Monroe L. Weber-Shirk S chool of Civil and Environmental Engineering HEC-RAS.
Monroe L. Weber-Shirk S chool of Civil and Environmental Engineering Gas Chromatography
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Transcript of Monroe L. Weber-Shirk S chool of Civil and Environmental Engineering Gas Chromatography
Monroe L. Weber-Shirk
School of Civil and
Environmental Engineering
Gas Chromatography
Gas Chromatography
Come to lab prepared to work on a variety of tasks
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 chromatographyApplicationsTheory of operationCalibrationDetectors
stationary bedfluid
What is “Chromatography”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 phasegas 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 (H2, N2, O2, Ar, CO2, CO, CH4) 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 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 flameall 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
Strip chart technique?
Gas Chromatograph
injection volume
Outputchromatogramconverted to peak areas and peak times
Convert peak area to mass using injection of known mass (standard)peak area is proportional to mass injectedmass 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
gasOctane
Acetone
Toluene
vapor pressureat 25 °C
1.88 kPa
24 kPa
3.8 kPa
MW
114.23 g
58.08 g
92.14 g
density
0.71 g/mL
0.79 g/mL
0.87 g/mL
Example Calibration: Octane
PVn
RT=
K298
KmolkPaL
8.31
L10 x 100 kPa 1.88n
6-
nmol 75.9 =mol 10 x 9.57n 9
Calculate moles, mass, and equivalent liquid volume of 100 µL headspace sample at 25 °C.
g 8.67g 10 x 8.67mol
114.23gmol 10 x 75.9 69
nL 12.2 = L 10 x .221g 0.71L 10
g 10 x 8.67 93
6
liquidoctane
gas
KmolkPaL
8.31R
moles
mass
volume
Table
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.
38
4
mg/m 4.3g/L 4.3PA10 x 2g 8.67
L 100PA 1x10
sample PA
calibration PAsample volume
mass injected for calibration
Syringe Technique
The Problem:VOC vapors sorb to glass barrel, Teflon plunger, and
stainless steel needleThe Solution:
Remove GC needle.Purge syringe 10 times with room air to remove any residual
VOCs.Put on sample needle. (continued)
Syringe Technique: solutionSyringe 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/m3)
LC50CAS# 111-65-9: Inhalation, rat: LC50 =118
g/m3/4H.
336-
6
g/m 86.5m
L 1000L 10 x 100g 10 x 8.67
concentration in octane source vial
500(1 m3 of air is approximately 1 kg)
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 DetectorSpecific 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
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
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
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 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