The prototype "253 Ultra” mass spectrometer. This is the machine that lets us make clumped-isotope measurements in organic molecules.
We are collaborating with John Eiler’s group here at Caltech to study the site-specific distribution of isotopes, and abundance of multiply-substituted isotopologues, in organic molecules. That’s quite a mouthful, so let me back up and explain. Traditional stable isotope measurements determine the average abundance of isotopes across all molecules, usually by burning or pyrolyzing them to some common form (like H2 or CO2). Thus a mixture of methane molecules consisting of CH4 and CD4 would appear to have exactly the same D/H ratio as a sample of pure CH2D2. But obviously these are different molecules, and so distinguishing between them should give us extra information about how it formed, where it came from, etc.
The key to distinguishing between these isotopic homologs (‘isotopologues’) is to measure the intact molecule, rather than turning it all into CO2 and H2. That introduces a few new headaches, but by far the biggest is that you are now trying to distinguish D from 13C in the same molecule, and they both add nominally 1 Da to the mass of the molecule. {For those of you more than 30 years old, the Dalton (Da) has replaced the atomic mass unit (amu)} The solution is to measure the mass of the molecule very accurately: a 13C substitution adds 1.0033 Da to the molecular mass, while a D substitution adds 1.0141 Da. Making high-precision stable isotope measurements with this kind of mass accuracy required a new approach. John and I commissioned Thermo to build a new mass spectrometer, which they call the “253-Ultra” (shown above), that allows us to measure organic isotopologues.
Using this new measurement technology, we are beginning to explore a wide range of scientific applications and questions. Some of these involve measuring the excess or dirth of molecules having two rare isotopes bonded to each other, for example in methane the species 13CDH3. This follows the same general principles as carbonate clumped isotope thermometry, a field that John also invented here at Caltech. The higher the temperature, the more ‘clumps’ exist. This turns out to have great utility for understanding the formation temperature of methane in natural gas deposits. Postdoc Peter Douglas is also using this property to study natural emission of biogenic methane in the Arctic. While methane is the most carefully studied molecule thus far, we can also make similar measurements in other volatile molecules, like ethane, propane, etc. Another type of measurement that uses the same basic techniques is determining the position of isotopic substitution (which I abbreviate as ‘isotopic ordering’, but is more formally known as position-specific isotopic analysis). For example, does a propane molecule have a 13C substitution at the terminal (methyl) positions, or at the central (methylene) position? Graduate student Brook Dallas has recently made the first position-specific carbon isotope measurements of the amino acid alanine (see figure below). This opens up a huge range of new biochemical and biogeochemical questions that can be addressed using these new techniques.
Carbon isotope ordering within two different samples of alanine measured from the intact molecular spectra. The differences in δ13C at C-2 and C-3 span a range of more than 40‰ between the two samples.
More generally, this is a field whose scope is expanding exponentially. Practically any molecule that can be vaporized at modest temperature is now fair game; we can study substitutions in the isotopes of H, C, O, N and S, potentially all at once; we can study both clumping and ordering of isotopologues; and we find evidence of both equilibrium and kinetic processes, making these measurements relevant to processes that range from modern biochemistry to atmospheric and marine chemistry to deep, hot geochemistry, and everything in between. Interested students and postdocs are encouraged to contact myself or John Eiler about opportunities for new research directions.
Recent papers on this subject:
Xie H, Ponton C, Formolo MJ, Lawson M, Peterson BK, Lloyd MK, Sessions AL, Eiler JM. (2018) Position-specific hydrogen isotope equilibrium in propane. Geochimica et Cosmochimica Acta, 238:193-207
Douglas PMJ, Stolper DA, Eiler JM, Sessions AL, Lawson M, Shuai Y, Bishop A, Podlaha OG, Ferreira AA, Santos Neto EV, Niemann M, Steen AS, Huang L, Chimiak L, Valentine DL, Fiebig J, Luhmann AJ, Seyfried WE Jr., Etiope G, Schoell M, Inskeep WP, Moran JJ, Kitchen N. (2017) Methane clumped isotopes: Progress and potential for a new isotopic tracer. Organic Geochemistry, 113, 262-282.
Douglas PMJ, Stolper DA, Smith DA, Walter Anthony KM, Paull CK, Dallimore S, Wik M, Crill PM, Winterball M, Eiler JM, Sessions AL (2016) Diverse origins of Arctic and Subarctic methane point source emissions identified with multiply-substituted isotopologues. Geochimica et Cosmochimica Acta 188, 163-188.
Stolper DA, Martini AM, Clog M, Douglas PM, Sessions AL, Shusta SS, Valentine DL, and Eiler JM (2015) Distinguishing and understanding thermogenic and biogenic sources of methane using multiply substituted isotopologues. Geochimica et Cosmochimica Acta 161, 219-247.
Stolper DA, Sessions AL, Ferreira AA, Santos Neto EV, Schimmelmann A, Shusta SS, Valentine DL, Eiler JM (2014) Combined 13C-D and D-D clumping in methane: methods and preliminary results. Geochimica et Cosmochimica Acta 126, 169-191.
Eiler JM, Clog M, Magyar P, Piasecki A, Sessions AL, Stolper D, Deerberg M, Schlueter H-J, Schwieters J (2013) A high-resolution gas-source isotope ratio mass spectrometer. International Journal of Mass Spectrometry, 335, 45-56.