61st Annual Report on Research 2016

Overview.

Lignite is important for electricity generation and may be used as a substrate for Fisher-Tropsch synthesis of liquid fuel. Its precursor is lignin the second most important plant biochemical, involved in structural support. Recently lignin in living trees has been shown to encode the D/H of precipitation, providing for the potential to record hydrological changes if those primary signals are preserved. The aims of this PRF funded project are to take the PIs research in new directions to understand the fundamental D/H systematics in living wood, sedimentary lignin in peats and on into lignites. Fundamental scientific research questions include: 1) Testing and optimizing analytical conditions towards rigorous isotopic determinations. 2) Assessing the magnitude of fractionations associated with biosynthesis, including variability between plant species. 3) Determining signals of diagenetic alteration and catagenic alteration. The ultimate motivation pertaining to PRF relevance is to assess the diagnostic potential of D/H of methoxyl groups within lignites.

Work done.

In the second award year, the PI and graduate student (Hyejung Lee) have worked on 1) optimizing analytical conditions and quantification of methoxyl groups. This was motivated by the observations that the primary limitations to the success of this isotopic technique (for objectives 2 and 3) may be the large uncertainties, associated with the cleavage of the lignin (geo)polymer in the HI acid reaction step. Addressing uncertainty is the most pressing issue to address, toward the successful implementation of this technique.

One solution to this problem would be to analyze intact lignin monomers by NMR, intact, rather than after cleavage by GC-IRMS. As part of these experiments Hyejung Lee learnt NMR techniques and she compared options for NMR and GC-IRMS. She started with phthalic acid which after esterification to yield or methyl or propyl esters can be analyzed by GC-IRMS and NMR, allowing for cross-comparisons of the potential of each technique. She conducted experiments to detect D-H exchange in aromatic ring structures. She discovered the main limitations, were the large sample sizes, long analysis times, and a mismatch between the chemistry insights and Earth Sciences applications. These test yielded useful insights but led us to solidify our direction working with GC-IRMS rather than NMR.

A prime consideration in Earth Sciences is small sample sizes, when sampling, especially when dealing with cores, that are sample limited, or tree ring wood within bores, or destructive analysis of unique geological specimens. We therefore experimented with sample mass, and found that smaller sample masses and reagent fluids (HI) resulted in higher yields. The explanation must be diffusion. Our method increased yield, decreased risks of fractionation, and lowered sample requirements from 10mg to 2mg. CH3I does have solubility in aqueous solutions at room temperature, and thus there may be incomplete escape of the reaction product CH3I from the HI once the vessel cools and after NOH is added to neutralize the aqueous surface may allow for more CH3I in solution, which presents another opportunity for fractionation. Our experiments will next target gas phase:liquid phase fractionation, and then proceed to quantify hypothesized progressive loss of methoxyl groups during maturation, to directly meet the needs of objective 3.

Scientific outcomes.

Our analytical results in year 2 indicate that quantitative conversion is key and highlight problems that need further testing. Results of work reported in the prior annual report in terms of the PI’s laboratory work on stem wood samples (sampling, water extraction, grinding) and their isotopic analyses (stem water and lignin methoxyl D/H composition) have been published in part in GCA (plant water and leaf wax results) (Feakins et al., 2016). The lignin methoxyl results will follow when we have constrained the cause of the large magnitude variability between species, that leads to a failure to record the altitudinal effect in precipitation isotopes, towards objective 2. This biosynthetic variability may explain the variability observed in lignite samples, rather than dia/catagenesis. We have established loss of methoxyl groups during catagenesis and are working on precise quantification.

Conference presentations acknowledging support from this award:

Lee, H., Feakins, S.J., Lu, Z., Tierney, J.E., Schimmelmann, A., Williams, T.J. Determination of isotope effects during methylation of aliphatic and aromatic compounds using GC-IRMS and 1-NMR, Gordon Research Conference on Organic Geochemistry, New Hampshire, July 2016.

Impact on career outcomes.

The PI received tenure during Spring 2015. Postdoc Camilo Ponton is now employed in a second postdoc at Caltech, working on multiply substituted isotopologues (C and H) in ethane and propane in John Eiler’s group, and recently interviewed for a faculty position. Graduate student Hyejung Lee is presenting the results of her work on this grant as part of her qualifying examinations in Fall 2016 to progress to PhD candidacy and hopes to continue her work on the award goals over the next 2 years, with one manuscript ready to be submitted and anticipation of additional publications and a PhD thesis.

Collaboration.

The PI has also been collaborating with a graduate student at Caltech, Max Lloyd working in John Eiler’s group, and with David Naafs a postdoc in Richard Pancost’s group at Bristol, for 1) analytical developments, 2) additional biomarker insights into the peats and lignites being examined here for paleoenvironmental reconstructions and 3) towards understanding catagenesis.