Isotope Ratio Monitoring Mass Spectrometry (IRmMS)

Biological formation of ethane and propane in the deep marine subsurface, K-U. Hinrichs 2006

Just as methane cycling has a biological component, ethane and propane may also have sources other than thermogenic production (“cracking”) from larger geomolecules. Samples of light hydrocarbons sorbed onto buried (0 to >300m) sediments recovered from several sites in the Equatorial Pacific and Peruvian Margin contained ethane and propane in abundance ratios and d¹³C compositions that are inconsistent with thermogenic production and migration. Dr. Hinrichs’ thermodynamic analysis of the data for each of the sites indicated isotope effects that supported the hypothesis of the biotic production of ethane (and propane) by reduction of acetate (plus formate), and that this pathway could be a potentially significant energy source for the sedimentary organisms (Hinrichs et al., 2006).

Gases in hydrothermal vent fluids, A. Cruse 2006

Since their discovery three decades ago, undersea hydrothermal vents have provided a new window into chemically important geological processes while reshaping our understanding of biology. The isotopic signatures imprinted on vent fluid compounds can be used to constrain their source, maturity, and history.

Early in her thesis work (advised by Dr. Jeff Seewald), Anna Cruse developed a GC method to cope with the wide range of CO₂ and hydrocarbon concentrations in vent fluids that had been sampled at in-situ seafloor pressures and stored to prevent degassing. She modified her methods for the Delta^plus where she was able to demonstrate these particular fluids had a thermogenic source of ethane and propane, a mixed thermogenic/biogenic source for methane, and that CO₂ was thermally produced from sedimentary organic material (Cruse & Seewald, 2006).

Photolytic CO₂ production from DOM, W. Wang 2009

The photolysis of colored dissolved organic material (CDOM) is potentially a very significant source of oceanic CO₂, however methods to determine the extent of this reaction are artifact-prone and of uncertain accuracy. Dr. Oliver Zafiriou was originally funded to develop an analysis involving the complete exchange of¹³CO₂ for ¹²CO₂in seawater, effectively reducing the background value of CO₂ so that very small (on a laboratory amount and time scale) increase of photolytically ¹²CO₂ produced is much more apparent. This project consist of three steps each of which must be accomplished without sample re-contamination by atmospheric ⁽¹²⁾CO₂: loading the exchanged seawater into quartz irradiation tubes, quantitative collection of the CO₂ produced, and introduction to the MS. Because the interexperimental exchange may not be reproducible, each exchange is used to fill 16 tubes; half to serve as a control, half to be irradiated for a uniform time. We have worked in reverse order, using each later step to assist in the development of the previous. CGJ has shown that the DeltaPlus MS operated in this mode is precise over time. The vacuum line preparation of samples routinely has a variation in blank which would contribute a minor imprecision to the final results. Final results from coastal water CO₂ photoproduction were in agreement with measurements obtained by other methods (Wang et al., 2009).

Isotopic discrimination in plant biomarker aerosols, M. H. Conte 2002

Establishing the relative proportions of atmospheric CO₂ moving into the oceans and terrestrial biosphere is crucial for modeling sinks for greenhouse gases. ¹³CO₂ and ¹²CO₂ move into the oceans to an equivalent extent, however terrestrial photosynthesis exhibits a strong preference for ¹²CO₂ . The globally integrated value of this carbon isotopic discrimination (D) is not easily measured since it is both spatially and temporally heterogeneous, changing with ecosystems which can themselves experience seasonal variation in plant types.

Dr. Maureen Conte’s (WHOI/MBL) approach to determining D has been to acquire time-series samples of aerosols reaching Bermuda, and determine the isotopic compositions of plant waxes as a spatially integrated signal from North America. Although only a few tens of nanograms of material are present in each sample, she has successfully used the Delta^plus to precisely determine D and has shown a strong seasonal signal (Conte & Weber, 2002).

Submicromolar d¹⁵N determinations in chlorophyll-a, L. A. Houghton 2000

Similar to the ¹³C/¹²C ratio, the ¹⁵N/¹⁴N ratio can also be used to follow diagenetic pathways. Nitrogen containing pigments such as chlorophyll can not only be isolated from fresh plantmaterial, but also from the water column and sediments where they may be transformed, but the basic phorphyrin ring structure remains intact. Unfortunately, only minute quantities are recovered from reasonable amounts of material.

Ms Leah Houghton and CGJ worked together to reduce the amount of material required by a factor about an order of magnitude for a bulk (EA) irMS d¹⁵N on the DeltaPlus. By reducing the scale of the combustion, rebuilding and simplifying the EA plumbing to eliminate all leaks, and using higher purity gases, the blank to signal ratio was dramatically decreased. Simultaneously, increasing the flow to the MS amplified the ratio of signal to instrumental noise and we demonstrated an accuracy of0.05?with 0.2? precision for samples containing as little as 150 nMol N2.

This method has been further refined by CGJ and is an available service that is used several times each year [Holtvoeth et al, 2005].

Carbon isotopic fractionation during reductive dechlorination of PCBs, N. J. Drenzek 2001

Bacterial degradation of polychlorinated biphenyls (PCBs) in sediments has been observed in many locations. Mass balance, patterns of chlorine loss and analogy to other compounds have been used to suggest dechlorination pathways.

Working with Dr. Chris Reddy (sponsor), Summer Student Fellow Mr. Nicolas Drenzek conducted a laboratory bacterial culture experiment using d¹³C values of PCBs as a probe for biochemical modifications to the carbon skeleton itself. Although ¹³C ratio clearly decreased with increasing chlorine content for several different starting lots of PCBs, only a minor ¹³C isotopic effect was observed during dechlorination for the individual congeners monitored. Thus, the overall effect in sediment is that the extent of degradation should be observed as a decrease in d¹³C ratio for each congener as the starting material disappeared only to be replaced by new material “growing in” from higher chlorination numbers (exhibiting lower d¹³C values) (Drenzek et al., 2001).

Temporal variations of petroleum fluids in subsurface reservoirs, J. K. Whelan 2001

The common notion of oil reservoirs as static entities during their production history has been challenged with recent findings.Dr. Jean Whelan (WHOI) has proposed the concept that reservoirs are much more dynamic systems, subject to episodic or continual filling and leakage.The implications for petroleum reserves in commercially exploited reservoirs are obvious.

CGJ developed a GCirmMS method on the Delta^plus to chromatographically resolve compounds in the C₆ to C₉ (gasoline) range found in a suite of whole oil samples obtained by Dr. Whelan and produced from a Gulf Coast oil field between 1985 to 1993. These data were compared with a biomarker analysis (determination of the abundance ratios of homologous and isomeric series of naphthalenes, benzothiophenes, phenanthrenes, hopanes, steranes/diasteranes and mono and triaromatic steranes) that CGJ provided her using the Facility’s high mass resolution Autospec-Q MS.

Some of the most abundant gasoline range hydrocarbons showed greater temporal than spatial variation. However, the covariation of isotopic d¹³C ratios and the uniformity of the biomarker ratios were indicative of “gas washing” and upwards migration of light hydrocarbons on a time frame of less than eight years (Whelan et al., 2001).

Methane-consuming archaebacteria, K-U. Hinrichs 1999

Large quantities of methane and methane hydrates are known to be present in marine sediments but a full biogeochemical knowledge of these materials is still developing. Sediment lipid extracts from an anaerobic methane seep were found to contain bis-O-phytanylglycerolether “archaeol”, a marker for the presence of archaea and particularly methanogens. Uniquely, when analyzed (Delta^plus), this material and a related hydroxyarchaeol had d¹³C values between -100 to -110 ? (Hinrichs et al., 1999; 2000). Dr. Kai-Uwe Hinrichs (WHOI, BremenUniv.) hypothesized that only by consuming methane could an organism produce such a ¹³C-depleted material. Anaerobic archaea were previously thought to only produce, and not consume, methane. This finding not only demonstrated the presence of one new species, but the ensuing phylogenetic analysis of ribosomal rRNA required expansion of the archaeal family to include anaerobic methanotrophic archaea.(Hinrichs et al., 1999).