Simultaneous characterization of methane and carbon dioxide produced by cultured methanogens using gas chromatography/isotope ratio mass spectrometry and gas chromatography/mass spectrometry

RATIONALE The stable carbon isotope ratios of methanogen‐produced CH 4 and CO 2 are useful information for identifying and quantifying methanogenic pathways. Isotope ratio mass spectrometry combined with gas chromatography (GC/IRMS) is a very attractive tool for performing high‐precision compound‐specific isotope analysis. However, no GC/IRMS techniques have yet been available or been validated that give baseline separation of methanogen‐produced CH 4 and CO 2 from N 2 /N‐oxides and H 2 O vapor at ambient temperature and compatibility with analysis by mass spectrometry. METHODS Microbe‐produced CH 4 and CO 2 in headspace gases were separated from N 2 /N‐oxides and H 2 O vapor in a single run on a GS‐CarbonPLOT column at 35°C and with a maximum operating temperature of 120–140°C. The simultaneous characterization of CH 4 and CO 2 was then performed by GC/IRMS using an optimized backflush time to eliminate the interference from N 2 /N‐oxides and H 2 O vapor, and by GC/MS due to there being no interference from O 2 gas in the culture. RESULTS GC/MS and GC/IRMS were used to calculate the ionization efficiency of CO 2 as 8.22–8.84 times that of CH 4 in GC/MS analysis, and it was deduced that the N‐oxides, which can potentially interfere with δ 13 C analysis, were probably produced mainly in the source of the isotope ratio mass spectrometer. We also determined the aceticlastic methanogenic pathway. CONCLUSIONS The established GC/MS and GC/IRMS techniques are suitable for characterizing the gaseous carbon‐containing compounds produced by microbial cultures. Through high‐precision carbon isotope analysis by GC/IRMS combined with low concentrations of 13 C‐labelled substrates, the technique has great potential for identifying and quantifying methanogen‐mediated carbon metabolic processes and pathways. Copyright © 2013 John Wiley & Sons, Ltd.