161
Total extracts and/or liquid chromatographic
subfractions are then analyzed by capillary column gas
chromatography using a flame ionization detector.
Except for hydrocarbon fractions, derivatization is
commonly applied to render polar lipids more volatile
in order to reduce gas chromatographic retention times
and to improve peak shape at the detector. Carboxylic
acids are usually transformed into their methyl esters,
and hydroxyl or amine groups into their trimethylsilyl
ether derivatives. Alternatively, both acid and hydroxyl
groups can be silylated. Acetate formation is another
common derivatization method. A variety of derivatization reagents are commercially available for this
purpose.
Only a few major compound series can be
recognized at the level of their molecular structures
based on relative retention times and distribution
patterns by gas chromatography alone. This applies
to n-alkanes in the nonaromatic hydrocarbon fraction,
n-fatty acids in the carboxylic acid fraction and in some
cases n-alkanols in the neutral polar fraction. High
abundance of a few single compounds (pristane,
phytane, long-chain alkenones) sometimes also allow
their direct identification from gas chromatograms.
The most powerful technique for assigning
molecular structures to constituents of complex
mixtures as they are found in the lipid extracts of
geological samples is the combination of capillary
column gas chromatography and mass spectrometry
(GC-MS). Although the expression “identification” is
frequently used, GC-MS alone is insufficient to fully
characterize a new compound whose gas chromatographic and mass spectrometric behavior has not been
described before. Normally, GC-MS analysis relies on
a comparison with GC and MS data published in the
(geochemical) literature or with data of standards,
commercially available or synthesized in the laboratory,
or on the interpretation of mass spectral fragmentation
patterns following common empirical rules (e.g.
McLafferty and Turecek 1993). Unfortunately, there is
no comprehensive compilation of mass spectra of
geochemically relevant organic compounds, although
a significant number of spectra was recently published
by de Leeuw (2004). Peters et al. (2005) have provided
a detailed coverage of hydrocarbons and selected polar
compounds of significance in petroleum geochemistry
and other fields of organic geochemistry.
The youngest, revolutionary development in
analytical organic geochemistry is the on-line coupling
of a gas chromatograph to an isotope ratio mass
spectrometer via a combustion interface (GC-irm-MS;
Hayes et al. 1990; Freeman et al. 1994). This instrument
allows the determination of stable carbon and hydrogen
isotope ratios of single organic compounds in complex
mixtures provided they are gas chromatographically
reasonably well separated. Chemotaxonomic relations
to specific precursor organisms are then possible if
these are distinct from other organisms in their carbon
isotope fractionation behavior during photosynthesis
and biosynthesis. Sample preparation and analysis are
similar to those for GC-MS analysis with the provision
that the isotopic composition of derivatizing agents is
accounted for in data interpretation.
4.6
The Future of Marine
Geochemistry of Organic
Matter
The evolution of organic geochemistry has always been
closely connected to the developments in instrumental
techniques for the analysis of organic compounds in
geological samples. Now that the instrumentation has
reached a high level of sophistication and particularly
sensitivity, one of the future targets will certainly be
higher stratigraphic (time) resolution which is particularly important for climate research. Advancement in
the fundamental understanding particularly of the early
part of the geological organic carbon cycle will depend
on the cooperation between organic geochemists and
microbiologists. They will have to refine the knowledge
of the biological effects on the early diagenesis of
organic matter arriving at the sediment-water interface
and becoming buried in the uppermost sediment. It will
be necessary to broaden the natural product inventory
of microorganisms, both of sedimentary bacteria and
archaea and of unicellular algae, protozoans and other
organisms at the lower end of the food chain in order
to arrive at solid chemotaxonomic relationships
between source organisms and molecular fossils.
Carefully designed laboratory simulation experiments together with high-resolution field studies will
refine the mass balance approaches of organic matter
exchange between sediment and water column related
to early diagenetic processes. Finally, mathematical
modeling of transport and reaction processes will
become an increasingly important tool in marine geochemistry of organic matter.
Acknowledgment
Dr. Rüdiger Stein (Alfred Wegener Institute of Polar
and Marine Research, Bremerhaven, Germany) and Dr.
Ute Güntner (ICBM, University of Oldenburg) kindly
4.6
The Future of Marine Geochemistry of Organic Matter
Total extracts and/or liquid chromatographic
subfractions are then analyzed by capillary column gas
chromatography using a flame ionization detector.
Except for hydrocarbon fractions, derivatization is
commonly applied to render polar lipids more volatile
in order to reduce gas chromatographic retention times
and to improve peak shape at the detector. Carboxylic
acids are usually transformed into their methyl esters,
and hydroxyl or amine groups into their trimethylsilyl
ether derivatives. Alternatively, both acid and hydroxyl
groups can be silylated. Acetate formation is another
common derivatization method. A variety of derivatization reagents are commercially available for this
purpose.
Only a few major compound series can be
recognized at the level of their molecular structures
based on relative retention times and distribution
patterns by gas chromatography alone. This applies
to n-alkanes in the nonaromatic hydrocarbon fraction,
n-fatty acids in the carboxylic acid fraction and in some
cases n-alkanols in the neutral polar fraction. High
abundance of a few single compounds (pristane,
phytane, long-chain alkenones) sometimes also allow
their direct identification from gas chromatograms.
The most powerful technique for assigning
molecular structures to constituents of complex
mixtures as they are found in the lipid extracts of
geological samples is the combination of capillary
column gas chromatography and mass spectrometry
(GC-MS). Although the expression “identification” is
frequently used, GC-MS alone is insufficient to fully
characterize a new compound whose gas chromatographic and mass spectrometric behavior has not been
described before. Normally, GC-MS analysis relies on
a comparison with GC and MS data published in the
(geochemical) literature or with data of standards,
commercially available or synthesized in the laboratory,
or on the interpretation of mass spectral fragmentation
patterns following common empirical rules (e.g.
McLafferty and Turecek 1993). Unfortunately, there is
no comprehensive compilation of mass spectra of
geochemically relevant organic compounds, although
a significant number of spectra was recently published
by de Leeuw (2004). Peters et al. (2005) have provided
a detailed coverage of hydrocarbons and selected polar
compounds of significance in petroleum geochemistry
and other fields of organic geochemistry.
The youngest, revolutionary development in
analytical organic geochemistry is the on-line coupling
of a gas chromatograph to an isotope ratio mass
spectrometer via a combustion interface (GC-irm-MS;
Hayes et al. 1990; Freeman et al. 1994). This instrument
allows the determination of stable carbon and hydrogen
isotope ratios of single organic compounds in complex
mixtures provided they are gas chromatographically
reasonably well separated. Chemotaxonomic relations
to specific precursor organisms are then possible if
these are distinct from other organisms in their carbon
isotope fractionation behavior during photosynthesis
and biosynthesis. Sample preparation and analysis are
similar to those for GC-MS analysis with the provision
that the isotopic composition of derivatizing agents is
accounted for in data interpretation.
4.6
The Future of Marine
Geochemistry of Organic
Matter
The evolution of organic geochemistry has always been
closely connected to the developments in instrumental
techniques for the analysis of organic compounds in
geological samples. Now that the instrumentation has
reached a high level of sophistication and particularly
sensitivity, one of the future targets will certainly be
higher stratigraphic (time) resolution which is particularly important for climate research. Advancement in
the fundamental understanding particularly of the early
part of the geological organic carbon cycle will depend
on the cooperation between organic geochemists and
microbiologists. They will have to refine the knowledge
of the biological effects on the early diagenesis of
organic matter arriving at the sediment-water interface
and becoming buried in the uppermost sediment. It will
be necessary to broaden the natural product inventory
of microorganisms, both of sedimentary bacteria and
archaea and of unicellular algae, protozoans and other
organisms at the lower end of the food chain in order
to arrive at solid chemotaxonomic relationships
between source organisms and molecular fossils.
Carefully designed laboratory simulation experiments together with high-resolution field studies will
refine the mass balance approaches of organic matter
exchange between sediment and water column related
to early diagenetic processes. Finally, mathematical
modeling of transport and reaction processes will
become an increasingly important tool in marine geochemistry of organic matter.
Acknowledgment
Dr. Rüdiger Stein (Alfred Wegener Institute of Polar
and Marine Research, Bremerhaven, Germany) and Dr.
Ute Güntner (ICBM, University of Oldenburg) kindly
4.6
The Future of Marine Geochemistry of Organic Matter