Hinrichs Lab - Metabolites

Gases

Biogeochemical studies of gases are more of an intellectual challenge rather than an analytical one. The chromatography of gas is relatively straightforward with barely a need of mass spectrometric assistance for identification, but retrieval of representative samples is challenging and interpretation of gas data often requires thermodynamic calculation and modeling.

In marine sciences, there are several important and complex models on gas biogeochemistry, such as the “Chung’s plot” for tracking the generation mechanisms of natural gases (Chung et al., 1988; Sherwood Lollar et al., 2002), or the theory of thermodynamic control of sedimentary hydrogen concentrations (Lovely and Goodwin, 1988; Hoehler et al., 1998).

Gas analysis in marine biogeochemistry

During an ocean coring or drilling project, gas sampling and measurement is normally the first analysis performed once a sediment core is retrieved, for several reasons.

First, gas is elusive. This is further compounded by the fact that there is a huge pressure difference between the deep ocean and sea surface, making accurate quantification of gas even more difficult. Therefore, in addition to rapid sampling, some pressure coring techniques have been invented for precise gas quantification.

Second, monitoring gas composition in the retrieved marine sediment, particularly hydrocarbon gases, provide a firsthand security control onboard.

Lastly, methane analysis provides a first approximation of the depth of sulfate-methane transition zone, a prominent biogeochemical feature in marine sediment. Such information usually helps guiding the strategy for subsequent fluid and solid-phase sampling.

Detailed sampling information for gas in marine sediment can be found in scientific literature as well as in ODP/IODP publications (see the Explanatory Notes of ODP Leg 201, Initial Reports for example).

The Hinrichs Lab is currently equipped with the facilities to analyze both the concentrations and carbon isotopic values of hydrocarbon gases and methylated sulfides, in combination with the classical headspace analysis. Gas chromatographs and detectors for molecular hydrogen (H₂) and carbon monoxide are also available.

Hydrocarbon gases

Due to their high distribution coefficients (concentration in the gaseous phase/concentration in the aqueous phase) and low solubility in aqueous solutions, volatile hydrocarbons can be relatively easily determined by the headspace technique. Depending on the purposes, samples can be heated shortly or slurried with solutions in a closed container prior to analysis (e.g., Ertefai et al., 2010). Analysis of hydrocarbon gases is a well-developed method. Separation of C1 to C6 can be achieved by the use of commercially available packed or capillary columns (e.g., Hinrichs et al., 2006), and quantification accomplished by a flame ionization detector. The carbon isotopic compositions of C1 to C6 can be determined by a gas chromatograph coupled to an isotope ratio mass spectrometer via a combustion interface (see “Analytical infrastructure” for more information on the instruments mentioned here).

Volatile methylated sulfides

Compared to volatile hydrocarbons, volatile methylated sulfides (e.g., methanethiol and dimethyl sulfide) have much lower distribution coefficients and higher solubility in aqueous solutions. Furthermore, some of them are sensitive to oxygen, adding further analytical difficulties. We currently determine only methylated sulfides from samples of laboratory incubation. For long-term sample storage, sample tubes containing sediment slurries are frozen directly at -20°C and heated shortly to 60°C prior to carbon isotopic analysis (Lin et al., 2010). The gas chromatograph, detector and isotope ratio mass spectrometer are identical to those for volatile hydrocarbons, but a capillary column with special packing materials (e.g., CP-PoraBOND Q) is necessary for good separation of methylated sulfides.

Molecular hydrogen (H₂)

There are two major approaches for determination of H₂ in anoxic sediment samples. One is the so-called “headspace equilibration technique”, which involves incubating sediment samples in a closed container with an oxygen-free headspace and analyzing the headspace H₂ concentration repeatedly until a steady state is reached (Hoehler et al., 1998). The steady-state H₂ concentration is supposed to be representative of the in situ H₂ concentration. The other approach aims at measuring the in situ H₂ concentration via an extraction step (Novelli et al., 1987; D’Hondt et al., 2009). After testing both methods, we propose the combined analysis by both approaches (Lin et al., 2012). H₂ is usually quantified by a reducing compound photometer, but other detectors, such as thermal conductivity detectors and pulsed discharge detectors, can also be used for H₂ determination.

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