3.3 Fractionation
Outlook
The second step of organic-geochemical analyses is the coarse fractionation of
the raw extracts. This process is commonly based on a chromatographic
separation. Principally, gas and liquid chromatography are used in Organic
Geochemistry.
3.3.1 Principles of Chromatography
As a result of extraction procedures, one obtains a complex mixture of organic
substances in a defined organic solvent. For further analytical measurements, these
mixtures are normally too complex to be analyzed directly. Therefore, a second main
sample treatment step is needed, the fractionation of the raw extract into several
subfractions. Basic method for the separation is the chromatography.
Chromatography works principally with two phases, the stationary and the
mobile phase (see Fig. 3.13). The latter one flows through or passes alongside the
stationary phase. The mixture of analytes is commonly dissolved in the mobile phase
and is injected at the starting point of the system. Then the analytes get transported
by convection with the mobile phase through the chromatographic system. Due to
this process, the mobile phase is also called eluent. The convection is superimposed
by dispersion of the analytes as the result diffusion of the individual molecules
within the mobile phase. The extent of dispersion depends on the flow time and
forms the typical shape of chromatographic signal, the peak. In average some of the
molecules have a summarized diffusion pathway with a direction parallel to mobile
phase flow resulting in a slight faster movement and forming the back tailing of the
peak. Molecules with a summarized pathway smaller than the average flow effect the
front tailing. As a result, chromatographic peaks look ideally like a symmetric
normal distribution. Finally, the most important effect of chromatography called
retention provokes the actual separation of different substances. The retention is
based on an interaction of the analytes with the stationary phase. These interactions
can vary widely and comprise e.g. ionic interaction, van-der-Waals forces or partition processes. If the analytes interact to a different extent with the stationary phase,
the analytes become retarded divergently and get separated. It is obvious, that the
interaction depends on the chemical and physico-chemical properties of the analytes
and both chromatographic phases.
Consequently, the selection of the chemical composition of stationary and mobile
phase is an important parameter to optimize the separation efficiency. All chromatographic techniques can be divided roughly by the state of matter of both phases (see
Fig. 3.14). Dominant classification parameter is the mobile phase defining gas and
liquid chromatography. A more special technique uses supercritical fluids as eluents
3.3 Fractionation
29
Outlook
The second step of organic-geochemical analyses is the coarse fractionation of
the raw extracts. This process is commonly based on a chromatographic
separation. Principally, gas and liquid chromatography are used in Organic
Geochemistry.
3.3.1 Principles of Chromatography
As a result of extraction procedures, one obtains a complex mixture of organic
substances in a defined organic solvent. For further analytical measurements, these
mixtures are normally too complex to be analyzed directly. Therefore, a second main
sample treatment step is needed, the fractionation of the raw extract into several
subfractions. Basic method for the separation is the chromatography.
Chromatography works principally with two phases, the stationary and the
mobile phase (see Fig. 3.13). The latter one flows through or passes alongside the
stationary phase. The mixture of analytes is commonly dissolved in the mobile phase
and is injected at the starting point of the system. Then the analytes get transported
by convection with the mobile phase through the chromatographic system. Due to
this process, the mobile phase is also called eluent. The convection is superimposed
by dispersion of the analytes as the result diffusion of the individual molecules
within the mobile phase. The extent of dispersion depends on the flow time and
forms the typical shape of chromatographic signal, the peak. In average some of the
molecules have a summarized diffusion pathway with a direction parallel to mobile
phase flow resulting in a slight faster movement and forming the back tailing of the
peak. Molecules with a summarized pathway smaller than the average flow effect the
front tailing. As a result, chromatographic peaks look ideally like a symmetric
normal distribution. Finally, the most important effect of chromatography called
retention provokes the actual separation of different substances. The retention is
based on an interaction of the analytes with the stationary phase. These interactions
can vary widely and comprise e.g. ionic interaction, van-der-Waals forces or partition processes. If the analytes interact to a different extent with the stationary phase,
the analytes become retarded divergently and get separated. It is obvious, that the
interaction depends on the chemical and physico-chemical properties of the analytes
and both chromatographic phases.
Consequently, the selection of the chemical composition of stationary and mobile
phase is an important parameter to optimize the separation efficiency. All chromatographic techniques can be divided roughly by the state of matter of both phases (see
Fig. 3.14). Dominant classification parameter is the mobile phase defining gas and
liquid chromatography. A more special technique uses supercritical fluids as eluents
3.3 Fractionation
29