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.  

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