Gas Chromatography

Gas Chromatography

Gas Chromatograph:an overview

What is “chromatography” History of chromatography Applications Theory of operation Detectors Syringe technique

stationary bedfluid

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 phase gas solid chromatography

gas

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

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 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

time (s)

dete

ctor

ou

tpu

t

Peak ____ proportional to mass of compound injected

Peak time dependent on ______ through column

area

velocity

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 Detector Specific for halogenated organics

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 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

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

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

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

Example Method