Samples are injected onto the 'top' of the column, through a sample injection
port containing a gas-tight septum. The two common sample injection
methods for capillary GC are:
- Split/splitless injector: in the split mode only a portion of the injected
sample (typically, 1 part in 50) reaches the column. The rest is vented to
waste. A split injector is used for concentrated samples (> 0.1 mgmL1 for FID). In the splitless mode all the sample volume injected
passes through to the column. It is used, in this mode, for trace samples
(< 0.1 mgmL1 for FID).
- Cold on-column injector: all the sample is injected onto the column. It is
used for thermally unstable compounds and high-boiling solvents.
In both cases a syringe (1 µL) is used to inject the sample. Examples of each
type of injection system are shown in Fig. 32.2. The procedure for injection
of a sample is shown in Fig. 32.3. In Fig. 32.3a the syringe is
filled with the sample/standard solution (typically 0.5 µL). Then the outside
of the syringe is wiped clean with a tissue (Fig. 32.3b). The syringe is placed
into the injector of the gas chromatograph (Fig. 32.3c) and, finally, the
plunger on the syringe is depressed to inject the sample (Fig. 32.3d).
|Fig. 32.2 Sample introduction in GC:
(a) split/splitless injector.
Fig. 32.2 Sample introduction in GC:
(b) on-column injector.
|Fig. 32.3 Sample injection in GC. (a) Fill the syringe, (b) wipe clean the outside of the syringe needle, (c) place the syringe needle into the injector and (d) depress the plunger on the syringe to inject the sample.
Modern GC uses capillary columns (internal diameter 0.1-0.5mm) up to
60m in length. The stationary phase is generally a cross-linked silicone
polymer, coated as a thin film on the inner wall of the fused silica (SiO2
capillary: at normal operating temperatures, this behaves in a similar manner
to a liquid film, but is far more robust. Common stationary phases for GC
are shown in Fig. 32.4. The mobile phase ('carrier gas') is usually nitrogen or
helium. Selective separation is achieved as a result of the differential
partitioning of individual compounds between the carrier gas and silicone
polymer phases. The separation of most organic molecules is influenced by
the temperature of the column, which may be constant during the analysis
('isothermal' - usually 50-250°C) or, more commonly, may increase in a preprogrammed
manner (e.g. from 50°C to 250°C at 10°C per minute).
Selecting an appropriate column for capillary GC is a difficult task and
one which is usually left to the technician. However, it is important to be
aware of some general issues and what influence they can have on the
separation. The column internal diameter can affect both resolution and
speed of analysis. Smaller internal diameters columns (0.25mm i.d.) can
provide good resolution of early eluting peaks (Fig. 32.5a). However, the
problem is that the analysis times of the eluting components may be longer
and that the linear dynamic range may be restricted. In contrast,
larger internal diameter columns (0.53 mm i.d.) provide less resolution for
early eluting compounds (Fig. 32.5b), but this is reflected in shorter analysis
times and a greater linear dynamic range. This type of column
may provide sufficient resolution for the analysis of complex mixtures. Fig.
32.5 illustrates the effects of column internal diameter.
|Fig. 32.4 Common stationary phases for capillary GC.
Fig. 32.5 Influence of GC column internal
diameter on separation: 1. phenol; 2. 2-
chlorophenol; 3. 2-nitrophenol; 4. 2,4-
dimethyl phenol; 5. 2,4-dichlorophenol; 6. 4-
chloro-3-methylphenol; 7. 2,4,6-trichlorophenol;
8. 2,4-dinitrophenol; 9. 4-nitrophenol; 10. 2-
Another important column effect is the length of the column and the
influence this can have on the resolution of eluting components. It was
previously shown that resolution was influenced by K'
, α and N
Substituting eqn [31.4] into eqn [31.6] produces the following equation:
|⇒ Equation [32.3]
||α − 1
||k' + 1
The importance of this equation can be shown by considering the influence
on resolution, R
, of column length, L
. Under isothermal analysis conditions,
i.e. the same column temperature, the retention of eluting compounds is more
dependent upon column length. For example, doubling the column length
doubles the analysis times and increases the resolution by 41%. This is shown
in Fig. 32.6 for the analysis of phenols. In contrast, under temperatureprogrammed
analysis, e.g. 130°C to 250°C at 4°C/min−1
, the retention time
of eluting components is more dependent on temperature. For example,
doubling the column length has minimal effect on analysis times. This is
shown in Fig. 32.7 for the analysis of bacterial acid methyl esters.
Fig.32.6 Influence of column length on analysis time. Analysis of phenols under isothermal conditions: 1. phenol; 2. a-cresol;
xylenol; 4. p-cresol; 5. n-cresol; 6. a-ethylphenol; 7. 2,4-xylenol; 8. 2,5-xylenol; 9. 2,3-xylenol; 10. p-ethylphenol;
11. m-ethylphenol; 12.
3,5-xylenol; 13. 3,4-xylenol.
|Fig.32.7 Influence of column length on analysis time. Analysis of bacterial acid methyl esters under temperature-programmed
The output from the GC column is monitored by a detector. The most
commonly used detectors for GC analysis of organic molecules are as
The flame ionization detector (FlD) is particularly useful for the analysis
of a broad range of organic molecules. It involves passing the exit gas stream
from the column through a hydrogen flame that has a potential of more than
100 V applied across it (Fig. 32.8). Most organic compounds, on passage
through this flame, produce ions and electrons that create a small current
across the electrodes, and this is amplified for measurement purposes. The
FID is very sensitive (typically down to ≈ 0.1 pg), with a linear response over
a wide concentration range. One drawback is that the sample is destroyed
The thermal conductivity detector (TCD) is based on changes in the
thermal conductivity of the gas stream brought about by the presence of
separated sample molecules. The detector elements are two electrically heated
platinum wires, one in a chamber through which only the carrier gas flows
(the reference detector cell), and the other in a chamber that takes the gas
flow from the column (the sample detector cell). In the presence of a constant
gas flow, the temperature of the wires (and therefore their electrical
resistance) is dependent on the thermal conductivity of the gas. Analytes in
the gas stream are detected by temperature-dependent changes in resistance
based on the thermal conductivity of each separated molecule; the size of the
signal is directly related to concentration of the analyte.
The advantages of the TCD include its applicability to a wide range of
organic and inorganic molecules and its non-destructive nature, since the
sample can be collected for further study. Its major limitation is its low
sensitivity (down to ≈ l0ng), compared with other systems.
The electron capture detector (ECD) is highly sensitive (Fig. 32.9) and is
useful for the detection of certain compounds with electronegative functional
groups, e.g. halogens, peroxides and quinones. The gas stream from the
column passes over a β
-emitter such as 63
Ni, which provides electrons
that cause ionization of the carrier gas (e.g. nitrogen). When carrier gas alone
is passing the β
-emitter, its ionization results in a constant current flowing
between two electrodes placed in the gas flow. However, when electroncapturing
sample molecules are present in the gas flow, a decrease in current
is detected. An example of the application of the ECD is in detecting and
quantifying chlorinated pesticides.
|Fig. 32.8 Components of a flame ionization detector
|Fig. 32.9 Components of an electron capture detector (ECD).
Mass spectrometry used in conjunction with GC provides
a powerful tool for identifying the components of complex mixtures, e.g.
environmental pollutants, synthetic products, etc. (Fig. 32.10). The procedure
requires computer control of the instrument and for data storage/analysis.
Compounds eluting from the column are bombarded by electrons (electron
impact, El, mode) causing fragmentation and production of charged species.
These charged species are separated by the mass spectrometer on the basis of
their mass-to-charge ratio. Ions passing through the mass spectrometer are
detected by an electron multiplier tube. The mass spectrometer can be used in
two modes: total ion and selected ion monitoring. In the former mode, the
|Fig. 32.10 Schematic diagram of a GCMS
complete mass spectrum of each of the components of the mixture eluting from the column is recorded. In the latter mode, only ions of specified massto- charge ratios are detected. Selected ion monitoring offers increased sensitivity and selectivity.