Section F – Combined techniques
F3 GAS CHROMATOGRAPHY-
MASS SPECTROMETRY
Principles The separation of mixtures by gas chromatography requires that they are
volatile within the operating temperature range of the instrument. Since stable
stationary phases are available for use up to 400°C and ovens may be tempera-
ture-programmed to operate from ambient to high temperatures, this allows the
separation of many samples, provided they do not decompose in the system.
The separated components may be classified according to their retention times
or by chromatographing spiked samples, but for unambiguous identification,
other techniques are required. Mass spectrometry, which is fully described in
Topic E14, is an important identification tool. Solutes may be ionized by electron
impact, or by softer techniques such as chemical ionization. This is very useful
in the identification of biological and less stable species.
Instrumentation The effluent gases from the gas chromatograph contain both the carrier gas and
the separated components at a pressure close to atmospheric. Detection by a
flame ionization detector (FID) or one of the other GC detectors is possible, and
effected by splitting the effluent stream at the column exit, allowing most of
the sample to enter the mass spectrometer. It is then necessary to reduce the
pressure to the operating pressure of the mass spectrometer, which is around
10 −^8 Nm−^2. With a capillary GC column, the flow of carrier gas is small, and the
effluent can be fed through a fine capillary directly into the mass spectrometer.
For a packed GC column, an interface between the GC and the MS is required.
This may either be a porous tube separator, or a jet separator, shown in Figure
1(b). In both of these, the low-mass carrier gas, usually helium, diffuses away
Key Notes
The use of chromatographic techniques to separate mixtures is one of the
most important analytical tools. The separated components may then be
identified by other techniques. Mass spectrometry is the most important
of these.Combining gas chromatography with mass spectrometry requires special
methods of interfacing since the two techniques operate under different
conditions.Gas chromatography-mass spectrometry has been used to study the
separation and identification of volatile mixtures such as natural
products, crude oils and environmental samples.Related topics Gas chromatography: principles Mass spectrometry (E14)
and instrumentation (D4)
Gas chromatography: procedures
and applications (D5)PrinciplesInstrumentationApplications