1. GAS LIQUID CHROMATOGRAPHY
Principles
Partition of molecules between gas (mobile phase) and liquid (stationary phase).

2. Gas Chromatography – Gas Chromatography Uses
Separation and analysis of organic compounds
Testing purity of compounds
Determine relative amounts of components in mixture
Compound identification
Isolation of pure compounds (microscale work)
Similar to column chromatography, but differs in 3 ways:
Partitioning process carried out between Moving Gas Phase and Stationary Liquid Phase
Temperature of gas can be controlled
Concentration of compound in gas phase is a function of the vapor pressure only.
GC also known as Vapor-Phase Chromatography (VPC) and Gas-Liquid Partition Chromatography (GLPC)
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3. Gas Chromatography – Gas Chromatograph Microliter Syringe
Heated injection port with rubber septum for inserting sample
Heating chamber with carrier gas injection port
Oven containing copper, stainless steel, or glass column.
Column packed with the Stationary Liquid Phase  a non-volatile liquid, wax, or low melting solid-high boiling hydrocarbons, silicone oils, waxes or polymeric esters, ethers, and amides
Liquid phase is coated onto a support material, generally crushed firebrick
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4. Gas Chromatography – Principals of Separation
Column is selected, packed with Liquid Phase, and installed.
Sample injected with microliter syringe into the injection port where it is vaporized and mixed into the Carrier Gas stream (helium, nitrogen, argon).
Sample vapor becomes partitioned between Moving Gas Phase and Stationary Liquid Phase.
The time the different compounds in the sample spend in the Vapor Phase is a function of their Vapor Pressure.
The more volatile (Low Boiling Point / Higher Vapor Pressure) compounds arrive at the end of the column first and pass into the detector
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5. Gas Chromatography – Principals of Detection Two Detector Types
Thermal Conductivity Detector (TCD)
Flame Ionization
TCD is electrically heated “Hot Wire” placed in carrier gas stream
Thermal conductivity of carrier gas (helium in our case) is higher than most organic substances.
Presence of sample compounds in gas stream reduces thermal conductivity of stream
Wire heats up and resistance decreases.
Two detectors used: one exposed to sample gas and the other exposed to reference flow of carrier gas.
Detectors form arms of Wheatstone Bridge, which becomes unbalanced by sample gas.
Unbalanced bridge generates electrical signal, which is amplified and sent to recorder
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6. Gas Chromatography – Factors Affecting Separation
Boiling Points of Components in Sample
Low boiling point compounds have higher vapor pressures.
High boiling point compounds have lower vapor pressures requiring more energy to reach equilibrium vapor pressure, i.e., atmospheric pressure.
Boiling point increases as molecular weight increases.
Flow Rate of Carrier Gas
Choice of Liquid Phase
Molecular weights, functional groups, and polarities of component molecules are factors in selecting liquid phase.
Length of Column
Similar compounds require longer columns than dissimilar compounds. Isomeric mixtures often require quite long columns
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7. Most Common Stationary Phases
1. Separation of mixture of polar compounds
Carbowax 20M (polyethylene glycol)
2. Separation of mixtures of non-polar compounds
OV101 or SE-30 (polymer of methylsilicone)
Methylester of fatty acids
DEGS (diethylene glycol succinate)

8. Gas Chromatography gas system inlet column detector data system
Filters/Traps
Data system H
RESET
Regulators
Syringe/Sampler
Air
Hydrogen
Gas Carrier
Inlets
Detectors
gas system
inlet
column
detector
data system
Column

9. Schematic Diagram of Gas Chromatography

10. Schematic Diagram of Gas Chromatography

11. 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.

12. Schematic Diagram of Flame Ionization Detector
Exhaust
Chimney
Igniter
Hydrogen
Inlet
Column
Effluent
Polarizing Electrode
Collector Electrode

13. Schematic Diagram of Flame Ionization Detector
Collector
Detector electronics
 volts
Flame
Chassis ground
Jet
Signal output
Column

14. Thermal Conductivity Detector
Measures the changes of thermal conductivity due to the sample (mg). Sample can be recovered.

15. Thermal Conductivity Detector
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.

16. Thermal Conductivity Basics
The TCD is a nondestructive, concentration sensing detector. A heated filament is cooled by the flow of carrier gas
When the carrier gas is contaminated by sample , the cooling effect of the gas changes. The difference in cooling is used to generate the detector signal.
Flow
Flow

17. Thermal Conductivity Detector
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

18. Thermal Conductivity Detector

19. Relative Thermal Conductivity
Compound
Relative Thermal Conductivity
Carbon Tetrachloride
0.05
Benzene
0.11
Hexane
0.12
Argon
Methanol
0.13
Nitrogen
0.17
Helium
1.00
Hydrogen
1.28

20. 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

21. Electron Capture Detector
For pesticide analysis (picogram).
Accept electrons of carrier gas.

22. Electron Capture Detector
ECD detects ions in the exiting from the gas chromatographic
column by the anode electrode.
3H or 63Ni which emits  particles.
Ionization : N2 (Nitrogen carrier gas) +  (e) = N2+ + 2e
These N2+ establish a “base line”
X (F, Cl and Br) containing sample +  (e)  X-
Ion recombination : X- + N2+ = X + N2
The “base line” will decrease and this decrease constitutes the signal.
Insecticides, pesticides, vinyl chloride, and fluorocarbons

23. Electron Capture Detector

24. Electron Capture Detector

25. Gas Chromatography Application

26. SEMI- QUANTITATIVE ANALYSIS OF FATTY ACIDS

27. TENTATIVE IDENTIFICATION OF UNKNOWN COMPOUNDS

28. Retention Times

29. GLC ADVANTAGES 1. Very good separation 2. Time (analysis is short)
3. Small sample is needed - ml
4. Good detection system
5. Quantitatively analyzed

30. DISADVANTAGES OF GAS CHROMATOGRAPHY
Material has to be volatilized at 250C without decomposition.

31. Gas Chromatogram of Methyl Esters of Fatty Acids

32. The Effects of OH groups of CarbohydratesHO CH 2 1 3 4 5 6 OH O HO CH 2 1 3 4 5 6

33. Derivation of Glucose with TrimethylchlorosilaneOH O HO CH 2 1 3 4 5 6 Si CH 3
5Cl +
Glucose
Trimethylchlorosilane 6 CH
O-Si(CH3)3 2 O 5 +
5HCl 4 1
O-Si(CH3)3
(CH3)3-Si-O
O-Si(CH3)3 3 2
O-Si(CH3)3

34. Effects of Derivation 1. Time consumption 2. Side reaction
3. Loss of sample

35. THIN LAYER CHROMATOGRAPHY
Stationary Phase > Silica Gel
Mobile Phase > Solvent (developing)

36. THIN LAYER CHROMATOGRAPHY

37. Thermal Conductivity Detector
The detector contains two filaments: one exposed only to carrier gas, while the other is exposed to the carrier gas for sample analysis.
When the gas for the sample analysis is only carrier gas , the two filaments can be balanced.
Instead of a direct measurement of filament temperature, the filament resistant, which is a function of temperature, is measured.

38. Thermal Conductivity Detector
The ability of a colliding molecule to carry off heat depending on its thermal conductivity. Hydrogen and helium have high thermal conductivity and therefore will be more efficient at “cooling” a heated filament than other gases will

39. Thermal Conductivity Detector
The TCD will respond to any substance
different from the carrier gas as long as its
concentration is sufficiently high enough.

40. Thermal Conductivity Detector

41. Thermal Conductivity Detector

42. Electron Capture Detector
Electron capture compound, X (highly electonegative element), tends to capture free electrons and increase the amount to ion recombination
X (F, Cl and Br) + e  X-
Ion recombination : X- + N2+ = X + N2
The current will decrease and this decrease constitutes the signal.
Halogens, lead, phosphorous, nitro groups, silicone and polynuclear aromatics.
Insecticides, pesticides, vinyl chloride, and fluorocarbons

43. Electron Capture Detector