Toward Radiocarbon Measurement of Individual Amino Acids in Marine Dissolved Organic Matter (DOM): Δ14C Blank Quantification for an HPLC Purification Method.

Abstract:

The presence of much of the marine dissolved organic nitrogen (DON) pool as uncharacterized, biologically recalcitrant molecules is a central mystery in the marine nitrogen cycle. Radiocarbon (Δ¹⁴C) isotopic measurements have been perhaps the most important data constraining the cycling of dissolved organic matter (DOM), but little Δ¹⁴C data specific to DON is available. Amino acids (AAs) are the major component of DON that can be isolated on a molecular level. Δ¹⁴C measurements for the operational “protein-like” fraction of DOM in the deep ocean indicate that this compound class has radiocarbon ages greater than several ocean mixing cycles, suggesting remarkable preservation of labile AAs exported from the surface. However, it is possible that the previously defined operational “protein-like” fraction may also contain non-AA material. Radiocarbon measurement of purified individual AAs would provide a more direct and reliable proxy for DON Δ¹⁴C age and cycling rate.

We present here Δ¹⁴C blank characterization of an AA purification method based on HPLC, with on-line fraction collection. This method allows the recovery of unmodified AAs, but accurate measurement of small AA samples that can be extracted from DOM requires a system with extremely low Δ ¹⁴C blanks. Here we assess the impact of HPLC purification on the Δ¹⁴C age of known amino acids standards. Individual AA standards with contrasting (modern vs. dead) and well- characterized Δ¹⁴C ages were processed in a range of sample sizes. The eluted peaks were collected and dried, and measurement of their post-chromatography Δ¹⁴C content allowed for determination of the Δ¹⁴C blank by method of additions. The same protocol was applied to a mixture of six AA standards, to evaluate tailing effects in consecutive AA peaks of contrasting Δ¹⁴C age. AA standards were selected to include both Δ¹⁴C modern and dead AAs that elute both early and late in the chromatographic solvent program. We discuss implications for future Δ¹⁴C analysis of AAs purified from DOM by this method.