An MIT-led team of researchers has developed an instrument capable of quickly and accurately analyzing samples of methane, pinpointing how they were formed. The breakthrough could give scientists a greater understanding of the role the gas plays in global warming.
Methane is a potent force in global warming. It is second only to carbon dioxide when it comes to trapping heat inside the atmosphere for extended periods of time, and can originate from a range of different sources, including lakes, livestock and natural gas pipelines.
The new study aimed to determine which of two common origins were responsible for any given sample of the gas. Specifically, it focused on determining whether they were thermogenic (produced by high-temperature decay of organic matter buried deep within the Earth) or microbial (formed as a metabolic byproduct by microorganisms living in the guts of animals) in origin.
The new method, catchily known as tunable infrared laser direct absorption spectroscopy, is designed to detect the ratio of methane isotopes in samples. Methane molecules consist of up to four hydrogen atoms combined with a single carbon atom, the latter of which can be one of two isotopes – carbon-12 or carbon-13. The hydrogen in the molecule is also found in two forms, one of which is deuterium – an isotope with an extra neutron.
In the study, the researchers focused on detecting molecules containing both an atom of carbon-13 and a deuterium atom, believing the rare molecule to be a signal of the formation temperature of methane, which is an important indicator of the origin of the molecule.
To detect the molecule, the team built an instrument that uses infrared spectroscopy to detect specific frequencies that correspond to motions within the methane molecules, highlighting the different isotopes. The method is portable, allowing for deployment out in the field.
The team put the new method to work, studying samples collected from various locations including deep ancient groundwater, natural gas reservoirs and the digestive tracts of cows. The results highlighted an inconsistency in theory regarding the link between the rare, doubly isotope-substituted methane, with one set of results calculating that a sample collected from a cow’s stomach formed at 400 ºC (752 ºF) – an obvious impossibility.
Reassessing the data, a new theory was arrived upon that links a characteristic of the bonds between the carbon and hydrogen atoms in the molecule – something the team refers to as "clumpiness" – with the rate of methane production.
"Cow guts produce methane at very high rates – up to 500 liters a day per cow. They’re giant methane fermenters, and they prefer to make less-clumped methane, compared to geologic processes, which happen very slowly," said graduate student David Wang. "We’re measuring a degree of clumpiness of the carbon and hydrogen isotopes that helps us get an idea of how fast the methane formed."
Further study backed up the theory, finding that the clumpier the bond, the slower the molecule forms. This in turn allowed the team to makes links between the type of isotope detected – the bonds of which exhibit different levels of clumpiness – with the rate of production and therefore the origin of the specific samples.
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