GAS CHROMATOGRAPHY
Gas Chromatography
• Chromatography used to separate and analyze
volatile components is called gas chromatography
Gas Chromatography
Gas – Solid (GSC) Gas – Liquid (GLC)
Stationary Phase
Sample MUST be volatile
GC and Column Chromatography
Ø Used in analytical chemistry for separating and
anlysing compounds that can be vaporised without
decomposition GC
Ø In GC mobile phase is a carrier gas (He or N2 gas
Ø Stationary phase is microscopic layer of liquid on
an inert solid support, inside a column
Ø Gaseous compounds analysed interact with the walls
of column which is coated with stationary phases
causes each compound to elute at different time
known as retention time
Ø When separated mixture directly injected into mass
spectrometer, then called GC-MS Column
Gas Liquid Chromatography
• Principles
Partition of molecules between gas (mobile
phase) and liquid (stationary phase).
Most Common Stationary Phases
1. Separation of mixture of polar compounds
Carbowax 20M (polyethylene glycol)
2. Separation of mixtures of non-polar compounds
OV101 or SE-30 (polymer of methylsilicone)
3. Methylester of fatty acids
DEGS (diethylene glycol succinate)
Gas Chromatography
Filters/Traps Data system
H
RESET
Regulators Syringe/Sampler
Inlets
Detectors
• gas system
Gas Carrier
Hydrogen
• inlet
Air
Column
• column
• detector
• data system
Schematic Diagram of Gas Chromatography
Flame Ionization Detector
(FID)
• Flame Ionization Detector (Nanogram - ng)
High temperature of hydrogen flame (H2 +O2 + N2)
ionizes compounds eluted from column into flame.
The ions collected on collector or electrode and were
recorded on recorder due to electric current.
§ Ions are attracted to the collector plate and upon
hitting produces current
§ Current measured with ammeter and feed the signal
to amplifier, integrator and display system
FID
Applications
• FID is excellent for the detections of hydrocarbons
and other easily flammable components
• i.e FID is excellent to detect methane in nitrogen
because it would response to methane not to
nitrogen
Drawbacks
• FID destroy most of the components it detecting
Thermal Conductivity Detector
(TCD)
Thermal conductivity is the property of a material’s ability to
conduct heat (k)
Measures the changes of thermal conductivity due to the sample
(mg).
Most compounds have less thermal conductivity than carrier
gases (He or H2), when analyte elutes from the column the
effluent thermal conductivity is reduced and a detectable signal is
produced
Sample can be recovered.
Relative Thermal Conductivity
Compound Relative Thermal Conductivity
Carbon Tetrachloride 0.05
Benzene 0.11
Hexane 0.12
Argon 0.12
Methanol 0.13
Nitrogen 0.17
Helium 1.00
Hydrogen 1.28
Thermal Conductivity Detector
• Responds to all compounds
• Adequate sensitivity for many compounds, Generally less
sensitive than FID
• Good linear range of signal
• Simple construction
• Called Universal Detector
Electron Capture Detector (ECD)
ECD is a device for detecting atoms and
molecules in a gas through the
attatchment of e- by electron capture
ionization
Most sensitive detector (10-12 gram)
Analyses for pesticide, Insecticides,
vinyl chloride, and fluorocarbons in
foods.
Electron Capture Detector
ECD detects positive ions of carrier gas by the anode electrode.
63Ni emits β particles.
Ionization : N2 (Carrier gas) + β (e) = N2+ + 2e. The N2+ establish a
“base line”
X (F, Cl and Br) containing sample + β (e) à X-
Ion recombination: X- + N2+ = X + N2, The “base line” due to the
N2+ will decrease and this decrease constitutes the signal.
The more the halogen containing X compounds in the sample, the
less the N2+ in the detector
Electron Capture Detector
Chromatogram of Compounds from Fermented Cabbage
Chromatogram of Orange Juice Compounds
Gas Chromatography Application
Semi-Quantitative Analysis of Fatty Acids
C18
10
C16 8
Peak Area
6
Response
Detector
C14 4
0.5 1.0 1.5 2.0 2.5 3.0
Sample Concentration (mg/ml)
Retention Time
C14
The content % of C14 fatty acids = ∗ 100
C14+ C16+ C 18
= the content % of C14 fatty acids
Tentative Identification of Unknown Compounds
Mixture of known compounds
Response
Octane Decane
1.6 min = RT
Hexane
GC Retention Time on Carbowax-20 (min)
Response
Unknown compound may be Hexane
1.6 min = RT
Retention Time on Carbowax-20 (min)
Retention Times
RT= 4.0 min on SE-30
Response
Hexane
GC Retention Time on SE-30
RT= 4.0 min on SE-30
Response
Unknown compound
GC Retention Time on SE-30
Retention index (RI)
Ø The retention indices of a certain chemical compound is its retention
time normalised to the retention indices of adjacent eluting n-alkanes
Ø Retention time may vary with chromatographic system (column length,
film thickness, diameter, carrier gas velocity, pressure) but RI remains
same
RI = [ n + log t` NX – log t` Nn ]
log t` N(n+1) – log t` Nn
t’N = Net retention time = tr – t0; t` NX = retention time for unknown;
t` Nn = smaller n-alkane; t` N(n+1) = larger n-alkane
and the analyte elutes between Cn and Cn+1
⎡ log(20.6 − 1.2) − log(16.2 − 1.2) ⎤
Iunk = 100⎢3 + ⎥
⎣ log( 25 .0 − 1.2 ) − log(16 . 2 − 1.2 ) ⎦
Iunk = 356
Advantages of Gas Chromatography
• Very good separation
• Time (analysis is short)
• Small sample is needed - µl
• Good detection system
• Quantitatively analyzed
Disadvantages of Gas Chromatography
Material has to be volatilized at 250 oC without decomposition.
Fatty Acids Methylester
O O
R C OH + CH3OH + H2SO4 R C O CH3
Reflux Volatile in Gas
Chromatography
O
CH2 O C R
O CH3ONa O
CH O C R + CH3OH 3 R C O CH3
Volatile in Gas
O Chromatography
CH2 O C R
Gas Chromatogram of Methyl Esters of Fatty Acids
Effects of OH groups of Carbohydrates
6
CH OH
2
O
5
4 1
OH
HO
OH
3 2
OH
Derivation of Glucose with Trimethylchlorosilane
6
CH2 OH
O CH 3
5
4
OH 1 + 5Cl Si CH 3
HO
3 2 OH CH 3
OH
Glucose Trimethylchlorosilane
6
CH2 O-Si(CH3)3
O
5
4 + 5HCl
O-Si(CH ) 1
3 3
(CH3)3-Si-O
3 2 O-Si(CH3)3
O-Si(CH3)3
Effects of Derivation
• Time consumption
• Side reaction
• Loss of sample
GC Limitations
Solutes must be:
1. Thermally Stable
2. Relatively Volatile
3. MW < 400
