Gas Chromatography Columns
Columns
Gas Chromatography.
columns are of two designs: packed or
capillary. Packed columns are typically a glass or stainless steel coil
(typically 1-5 m total length and 5 mm inner diameter) that is filled with the
stationary phase, or a packing coated with the stationary phase. Capillary
columns are a thin fused-silica (purified silicate glass) capillary (typically
10-100 m in length and 250 µm inner diameter) that has the stationary phase
coated on the inner surface. Capillary columns provide much higher separation
efficiency than packed columns but are more easily overloaded by too much
sample
Stationary
Phases
The most common stationary phases in
gas-chromatography columns are polysiloxanes, which contain various substituent
groups to change the polarity of the phase. The nonpolar end of the spectrum is
polydimethyl siloxane, which can be made more polar by increasing the
percentage of phenyl groups on the polymer. For very polar analytes,
polyethylene glycol (a.k.a. carbowax) is commonly used as the stationary phase.
After the polymer coats the column wall or packing material, it is often
cross-linked to increase the thermal stability of the stationary phase and
prevent it from gradually bleeding out of the column.
Small gaseous species can be
separated by gas-solid chromatography. Gas-solid chromatography uses packed
columns containing high-surface-area inorganic or polymer packing. The gaseous
species are separated by their size, and retention due to adsorption on the
packing material.
GC Columns
Stationary
Phases
The most common stationary phases in
gas-chromatography columns are polysiloxanes, which contain various substituent
groups to change the polarity of the phase. The nonpolar end of the spectrum is
polydimethyl siloxane, which can be made more polar by increasing the percentage
of phenyl groups on the polymer. For very polar analytes, polyethylene glycol
(a.k.a. carbowax) is commonly used as the stationary phase. After the polymer
coats the column wall or packing material, it is often cross-linked to increase
the thermal stability of the stationary phase and prevent it from gradually
bleeding out of the column.
Small gaseous species can be
separated by gas-solid chromatography. Gas-solid chromatography uses packed
columns containing high-surface-area inorganic or polymer packing. The gaseous
species are separated by their size, and retention due to adsorption on the
packing material.
GC Columnsadsorbent, are mostly used
for gas analysis. As a result of the simpler injection procedure and the more
precise sampling method, the packed column tends to give greater quantitative
accuracy and precision. However, despite its problems with sample injection,
the open tubular column is seen as the 'state of the art' column and is by far
the most popular column system in general use. The length of open tubular
columns range from about 10 m to 100 m and can have internal diameters from 100
mm to 500 mm. The stationary phase is coated on the internal wall of the
column as a film 0.2 mm to 1 mm thick.
The Packed GC Column
Packed columns are usually constructed from stainless steel or
Pyrex glass. Pyrex glass is favored when thermally labile materials are being
separated such as essential oils and flavor components. However, glass has
pressure limitations and for long packed columns, stainless steel columns are
used as they can easily tolerate the necessary elevated pressures. The sample
must, of course, be amenable to contact with hot metal surfaces. Short columns
can be straight, and installed vertically in the chromatograph. Longer columns
can be U-shaped but columns more than a meter long are usually coiled. Such
columns can be constructed of any practical length and relatively easily
installed. Pyrex glass columns are formed to the desired shape by coiling at
about 700˚C and metal columns by bending at room temperature. Glass columns
are sometimes treated with an appropriate silanizing
reagent to eliminate the surface hydroxyl groups which can be catalytically
active or produce asymmetric peaks. Stainless steel columns are usually washed
with dilute hydrochloric acid, then extensively with water followed by
methanol, acetone, methylene dichloride and n-hexane. This washing procedure
removes any corrosion products and traces of lubricating agents used in the
tube drawing process. The columns are then ready for packing.
walls and then initiating polymerization either by heat or an
appropriate catalyst. This locks the stationary phase to the column wall and is
thus completely immobilized. Polymer coatings can be formed in the same way
using dynamic coating. The techniques used for immobilizing the stationary
phases are also highly proprietary and little is known of the methods used by
companies that manufacture the columns. In any event, most chromatographers do
not want the trouble of coating their own columns and prefer to purchase
proprietary columns.
Very difficult separations can be achieved using the capillary
column, and in a relatively short time. An example of the separation of a
complex mixture on a capillary column is shown in figure 17. The column used
was designated as a VOCOL column and was 60 m long, 0.75 mm I.D. and carried a
film of stationary phase 1.5 micron thick. The column was held a 10˚C
for 6 minutes and then programmed to 170˚C at 6˚C per minute. The carrier gas was helium at a flow
rate 10 ml/min. The detector employed was the FID. This chromatogram
demonstrates the clear advantages of capillary columns over packed column. Not
only does the column produce exceeding high efficiencies but they are also achieved
with reasonable separation times.
Open Tubular Column Types
Open Tubular columns are broadly split into two classes, the wall
coated open tubular columns or WCOT Columns (which have already been
described and are by far the mot popular,) and the porous layer open tubes
or PLOT Columns. The two types of column are shown diagramatically in figure
18. The PLOT columns are largely used for gas analysis and the separation of
low molecular weight
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