Pharma information Zone: Types of column in Gas Chromatography

In the gas chromatography gas mixture is passed through a glass or metal tube which is known as column.
2 types of columns used in Gas chromatography:
Packed columns
Open tubular columns

Packed columns

Columns which contain particles of stationary phase packed into metallic or glass tube.
Metallic tubes are of stainless steel, Cu, Al.
Column diameter: 2 – 10 mm
Column length: 1 - 4m and it is dependent upon required number of theoretical plates.
It must be carefully bent or coiled to fit into temperature controlled oven in GC.
Bending or coiling is done such that it does not impede flow rate of carrier gas.

Stationary phase:

Either small particles of solid adsorbent (GSC). or liquid supported on small particles of solid (GLC).
Solid stationary phase Provide inert surface on which liquid phase can be coated.
Most common solid supports are derived from diatomaceous earths.
Diatomaceous earth primarily consists of hydrated silica groups.
Other substance used as solid supports are powdered teflon, graphited carbon black, calcium chloride,volcanic scoria, Activated carbon, organic polymer and glass beads.
Keiselguhr and embacel are widely used.

Liquid Stationary Phase
They are classified according to polarity.
Polar samples are retained longer in stationary polar liquid phases than are non polar
components and separation is at least partially based on polarity of sample components.
With non polar stationary liquid phase, components are separated according to their relative solubility in liquid and gaseous.
As component increases in molecular weight, volatility or solubility decreases and time spend in column increases.
I.e. on non polar columns retention time or retention volume are related to molecular weight or boiling point of sample components.
Retention time increases with increase in molecular weight and with increase in boiling point.

Open tubular column (OTC) or capillary Column

There are two types of open tubular column gas chromatography
- Support coated open tubular column.
- Wall coated open tubular column.

· Support coated open tubular column

In this type a thin layer of solid support, previously coated with liquid stationary phase, is coated on inner wall of column.
Stationary phase is coated on solid support. Solid support material used are- diatomaceous earth, silica gel, graphite, metal oxides and silicates.
Because of solid support internal surface area of column increases so ability to handle sample is also increase.

Wall coated open tubular column

In this thin layer of liquid stationary phase is coated directly on inner wall of glass capillary or fused silica column
Thickness of film should be ≤ 1μm to prevent bleeding of column. (When stationary phase eluted from column from mobile phase is called as bleeding of column

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