How to Buy GC Columns
26 June 2017

Introduction to gas chromatography

Gas chromatography is an analytical separating technique used to separate volatile and semivolatile substances from complex mixtures. The method consists of a gaseous mobile phase, usually an inert carrier gas, such as helium, and a liquid stationary phase adsorbed onto an inert cylindrical solid support called a column. Once the sample to be analyzed is injected into the GC system, an isothermal oven temperature or temperature gradient is applied to the column. The compounds in the mixture interact with the stationary phase. Depending on the type of interaction and the boiling points of the compounds, separation occurs.

GC columns are broadly classified into two types: packed and capillary (Figure 1). The packed columns are filled with inert solid material coated with the liquid stationary phase, while the capillary columns maintain a hollow interior with the stationary phase coated along its inner walls. Packed columns are preferred for analysis of gas samples, but for most analytical separations, capillary columns are more efficient and provide good peak separation and consistent results.

Figure 1: Packed GC columns (left) and capillary GC columns (right). Stationary phase appears in blue.

The cross section of a capillary GC column
There are three distinct layers that make up the cross section of a capillary GC column (Figure 2):

Figure 2: The cross section of a capillary GC column. Image courtesy of Phenomenex.

  1. Polyimide coating: This is a protective coating applied to the outer surface of the column. Polyimide is most commonly used as a coating. It not only gives the column its distinctive brownish appearance but also makes the column flexible and resistant to temperature.
  2. Fused silica: The main material with which the column is built needs to be inert, with the compounds being separated. Fused silica, which is synthetic quartz of high purity, has been a reliable and routinely used material for GC columns. It is supported with an outer layer of polyimide coating to add strength and an inner coating of stationary phase that performs the separation.
  3. Stationary phase: A thin film of stationary phase coated on the inner wall of the capillary column serves as the most important factor in selecting a GC column. The fundamental rule in choosing the stationary phase for your application is ‘like dissolves like’. Analytes interact better with stationary phases of similar chemical nature, yielding a better separation.

The structural characteristics of the stationary phase divide them into three categories (Figure 3): (i) Wall-coated open tubular column (WCOT), (ii) Porous-layer open tubular column (PLOT), and (iii) Support-coated open tubular column (SCOT). A wall-coated open tubular column (WCOT) consists of a thin film coated on the inner wall. The porous-layer open tubular column (PLOT) is a porous solid layer on the capillary’s inner wall, while the support-coated open tubular column (SCOT) includes a liquid stationary phase in addition to a porous support.