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