“You put your GC on my ICPMS!”. Our home-built system for measuring compound-specific 34S.
Measuring the abundance of stable isotopes in individual organic compounds is a very useful analytical technique that can provide detailed geochemical information about the origins of the molecules, geochemical processes that have altered them, and even the nature of environmental conditions millions or billions of years ago. Traditionally, these measurements are accomplished by connecting a standard gas chromatograph (GC) to an isotope-ratio mass spectrometer (IRMS) by way of a chemical reactor. This reactor either oxidizes or reduces organic species to simple molecular forms (H2, CO2, N2) that can then be analyzed by the IRMS (for technical details, see this paper). This is the technique that was pioneered by John Hayes and others in the 1980's, and is still used widely today. Its commonly called "CSIA", or 'compound-specific isotope analysis'. If you look at some of the other projects going on in the lab, you'll see that we use this method commonly to measure 13C and 2H. Unfortunately, it doesn’t work well for sulfur.
To get around this problem, we have have been collaborating with Jess Adkins to couple our gas chromatograph to a multicollector inductively-coupled plasma (ICP) mass spectrometer. The ICP uses a super-hot argon plasma to disintegrate organic compounds into their constituent atoms, including S. The idea is that this hot plasma bypasses all of the problems with chemical conversion mentioned above, and allows us to make isotopic measurements directly on atomic S ions. The system that we have developed to make this work is shown above. Using this, our lab has reported the first-ever compound-specific δ34S measurements, and is still (as of 2015) one of only 3 labs in the world making these measurements. At the same time, we have discovered that ICP-MS is a wonderful way to measure the isotopes of inorganic sulfur species, like sulfate, sulfide, etc. The big advantages here over traditional methods are a ~1000-fold increase in sensitivity, and the ability to measure δ33S to high precision (relative to EA/IRMS). We have also ventured into making S isotope measurements of individual organic kerogen blebs in ~3-billion year old rocks using the two Ion Probe instruments in the division.
These new analytical capabilities open up a huge array of new scientific questions that can be addressed. Graduate student Morgan Raven is tackling the question of how sulfur becomes incorporated into organic matter during diagenesis, and the role that sulfur plays in the preservation of organic matter as part of the global carbon cycle. Postdoc Richard Schinteie has worked on thiophene molecules in crude oils, and the relationship between d34S and thermal maturity. Postdoc Alon Amrani has worked on the process of thermal sulfate reduction in the subsurface, and dimethyl sulfide emissions from the surface ocean. On the inorganic side, Guillaume Paris has worked on new microscale measurements of carbonate-associated sulfate, and applied them to the first good estimates of seawater sulfate δ34S and Δ33S in the Archean.
Recent papers on this subject:
Rennie VCF, Paris G, Sessions AL, Abramovich S, Turchyn AV, Adkins JF (2018) Cenozoic record of δ34S in foraminiferal calcite implies an early Eocene shift to deep-ocean sulfide burial. Nature Geoscience, 1–5.
Sim MS, Paris G, Adkins JF, Orphan VJ, and Sessions AL (2017) Quantification and isotopic analysis of intracellular sulfur metabolites in the dissimilatory sulfate reduction pathway. Geochimica et Cosmochimica Acta 206, 57–72.
Raven MR, Sessions AL, Adkins JF, Thunell RC (2016) Rapid organic matter sulfurization in sinking particles from the Cariaco Basin water column. Geochimica et Cosmochimica Acta 190, 175-190
Raven MR, Adkins J.F, Werne JP, Lyons TW and Sessions AL (2015) Sulfur isotopic composition of individual organic compounds from Cariaco Basin sediments. Organic Geochemistry 80, 53–59.
Crowe SA, Paris G, Katsev S, Jones CA, Kim S-T, Zerkle AL, Nomosatryo S, Fowle DA, Adkins JF, Sessions AL, Farquhar J, Canfield DE (2014) Sulfate was a trace constituent of Archean seawater. Science 346, 735-739.
Paris G, Adkins JF, Sessions AL, Webb SM, Fischer WW (2014) Neoarchean carbonate-associated sulfate records positive Δ33S anomalies.Science 346, 739-741.
Greenwood PF, Amrani A, Sessions AL, Raven MR, Holman A, Dror G, Grice K, McCulloch MT, and Adkins JF (2014) Chapter 10: Development and initial biogeochemical applications of compound-specific sulfur isotope analysis. in Principles and Practice of Analytical Techniques in Geosciences, ed K. Grice, Royal Society of Chemistry, pp 285-312.
Paris G, Fehrenbacher JS, Sessions AL, Spero HJ, Adkins JF (2014) Experimental determination of carbonate-associated sulfate δ34S in planktonic foraminifera shells. Geochemistry, Geophysics, Geosystems 15, 1452-1461.
Paris G, Sessions AL, Subhas AV, Adkins JF (2013) MC-ICP-MS measurement of δ34S and Δ33S in small amounts of dissolved sulfate. Chemical Geology 345, 50-61.
Amrani A, Deev A, Sessions AL, Tang Y, Adkins JF, Hill RJ, Moldowan JM, Wei Z (2012) The sulfur-isotopic compositions of benzothiophenes and dibenzothiophenes as a proxy for thermochemical sulfate reduction. Geochimica et Cosmochimica Acta 84, 152-164.
Bontognali TRR, Sessions AL, Allwood AC, Fischer WW, Grotzinger JP, Summons RE, Eiler JM (2012) Sulfur isotopes of organic matter preserved in 3.45-billion-year-old stromatolites reveal microbial metabolism. Proceedings of the National Academy of Sciences 109, 15146-15151.
Li C, Love GD, Lyons TW, Fike DA, Sessions AL, and Chu X (2010) A stratified redox model for the Ediacaran Ocean. Science 328, 80-83.
Amrani A, Sessions AL, Adkins JF (2009) Compound-specific δ34S analysis of volatile organics by coupled GC/ICPMS. Analytical Chemistry 81, 9027-9034.