Transcript Gas Chromatography

Gas
Chromatography
Gas Chromatograph:
an overview
 What
is “chromatography”
 History of chromatography
 Applications
 Theory of operation
 Detectors
 Syringe technique
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 ______________
stationary bed of large surface area,
the other being a ______
fluid 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
Applications
Compound must exist as a ____
gas 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 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
11. 2-Methylhexane
12. 3-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
______
mass of compound
adsorbed to stationary
phase
Carrier gas
Gas phase concentration
Process Flow Schematic
Sample injection
Carrier gas
(nitrogen or
helium)
Long Column (30 m)
Detector (flame
ionization
detector or FID)
Air
Hydrogen
Gas Chromatograph Components
top view
Injection Port
Column
Oven
front view
Flame
Ionization
Detector
Flame Ionization Detector
Teflon insulating ring
Gas outlet
Collector
Sintered disk
Coaxial cable to
Analog to Digital
converter
Ions
Flame
Platinum jet
Air
Hydrogen
Capillary tube (column)
Why do we need
hydrogen?
Flame Ionization Detector
 Responds
to compounds that produce ____
ions
when burned in an H2-air flame
 all
organic compounds
 Little
or no response to (use a Thermal
Conductivity Detector for these gases)
 CO,
 Linear
CO2, CS2, O2, H2O, NH3, inert gasses
from the minimum detectable limit
107 times the
through concentrations ____
minimum detectable limit
Gas Chromatograph Output
____
area proportional to mass of compound
injected
 Peak time dependent on velocity
______ through column
detector
output
 Peak
time (s)
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 ____
low thermal
conductivity (He)
 Electron
Capture Detector
 Specific
for halogenated organics
TCE
methane
time
ECD output
Mixture containing
lots of methane and a
small amount of TCE
FID output
Advantage of Selective Detectors
time
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 ( Gas
__________________
chromatograph )

separate the ions in space or time based on their massto-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

N2
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
Example Method
Column: Petrocol DH, 100m
x 0.25mm ID, 0.5µm film
Cat. No.: 24160-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