MUHAMMAD NADEEM ASHRAF

SOIL & ENVIRONMENTAL SCIENCES

SUBMIYYED TO :SIR NASIR RAHEEM

Faculty of Agriculture, Rawalakot

GAS LIQUID CHROMATOGRAPHY

Principles

Partition of molecules between gas (mobile phase) and liquid (stationary phase).

In gas chromatography (GC), the sample is vaporized and injected onto the head of a chromatographic column. Elution is brought about by the flow of an inert gaseous mobile phase. The mobile phase does not interact with molecule of the analyte; its only function is to transport the analyte through the column.Gas-liquid chromatography is based upon the partition of the analyte between a gaseous mobile phase and a liquid phase immobilized on the surface of an inert solid.

Filters/Traps

Air

Hyd

rog

en

Gas C

arrier

Column

gas system

inlet column detect

or data

system

Data system

Syringe/Sampler

Inlets

Detectors

Regulators

H

RESET

Schematic Diagram of Gas Chromatography

Detection Systems

Characteristics of the Ideal Detector: The ideal detector for gas chromatography has the following characteristics:1. Adequate sensitivity2. Good stability and reproducibility.3. A linear response to solutes that extends over several orders of magnitude.4. A temperature range from room temperature to at least 400oC.

Characteristics of the Ideal Detector

5. A short response time that is independent of flow rate.

6. High reliability and ease of use.

7. Similarity in response toward all solutes or a highly selective response toward one or more classes of solutes.

8. Nondestructive of sample.

DETECTORS

Flame 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 were recorded on recorder due to electric current.

Exhaust

Chimney

Igniter

Hydrogen Inlet

Column Effluent

Polarizing Electrode

Collector Electrode

Schematic Diagram of Flame Ionization DetectorSchematic Diagram of Flame Ionization Detector

Thermal Conductivity Detector

Measures the changes of thermal conductivity due to the sample (g). Sample can be recovered.

 

Principal: The thermal balance of a heated filament

Electrical power is converted to heat in a resistant filament and the temperature will climb until heat power loss form the filament equals the electrical power input.

The filament may loose heat by radiation to a cooler surface and by conduction to the molecules coming into contact with it.

When a compound elutes, the thermal conductivity of the gaseous mixture of carrier gas and compound gas is lowered, and the filament in the sample column becomes hotter than the other control column.

Its resistance increased, and this imbalance between control and sample filament resistances is measured by a simple gadget and a signal is recorded

Thermal Conductivity DetectorThermal Conductivity Detector

CompoundRelative Thermal

Conductivity

Carbon Tetrachloride 0.05

Benzene 0.11

Hexane 0.12

Argon 0.12

Methanol 0.13

Nitrogen 0.17

Helium 1.00

Hydrogen 1.28

• 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

Thermal Conductivity DetectorThermal Conductivity Detector

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 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.

Electron Capture DetectorElectron Capture Detector

Thermionic Detectors (TID)

The thermionic detector is selective toward organic compounds containing phosphorus and nitrogen. Its response to a phosphorus atom is approximately ten times greater than to a nitrogen atom and 104 to 106 larger than a carbon atom. Compared with the flame ionization detector, the thermionic detector is approximately 500 times more sensitive to phosphorus-containing compounds and 50 times more sensitive to nitrogen bearing species. These properties make thermionic detection particularly useful for detecting and determining the many phosphorus-containing pesticides.

Particle Size of Supports

The efficiency of a gas-chromatographic column increases rapidly with decreasing particle diameter of the packing. The pressure difference required to maintain a given flow rate of carrier gas, however, varies inversely as the square of the particle diameter.

GLC ADVANTAGES

1. Very good separation

2. Time (analysis is short)

3. Small sample is needed - l

4. Good detection system

5. Quantitatively analyzed