By Craig Schiffries, DCO Director; Geophysical Laboratory, Carnegie Institution of Washington, USA
When the Deep Carbon Observatory was launched, it recognized that new scientific instruments are keys to discovery. DCO emphasized that developing next-generation instrumentation early in the decadal program would advance the study of deep carbon in the program’s later stages. This strategy is paying dividends. Recent instrumentation breakthroughs by research groups at Caltech, MIT, and UCLA have positioned DCO to achieve a major decadal goal regarding a deeper understanding of methane formation temperatures and sources. Data from their new instruments will enable researchers to test hypotheses about biotic versus abiotic origins of methane.
A molecule of methane (CH4) has only five atoms and seems remarkably simple, but until recently there were few attempts to measure multiply substituted isotopologues (“clumped isotopologues”) of methane in which two or more rare isotopes substitute for two or more common isotopes (e.g., 13C for 12C or D for H, where D is 2H). Measuring rare doubly substituted methane isotopologues (13CDH3 and 12CD2H2) poses extraordinary analytical challenges but may provide a wealth of new information. The formation of doubly substituted methane isotopologues is favored at lower temperatures and the new techniques can be used as a geothermometer to calculate methane formation temperatures. Mixing, kinetics, and other processes can also affect isotopic ratios and measurements of doubly substituted isotopologues of methane can provide insights into these processes and methane provenance.
Analytical challenges arise because the two doubly substituted isotopologues of methane are exceedingly rare and have similar physical properties, requiring instruments that have both high sensitivity and high resolving power. The doubly substituted isotopologues of methane have mixing ratios of approximately 10-6 to 10-8 and differ in mass by approximately 0.003 atomic mass units (amu). Until recently, most gas-source isotope ratio mass spectrometers could only resolve species that differ in mass by one or more amu.
The Deep Carbon Observatory is simultaneously pursuing two radically different approaches for measuring doubly substituted methane isotopologues. One approach involves mass spectrometry and the other involves absorption spectroscopy. Mass spectrometry and absorption spectroscopy are based on different physical principles and each approach has distinct advantages and disadvantages. For example, mass spectrometry is subject to isobaric interferences (species with similar masses, such as 18O+ and H216O+, that may be difficult to resolve) and the formation of species in the ion source (e.g., fragments, such as17OH+, and hydrogen aducts, such as CH5+). Absorption spectroscopy is not subject to isobaric interferences because it does not measure mass, but it is subject to spectral interference. Absorption spectrometry does not involve an ion source and is not affected by fragmentation and related processes in the ion source of a mass spectrometer. Mass spectrometers require a heavy magnet, a long flight tube, multiple collectors, and a high vacuum system. In contrast, the absorption spectrometer requires at least two tunable quantum cascade lasers, an absorption cell with aligned mirrors, and a sensitive detector. The two newly developed mass spectrometers are more expensive than the absorption spectrometer but they are more versatile because they can measure a large range of species beyond methane. The absorption spectrometer is smaller, lighter, and potentially field deployable.
The Caltech group, led by John Eiler and Daniel Stolper, worked in collaboration with Thermo Fisher to design and build a new mass spectrometer by mixing and matching existing technologies to piece together a new platform (pictured above). Their recent paper on formation temperatures of thermogenic and biogenic methane  is a milestone in geochemistry. Stolper said, "In making these measurements of temperature, we are able to really, for the first time, say in an independent way, 'We know the temperature, and thus the environment where this methane was formed.'"
The MIT group, led by Shuhei Ono, worked in collaboration with Aerodyne Research to design and build a new instrument for measuring a doubly substituted methane isotopologue (13CDH3) by tunable infrared laser direct absorption spectroscopy (pictured left). Their recent paper  demonstrates the success of this high-risk, high-reward approach. Shuhei Ono said, “Now we have the technique sorted out, we are in the middle of an exciting time to explore sources of methane using this new tool. The reports so far are only the beginning. There is a lot more to study.” Their research opens the door for measuring other rare isotopologues by absorption spectroscopy.
The UCLA group, led by Edward Young and Douglas Rumble, is working in collaboration with Nu Instruments to design and build a new mass spectrometer, called Panorama, that is the largest gas-source mass spectrometer in the world and has the greatest resolving power of any such multiple-collector instrument (pictured right). A recent poster presentation  at the International Symposium on Isotopomers demonstrates that this instrument, which is still under development, has the potential to resolve one doubly substituted methane isotopologue (13CDH3) from another (12CD2H2). If both doubly substituted species can be measured separately, which the Panorama mass spectrometer is designed to achieve, then it would be possible to expand the isotopic parameter space and improve our ability to discriminate between the effects of temperature and other processes.
Recent instrumentation breakthroughs are opening vast frontiers for new research on hydrocarbons. Advances in our study of methane will be complemented by research into multiply substituted rare isotopologues of other hydrocarbons, furthering the decadal goals of DCO, and deepening our understanding of our planet.
1. Stolper DA, Lawson M, Davis CL, Ferreira AA, Santos Neto EV, Ellis GS, Lewan MD, Martini AM, Tang Y, Schoell M, Sessions AL, Eiler JM (2014) Formation temperatures of thermogenic and biogenic methane. Science 344:1500-1503
2. Ono S, Wang DT, Gruen DS, Sherwood Lollar B, Zahniser M, McManus BJ, Nelson DD (2014) Measurement of a doubly-substituted methane isotopologue, 13CH3D, by tunable infrared laser direct absorption spectroscopy. Analytical Chemistry, 86:6487-6494
3. Young ED, Freedman P, Rumble D, Schauble E (2014) Panorama, a new gas source, electron impact, double-focusing, multi-collector mass spectrometer for the measurement of isotopologues in geochemistry. The 7th International Symposium on Isotopomers (ISI2014), Tokyo, Japan