4.2.3 Stable Isotope Mass Spectrometry
A special type of mass spectrometry has been applied intensively in Organic
Geochemistry, that is neither focused on getting structural information on the
compounds nor is used for quantification. This approach aims at measuring the
isotope composition of the organic matter or even of individual compounds, the
so-called stable isotope mass spectrometry.
In Organic Geochemistry a general interest exists to detect the relative composition of stable isotopes of carbon and to a minor extent of hydrogen, oxygen and
nitrogen. The isotope composition is influenced by various geo- and biochemical
processes as well as environmental conditions. As the main clue, for detection of the
isotopes the samples are converted into one representative species via combustion or
pyrolysis. For stable carbon isotope analyses an exhaustive combustion of the
organic matter produces CO 2 as analyte for further mass spectrometric measurements. The mass spectrometer just measures the amount of
12 CO 2 with 44 m/z and
13 CO 2 with 45 m/z. From these quantitative values the carbon isotope ratio (δ
13 C)
can be easily calculated. The same approach analyses organically bound nitrogen
and hydrogen isotopes by conversion into N 2 and H 2 , respectively, and measuring of
H 2 /HD or of the
14 N and
15 N signals.
This approach is applied to complete samples revealing insights into the isotope
composition of bulk material. However, it is also of high interest to get information
about the isotope composition of individual compounds (e.g. n-alkanes, isoprenoid
biomarker, BTEX pollutants). For this purpose, the so-called Compound Specific
Isotope Analysis CSIA has been developed. Here, the combustion of organic
compounds is directly performed at the outlet of a gas chromatograph. The converted
products (e.g. CO 2 ) are directly transferred into the mass spectrometer and the
isotopes are measured. A principal scheme of such a isotope-ratio-monitoring
gas-chromatographic mass-spectrometric system (irm-GC/MS) is given in Fig. 4.32.
4.3 Spectroscopy
Outlook
Information about molecular properties and structural moieties can be
obtained by spectroscopic analyses. Based on the interaction of electromagnetic radiation with molecules various energies become excited and the
corresponding absorption is measured. The physical basics, the technical
realization but especially the interpretation of spectra are described in this
chapter.
72
4 Instrumental Analysis
A special type of mass spectrometry has been applied intensively in Organic
Geochemistry, that is neither focused on getting structural information on the
compounds nor is used for quantification. This approach aims at measuring the
isotope composition of the organic matter or even of individual compounds, the
so-called stable isotope mass spectrometry.
In Organic Geochemistry a general interest exists to detect the relative composition of stable isotopes of carbon and to a minor extent of hydrogen, oxygen and
nitrogen. The isotope composition is influenced by various geo- and biochemical
processes as well as environmental conditions. As the main clue, for detection of the
isotopes the samples are converted into one representative species via combustion or
pyrolysis. For stable carbon isotope analyses an exhaustive combustion of the
organic matter produces CO 2 as analyte for further mass spectrometric measurements. The mass spectrometer just measures the amount of
12 CO 2 with 44 m/z and
13 CO 2 with 45 m/z. From these quantitative values the carbon isotope ratio (δ
13 C)
can be easily calculated. The same approach analyses organically bound nitrogen
and hydrogen isotopes by conversion into N 2 and H 2 , respectively, and measuring of
H 2 /HD or of the
14 N and
15 N signals.
This approach is applied to complete samples revealing insights into the isotope
composition of bulk material. However, it is also of high interest to get information
about the isotope composition of individual compounds (e.g. n-alkanes, isoprenoid
biomarker, BTEX pollutants). For this purpose, the so-called Compound Specific
Isotope Analysis CSIA has been developed. Here, the combustion of organic
compounds is directly performed at the outlet of a gas chromatograph. The converted
products (e.g. CO 2 ) are directly transferred into the mass spectrometer and the
isotopes are measured. A principal scheme of such a isotope-ratio-monitoring
gas-chromatographic mass-spectrometric system (irm-GC/MS) is given in Fig. 4.32.
4.3 Spectroscopy
Outlook
Information about molecular properties and structural moieties can be
obtained by spectroscopic analyses. Based on the interaction of electromagnetic radiation with molecules various energies become excited and the
corresponding absorption is measured. The physical basics, the technical
realization but especially the interpretation of spectra are described in this
chapter.
72
4 Instrumental Analysis