Gas chromatography is an analytical technique for separation and analyzing compounds that can be vaporized without decomposition or it is an analytical technique that separates the chemical components (organic molecules or gases) of a sample mixture and detect them to determine their presence or absence and how much is present.
In this technique, the mobile phase (moving phase) is a carrier gas, usually an inert gas such as helium, nitrogen etc.
Stationary phase is a microscopic layer of a liquid a polymer on an inert solid support, called column.
The instrument used for this technique is known as gas chromatograph or gas separator.
The main purpose of Gas chromatography technique is to separate the compound that posses:
High volatility
Low molecular weights
Thermal stability
It is at present the most widely used and powerful technique of separation. The main reason for this is speed, resolving power, and extreme sensitivity of the technique to separate.
In gas- liquid chromatography (GLC) the stationary phase is a thin layer of a non volatile liquid bound to the solid support whereas inn gas solid chromatography (GSC), utilizes a solid adsorbent as the stationary phase and separation takes place by adsorption process. The gas liquid chromatography is more popular than gas solid chromatography. The GLC was developed by A.P.J. Martin in 1951, together with A.T. JAMES.
PRINCIPLE OF GAS CHROMATOGRAPHY
The principle of Gas Chromatography is based on “ partition chromatography ”. The mixture of component to be separate is converted to vapor and mixed with the carrier gas or mobile phase. The component which is more soluble in stationary phase travel slow and elute later than compared to the component which Is less soluble travels faster and elute out first. No two compound have the same partition co efficiency. Different species of molecule spend different time period in the moving phase, affects the length of the time of a gas takes places through and emerges from a chromatographic column.
INSTRUMENTATION OF GAS CHROMATOGRAPHY
The 5 basic components of the gas chromatography:
- CARRIER GAS: the carrier gas play an important role in this technique. The gas should be free from oxygen, dry, and chemically inert. The inert gas should be supplied at high pressure . The most commonly used carrier gas are helium, nitrogen, hydrogen. The purity ranges of all carrier gas varies should be in the range of 99.995%- 99.9995% . The pressure of gas should be 40-80 psi. A molecular sieve is present in the carrier gas system which removes water and other impurities.
A carrier gas in Gas Chromatography system flow through the injector and push the gaseous constituents of the sample onto the GC column. The carrier gas should be: Inert, suitable for detector used, cheap, high purity, and no cause of fires or explosion hazards. SAMPLE INTRODUCTION SYSTEM/ SAMPLE INLET : the sample injection port plays an important role as very small and reproducible amount of sample Is introduced into the column. Liquid sample introduced through hypodermic syringe and solid sample must dissolve in volatile liquids. The gas samples are injected by a gas –tight syringes or by valves. The sample volume ranges from 0.1 to 0.2 ml.
2. SEPARATING COLUMNS: A column is the heart of the technique. A column can be either packed or open tubular. The column and sample inlet portion is enclosed in a chamber which is oven. The sample is elute throughout the column by using gas as a mobile phase. The various shapes of column is available for gas chromatography are straight, coiled and u-shaped columns.
Packed column are made from glass, stainless steel, copper or other suitable tubes.
The open capillary columns are of two types: WCOT ( wall coated tubular column) or SCOT ( support coated tubular column). WCOT columns are capillary tubes that have a thin layer of stationary phase coated along the column walls. In SCOT the walls of column is coated with a thin layer of solid adsorbent. One of the most popular SCOT column is fused silica coated in which walls are coated with the purified silica with a small amount of metal oxides.
3. OVEN: the column is enclosed by a column oven which is responsible for maintaining a constant temperature during isothermal operation. The length of the column varies from 1-300m and the temperature of oven can be change up to 400degree Celsius. The higher the oven temperature, the faster the samples moves through the column.
Temperature programming is the gradual changes in temperature during analysis.
4. DETECTING SYSTEM: the detector is the device that is located at the end of the column that provides a quantitative measurement of the sample mixture. Each detector has two main parts. The first part of detector is the sensor which is placed as close to the column exit to optimize detection. The second part is the electronic equipment that convert the detected property changes into electrical signals to the computer may analyze the acquired chromatogram.
There are various types of detectors used in Gas chromatography are:
- Thermal conductivity detector (TCD)
- Flame ionization detector (FID)
- Electron capture detector (ECD)
- Flame photometric detector (FPD)
- Photo-ionization detector (PID)
- Mass –spectral detector (MSD).
5. AMPLIFICATION AND RECORDING SYSTEM: these are final component of Gas Chromatography that records the signals from the detector system. They use electronic circuits to process and amplify these signals so as to display in a graph format, representing peaks of the sample in analysis.
ADVANTAGES OF GC
- Low cost of instrument with a long life span
- Sensitive technique with few mg of sample is sufficient for analyzing
- Has good resolving power and even complex samples can be separated into several component
- Good precision and accuracy
- The analysis can be completed in a short time.
APPLICATION OF GAS CHROMATOGRAPHY
There are various applications of GC:
Forensic applications:
- In toxicology analysis of alcohol and drugs in drivers, markers of alcohol abuse.
- In poisoning cases such as carbon monoxide poisoning, and other poisoning.
- Analysis of ethanol in blood, analysis of drugs, chemical identification of individual substances.
- Analysis of substances in arson and explosives cases.
Other applications
- In analysis of food, the separation and identification of lipids, proteins, colorants, preservatives and traces elements are involved.
- In analysis of dairy products for aldehyde, ketones and milk sugars.
- Detection of steroids drugs used by athlete in sports competition.
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