Gas Chromatography

1. Gas Chromatography

2. Come to lab prepared to work on a variety of tasks • Map the location of a VOC spill [9 total] • Begin assembling your research apparatus

3. Gas Chromatograph:an overview • What is “chromatography” • History of chromatography • Applications • Theory of operation • Calibration • Detectors

4. 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) stationary bed fluid

5. 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

6. 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 Depending on the column

7. 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)

8. Chromatogram 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

9. 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

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

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

12. 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)

13. Flame Ionization Detector ions • 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 107

14. Gas Chromatograph Output area • Peak ____ proportional to mass of compound injected • Peak time dependent on ______ through column Strip chart technique? velocity detector output time (s)

15. Gas Chromatograph • Output • chromatogram • converted to peak areas and peak times • Convert peak area to mass using injection of known mass (standard) • peak area is proportional to mass injected • mass injected can be converted to concentration given _________ _________ • Alternately use peak area (PA) as surrogate for mass injection volume (If a calibrated mass isn’t required)

16. vapor pressure at 25 °C MW density Octane 1.88 kPa 114.23 g 0.71 g/mL Acetone 24 kPa 58.08 g 0.79 g/mL Toluene 3.8 kPa 92.14 g 0.87 g/mL Gas Chromatograph Calibration • We can use the headspace sample from source vials to calibrate the GC. • We will use the ideal gas law and the vapor pressure of the VOCs. gas liquid

17. gas liquid octane Example Calibration: Octane Calculate moles, mass, and equivalent liquid volume of 100 µL headspace sample at 25 °C. Table moles mass volume

18. VOC Contaminated Site Map • Report gas concentrations in mg/m3. • Example: Given a peak area of 1 x 104 from an injection volume of 100 µL, calculate the concentration in mg/m3. Assume the peak area from the source vial injections was 2 x 108. sample PA mass injected for calibration sample volume calibration PA

19. Syringe Technique • The Problem: • VOC vapors sorb to glass barrel, Teflon plunger, and stainless steel needle • The Solution: • Remove GC needle. • Purge syringe 10 times with room air to remove any residual VOCs. • Put on sample needle. (continued)

20. Syringe Technique: solution • Insert into sample bottle (with syringe at zero volume). • Fill syringe fully with gas and purge syringe contents back into the source bottle (repeat 3 times). • Fill syringe and adjust to 100 µL. • Close syringe valve and remove syringe from sample vial and remove sample needle. • Put on GC needle. • Instruct GC to measure sample. • Insert needle in injection port, open syringe valve, inject sample, hit enter button all as quickly as possible. • Remove syringe from the GC injection port. Equilibrate with headspace Eliminate needle carryover

21. Octane Exposure Limits • OSHA PEL (Permissible exposure level) • 500 ppm TWA (approximately ____ mg/m3) • LC50 • CAS# 111-65-9: Inhalation, rat: LC50 =118 g/m3/4H. 500 (1 m3 of air is approximately 1 kg) concentration in octane source vial

22. 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

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

24. 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

25. 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

26. 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

27. 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…

28. Column: Petrocol DH, 100m x 0.25mm ID, 0.5µm filmCat. No.: 24160-UOven: 35°C (15 min) to 200°C at 2°C/min, hold 5 minCarrier: helium, 20cm/sec (set at 35°C)Det.: FID, 250°CInj.: 0.1µL premium unleaded gasoline, split (100:1), 250°C Temperature Control Options