Monroe L. Weber-Shirk S chool of Civil and Environmental Engineering Gas Chromatography 

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

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

stationary bed fluid 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  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 Gas Chromatography 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 (H 2, N 2, O 2, Ar, CO 2, CO, CH 4 )  Hydrocarbons  Pesticides, Herbicides, PCBs, and Dioxins  Solvents Depending on the column

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 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

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 concentration Carrier 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 converter Ions Why do we need hydrogen?

Flame Ionization Detector  Responds to compounds that produce ____ when burned in an H 2 -air flame  all organic compounds  Little or no response to (use a Thermal Conductivity Detector for these gases)  CO, CO 2, CS 2, O 2, H 2 O, NH 3, inert gasses  Linear from the minimum detectable limit through concentrations ____ times the minimum detectable limit ions 10 7

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

Gas Chromatograph injection volume  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 (If a calibrated mass isn’t required)

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. liquid gas Octane Acetone Toluene vapor pressure at 25 °C 1.88 kPa 24 kPa 3.8 kPa MW g g g density 0.71 g/mL 0.79 g/mL 0.87 g/mL

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

VOC Contaminated Site Map  Report gas concentrations in mg/m 3.  Example: Given a peak area of 1 x 10 4 from an injection volume of 100 µL, calculate the concentration in mg/m 3. Assume the peak area from the source vial injections was 2 x sample PA calibration PA sample volume mass injected for calibration

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)

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

Octane Exposure Limits  OSHA PEL (Permissible exposure level)  500 ppm TWA (approximately ____ mg/m 3 )  LC50  CAS# : Inhalation, rat: LC50 =118 g/m 3 /4H. concentration in octane source vial 500 (1 m 3 of air is approximately 1 kg)

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

Gas chromatograph 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

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 N2N2 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…

Temperature Control Options 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