Gas Chromatography - Download Now PowerPoint by linzhengnd

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									   GAS LIQUID CHROMATOGRAPHY


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

3. Methylester of fatty acids
   DEGS (diethylene glycol succinate)
                          Gas Chromatography
                               Filters/Traps                                 Data system
                                                                              H




                                                                                   RESET




  Regulators                                   Syringe/Sampler


                                               Inlets

                                                                 Detectors
                                                                                   gas
                 Gas Carrier
      Hydrogen




                                                                                    system
Air




                                                   Column
                                                                                   inlet
                                                                                   column
                                                                                   detector
                                                                                   data
                                                                                    system
Schematic Diagram of Gas Chromatography
Schematic Diagram of Gas Chromatography
                    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.
Schematic Diagram of Flame Ionization Detector


                      Exhaust



                      Chimney

          Igniter                 Collector Electrode
                                    Polarizing Electrode


           Hydrogen             Column
           Inlet                Effluent
Schematic Diagram of Flame Ionization Detector

                  Collector
                                   Detector electronics




                  - 220 volts

    Flame
                  Chassis ground


    Jet

               Column                  Signal output
Thermal Conductivity Detector


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

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




                                                     Flow
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
Thermal Conductivity Detector
Relative Thermal Conductivity
                       Relative Thermal
     Compound
                        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
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
   Electron Capture Detector

For pesticide analysis (picogram).


Accept electrons of carrier gas.
                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
Electron Capture Detector
Electron Capture Detector
Gas Chromatography Application
SEMI- QUANTITATIVE ANALYSIS OF FATTY ACIDS


    Detector Response
                                             C18        Peak Area (cm2 )
                                                            10

                          C 16                               8

                                                             6

                    C14                                     4

                                                             2

                                                                   0.5     1.0   1.5   2.0   2.5   3.0
                                                                   Sample Concentration (mg/ml)
                            Retention Time

                                                            C
              The content % of C fatty acids =
                                14                                          
                                                           +
                                                     C + C C        
                                                      

                                                   = the content % of C fatty acids
                                                                       14
TENTATIVE IDENTIFICATION OF UNKNOWN COMPOUNDS


            Response

                              Mixture of known compounds


                                             Octane
                                                                  Decane
                       1.6 min = RT

                          Hexane




                         GC Retention Time on Carbowax-20 (min)




            Response



                         Unknown compound may be Hexane




                           1.6 min = RT




                       Retention Time on Carbowax-20 (min)
Retention Times
Response




                 RT= 4.0 min on SE-30
                       Hexane




           GC Retention Time on SE-30




Response



                 RT= 4 min on SE-30

                    Unknown compound




           GC Retention Time on SE-30
     GLC ADVANTAGES


1.   Very good separation
2.   Time (analysis is short)
3.   Small sample is needed - ml
4.   Good detection system
5.   Quantitatively analyzed
DISADVANTAGES OF GAS CHROMATOGRAPHY


 Material has to be volatilized at 250C without decomposition.


      Fatty Acids               Methylester



          O                                        O
      R   C OH + CHOH + H SO4
                  3      2                     R   C O   CH
                                                          3
                                    Reflux     Volatile in Gas
                                               Chromatography
                    O
        CH 2   O C      R

                    O                         CH3 ONa         O
        CH     O C      R   +   CH 3 OH                  3R   C O   CH
                                                                     3
                                                          Volatile in Gas
                    O                                     Chromatography
        CH 2   O    C R
Gas Chromatogram of Methyl Esters of Fatty Acids
The Effects of OH groups of Carbohydrates


           6
           CH2 OH
                 O
             5            6
        4                 CH2 OH
            OH       1
         HO                     O
                            5
             3    2 OH 4
                 OH        OH       1
                        HO
                            3    2 OH
                                OH
Derivation of Glucose with Trimethylchlorosilane

       6
       CH2 OH
                O                       CH 3
        5
 4
       OH            1    + 5Cl         Si     CH 3
  HO
        3        2 OH                   CH 3
                OH
       Glucose                 Trimethylchlorosilane
                          6
                          CH2 O-Si(CH3)3
                                 O
                            5
                    4                                 +   5HCl
                          O-Si(CH ) 1
                                  3 3
            (CH3)3-Si-O
                           3      2 O-Si(CH3)3
                                 O-Si(CH3)3
Effects of Derivation

 1.   Time consumption
 2.   Side reaction
 3.   Loss of sample
  THIN LAYER CHROMATOGRAPHY


Stationary Phase --------->   Silica Gel


Mobile Phase ------------->   Solvent
(developing)
THIN LAYER CHROMATOGRAPHY



                                         DEVELOPING
                                         CHAMBER


                                           SPOT



                          SOLVENT



                                Solvent
                                Front




              1.1 cm


                                            5.5 cm



                                Origin




             Distance from starting origin to center of zone
      Rf =
             Distance from starting origin to solvent front

               5.5      = 0.5
        =
               11
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.
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
 Thermal Conductivity Detector


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

								
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