We have developed several isotope-ratio-monitoring gas-chromatography/mass-spectrometry (irm-GC/MS) techniques that allow highly sensitive determination of the concentration and stable isotopic ratios of methane and nitrous oxide from a wide variety of natural aqueous and gaseous samples.
These sample introduction systems have been optimized for specific methane concentration ranges and sample matrices, and thus provide considerable flexibility in the types of samples that can be analyzed. The analytical system consists of:
(1) a sample introduction system optimized for a specific sample type,
(2) a micro-scale cryofocusing system,
(3) a porous layer open-tubular capillary analytical column,
(4) an in-line NiO2/Pt oxidation reactor (only used for methane), and
(5) a magnetic sector isotope-ratio-monitoring mass spectrometer with an open-split interface.
The detection limit of the technique is
200 pmol of methane in
either
aqueous or gaseous samples. The precision of isotopic measurements of
replicate
samples is 0.8 per mil for water, and 0.9 per mil when analyzing
200-pmol
gas samples, improving to less than 0.1 per mil with 4-nmol or larger
gas
samples.
How the equipment is configured:
Figure showing the general layout of the irm-GC/MS equipment we have developed for these analyses.
Figure showing the inlet system used for analyzing low-level water samples (0.5 - 50 nM).
Figure showing the inlet system used for analyzing sub-atmospheric concentrations of methane in gas (45 ppb - 2 ppm v/v).
Further information can be found in the following articles:
F.J. Sansone, B.N. Popp, and T.M. Rust. 1997. Stable carbon isotopic analysis of low-level methane in water and gas. Anal. Chem. 69: 40-44.
B.N. Popp, F.J. Sansone, T.M. Rust, and D.A. Merritt. 1995. Rapid method for determining abundance and carbon isotopic composition of dissolved methane in sediments and nearshore waters. Anal. Chem. 67: 405-411.
Dore, J.E., B.N. Popp, D.M. Karl, and F.J. Sansone. 1998. A large source of atmospheric nitrous oxide from subtropical North Pacific surface waters. Nature 395: 63-66.
Reprints are available - see my Publications page, or send me an email to request copies: sansone(at)hawaii.edu
Collecting samples - field work in the frozen north:
Deadhorse, Alaska (aka Prudhoe Bay) -- We were up there to collect water, ice and sediment samples in the Beaufort Sea (seen as a offcolored area in thebackground) for methane concentrationa and stable isotope analyses. These data would be used to determine if methane hydrates in the surface sediments were decomposing and releasing methane to the overlying seawater. Some scientists think that this phenomena would be a early indicator of global warming.
Of course, the first thing to do is to haul your gear to the field site. Here is Tom Lorenson doing it the old fashioned way. However, there are also higher tech ways to do this!
Here's how we collected the ice samples using a corer. The seasonal ice gets about 2 meters thick here.
Finally, Prudhoe Bay can be impressively beautiful. This photo shows the low-lying sun reflecting on water from the first spring river flow -- the water is overlying the ice on the bay.
