Gas Chromatography
427 PHC
Gas Chromatograph
• Gas chromatography (GC), is a
common type of chromatography
used in analytic chemistry for
separating and analyzing compounds
that can be vaporized without
decomposition.
Principle:
• A method of analysis by which the analyte is
vaporized and introduced into a stream of
carrier gas.
• It is conducted through a chromatographic
column and separated into its constituents.
• These fractions pass through the column at
characteristic rates, and are detected as they
emerge in a time sequence.
• The detecting cell responses are recorded on a
chart, from which the components can be
identified both qualitatively and quantitatively.
• In GC, the moving phase (mobile phase) is a
carrier gas, usually an inert gas such as helium
or nitrogen.
• The stationary phase is a microscopic layer of
liquid or polymer on an inert solid support,
inside a piece of glass or metal tubing called a
column.
• The instrument used to perform GC is called a
gas chromatograph.
• The gaseous compounds being analyzed interact
with the walls of the column, which is coated
with different stationary phases.
• This causes each compound to elute at a
different time, known as the retention time of
the compound.
• The comparison of retention times is what gives
GC its analytical usefulness.
• Gas chromatography is in principle similar to
column chromatography , but has several notable
differences:
▫ The process of separating the compounds in a mixture
is carried out between a liquid stationary phase and a
gas moving phase, whereas in column chromatography
the stationary phase is a solid and the moving phase is
a liquid.
▫ The column through which the gas phase passes is
located in an oven where the temperature of the gas
can be controlled, whereas column chromatography
has no such temperature control.
▫ The concentration of a compound in the gas phase is
solely a function of the vapor pressure of the gas.
Typical uses of GC include:
• Testing the purity of a particular substance.
• Separating the different components of a
mixture (the relative amounts of such
components can also be determined).
• In some situations, GC may help in identifying a
compound.
• In preparative chromatography, GC can be used
to prepare pure compounds from a mixture.
GC Instrument
Schematic Diagram of Gas Chromatograph
Carrier gas:
• The carrier gas must be chemically inert.
Commonly used gases include nitrogen,
helium, argon, and carbon dioxide.
• The choice of carrier gas is often
dependant upon the type of detector which
is used.
• The carrier gas system also associated
with pressure regulators and flow meters.
• In addition, it contains a molecular sieve
to remove water and other impurities.
Flow control:
• Flow rates are controlled by a 2 stage pressure
regulator:
▫ At the gas cylinder.
▫ Mounted in the chromatograph.
• Inlet pressure 10-50 psi →
F = 25-150 ml/min with packed column
F = 1-25 ml/min with capillary column
The flow rate will be constant if the inlet pressure
remains constant.
Sample injection port:
• The injector is a piece of hardware attached to
the column head.
• It provides the means to introduce a sample into
a continuous flow of carrier gas.
• For optimum column efficiency, the sample
should not be too large, and should be
introduced onto the column as vapor.
• Slow injection of large samples causes band
broadening and loss of resolution.
Common injector types are:
• Microflash vaporizer direct injector:
▫ It involves the use of a microsyringe to inject the
sample through a rubber septum into a flash
vaporizer port located at the head of the column.
▫ The temperature of the sample port is usually
about 50°C higher than the boiling point of the
least volatile component of the sample.
▫ Used for packed columns, where the sample size
vary from a few tenth of microliter to 20 ul.
Capillary columns require much smaller
-3
samples ( 10 ul), so a sample splitter
system is used to deliver only a small
fraction of the injected sample to the
column head, with the rest going to waste.
• Sample splitter (Split/Splitless) injector:
▫ The injector can be used in one of two modes; split
or splitless.
▫ a sample is introduced into a heated small
chamber via a syringe through a septum (the heat
facilitates volatilization of the sample).
▫ The vaporized sample/carrier gas mixture then
either sweeps entirely (splitless mode) or as
portion (split mode) into the column.
▫ In split mode, a part of the sample/carrier gas
mixture in the injection chamber is exhausted
through the split vent.
▫ Split injection is preferred when working with
samples with high analyte concentrations (>0.1%).
▫ Splitless injection is best suited for trace analysis
with low amount of analyte (<0.01%).
For quantitative work, more reproducible
sample size are required and this can be
obtained by a rotary sample valve.
• Rotary sample valve:
▫ gaseous samples in collection bottles are
connected to what is most commonly a sixport switching valve.
▫ The carrier gas flow is not interrupted while a
sample can be expanded into a previously
evacuated sample loop.
▫ Upon switching, the contents of the sample
loop are inserted into the carrier gas stream.
Column:
• There are two general types of column:
▫ Packed column
▫ Capillary column (open tubular).
• All the GC studies in the early 1950s were
carried out on packed column.
• In the late 1950s capillary column were
constructed that much superior in speed and
column efficiency (≈ 300000 plates).
• Capillary columns did not gain widespread until
the late 1970s due to several reasons:
▫ Small sample capacity.
▫ Difficulties in coating the column.
▫ Tendencies of columns to clog.
▫ Short lifetimes of poorly prepared columns.
▫ Fragileness of columns.
▫ Mechanical problems in sample introduction
and connection to the detector.
Capillary column:
• Capillary columns have an internal diameter of a few
tenths of a millimeter.
• They were constructed of stain-less steel, aluminum,
copper, plastic, or glass.
• They can be one of two types; wall-coated open tubular
(WCOT) or support-coated open tubular (SCOT).
• Wall-coated columns consist of a capillary tube whose
walls are coated with liquid stationary phase.
• In support-coated columns, the inner wall of the
capillary is lined with a thin layer of support material,
onto which the stationary phase has been adsorbed.
• SCOT columns are generally less efficient than WCOT
columns, but both types are more efficient than packed
columns.
• In 1979, a new type of WCOT column appeared,
the Fused Silica Open Tubular (FSOT) column.
• It was drawn from specially purified silica that
contains metal oxides.
• These have much thinner walls than the glass
capillary columns, and are given strength by an
outside protective polyimide coating.
• These columns are flexible and can be bent into
coils.
• They have the advantages of physical strength,
flexibility and low reactivity.
Packed column:
• Packed columns contain a
finely divided, inert, solid
support material coated
with a thin layer of liquid
stationary phase.
• Most packed columns are
1.5 - 10m in length and
have an internal diameter
of 2 - 4mm.
• They are made from glass,
metals, or Teflon.
Solid support materials:
• Hold the liquid stationary phase.
• Consists of small, uniform, spherical particles
with good mechanical strength.
• It should be inert at elevated temperature.
• The most widely used is prepared from the
naturally occurring diatomaceous earth
(skeleton of thousands of single-celled plantsdiatoms- lives in lakes and seas).
• The efficiency of a GC column increases with
decreasing particle size of the solid support.
Properties and characteristics of GC columns
FSOT
WCOT
SCOT
Packed
Length, m
10-100
10-100
10-100
1-6
Inside
diameter, mm
0.1-0.53
0.25-0.75
0.5
2-4
Efficiency,
plates/m
2000-4000
1000-4000
600-1200
500-1000
Sample size, ng
10-75
10-1000
10-1000
10-106
Relative back
pressure
Low
Low
Low
high
Relative speed
Fast
Fast
Fast
slow
Chemical
inertness
Best
→
→
Poorest
Flexible
Yes
No
No
No
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