Coupled ultrafiltration and solid phase extraction approach for the targeted study of semi-labile high molecular weight and refractory low molecular weight dissolved organic matter

Abstract

Abstract Only a small fraction of dissolved organic matter (DOM) can be characterized at the molecular level by direct seawater analysis. Thus, the study of DOM requires isolation of extremely dilute organics from orders of magnitude greater concentrations of inorganic salts. Traditional isolation approaches have sought to isolate representative DOM fractions, however, currently available isolation methods all have selective chemical or physical biases. Recent work has indicated that DOM exists in a functional continuum of molecular size and 14C age. High molecular weight (HMW) DOM is primarily composed of younger, semi-labile material, while much older, low molecular weight (LMW) DOM dominates the refractory background pool. Here we describe a new large volume DOM isolation approach that selectively isolates HMW and LMW DOM fractions with distinct 14C ages, a proxy for reactivity. The method uses ultrafiltration (UF) to isolate HMW DOM (UDOM), and then solid phase extraction (SPE) to isolate LMW DOM permeating the UF system. We first assess two SPE sorbents (Agilent Bond Elute PPL and Diaion HP-20) for DOM chemical and isotopic selectivity. Second, we evaluate our UF/SPE approach in the context of DOM recovery, elemental (C/N) and isotopic (δ13C, δ15N, Δ14C) composition of 8 HMW and LMW sample pairs, isolated from the North Central Pacific Ocean. Radiocarbon (Δ14C) analysis shows major differences in the Δ14C value of HMW (Δ14C = − 37 to − 380‰) and LMW (Δ14C = − 343 to − 578‰) DOM fractions. We also observe elemental and stable isotopic offsets between HMW and LMW DOM at all depths. HMW UDOM (C/N = 11.5 to 13.1, δ13C = − 22.5 to − 21.1‰, δ15N = 6.2 to 7.1‰) has significantly lower C/N ratios and higher δ13C and δ15N values than LMW SPE-DOM (C/N = 24.2 to 28.5, δ13C = − 23.3 to − 22.2‰, δ15N = 3.1 to 4.0‰), with the exception of surface δ13C, which is equivalent in both size fractions. Together, these results indicate that our combined UF/SPE method successfully isolates separate young (semi-labile, HMW) and old (refractory, LMW) DOM fractions, each with distinct chemical and isotopic composition. Ultimately, by limiting the influence of DOM reactivity mixtures, our method provides an alternative approach for understanding DOM sources and cycling.