1. Gas Chromatography

2. Applications
gas
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

3. 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)

4. 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 Methylhexane 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

5. 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
mass
Carrier gas
Gas phase concentration

6. Process Flow Schematic
Detector (flame ionization detector or FID)
Sample injection
Carrier gas (nitrogen or helium)
Air
Hydrogen
Long Column (30 m)

7. Gas Chromatograph Components
top view
Flame Ionization Detector
Injection Port
Column
Oven
front view

8. Flame Ionization Detector
Teflon insulating ring
Coaxial cable to Analog to Digital converter
Gas outlet
Collector
Ions
Flame
Sintered disk
Platinum jet
Air
Hydrogen
Why do we need hydrogen?
Capillary tube (column)

9. 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

10. Gas Chromatograph Output
Peak ____ proportional to mass of compound injected
Peak time dependent on ______ through column
area
velocity
detector
output
time (s)

11. Other 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
low

12. Advantage of Selective Detectors
TCE
Mixture containing lots of methane and a small amount of TCE
FID output
methane
time
ECD output
time

13. Mass 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
Gas chromatograph

14. Mass Spec Output
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

15. 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

16. 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…

17. Temperature Control Options
Example Method
Column: Petrocol DH, 100m x 0.25mm ID, 0.5µm film Cat. No.: U Oven: 35°C (15 min) to 200°C at 2°C/min, hold 5 min Carrier: helium, 20cm/sec (set at 35°C) Det.: FID, 250°C Inj.: 0.1µL premium unleaded gasoline, split (100:1), 250°C