GAS CHROMATOGRAPHY
(GC)
GAS CHROMATOGRAPHY
He
Injection
port
Oven
Detector
Recordercomputer
Carrier Gas
Column
Sample is injected (using a syringe) into the injection port.
Sample vaporizes and is forced into the column by the carrier gas ( = mobile
phase which in GC is usually helium)
Components of the sample mixture interact with the stationary phase so that
different substances take different amounts of time to elute from the column.
The separated components pass through a detector. Electronic signals, collected
over time, are sent to the GC software, and a chromatogram is generated.
Capillary Tube
GAS CHROMATOGRAPHY
Liquid Stationary Phase
A ABA
A
A
A A
BA B B
B
He Carrier gas
AA B
A BA
B
He Carrier gas
A
A
A A
B
B
B B
B
B
B
B B
0
Immediately after injection
A
After several minutes
Time
Resulting chromatogram
Compounds A and B interact with the stationary phase through intermolecular forces:
(van der Waals or dipole-dipole forces, including hydrogen bonding).
A interacts more strongly with the stationary liquid phase and is retained relative to B,
which interacts weakly with the stationary phase. Thus B spends more time in the gas
phase and advances more rapidly through the column and has a shorter retention
time than A.
Typically, components with similar polarity elute in order of volatility. Thus alkanes
elute in order of increasing boiling points; lower boiling alkanes will have shorter
retention times than higher boiling alkanes.
Pentane
GC - Alkane Standards
Isooctane
Gas Chromatograph of alkane standard mixture containing equimolar amounts of:
n-hexane,
3-methyl-pentane,
3-methylhexane,
3-methylheptane,
all dissolved in pentane.
2,3-dimethylbutane,
n-heptane,
n-octane,
2,2,4-trimethyl-pentane
(= isooctane),
GC of Alkane Standards vs. distillation fraction #1
Standards
Distillation Fraction #1
GC – Peak Areas and Resolution
GC – Isothermal vs Temperature Programming
GC – Example Chromatograms
GC – Packed vs. Capillary Columns
Alltech Chromatography catalog, 550
GC – Packed vs. Capillary Columns
GC – Stationary Phase
Experimental Organic Chemistry D. R. Palleros, Wiley, NY, 2000
GC – Stationary Phase
Alltech Chromatography catalog, 550
GC – Elution order vs Stationary Phase
Alltech Chromatography catalog, 550
GC - Derivatization
Why is chemical derivatization needed?
GC is best for separation of volatile compounds which are thermally stable.
Not always applicable for compounds of high molecular weight or containing polar
functional groups. These groups are difficult to analyze by GC either because they
are not sufficiently volatile, tail badly, are too strongly attracted to the stationary
phase, thermally unstable or even decomposed.
Chemical derivatization prior to analysis is generally done to:
•
•
•
•
increase the volatility and decrease the polarity of compounds;
reduce thermal degradation of samples by increasing their thermal stability;
increase detector response
improve separation and reduce tailing
Derivatizing Reagents Common derivatization methods can be classified into 4
groups depending on the type of reaction applied:
•
•
•
•
Silylation
Acylation
Alkylation
Esterification
GC - Derivatization
Derivatizing Reagents Common derivatization methods can be classified into 4
groups depending on the type of reaction applied:
•
Silylation
•
Acylation
•
Alkylation
•
Esterification
Alltech.com
GC – Resolution and Efficiency
Skoog and Leary: Principals of Instrumental Analysis, 4 th ed. Suanders, 1992
GC – Resolution vs Column Efficiency (N, H)
H=L/N
van Deemter Equation
H = A + B/u +(Cs + Cm)u
Skoog and Leary: Principals of Instrumental Analysis,
5th ed. Suanders, 1998
CHROMATOGRAPHY
Preparative vs Resolution vs Speed vs Expense