DETERMINACIÓN DE ESTRUCTURAS ORGÁNICAS
(ORGANIC SPECTROSCOPY)
GAS CHROMATOGRAPHY
Hermenegildo García Gómez
Departamento de Química
Instituto de Tecnología Química
Universidad Politécnica de Valencia
46022 Valencia
E-mail: [email protected]
Telephone: +34 96 387 7807 or ext. 78572/73441
Fax: + 34 96 387 7809
Gas Chromatograph:
an overview
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What is “chromatography”
History of chromatography
Applications
Theory of operation
Detectors
Syringe technique
What is “Chromatography”
• “color writing”
• “Chromatography is a physical method of
separation in which the components to be
separated are distributed between two phases,
one of the phases constituting a stationary bed
of large surface area, the other being a fluid that
percolates through or along the stationary bed.”
(Ettre & Zlatkis, 1967, “The Practice of Gas
Chromatography)
History of Chromatography
• 1903 - Mikhail Tswett separated plant
pigments using paper chromatography
– liquid-solid chromatography
• 1930’s - Schuftan & Eucken use vapor as
the mobile phase
– gas solid chromatography
Limitation
• Compound must exist as a in the vapor phase
at a temperature that can be produced by the
GC and withstood by the column (up to 450°C)
Applications
• Alcohols in blood
• Aromatics (benzene, toluene, ethylbenzene,
xylene)
• Flavors and Fragrances
• Permanent gases (H2, N2, O2, Ar, CO2, CO,
CH4)
• Hydrocarbons and fuels
• Pesticides, Herbicides, PCBs, and Dioxins
• Solvents
Advantages of Gas
Chromatography
• Requires only very small samples with little
preparation
• Good at separating complex mixtures into
components
• Results are rapidly obtained (1 to 100 minutes)
• Very high precision
• Only instrument with the sensitivity to detect
volatile organic mixtures of low concentrations
• Equipment is not very complex (sophisticated
oven)
Chromatogram of Gasoline
1. Isobutane
2. n-Butane
3. Isopentane
4. n-Pentane
5. 2,3-Dimethylbutane
6. 2-Methylpentane
7. 3-Methylpentane
8. n-Hexane
9. 2,4-Dimethylpentane
10. Benzene
11. 2-Methylhexane
12. 3-Methylhexane
13. 2,2,4-Trimethylpentane
14. n-Heptane
15. 2,5-Dimethylhexane
16. 2,4-Dimethylhexane
17. 2,3,4-Trimethylpentane
18. Toluene
19. 2,3-Dimethylhexane
20. Ethylbenzene
21. m-Xylene
22. p-Xylene
23. o-Xylene
Theory of Operation
• Velocity of a compound through the
column depends upon affinity for the
stationary phase
Area under curve is
______
mass of compound
adsorbed to stationary
phase
Carrier gas
Gas phase concentration
Process Flow Schematic
Sample injection
Carrier gas
(nitrogen or
helium)
Long Column (30 m)
Detector (flame
ionization
detector or FID)
Air
Hydrogen
Flame Ionization Detector
Teflon insulating ring
Gas outlet
Collector
Sintered disk
Coaxial cable to
Analog to Digital
converter
Ions
Flame
Platinum jet
Air
Hydrogen
Capillary tube (column)
Why do we need
hydrogen?
Flame Ionization Detector
• Responds to compounds that produce
____
ions when burned in an H2-air flame
– all organic compounds
• Little or no response to (use a Thermal
Conductivity Detector for these gases)
– CO, CO2, CS2, O2, H2O, NH3, inert gasses
• Linear from the minimum detectable
limit through concentrations ____
107 times
the minimum detectable limit
Gas Chromatograph Output
detector
output
area proportional to mass of
• Peak ____
compound injected
velocity through
• Peak time dependent on ______
column
time (s)
Other Detectors
• Thermal Conductivity Detector
– Difference in thermal conductivity between the
carrier gas and sample gas causes a voltage
output
low thermal
– Ideal carrier gas has a very ____
conductivity (He)
• Electron Capture Detector
– Specific for halogenated organics
TCE
methane
time
ECD output
Mixture containing
lots of methane and a
small amount of TCE
FID output
Advantage of Selective
Detectors
time
Purge and Trap
• Way to measure dilute samples by concentration
of constituents
• Trap constituents under low temperature
• Heat trap to release constituents and send to
GC column
N2
Trap