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What is Gas Chromatography?
Chromatography is the separation of a mixture
of compounds (solutes) into separate components. By separating the sample
into individual components, it is easier to identify (qualitate) and
measure the amount (quantitate) of the various sample components. There
are numerous chromatographic techniques and corresponding instruments. Gas
chromatography (GC) is one of these techniques. It is estimated that
10-20% of the known compounds can be analyzed by GC. To be suitable for GC
analysis, a compound must have sufficient volatility and thermal
stability. If all or some of a compound’s molecules are in the gas or
vapor phase at 400-450°C or below, and they do not decompose at these
temperatures, the compound can probably be analyzed by GC.
The main parts of a basic GC system are shown in Figure 1. One or more
high purity gases are supplied to the GC. One of the gases (called the
carrier gas) flows into the injector, through the column and then into the
detector. A sample is introduced into the injector usually with a syringe
or an exterior sampling device. The injector is usually heated to 150-250°C
which causes the volatile sample solutes to vaporize. The vaporized
solutes are transported into the column by the carrier gas. The column is
maintained in a temperature controlled oven. The solutes travel through
the column at a rate primarily determined by their physical properties,
and the temperature and composition of the column. The various solutes
travel through the column at different rates. The fastest moving solute
exits (elutes) the column first then is followed by the remaining solutes
in corresponding order. As each solute elutes from the column, it enters
the heated detector. An electronic signal is generated upon interaction of
the solute with the detector. The size of the signal is recorded by a data
system and is plotted against elapsed time to produce a chromatogram.
Figure 1. The Basic Components of a GC System
The ideal chromatogram has closely spaced peaks with no overlap of the
peaks. Any peaks that overlap are called coeluting. The time and size of a
peak are important in that they are used to identify and measure the
amount of the compound in the sample. The size of the resulting peak
corresponds to the amount of the compound in the sample. A larger peak is
obtained as the concentration of the corresponding compound increases. If
the column and all of operating conditions are kept the same, a given
compound always travels through the column at the same rate. Thus, a
compound can be identified by the time required for it to travel through
the column (called the retention time). The identity of a compound cannot
be determined solely by its retention time. A known amount of an
authentic, pure sample of the compound has to be analyzed and its
retention time and peak size determined. This value can be compared to the
results from an unknown sample to determine whether the target compound is
present (by comparing retention times) and its amount (by comparing peak
sizes). If any of the peaks overlap, accurate measurement of these peaks
is not possible. If two peaks have the same retention time, accurate
identification is not possible. Thus, it is desirable to have no peak
overlap or co-elution.
Inside a Capillary GC Column
capillary GC column is comprised of two major
parts - tubing and stationary phase. A thin film (0.1-10.0 µm) of a high
molecular weight, thermally stable polymer is coated onto the inner wall
of small diameter (0.05-0.53 mm I.D.) tubing. This polymer coating is
called the stationary phase. Gas flows through the tubing and is called
the carrier gas or mobile phase.
Upon introduction into the column, solute molecules distribute between
the stationary and mobile phases. The molecules in the mobile phase are
carried down the column; the molecules in the stationary phase are
temporarily immobile and do not move down the column. As the molecules in
the mobile phase move through the column, some of them eventually collide
with and re-enter the stationary phase. During the same time span, some of
the solute molecules leave the stationary phase and enter the mobile
phase. This occurs thousands of times for each solute molecule as it
passes through the column. All of the molecules corresponding to a
specific compound travel through the column at nearly the same rate and
appear as a band of molecules (called the sample band). The goal is to
have no overlap between adjacent sample bands as they exit the column.
This is accomplished by making each sample band travel at a different rate
and by minimizing the width of the sample band. The rate at which each
sample band moves through the column depends on the structure of the
compound, the chemical structure of the stationary phase and the column
temperature. The width of the sample band depends on the operating
conditions and the dimensions of the column. The proper column and
operating conditions are critical in obtaining no, or the least amount of,
peak co-elution.
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