GAS CHROMATOGRAPHY
ORG I LAB
Dr. W. J. KELLY
THE GAS CHROMATOGRAPH
THE GAS CHROMATOGRAPH
•
A gas mixture,called the Mobile
Phase, composed of an inert
“carrier” gas (usually Helium) and
various other chemical components
in the vapor phase introduced at the
Injecton Port pass through a nonmoving material, called the...
•
Stationary Phase, usually tightly
packed into a copper tube called the
Column…
–
•
which separates the gas stream into
its particular components based on
how “quickly” they move through the
stationary phase. The components
reach a...
Detector at different times, the
detector generates an electrical signal
which is sent to a...
•
Recorder to record the signal.
• Since many chemical substances are solids or
liquids at room temperature, the stationary
phase-mobile phase must be heated to higher
temperature in order to “vaporize” the
components of the mobile phase. Thus an
external heating device or...
• Oven is needed. The temperature of the
oven need not be above the boiling points of the
components but rather above the “dew point” of
the least volatile component
THE GC PROCESS
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THE STATIONARY PHASE
•
The mobile phase can be separated out by the stationary phases through several different
processes. The two most common processes are Liquid-Partition Chromatography and
Adsorption Chromatography. We will initially consider the former case.
•In Liquid-Partition Chromatography,
the stationary phase consists of a very
porous, inert, finely powered material
called the Solid Support. The solid
support is coated with a thin film of
material called the Liquid Phase.
TYPICAL LIQUID PHASES
•Most liquid phases are very high
boiling liquids, some are waxes at
room temperature.
THE SEPARATION PROCESS
•
As the mobile phase passes
through the stationary phase, the
different components of the
mobile phase will dissolve into
the liquid film of the stationary
phase. The solubility of the
gasses in the liquid phases
depends on a number of factors,
but one of the most important is
the volatility of the liquid from
which the component vapor
drives. Gasses from low boiling
liquids will spend less time
dissolved in the liquid phase,
whereas gasses from less
volatile liquids will spend more
time in solution with the liquid
phase. The less time a gas
spends dissolved in the liquid
phases, the more time it spends
flying along with the carrier
gas. Thus it moves through the
stationary phase more quickly.
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THERMAL CONDUCTIVITY DETECTOR
PEAK RESOLUTION
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•
•
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Temperature
Carrier Gas Pressure
Column Length
Column Diameter
Column Efficiency and HETP
•
The resolution in a gas
chromatogram can be expressed, as
in the case of distillaton, in terms
of the number of theoretical plates.
A simple formula can be used to
derive the number of plates from
the position and width of the GC
peak:
• n = 16(d/w)2
•
Where the number of plates (n) is
given by the width (w) and the
retention time (d) measure in the
same units ( mm, cm ,inch or
minuits, etc.). Like distillation, the
overall efficiency of a stationary
phase can be expressed in terms of
HETP where:
• HETP = n/l
Peak Analysis