Gas chromatography (GC) is another method that can be used for qualitative separation. GC, also
known as vapor-phase chromatography (VPC), is similar to the other types of chromatography and
is shown in Figure 12.7. The main conceptual difference is that the eluent is a gas (usually helium or
nitrogen) instead of a liquid. The adsorbent is a crushed metal or polymer inside a 30-foot column.
This column is coiled and kept inside an oven to control its temperature. The mixture is then
injected into the column and vaporized. The gaseous compounds travel through the column at
different rates because they adhere to the adsorbent in the column to different degrees and will
separate in space by the time they reach the end of the column. The injected compounds must be
volatile: low melting-point, sublimable solids or vaporizable liquids. The compounds are registered
by a detector, which records them as a peak on a chart.
Figure 12.7. Gas Chromatography
The sample is injected into the column and moves with the gaseous mobile phase through a
stationary liquid or solid phase; a computer identifies the sample components.It is common to separate molecules using GC and then to inject the pure molecules into a mass
spectrometer for molecular weight determination. Mass spectrometry involves the ionization and
fragmentation of compounds; these fragments are then run through a magnetic field, which
separates them by mass-to-charge ratio. The total molecular weight can thus be determined, or the
relative concentrations of the different fragments can be calculated and compared against
reference values to identify the compound.