RESUMO
We describe a simple, sensitive, and robust method for sulfur isotope ratio (34S/32S) analysis of ppm-level organic sulfur compounds (OSCs) in the presence of percent-level H2S. The method uses a gas chromatograph (GC) coupled with a multicollector inductively coupled plasma mass spectrometer (MC-ICPMS). The GC, equipped with a gas inlet and a valve that transfers the H2S to a thermal conductivity detector (TCD), enables a precise heart cut and prevents the saturation of the MC-ICPMS. The sensitivity and accuracy of the method are better than 0.3 for OSCs at a concentration of 25 pmol or 1.4 ppm, and better than 0.5 for concentrations ≥0.7 ppm of OSCs. An order of magnitude increase in sensitivity, with no effect on accuracy, can be achieved if the loop volume (0.5 mL) is changed to 5 mL. High concentrations of methane (95% v/v) and/or H2S (20% v/v) had no effect (within 0.5) on the precision and accuracy of the gas sample containing 2 ppm of OSCs after heart cut. The applicability and robustness of this method are demonstrated on a gas sample (10% v/v H2S) that was produced by pyrolysis of sulfur-rich kerogen. The results show good precision and reveal sulfur isotope variability between individual OSCs that may represent key processes during formation and degradation of OSCs.
RESUMO
We describe a trapping and chromatography system that cryogenically removes CO(2) and N(2) generated from sample combustion in an elemental analyzer (EA) and introduces these gases into a low-flow helium carrier stream for isotopic analysis. The sample size required for measurement by this system (termed nano-EA/IRMS) is almost 3 orders of magnitude less than conventional EA analyses and fills an important niche in the range of analytical isotopic methods. Only 25 nmol of N and 41 nmol of C are needed to achieve 1.0 per thousand precision (2sigma) from a single measurement while larger samples and replicate measurements provide better precision. Analyses of standards demonstrate that nano-EA measurements are both accurate and precise, even on nanomolar quantities of C and N. Conventional and nano-EA measurements on international and laboratory standards are indistinguishable within analytical precision. Likewise, nano-EA values for international standards do not differ statistically from their consensus values. Both observations indicate the nano-EA measurements are comparable to conventional EA analyses and accurately reproduce the VPDB and AIR isotopic scales. Critical to the success of the nano-EA system is the procedure for removing the blank contribution to the measured values. Statistical treatment of uncertainties for this procedure yields an accurate method for calculating internal and external precision.
RESUMO
A series of molecular and geochemical studies were performed to study microbial, coal bed methane formation in the eastern Illinois Basin. Results suggest that organic matter is biodegraded to simple molecules, such as H(2) and CO(2), which fuel methanogenesis and the generation of large coal bed methane reserves. Small-subunit rRNA analysis of both the in situ microbial community and highly purified, methanogenic enrichments indicated that Methanocorpusculum is the dominant genus. Additionally, we characterized this methanogenic microorganism using scanning electron microscopy and distribution of intact polar cell membrane lipids. Phylogenetic studies of coal water samples helped us develop a model of methanogenic biodegradation of macromolecular coal and coal-derived oil by a complex microbial community. Based on enrichments, phylogenetic analyses, and calculated free energies at in situ subsurface conditions for relevant metabolisms (H(2)-utilizing methanogenesis, acetoclastic methanogenesis, and homoacetogenesis), H(2)-utilizing methanogenesis appears to be the dominant terminal process of biodegradation of coal organic matter at this location.
