Gas Chromatography

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

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 ______ of compound adsorbed to stationary phase mass Carrier gas Gas phase concentration

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

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

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)

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

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

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

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

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

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

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…

Temperature Control Options Example Method Column: Petrocol DH, 100m x 0.25mm ID, 0.5µm film Cat. No.: 24160-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