Gas chromatography



Chromatography is a technique used in separating multi-component mixtures of gas and liquids. This is achieved by the relative speed of the component mixture’s velocity. though this may not be common in the industry but it is useful i laboratories.

Gas chromatography is a type of separation process use in analytical chemistry for separating and analyzing mixtures that can vaporize with decomposition or loss of components. It also helps in identification of compound from a mixture. Chromatography is classified by the nature of the mobile and stationary phases. In gas chromatography (GC), which includes gas solid chromatography having gas as the mobile phase and solid as the stationary phase and gas liquid chromatography (GlC) which has gas as the mobile phase and liquid as the stationary phase kept in a porous solid? In both types of gas chromatography, the mobile gas is the carrier gas which is usually an inert gas such as Helium or a nonreactive gas such as nitrogen depending on the component to be separated but in all Helium remains the most commonly used carrier gas in most instruments because of the low concentration of other components which can be measured by their thermal conductivity or density and also by using flame ionization detector. Also Hydrogen is preferred in for some improved separation processes. In gas solid chromatography, if one component is strongly adsorbed with non linear isotherm, then, the resulting peck is asymmetric with a long tail which makes it difficult for other components to separate.

For optimum column efficiency, the gaseous compound or sample being analyzed should not be too large and should be introduce into the column as a plug of vapor in order to interact with the walls of the column which is coated with a stationary phase. This causes each component to eluate at different time (known as retention time of the sample). A micro syringe is mostly used to inject the sample through a rubber septum into a flash vaporizer port at the head of the column.

Gas chromatography works in a similar principle as column chromatography as well as other forms of chromatography such as high performance liquid chromatography (HPLC) which is applied in separation of component of a sample under high pressure of finely particles and high flow rates there are two types of column: packed and capillary column. Packed column contains a finely divided inert solid supported materials coated in stationary phase. Capillary column have an internal diameter of tenths of millimeters. The capillary column can be open tabular or supported coated open tabular.

Gas chromatography has a likened industrial application of fractional distillation, since both processes deals on separation of components of compounds and it is on a larger scale that fractional distillation operates. Gas chromatography is sometimes called gas liquid partition chromatography where each component of the feed or sample moves through the column at a rate proportional to the velocity of the carrier gas and inversely proportional to the volume of the stationary phase and mobile phase. Mathematically the equilibrium relationship for a component B of a mixture express in mole fraction is given as:

yB = ΥB BPB/ P

Where: yB is the vapor composition of B in the mixture

B is the mole fraction of component B in the mixture

PB  is the partial pressure  of component B in the mixture

P is the total pressure of the mixture.

the above equation can also be converted to mass concentration.

For gas chromatography the column temperature must be kept constant, the optimum column temperature is dependent upon the boiling point of the sample, when there is a temperature above the average boiling point of the sample; increase in the eluation time is to the range of two to thirty minutes while minimal temperature gives good resolution.

There are many detectors which can be used I gas chromatography and different types of detectors gives different selectivity type. They include:

  • Thermal conductivity detector: this depends on the thermal conductivity of the sample passing around a tungsten rhenium filament with current passing through it. It works over concentration type range.
  • Flame ionization detector: this is sensitive to a wide range of concentration. It has a type on mass flow, having a supported gas of hydrogen or air and has a selectivity of most organic compounds.
  • Electron capture detector: this makes use of radioactive beta particle source to measure the electron capture. It is used in detecting electromotive elements and some organic compounds functional group example, carbonyl and halogens as selectivity.
  • Nitrogen-phosphorus: in this type of detector, nitrogen and phosphorus is used to alter the work function on a specific coated bead and the resulting current is measured. It has a mass flow type and selectivity element of nitrogen and phosphorus.
  • Catalytic combustion detector: this measures the combustibility of hydrocarbon and hydrogen.

Others forms of detector include:

  • Dry electrolytic conductivity detector
  • Flame photometric detector
  • Atomic emission detector
  • Mass spectrometer detector
  • Infrared detector
  • Hall electrolytic conductivity
  • Pulse discharge ionization
  • Vacuum ultraviolet detector

                                Application of gas chromatography

Generally, substances with vaporization temperature below 300˚c can be measured quantitatively. The substance should be salt and ion free. Gas chromatography can be used in chemical industry in determining the chemical content of products in order to ascertain the quality products. It can be used to determine the toxicity of the soil, soil and water.

In forensic science, gas chromatography is used in identification and quantification, arson identification, paint chip analysis and identification of biological specimen.


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