In gas chromatography, the column is the heart of the system where the separation of sample components takes place. They are classified in terms of tubing dimensions and type of packing material. Packed columns are generally 1.5 – 10m in length and 2 – 4mm id. These are generally made of stainless steel or glass. On the other hand capillary columns are 0.1 – 0.5 mm id and can be 10 – 100m long.
Three types of capillary columns are commonly used in gas chromatography:
Wall Coated Open Tubular (WCOT)
Internal wall of capillary is coated with a very fine film of liquid stationary phase.
Surface Coated Open Tubular (SCOT)
Capillary tube wall is lined with a thin layer of solid support on to which liquid phase is adsorbed. The separation efficiency of SCOT columns is more than WCOT columns because of increased surface area of the stationary phase coating.
Fused Silica Open Tubular (FSOT)
Walls of capillary fused silica tubes are strengthened by a polyimide coating. These are flexible and can be wound into coils.
Fused silica and stainless steel columns offer high degree of inertness and flexibility. When breakage is not of much concern fused silica is the best choice.
Sample concentration is the deciding factor for the internal diameter of the column. Loss of resolution, poor reproducibility and peak distortion result if sample concentration exceeds column capacity. Typical sample loading ranges around 10ng for 0.1mm id columns to up to 2,000ng for 0.53mm id columns.
Longer columns provide greater resolution of sample components. However, increasing column length increases analysis time.
Film thickness determines the retention and elution temperature of each sample component. Thick films increase the time a compound stays on the stationary phase and thinner films reduce retention time. Compounds having high volatility require more residence time for better separation and should be analyzed on thicker films. The commonly used film thickness in gas chromatography columns ranges from 0.1 to 5.0µm.
Columns are selected for use in a particular application based on column length and type of packing. Guidelines on selection of columns are provided in more detail in the certificate programme which will be launched in due course.
The most important criteria in selection of column is the stationary phase packing which will be discussed in greater detail in the next module.