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Study Notes: The GC Column

In GC, the column is the most important component as it is the interaction of the analyte with the column’s stationary phase (and not the carrier gas) that permits separation of the analytes in the sample.

There are two types of gas chromatography based on column characteristics:

  • Gas-Solid Chromatography (GSC) in which a solid stationary phase physically adsorbs analytes leading to retention of the analyte on the column. This GC has limited use due to long retention of active or polar molecules and severe tailing of elution peaks. Its only real application is for specific low molecular weight species and it will not be discussed further in this section.
  • Gas-Liquid Chromatography (GLC) in which a thin layer of a liquid stationary phase is immobilised inside the column. The sample analytes partition between the gaseous mobile phase and the liquid phase. GLC has been adopted as the most useful method and is generally now called Gas Chromatography (GC) although GLC is still mentioned in older textbooks.
Columns are normally coiled so that they can fit inside the oven of a GC. A GC column showing its characteristic coiled shape.

The GC column must have the following characteristics.

  • Be rigid and able to withstand moderate pressures (up to 50 psi [~145 kpa]).
  • Contains a stationary phase that is temperature stable and does not chemically react with the analyte components.
  • Contains a stationary phase that will cause the separation of the analytes of interest.

Two general types of column are used in GC (ie GLC):

  • Packed columns in which the inside of the column tube is densely packed with a solid support material onto which is coated a liquid stationary phase. The stationary phase forms a thin liquid film on the surface of a finely divided, inert support material (typically derived from diatomaceous earth which has a very high surface area). The mobile phase moves over and around the coated material as it travels down the column.
  • Open tubular columns, known as capillary columns, are characterised by a narrow opening running down the centre (a capillary) through which the mobile phase travels as it moves past the stationary phase.

Typical physical characteristics of both column types are shown in the table below (columns are formed into coils to enable them to fit into the oven of a GC). Note that capillary columns are narrower and longer.

Characteristic
Packed Column
Capillary Column
Length (m) 1 - 6 10 - 100
Inside Diameter (mm) 2 - 4 0.1 – 0.3
Coil Diameter (cm) 15 10 - 30

Packed columns are normally made from stainless steel but can be glass if a less reactive surface is desired. The material of choice for the construction of capillary columns is fused silica, a highly purified and inert material. An outside protective coating, called polyimide, affords strength and flexibility. A cross section of a capillary column showing the outer polyimide layer, the middle fused silica layer and the inner stationary phase.

The most widely used columns in gas chromatography are referred to as WCOT (wall coated open tubular). These are capillary columns in which a liquid stationary phase is coated directly onto the column wall forming a thin film. 0.25 – 0.5 µm thick. Other types of capillary columns exist with the stationary phase contained in different formats.

The table below provides a further comparison of capillary (WCOT) and packed columns. For capillary columns, the greater efficiency (narrower peaks and therefore better separation) and smaller sample sizes means that they are used widely today particularly for new or updated methods.

Parameter
Capillary
Packed
Efficiency (plates/m)
2 000 – 4 000
500 - 1 000
Sample size (ng)
10 - 75
10 – 1 000 000
Relative pressure
Low
High
Relative speed
Fast
Slow
Chemical inertness
Best
Poorest
Flexible column
Yes
No

Adapted from Skoog, D. and Leary, J. (1992), Principles of Instrumental Analysis, Fort Worth: Saunders College Publishing.

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