Gas chromatography (GC) is probably the most utilized of all the chromatographic techniques. Since the early 1950s, when the technique was first used for the separation of amino acids, gas chromatography now finds thousands of applications in virtually all spheres of chemistry and biochemistry. The technique is used in the separation and identification of constituents of the atmosphere, drugs, foodstuff, petrochemicals, pesticides, etc.
Gas chromatography is similar to column chromatography, except that the gas is used as the mobile phase instead of a liquid. In gas–liquid chromatography (GLC), the stationary phase is a thin layer of a nonvolatile liquid bound to a solid support. Separation takes place by the process of partition.
On the other hand, gas-solid chromatography (GSC), utilizes a solid adsorbent as the stationary phase and separation take place by adsorption process. Gas-liquid chromatography is more popular than gas-solid chromatography and has relatively more applications. Gas-Liquid chromatography was developed by A.P.J. Martin in 1951, together with A.T. James. In 1952, Martin and Synge were awarded the Nobel Prize in chemistry for their work on the development of partition chromatography.
Principle of Gas Chromatography
The principles of gas chromatography can be explained in terms of the following experiments.
A gas that is flowing smoothly at the rate of 3 ft/min down an empty tube that is 6ft long takes 6/3 = 2 min to flow from one end of the tube to the other. If such a tube were filled with sand, the gas would flow through it more slowly. If the rate at which the gas flows in the sand-filled tube is 2ft/min, it will take the gas 6/2 = 3 min to traverse the tube. The sand-filled tube in this example has some properties of a gas chromatography column.
The gas is the moving mobile phase. The sand is the stationary phase. The gas that emerges after it has passed through the column is called the eluent. In practice the mobile phase should be relatively insoluble in the stationary phase; otherwise, the stationary phase becomes overloaded.
Basically all gas chromatographs, whether GLC or GSC, consist of six basic components:
• A carrier gas: An inert gas is supplied at high pressure and is passed to the instrument at a rapid and reproducible flow rate.
• A sample injection system.
• The separation column.
• One or more Detectors.
• Thermostat chambers for temperature regulation of column and detectors.
• An amplification and recorder system.
• Thermal Conductivity Detectors
All gases conduct the heat but hydrogen and helium are by far the best thermal conductor when either of these useless Carrier gas anything else present caused a decrease in the thermal activity of the gas.
When the voltage is applied to a filament it heats up and the sturdy stage depends upon the applied voltage and the resistance of the filament. It is also affected by the rate at which it loses the temperature to the surroundings. If this phenomenon is placed in this temperature of gas anything is changed in thermal activities of the gas changes the temperature of the filament with changes the resistance of the filament as per the diagram.
The detector uses the single filament rapid switching valve causes it to sample a carrier and a reference gas alternatively. If two gas are identical and there is no component present, the resistance does not change by switching the gas. If a component is present or enters the detector, the temperature of the filament is dropped and is recovered when the reference gas is switched again. Change in current keeps the temperature is constantly provide the qualification of that component.
• Flame Ionization Detector
Air hydrogen flame creates very few ionized particles however a carbon-containing material enters the flame ion production increase Carrier gas from the column mixes with hydrogen and burn in air.
The flame ionization detector uses two electrons one of which is the jet where the flame is burning and the polarized voltage cooker and the polarized voltage to collect the ions from the flame. When the component appears the collected current increases. This increase of decrease in current helps in the formation of a chromatogram. The flame ionization detector responds to anything that creates ions in flame. Most all organic compounds can create ions in hydrogen-air flames.
• Nitrogen Phosphorus Detectors
The compounds are burned in the plasma surrounding the rubidium supplied with hydrogen and air. Nitrogen phosphorus-containing compounds produce ions that are attracted to the collector. The number of ions is major and the signal is generated. The detector is nitrogen phosphorus-containing as a sensitivity of 1 to 10 picogram and the temperatures range from 350 to 300ﾟC.
• Flame Photometric Detector
Compounds are burned in hydrogen-air flames sulfur and phosphorus-containing compounds produce radiation at 394 and 526nm respectively. A Photomultiplier tube with the help of a monochromator measures the amount of light and generates the signal sensitive to sulfur or phosphorus-containing compounds one at a time. Sensitivity is 10 picogram and the temperature ranges from 250 to 300ﾟC.
• Photoionization Detector
Compounds eluting into a cell are bombarded with high-energy photons. Compound with ionizing potential less than the photon energy is ionized. The resulting ions are attracted to an electrode measured and a signal is generated. Especially, used for aromatics and the sensitivity varies from 25 to 200 picogram and the temperature is 200ﾟC.
Applications of GC-MS
Gas Chromatography (GC-MS) is a sophisticated separation technique that cannot be compared with other modern analytical types of equipment but can be complemented by a mass spectrophotometer to achieve GC-MS/MS. It has a wide variety of applications that caters to academic research, quality control as well as industrial applications. Its concise, efficient, automated system gives fast, reproducible, and effective results that serve a key role in the advancement of Science and Technology. This resourceful analytical technique could be explored for better prospects in the future.
It has established applications in various fields such as:
• Environmental monitoring
• Food, beverage, flavor, and fragrance analysis
• Biological and pesticides detections
• Security and chemical warfare agent detection
• Chemistry and Geochemical Research
• Medicine and Pharmaceutical Applications
• Petrochemical and hydrocarbons analysis
• Doping of drugs
• The applications of Gas chromatography in the forensic sciences, including those in forensic toxicology, which include alcohol and drugs in drivers, markers of alcohol abuse, volatiles, and anesthetics, carbon monoxide (CO) poisoning.
Advantages of Gas Chromatography
The main advantages of gas chromatography are given below:
• The technique has good resolving power and even complex mixture can be separated into constituents.
• It is a sensitive technique with a few mg of the sample being sufficient for analysis.
• It provides good precision and accuracy.
• The analysis can be completed in a short time.
• The cost of the instrument is relatively low with a long life span.
• The technique is suitable for routine analysis as the operation of a gas chromatograph and related calculations do not require highly skilled operators.