GC – Gas Chromatography

Gas chromatography (GC) is a common type of chromatography used in analytical chemistry for separating and analyzing compounds that can be vaporized without decomposition. In simple words it is separation of gaseous and volalite substance. The father of modern gas chromatography is Noble prize winner John Porter Martin who also developed the first liquid gas chromatography in 1950. Gas chromatography is the process of separating compounds in a mixture by injecting a gaseous or liquid sample into a mobile phase, typically called the carrier gas, and passing the gas through a stationary phase.

There are major two types of gas chromatography 

• Gas – Solid Chromatography – The mobile phase is a gas while stationary phase is a solid. This is uses for separation of low molecular gases. e.g air components, H2S, CS2, CO2, rare gases, CO and oxides of nitrogen.
• Gas – Liquid Chromatography – The mobile phase is a gas while the stationary phase is as liquid retained on the surface as an inert solid by adsorption or chemical bonding.

Principles

The principle of separation in gas chromatography is Adsorption and partition. The mixture of component to be separated is converted to vapour and mixed with gaseous mobile phase. The component which is more soluble in stationary phase travel slower and eluted later. The component which is less soluble in stationary phase travels faster and eluted out first. No two component has same partition coefficient conditions. So the components are separated according to their partition coefficient. Partition coefficient is the ratio of solubility of a substance distributed between two immiscible liquids at a constant temperature.

Instrumentation

• Carrier gas He (common), N2, H2, Argon
• Sample injection port – micro syringe
• Columns
• Detectors

Carrier Gas

The cylinder/gas tank is fitted with a pressure controller to control the pressure of gas, a pressure gauge that indicates the pressure, molecular sieve to transfer filtered dry gas and a flow regulator to ensure a constant rate of flow of mobile phase to the column.

While selecting the carrier gas it should meet the following criteria-

• Should be chemically inert
• Should be cheap and readily available
• Should be of high quality and not cause any fire accidents
• Should give best possible results
• Should be suitable for the sample to be analysed and for the detector

Hydrogen, helium, nitrogen and carbon dioxide are commonly used. Hydrogen has low density and better thermal conductivity. However, it reacts with unsaturated compounds and is inflammable and explosive in nature. Nitrogen is inexpensive but it gives reduced sensitivity. Helium is most preferred gas.

• Inlet pressure ranges from :10 – 15 psi.
• Flow rate: 25 – 150 mL/min for packed columns
• Flow rate: 2-25 mL/min for open tubular column

Sampling Unit

Sampling unit or injection port is attached to the column head. Since the sample should be in vaporized state, the injection port is provided with an oven that helps to maintain its temperature at about 20-50 C above the boiling point of the sample. Gaseous samples may be introduce by use of the gas tight hypodermic needle of 0.5 – 10 ml capacity. For liquid samples, micro syringes of 0.1 100µL capacity may be used.

The flowing types of injector are used –

• Spilt injections
• Splitless injectors
• On column injector
• Automatic injector

Column Unit

Columns are of different shapes and sizes that includes: “U” tube type or coiled helix type. They are mainly made of copper, stainless steel, aluminium, glass, nylon and other synthetic plastics. Column unit contain two phase Support material – it’s main function is to provide mechanical support to the liquid phase. An ideal support should have a large surface area, chemically inert, should get informally wet with liquid phase, should be thermostable. Commonly used solid phases are: Diatomaceous earth or kieselguhr, glass beads, porous polymers, sands, etc. Other is Liquid phase – it should have the following requirements. It should be non-volatile. Should have high decomposition temperature and chemically inert. Should possess low vapour pressure at column temperature. Liquid phases are – paraffin oil, silicon oil, glycol, glycerol, hydroxy acids, polyglycols, etc.

There are two types of columns – Packed columns and Capillary columns.

Detectors

The eluted solute particles along with the carrier gas exit from the column and enter the detector. The detector then produces electrical signals proportional to the concentration of the components of solute. The signals are amplified and recorded as peaks at intervals on the chromatograph.

The properties of an ideal detector –

• Sensitive
• Operate at High temperature (0-400° C)
• Stable and reproducible
• Linear response
• Wide dynamic range
• Fast response
• Simple (reliable)
• Non-destructive
• Uniform response to all analytes

Detectors which are used in GC are – Thermal conductivity detector (TCD), Electron capture Detector (ECD), Flame ionization detector (FID), Flame photometric detector.

Working

1. Fill the syringe with sample.
2. Record the setting i.e., column temperature, detector temperature and injection port temperature.
3. Introduce sample into the injection port by completely inserting the needle into the rubber septum. Note down the injection time.
4. The sample gets vaporized due to higher temperature of the injection port and is swept into column by carrier gas.
5. This sample component with minimal solubility move faster and those with maximum solubility travel slowly.
6. The components leaving the column activate detector and recorder to give a plot.
7. The components that slowly leave the column give a bell shaped curve of shorter peak while the one which travel faster gives a bluntly pointed curve of larger peak.
8. The graph determined in order to obtain the percentage composition of the mixture.
9. Once graph is obtain the Evaluation is done by analysis of Theoretical Plate, Retenation time and  Retention Volumne.

Applications

• Qualitative analysis – by comparing the retention time or volume of the sample to the standard/ by collecting the individual components as they emerge from the chromatograph and identifying these components by other methods like UV, IR, NMR.
• Analysis of drugs.
• In determining the levels of metabolites in body fluids like plasma, serum, urine, etc.
• Analysis of Food – protiens, lipids, dairy products.
• Since gas chromatography is useful in identifying the individual elements and molecules present in a compound, it has been applied in forensic pathology to determine which fluids and compounds are present inside a human body after death.
• Gas chromatography can also be used to test samples found at a crime scene, whether these be blood samples or fibre samples from clothing or other materials.
• Use in Arson investigation.