seen between the analytes and the stationary phase are dependent on polarity. Non-polar stationary phase e.g. SCION-1 column tend to have only dispersive interactions dominated by Van der Waals forces. The intermolecular interactions increase with the size of the analyte. The larger a compound is and higher the boiling point it has will increase retention time. Polar compounds and stationary phases can have other intermolecular interactions such as hydrogen bonding, pi-pi, dipole and acid-base interactions.
Column Internal Diameter
Choosing the internal diameter (I.D.) of the column is a balancing act between efficiency and sample load capacity. The efficiency of the column relates to the number of theoretical plates within the column, N. The theoretical number of plates needed in the column for an application will be dependant on your sample as by increasing N the sharpness and resolution of peaks will increase. Decreasing the diameter will also increase column head pressure and decrease sample capacity. Increasing the internal diameter will increase the amount of sample which can injected onto the column without overloading the column which leads to distorting peak shape and decreasing resolution. 0.25 mm id columns are regarded generally as the best choice for most applications as a good balance between efficiency and sample capacity is achieved.
Other factors to take into consideration when choosing column internal diameter is the instrument set up you have. Smaller I.D. such as 0.10 and 0.18 mm may require equipment to allow a higher column head pressure or 0.53 mm id column will require a much higher flow rate that may be unsuitable for MS.
Column Film Thickness
Film thickness must be considered in your column selection process. When considering a film thickness the factors which can be taken into account are operating temperature of your method, column bleed, retention time and separation.
The more stationary phase you have, the more stationary phase degradation which will result in greater column bleed but you will also increase inertness by reducing analyte-tubing interactions. The retention time and sample capacity will increase with film thickness which allows very volatile compounds at a higher concentration to be analysed. Although by increasing film thickness, peak width is increased which can decrease resolution and the operating temperature of your column will also be lower. There are two maximum temperature limits on your column, one is the maximum isothermal temperature and the other is the maximum programmed temperature e.g. 325 °C (350 °C). See our column care technical note for more information.
Column Length
To increase retention time and resolution you can increase column length but this will increase back pressure and cost. It must be noted that increasing column length by double will not double the resolution as it is not a linear relationship. Column I.D. should be reduced before increasing length. Typically a 30 m column will suit most applications for a balance of analysis time, resolution and pressure requirements but you must look at your method requirements.