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Gas Chromatograph:an overview
What is “chromatography” History of chromatography Applications Theory of operation Detectors Syringe technique
stationary bedfluid
What is “Chromatography”
“color writing” the separation of mixtures into their constituents by
preferential adsorption by a solid” (Random House College Dictionary, 1988)
“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 ______________ of large surface area, the other being a ______ 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
gas
Applications
Compound must exist as a ____ at a temperature that can be produced by the GC and withstood by the column (up to 450°C)
Alcohols in blood Aromatics (benzene, toluene, ethylbenzene, xylene) Flavors and Fragrances Permanent gases (H2, N2, O2, Ar, CO2, CO, CH4) Hydrocarbons 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 GasolineChromatogram of Gasoline
1. Isobutane2. n-Butane3. Isopentane4. n-Pentane5. 2,3-Dimethylbutane6. 2-Methylpentane7. 3-Methylpentane8. n-Hexane9. 2,4-Dimethylpentane10. Benzene11. 2-Methylhexane12. 3-Methylhexane13. 2,2,4-Trimethylpentane14. n-Heptane15. 2,5-Dimethylhexane16. 2,4-Dimethylhexane17. 2,3,4-Trimethylpentane18. Toluene19. 2,3-Dimethylhexane20. Ethylbenzene21. m-Xylene22. p-Xylene23. o-Xylene
Theory of Operation
Velocity of a compound through the column depends upon affinity for the stationary phase
Area under curve is ______ of compound adsorbed to stationary phase
Gas phase concentrationCarrier gas
mass
Process Flow Schematic
Carrier gas (nitrogen or helium)
Sample injection
Long Column (30 m)
Detector (flame ionization detector or FID) Hydrogen
Air
Gas Chromatograph Components
Flame Ionization Detector
Column
Oven
Injection Port
top view
front view
Flame Ionization Detector
Hydrogen
Air
Capillary tube (column)
Platinum jet
Collector
Sintered disk
Teflon insulating ring
Flame
Gas outlet
Coaxial cable to Analog to Digital converterIons
Why do we need hydrogen?
Flame Ionization Detector
Responds to compounds that produce ____ 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 ____ times the minimum detectable limit
ions
107
Gas Chromatograph Output
time (s)
dete
ctor
ou
tpu
t
Peak ____ proportional to mass of compound injected
Peak time dependent on ______ through column
area
velocity
Other DetectorsOther Detectors
Thermal Conductivity Detector Difference in thermal conductivity between the
carrier gas and sample gas causes a voltage output
Ideal carrier gas has a very ____ thermal conductivity (He)
Electron Capture Detector Specific for halogenated organics
Thermal Conductivity Detector Difference in thermal conductivity between the
carrier gas and sample gas causes a voltage output
Ideal carrier gas has a very ____ thermal conductivity (He)
Electron Capture Detector Specific for halogenated organics
low
Advantage of Selective DetectorsAdvantage of Selective Detectors
methane
TCE
time
time
FID
ou
tpu
tEC
D
ou
tpu
t
Mixture containing lots of methane and a small amount of TCE
Gas chromatograph
Mass SpectrophotometerMass Spectrophotometer
Uses the difference in mass-to-charge ratio (m/e) of ionized atoms or molecules to separate them from each other.
Molecules have distinctive fragmentation patterns that provide structural information to identify structural components.
The general operation of a mass spectrometer is: create pure gas-phase ions ( __________________ ) separate the ions in space or time based on their mass-to-
charge ratio measure the quantity of ions of each mass-to-charge ratio
Uses the difference in mass-to-charge ratio (m/e) of ionized atoms or molecules to separate them from each other.
Molecules have distinctive fragmentation patterns that provide structural information to identify structural components.
The general operation of a mass spectrometer is: create pure gas-phase ions ( __________________ ) separate the ions in space or time based on their mass-to-
charge ratio measure the quantity of ions of each mass-to-charge ratio
Mass Spec OutputMass Spec Output
Each peak of a chromatogram becomes a “fingerprint” of the compound
The fingerprints are compared with a library to identify the compounds
Each peak of a chromatogram becomes a “fingerprint” of the compound
The fingerprints are compared with a library to identify the compounds
mass-to-charge ratio
Purge and TrapPurge 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
Way to measure dilute samples by concentration of constituents Trap constituents under low temperature Heat trap to release constituents and send to GC column
N2N2
Trap
Techniques to Speed Analysis
Problem: some components of a mixture may have very high velocities and others extremely low velocities.
slow down fast components so they can be separated
speed up slow components so analysis doesn’t take forever
Solution…