Q.5. (b) Discuss the principle of GLC and its column configuration, detection system & stationary phases.
Ans.5.(b) Principle of GLC Gas chromatography is probably the most utilized of all the chromatographic techniques.
The primary limitation is that the sample must be capable of being volatilized without undergoing decomposition. Because of this limitation it is now being replaced, to a large extent, by high performance liquid chromatography.
In gas liquid chromatography, GLC,the mobile phase is a gas and the stationary phase is a thin layer of a non-volatile liquid bound to a solid support. A partition process occurs. On the other hand, gas solid chromatography, GSC, utilizes a solid adsorbent as the stationary phase and an adsorption process takes place.
When a gas or vapour comes in contact with an adsorbent, certain amount of it gets adsorbed on the solid surface. The phenomenon takes place according to the well-known laws of Freundlich, i.e., x/m = Kc1/n or Langmuir, i.e., x/m = K1c + K2c where x is the mass of the gas or vapour sorbed in mass m of the sorbent and c is the vapour concentration in the gas phase and K, K1& K2 are constants, similarly if vapour or gas comes in contact with a liquid, a fixed amount of it gets dissolved in the liquid. The phenomenon takes place according to Henry’s law of partition, i.e., x/m = Kc. Now both the phenomena are selective and there are different K values for different vapour-sorbent pairs
Column configuration: the column can be constructed of glass or metal tubing and for analytical work it has 4.8 mm diameter. It can be of any length from a few centimeters to over a hundred meters and can be coiled, bent or straight. Three types of analytical columns are generally used in gas chromatography. These are described below:
(i) Packed column: These are prepared by packing metal or glass tubings with granular stationary phase. For GSC the columns are packed with size graded adsorbent or porous polymers, whereas for GLC the packing is prepared by coating the liquid phase over a size graded inert solid support.
(ii) Open Tubular columns: These columns are also referred to capillary or Golay columns and are made of long capillary tubing (30-90 meters) having uniform & narrow internal diameter (0.025-0.075 cm). They are made of stainless steel, copper, nylon or glass etc., stainless steel being the most popular. The inside wall of the capillary tubing is coated with the liquid phase in the form of a thin (0.5-1 micron) and uniform film. The carrier gas flow faces least resistance because there is no packing in the column.
(iii) Support coated open tubular columns: These columns are made by depositing a micron size porous layer of support material on the inside wall of a capillary column and then coating with a thin film of liquid phase. These columns have more sample capacity and an inlet splitter may not be required. SCOT columns are proffered for trace analysis.
Detectors: Almost all the detectors monitor the GLC column effluent by measuring the changes in the composition arising from the variations in the eluted components. When the carrier gas alone is passing they gave a zero signal. When a component is eluted it is detected and a signal proportional to the concentration of that compound is produced. Integrated detectors which give signals proportional to the amount of the eluted component are also available. Some commercially available detectors are described below:
1. Differential Thermal Conductivity Detector: One of the first detectors used was the differential thermal conductivity detector. The principle of the detector is that the temperature and thus the resistance of a wire through which a current is flowing are dependent upon the thermal conductivity of a gas in which it is immersed. The thermal conductivity of a gas is a function of its composition.
2. Flame Ionization Detector: The ionization detectors are based on the electrical conductivity of gases. At normal temperatures and pressure gases act as insulators but will become conductive of ions if electrons are present. If the conditions are such that the gas molecules themselves do not ionize, the change in conductivity due to the presence of a very small number of ions can be detected.
3. Electron Capture Detector: The electron affinity of different substances can be used as the basis for an ionization detector known as the electron capture detector. It responds to only those compounds, whose molecules have an affinity for electrons, e.g., chlorinated compounds, alkyl lead etc. On the contrary it responds very little to compounds such as hydrocarbons. The ECD is most frequently used for detection and measurement of trace environmental pollutants. It has a high sensitivity for halogenated compounds and is therefore used for the detection of herbicides, pesticides, SF6 traces in fuel gases, organometallics (e.g., lead tetraethyl), polynuclear aromatic carcinogens, NO2 and SO2 in chimney stack gases
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Stationary Phase: The stationary phase is a microscopic layer of liquid or polymer on an inert solid support, inside a piece of glass or metal tubing called a column (a homage to the fractionating column used in distillation).The gaseous compounds being analyzed interact with the walls of the column, which is coated with a stationary phase. This causes each compound to elute at adifferent time, known as the retention time of the compound. The comparison of retention times is what gives GC its analytical usefulness.
The polarity of the solute is crucial for the choice of stationary compound, which in an optimal case would have a similar polarity as the solute. Common stationary phases in open tubular columns are cyanopropylphenyl
dimethyl polysiloxane, carbowax polyethyleneglycol, biscyanopropyl cyanopropylphenyl polysiloxane and diphenyl dimethyl polysiloxane. For packed columns more options are available.
POLYSILOXANES:
Polysiloxanes are the most common stationary phases. They are available in the greatest variety and are the most stable, robust and versatile. The most basic polysiloxane is the 100% methyl substituted.
POLYETHYLENE GLYCOLS:
Polyethylene glycols (PEG) are widely used as stationary phases. Stationary phases with "wax" or "FFAP" in their name are some type of polyethylene glycol. Polyethylene glycols stationary phases are not substituted, thus the polymer is 100% of the stated material. They are less stable, less robust and have lower temperature limits than most polysiloxanes.
BONDED AND CROSS-LINKED STATIONARY PHASES:
Cross-linked stationary phases have the individual polymer chains linked via covalent bonds.Bonded stationary phases are covalently bonded to the surface of the tubing.Both techniques impart enhanced thermal and solvent stability to the stationary phase. Also,columns with bonded and cross-linked stationary phases can be solvent rinsed to removecontaminants. Most polysiloxanes and polyethylene glycol stationary phases are bonded andcross-linked. A few stationary phases are available in a no-bonded version; some stationaryphases are not available in bonded and cross-linked versions. Use a bonded and cross-linked stationary phase if one is available.
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