Mass defects for natural history

Isotopic mass defects are the topic du jour around here. Those tiny mass differences form the basis of a nice multiplexing scheme for analytical chemists – if you assume that different stable isotopes of the same element share exactly the same chemical reactivity.

That assumption of equal reactivity works well enough for many applications, but there’s an entire field of chemistry dedicated the fact that it isn’t always true: isotopic geochemists live in a world where isotopes react differently. Our knowledge of the ancient environment on Earth — temperature, atmospheric composition, seawater chemistry, and many other climate variables — comes from careful measurements of the average abundance of say carbon-13 vs. carbon-12 in samples of limestone, or from the abundance of deuterium (2H) relative to protium (1H) in carefully extracted samples of fossil biomarkers.

Mass spectrometers for geochemistry have been improving rapidly, just as they have been in proteomics or metabolomics, and now geochemists can measure mass defects too. Their primary question is, what is the internal distribution of heavy isotopes in the molecules of the sample? “I know this methane (CH4) contains X% carbon-13 and Y% deuterium,” says today’s geochemist, “but how much of that deuterium is clumped with the carbon-13 in molecules like 13CDH3, and how much is separate in molecules like 13CH4 and 12CDH3?”.

Late last year, one group reported on their mass spec technique for measuring isotope clumping in methane, and found that clumping in their samples was controlled by thermodynamics. (Yes, thermodynamics has an opinion on how isotopes should clump, just as it does on nearly any question in science.) But mass spec isn’t the only game in town. Another group developed a laser spectroscopy technique to measure isotope clumping in methane, and they found that clumping in their methane samples was kinetically controlled.

The two groups weren’t looking at the same samples, so it could well be that they are both right. But newfound analytical techniques to measure mass defects are going to keep geochemists busy for a long time to come.


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