diameters of 10–20 cm enabling an easy handling of the long columns and a
positioning of them in an oven.
In gas chromatography normally a liquid phase is used as stationary phase. In
order to obtain an immobilized fluid, not a true liquid but a pseudo-liquid is utilized.
For this purpose, polymer molecules are fixed (mostly chemically) at the inner wall
of the column. Due to its length and the resulting mobility similar to liquid molecules, these polymer films act physically similar to fluids. The retention interaction
of the by far highest proportion of GC columns used in Organic Geochemistry is
based on the partition between liquid and gas phase. Hence, the chemical properties
of the film determine the retention behavior. A major part of GC columns is
equipped with polysiloxane based films. Chemical derivatization of the
polysiloxanes with aliphatic moieties (e.g. methyl groups) produce nonpolar films,
insertion of more polar moieties such as nitrile groups or phenyl rings enhance the
polarity, whereas films composed of ethylene glycol moieties represent very polar
films (see Fig. 4.3).
Dimension and type of film are highly influencing the gas chromatographic
separation. The thicker the film, the higher the retention and, as a consequence, the
retention time but also the separation capacity increases (see Fig. 4.4). Changing the
type of film influences the separation capacity but also the retention order of the
individual substances. This becomes visible especially for compounds of different
chemical properties as illustrated in Fig. 4.4. On a similar way, the dimension of the
column systematically alters the separation efficiency but also the analysis time. A
summary of the quantitative impact of column length and diameter is given in
Table 4.1.
Noteworthy, the huge success of gas chromatographic analyses is related also to a
second interaction superimposing drastically the already introduced partition processes. In former times, gas chromatographic separations have been performed at a
constant temperature (isothermal), but in modern times a ramped temperature program is applied during the separation process. As a result, a type of distillation is
additionally overlaid. Hence, the separation depends on both boiling point and
polarity of the analytes. The influence of temperature on the separation efficiency
is exemplified in Fig. 4.5. Typical temperature programs start in the region of solvent
boiling points (50–80
C), have a short isothermal time around 2–5 min and ramp
following the temperature to endpoints around 300–350
C with a heating rate
between 2
C and 15
C/min. At the end, the final temperature is hold for several
minutes. Taking such common values in mind, analysis times between 30 and
100 min are typical.
Beside the stationary also the mobile phase is an important element of chromatography. In contrast to liquid chromatography, the role of the mobile gas phase in
GC is less important. One key aspect is the demand that the carrier gas must not react
with the analytes or the solvent. Hence, only chemically inert gases can be used and,
therefore, the spectrum of gases is very limited comprising solely hydrogen nitrogen
and helium. For these gases one parameter has an influence on the separation
efficiency, the carrier gas velocity. Theoretical description of this influence is
given by the Van-Deemter equation, where the term H (separation step, an inverse
4.1 High Performance Chromatography: GC, HPLC
41
positioning of them in an oven.
In gas chromatography normally a liquid phase is used as stationary phase. In
order to obtain an immobilized fluid, not a true liquid but a pseudo-liquid is utilized.
For this purpose, polymer molecules are fixed (mostly chemically) at the inner wall
of the column. Due to its length and the resulting mobility similar to liquid molecules, these polymer films act physically similar to fluids. The retention interaction
of the by far highest proportion of GC columns used in Organic Geochemistry is
based on the partition between liquid and gas phase. Hence, the chemical properties
of the film determine the retention behavior. A major part of GC columns is
equipped with polysiloxane based films. Chemical derivatization of the
polysiloxanes with aliphatic moieties (e.g. methyl groups) produce nonpolar films,
insertion of more polar moieties such as nitrile groups or phenyl rings enhance the
polarity, whereas films composed of ethylene glycol moieties represent very polar
films (see Fig. 4.3).
Dimension and type of film are highly influencing the gas chromatographic
separation. The thicker the film, the higher the retention and, as a consequence, the
retention time but also the separation capacity increases (see Fig. 4.4). Changing the
type of film influences the separation capacity but also the retention order of the
individual substances. This becomes visible especially for compounds of different
chemical properties as illustrated in Fig. 4.4. On a similar way, the dimension of the
column systematically alters the separation efficiency but also the analysis time. A
summary of the quantitative impact of column length and diameter is given in
Table 4.1.
Noteworthy, the huge success of gas chromatographic analyses is related also to a
second interaction superimposing drastically the already introduced partition processes. In former times, gas chromatographic separations have been performed at a
constant temperature (isothermal), but in modern times a ramped temperature program is applied during the separation process. As a result, a type of distillation is
additionally overlaid. Hence, the separation depends on both boiling point and
polarity of the analytes. The influence of temperature on the separation efficiency
is exemplified in Fig. 4.5. Typical temperature programs start in the region of solvent
boiling points (50–80
C), have a short isothermal time around 2–5 min and ramp
following the temperature to endpoints around 300–350
C with a heating rate
between 2
C and 15
C/min. At the end, the final temperature is hold for several
minutes. Taking such common values in mind, analysis times between 30 and
100 min are typical.
Beside the stationary also the mobile phase is an important element of chromatography. In contrast to liquid chromatography, the role of the mobile gas phase in
GC is less important. One key aspect is the demand that the carrier gas must not react
with the analytes or the solvent. Hence, only chemically inert gases can be used and,
therefore, the spectrum of gases is very limited comprising solely hydrogen nitrogen
and helium. For these gases one parameter has an influence on the separation
efficiency, the carrier gas velocity. Theoretical description of this influence is
given by the Van-Deemter equation, where the term H (separation step, an inverse
4.1 High Performance Chromatography: GC, HPLC
41