Helium-flushed sheathed nickel tube reactor for continuous flow oxygen stable isotope compound-specific analysis.

RATIONALE: Compound-specific oxygen stable isotope analysis has been commercially available for about 20 years but the technique has been infrequently applied and practical operating knowledge is limited. Existing reactor designs and chemistry fail to fully exclude contaminating oxygen, are chromatographically flawed, and are generally fabricated from expensive platinum tubing. METHODS: As a new interface between the gas chromatograph and isotope ratio mass spectrometer, we used a nickel tube reactor (containing no platinum wires) in a ceramic sheath with countercurrent helium flushing, operated at 1250°C. We analyzed suites of four long-chain alcohols (LCAs), defining the least and most 18 O-enriched LCAs as internal standards. The other two LCAs, co-injected in varying amounts, were treated as unknowns and amplitude-corrected during data reduction. RESULTS: With this design, signals from non-oxygenated compounds are minimized with a hydrogen trickle, while excessive graphitization is counterproductive. A balance of hydrogen and graphite is required to maintain a proper, stable redox state in the reactor. With low extraneous oxygen, the exact hydrogen flow has little effect on final δ18 O data quality. Overall average δ18 O values precisions and errors for the unknowns were better than 0.4 ‰ for injections between 2 and 7 nmol per component. CONCLUSIONS: Our actively flushed sheath reduces oxygen infiltration to negligible levels and results in a chromatographic flow devoid of dead or poorly swept volumes. The improved design allows measurement of LCAs with accuracy and precision sufficient to enable investigating real processes, and may also have wider applications to other isotopes.

Spectral characterization of two bioaccumulated methoxylated polybrominated diphenyl ethers.

Two methoxylated polybrominated diphenyl ethers (MeO-PBDEs) were isolated from a True's beaked whale (Mesoplodon mirus) and identified by NMR (1H, 1H-1H and 1H-13C) and high resolution mass spectrometry as 2-(2',4'-dibromophenoxy)-3,5-dibromoanisole (6-MeO-BDE47) and 2-(2',4'-dibromophenoxy)-4,6-dibromoanisole (2'-MeO-BDE68). Previously the structures of these bioaccumulated compounds have been determined by comparison of their mass spectra and gas chromatographic (GC) retention times with those of authentic standards. While this method is accepted and generally successful, NMR of the isolated compounds allows us to definitively identify the congeners. Our characterizations are consistent with those made for MeO-PBDEs in other organisms, identified by chromatographic methods.

Measuring the photochemical production of carbon dioxide from marine dissolved organic matter by pool isotope exchange.

CO(2) is the major known product of solar photolysis of marine dissolved organic matter (DOM). Measuring the rate of this globally significant process is hindered by low rates per unit volume, high background CO(2) in seawater, and ubiquitous contamination. Current methods utilize CO(2)-free seawater matrices, possibly introducing artifacts. Alternatively, pool isotope exchange (PIE) replaces most of the sample's DI(12)C with DI(13)C at natural pH and temperature, so that (12)CO(2) from DOM photooxidation elevates (12)CO(2)/(13)CO(2) ratios in irradiated samples compared to dark controls. (12)CO(2)/(13)CO(2) ratios are then measured using a modified GC-IRMS. The minimum detectable concentration change (three standard deviations) is 300 nmol DI(12)C/kg. Methods for minimizing contamination while exchanging, transferring, sealing, and irradiating samples, and for recovering and purifying CO(2) are presented. Results from PIE agree within uncertainties with those from CO(2)-free coastal seawater, suggesting that both methods apply to river-dominated coastal waters. However, photooxidation in the open ocean, which likely dominates the global flux despite lower rates per unit volume, involves DOM that differs from coastal DOM, so that coastal agreement cannot validate open-ocean studies. Major advantages of PIE are use of nearly unperturbed seawater matrices, potential to incubate samples in situ to obtain depth-integrated rates directly, and potential to use larger samples to measure open-ocean waters.

Biological formation of ethane and propane in the deep marine subsurface.

Concentrations and isotopic compositions of ethane and propane in cold, deeply buried sediments from the southeastern Pacific are best explained by microbial production of these gases in situ. Reduction of acetate to ethane provides one feasible mechanism. Propane is enriched in (13)C relative to ethane. The amount is consistent with derivation of the third C from inorganic carbon dissolved in sedimentary pore waters. At typical sedimentary conditions, the reactions yield free energy sufficient for growth. Relationships with competing processes are governed mainly by the abundance of H(2). Production of C(2) and C(3) hydrocarbons in this way provides a sink for acetate and hydrogen but upsets the general belief that hydrocarbons larger than methane derive only from thermal degradation of fossil organic material.

Two chemically distinct pools of organic nitrogen accumulate in the ocean.

The chemical dynamics of marine dissolved organic nitrogen (DON), a reservoir featuring surface accumulations even in areas where nitrogen limits productivity, have yet to be resolved. We exploited differences in the acid lability of amide bonds within high-molecular-weight (HMW) DON to show that vertical DON profiles result in part from the presence of two chemically distinct pools of amide. Half of HMWDON in surface waters is present as N-acetyl amino polysaccharides. In contrast, nearly all deep-sea HMWDON, and therefore, most HMWDON, is present in amides that resist both chemical hydrolysis and biological degradation.