A new method for compound-specific radiocarbon analysis of amino acids

Radiocarbon (14C) measurements have been used to provide reliable age estimates extensively in the fields of climatology, atmospheric science, biogeochemistry, and paleoceanography. However, most radiocarbon data is generated by measuring a bulk sample (e.g., bulk sediments), averaging 14C content of all carbon (C)-containing organic compounds (after inorganic C removal) rather than the specific organic compound produced by the organisms of interest during their life. Compound-specific radiocarbon analysis (CSRA) of amino acids (AA) can thus be a useful tool to precisely determine the age when the AAs were produced by the organisms. Robust methods for CSRA-AA are not yet widely available, partly due to the relatively small amount of C in AAs. Here we present a new approach of CSRA-AA combining semi-preparative ion- exchange chromatography (IC) and Accelerator Mass Spectrometry (AMS). Phenylalanine (Phe), glutamic acid (Glu), and methionine (Met) with their corresponding blanks were separated and collected using the IC as individual fractions from a commercial AA mixture, followed by graphitization and AMS analysis for 14C measurements. Current blank contribution was 5.2 and 16.6 g C with corresponding yields of 132 and 71.1 g C for Phe and Glu respectively. The obtained fraction modern (FM) with blank subtraction and error propagation was 0.9937±0.0065 for Phe and 1.1362±0.0271 for Glu, which is in line with the modern origins of the AA standards (Phe: 1.0276±0.0017; Glu: 1.0293±0.0005). Initial result for 14C-free Met standard was 0.0108±0.0062 (FM) with a blank contribution of 12.3 g C, consistent with its origin (0.0024±0.0005). The next step is to include additional AA standards and test real samples. Our ultimate goal is to conduct multiple-isotope measurements (13C, 14C, and 15N) of AAs preserved in sediments and separate the N cycling signals produced by surface plankton community from influences induced by diagenesis, microbial reworking, and horizontal transfer.