MUHAMMAD NADEEM ASHRAF SOIL & ENVIRONMENTAL SCIENCES SUBMIYYED TO : SIR NASIR RAHEEMFaculty of Agriculture, Rawalakot
GAS LIQUID CHROMATOGRAPHYPrinciplesPartition of molecules between gas (mobilephase) and liquid (stationary phase).
In gas chromatography (GC), the sample is vaporizedand injected onto the head of a chromatographiccolumn. Elution is brought about by the flow of an inertgaseous mobile phase.The mobile phase does not interact with molecule of theanalyte; its only function is to transport the analytethrough the column.Gas-liquid chromatography is based upon the partitionof the analyte between a gaseous mobile phase and aliquid phase immobilized on the surface of an inertsolid.
Filters/Traps Data system H RESET Regulators Syringe/Sampler Inlets Detectors gas system Gas Carrier Hydrogen inletAir Column column detector data system
Detection SystemsCharacteristics of the Ideal Detector: The idealdetector for gas chromatography has thefollowing characteristics:1. Adequate sensitivity2. Good stability and reproducibility.3. A linear response to solutes that extends overseveral orders of magnitude.4. A temperature range from room temperatureto at least 400oC.
Characteristics of the Ideal Detector5. A short response time that is independent offlow rate.6. High reliability and ease of use.7. Similarity in response toward all solutes or ahighly selective response toward one or moreclasses of solutes.8. Nondestructive of sample.
DETECTORSFlame Ionization Detector (Nanogram - ng)High temperature of hydrogen flame (H2 +O2 + N2)ionizes compounds eluted from column into flame.The ions collected on collector or electrode and wererecorded on recorder due to electric current.
Thermal Conductivity DetectorMeasures the changes of thermal conductivity dueto the sample (µg). Sample can be recovered.
Principal: The thermal balance of a heated filamentElectrical power is converted to heat in a resistantfilament and the temperature will climb until heatpower loss form the filament equals the electricalpower input.The filament may loose heat by radiation to a coolersurface and by conduction to the molecules cominginto contact with it.
Thermal Conductivity DetectorWhen a compound elutes, the thermalconductivity of the gaseous mixture of carrier gasand compound gas is lowered, and the filament inthe sample column becomes hotter than the othercontrol column.Its resistance increased, and this imbalancebetween control and sample filament resistances ismeasured by a simple gadget and a signal isrecorded
Thermal Conductivity Detector• Responds to all compounds• Adequate sensitivity for many compounds• Good linear range of signal• Simple construction• Signal quite stable provided carrier gas glow rate,block temperature, and filament power are controlled• Nondestructive detection
For pesticide analysis (picogram).Accept electrons of carrier gas.
Electron-Capture Detectors(ECD) The electron-capture detector has become one of the most widely used detectors for environmental samples because this detector selectivity detects halogen containing compounds, such as pesticides and polychlorinated biphenyls. The effluent from the column is passed over a β emitter, usually nickel-63. An electron from the emitter causes ionization of the carrier gas and the production of a burst of electrons. In the absence of organic species, a constant standing current between a pair of electrodes results from this ionization process. The current decreases markedly, however, in the presence of those organic molecules that tend to capture electrons.
Electron-Capture Detectors(ECD) The electron-capture detector is selective in its response being highly sensitive to molecules containing electronegative functional groups such as halogens, peroxides, quinones, and nitro groups. It is insensitive to functional groups such as amines, alcohols, and hydrocarbons. An important application of the electron- capture detector has been for the detection and determination of chlorinated insecticides.
Thermionic Detectors (TID)The thermionic detector is selective toward organiccompounds containing phosphorus and nitrogen. Itsresponse to a phosphorus atom is approximately tentimes greater than to a nitrogen atom and 104 to 106 largerthan a carbon atom. Compared with the flame ionizationdetector, the thermionic detector is approximately 500times more sensitive to phosphorus-containingcompounds and 50 times more sensitive to nitrogenbearing species. These properties make thermionicdetection particularly useful for detecting anddetermining the many phosphorus-containing pesticides.
Particle Size of SupportsThe efficiency of a gas-chromatographiccolumn increases rapidly with decreasingparticle diameter of the packing. The pressuredifference required to maintain a given flowrate of carrier gas, however, varies inversely asthe square of the particle diameter.
GLC ADVANTAGES1. Very good separation2. Time (analysis is short)3. Small sample is needed - µl4. Good detection system5. Quantitatively analyzed