Gas chromatography (GC)

 

Introduction:

Gas chromatography - specifically gas-liquid chromatography - involves a sample being vapourised and injected onto the head of the chromatographic column. The sample is transported through the column by the flow of inert, gaseous mobile phase. The column itself contains a liquid stationary phase which is adsorbed onto the surface of an inert solid. Gas-Solid Chromatography. GSC is a type of GC in which the same material acts as both the stationary phase and the support. In this method, chemicals are retained by their adsorption to the surface of the support. This support is often an inorganic material such as silica or alumina.

Principle of gas chromatography:

The sample solution injected into the instrument enters a gas stream which transports the sample into a separation tube known as the "column." (Helium or nitrogen is used as the so-called carrier gas.) The various components are separated inside the column.

 Basic Flow diagram of a gas chromatograph:

                   
Apparatus:

Carrier Gas:

Carrier gas used in the gas chromatography should be inert chemically. Commonly used gases are Nitrogen, Helium, Argon and Carbon dioxide. The choice of carrier gas is often dependent upon the type of detector which is used. The carrier gas system also contains a molecular sieve to remove water and other impurities. And the gas selection is based on the compound nature to be analyzed. Hydrogen gas is commonly used as the fuel gas in FID detectors. Zero air is used as the makeup gas in many analysis

Gas control Panel:

Gas control panel is must to control the gas flow and regulate the correct flow to the instrument, which avoids the solenoid valve damage. Direct flow from the gas cylinder gives high pressure and it may contain some amount of moisture too. Gas control panels having the moisture traps whish is used to absorb the moisture present in the gas.

Injector:

The injector can be used in one of two modes; split or splitless. The injector contains a heated chamber containing a glass liner into which the sample is injected through the septum. The carrier gas enters the chamber and can leave by three routes (when the injector is in split mode). The sample vapourises to form a mixture of carrier gas, vapourised solvent and vapourised solutes. A proportion of this mixture passes onto the column, but most exits through the split outlet. The septum purge outlet prevents septum bleed components from entering the column.

                             
Columns:

There are two types of columns used in GC analysis,

1.      Packed Column

2.      Capillary Column or Open tubular Column

Packed Column: contain a finely divided, inert, solid support material (commonly based on diatomaceous earth) coated with liquid stationary phase. Most packed columns are 1.5 - 10m in length and have an internal diameter of 2 - 4mm.

Capillary Column: These columns having an internal diameter of a few tenths of a millimetre. They can be one of two types; wall-coated open tubular (WCOT) or support-coated open tubular (SCOT). Wall-coated columns consist of a capillary tube whose walls are coated with liquid stationary phase. In support-coated columns, the inner wall of the capillary is lined with a thin layer of support material such as diatomaceous earth, onto which the stationary phase has been adsorbed. SCOT columns are generally less efficient than WCOT columns. Both types of capillary column are more efficient than packed columns.

Column Oven:

This column oven plays a main role in gas chromatography analysis. Temperature is main component in the GC to evaporate and separate the analyte base on their vaporisations point. Minimal temperatures give good resolution, but increase elution times. If a sample has a wide boiling range, then temperature programming can be useful.

By reducing the temperature in the column oven leads to the good separation and resolution between the analytes, where as increasing the temperature shows the fast elution of the components.

Detector:

There are many detectors which can be used in gas chromatography. Different type of detectors having different applications and sensitivity. A non-selective detector responds to all compounds except the carrier gas, a selective detector responds to a range of compounds with a common physical or chemical property and a specific detector responds to a single chemical compound. Blow mentioned detectors are used commonly,

1.      Flame ionization (FID)

2.      Thermal conductivity (TCD)

3.      Electron capture (ECD)

4.      Nitrogen-phosphorus

5.      Flame photometric (FPD)

6.      Photo-ionization (PID)

7.      Hall electrolytic conductivity

FID is most commonly used in all pharma industries. The effluent from the column is mixed with hydrogen and air, and ignited. Organic compounds burning in the flame produce ions and electrons which can conduct electricity through the flame. A large electrical potential is applied at the burner tip, and a collector electrode is located above the flame. The current resulting from the pyrolysis of any organic compounds is measured. FIDs are mass sensitive rather than concentration sensitive; this gives the advantage that changes in mobile phase flow rate do not affect the detector's response. The FID is a useful general detector for the analysis of organic compounds; it has high sensitivity, a large linear response range, and low noise. It is also robust and easy to use, but unfortunately, it destroys the sample.

Diagram of FID: