Gas Chromatographic Columns

as Chromatographic Columns

In gas chromatography the mobile phase is a gas. Gas chromatographic columns are usually between 1 and 100 meters long.

Gas liquid chromatography(GLC): The liquid stationary phase is bonded or adsorbed onto the surface of an open tubular (capillary) column, or onto a packed solid support inside the column.

Gas solid chromatography (GSC): The stationary phase is a solid and the analyte absorbs onto the solid. GSC uses a packed or open capillary column. GSC is used for analytes that do not absorb onto a liquid stationary phase. GSC is a popular method for analyzing carbon disulfide, hydrogen sulfide, carbon monoxide, carbon dioxide, nitrogen oxide, volatile hydrocarbons, halo-carbons, solvents, and other gases found in the air. The solid stationary phase is usually made of graphite carbon blocks, alumina, silica, molecular sieves, or porous polymers beads. Molecular sieves are composed of aluminum and silicate ion exchangers. Porous polymer beads are made up of styrene cross-linked by divinylbenzene. 

Open Tubular (capillary) Columns

When the stationary phase is uniformly distributed on the interior surface of column it is called an open tubular (capillary) column. Open tubular columns are longer, smaller in diameter, and more efficient than packed columns. Open tubular columns have less flow resistance which allows for them to be longer and have a lot of theoretical plates. Capillary columns are between 3 and 100 meters long and form a helical shape. The most common stationary phases used for open tubular columns are polysiloxanes. Polysiloxanes are silicon atoms which have attached oxygen and R groups. The R groups can vary, which makes polysiloxanes very versatile (see Figure 2). There are three types of open tubular columns: wall-coated (WCOT), support-coated (SCOT), and porous-layer (PLOT). WCOT is the most popular type of open tubular column.

The wall coated open tubular column consists of a capillary tube with its interior surface coated in a tiny layer of stationary phase. The most common type of wall coated open tubular column used is fused-silica, because it is stronger, inert, reliable, easy to use, and flexible. Fused silica capillary tubes are made from purified silica that has a small quantity of metal oxides dispersed throughout the silica. The fused silica column also has a layer of polyimide on the outside of the column, which makes the column flexible and extends the life of the column. Wall-coated open tubular columns can also be made out of plastic, glass, stainless steel, aluminum, or copper. 

Figure 1: Diagram of a fused-silica open tubular column. This figure was created with Microsoft Paint.

A support-coated open tubular column has a thin layer (approximately 30 µm) of liquid support matter. This type of open tubular column has a greater amount of stationary phase than the wall coated column, so it can handle a larger quantity of sample. 

A porous-layer open tubular (PLOT) column is very similar to a support-coated open tubular column. The only difference between the two types of columns is that a PLOT does not have a liquid stationary phase. PLOT columns are used for gas solid chromatography. PLOT columns have a solid layer of carbon, molecular sieves, cyclodextrins, inorganic oxides, or porous polymers, coating the inner wall of the column. PLOT columns can be up to 100 meters long. The inner diameter of a PLOT column is between 0.25 and 0.53 mm. The stationary phase coating is between 5 and 50 micrometers thick.

Packed Columns (also applies to liquid chromatographic columns)

A packed column is dense and evenly packed by solid support. The solid support usually has a liquid stationary phase bonded to it. The solid support allows for the liquid stationary phase to be exposed to the maximum amount of the mobile phase. The solid support and stationary phase must be inert at high temperatures and allow for the mobile phase to be evenly distributed as it moves through the column. The packed columns are shorter in length and wider in diameter than the open tubular columns. The diameter of a packed column is usually between 2 and 4 mm. Packed columns are typically 1 to 5 meters long and also form a helical shape.The packing particles typically have a diameter of 100 to 250 micrometers. Micro-packed columns are packed capillary tubes and are packed with the same material as larger packed columns.  The most common stationary phase used for packed columns is diatomaceous earth (diatomite). Diatomite is made up of  diatom (single-celled algae) skeletons. The skeletons are composed of mostly silica, and small quantities of alumina and metallic oxides. Other popular stationary phases are pure silica (SiO₂) and alumina (Al₂O₃). Alumina is great for separating aromatic hydrocarbons.

Choosing a Stationary Phase for Gas and Liquid Chromatographic Columns

The liquid stationary phase must be inert, thermally stable, and not volatile at high temperatures. The right type of stationary phase is necessary for separation of molecules. Remember “like dissolves like”, because the analyte must be somewhat soluble into stationary phase. In other words, the polarity of the analyte must be equivalent or closely resemble the polarity of the stationary phase.

Column Stationary Phase	Polarity
Diatomaceous Earth (SiO2, Al2O3, Fe2O3, CaO, MgO, Na2O, and K2O)	relatively non-polar
Clay	relatively non-polar
Celite	relatively non-polar
Squalane (C30H62) (purified with charcoal and alumina)	non-polar
Hexadecane	non-polar
Apolane-87 (C87H176)	non-polar
Apiexon Greases (purified with charcoal and alumina)	non-polar
Dialkyl Phthalates	moderately polar
Tetrachlorophthalates	moderately polar
Polyethylene Glycol (Carbowax 20M)	moderately polar
Polysiloxanes	non-polar-polar (depending on the R group attached)

Table 1: List of common stationary phases and their polarities. The polarity of the stationary phase should resemble the polarity of the analyte.

Stationary Phase Functional Group	Polarity
Polyester Phases	Highly Polar
-OH	Polar
-CN	Polar
-CO	Polar
Hydrocarbons	Non-polar
Dialkyl Siloxanes	Non-polar

Table 2: List of functional groups attached to the stationary phase and their polarities. These polarities should resemble the polarities of the analyte and can be compared to the analyte functional groups in Table 3.

Analyte Functional Groups Ranked From Most Polar to Non-polar

Water

Acids

Alcohols

Amines

Amides

Aldehydes

Ketones

Esters

Ethers

Aromatics

Hydrocarbons

Table 3: This table shows the order of relative polarity of different functional groups. The most polar compounds are at the top of the table and the least polar compounds are at the bottom. The polarity of the analyte should resemble the polarity of the stationary phase in Table 2.

Matching the polarities of the analyte and stationary phase is not an exact science. The two should have similar polarities. The thickness of the stationary phase ranges between 0.1 and 8 µm. The thicker the layer the more volatile the analyte can be.