A simple method to isolate fluorescence spectra from small dissolved organic matter datasets

Urban J Wünsch,Kathleen Murphy

doi:10.1016/j.watres.2020.116730

Abstract

Dissolved organic matter (DOM) is a complex pool of compounds with a key role in the global carbon cycle. To understand its role in natural and engineered systems, efficient approaches are necessary for tracking DOM quality and quantity. Fluorescence spectroscopy combined with parallel factor analysis (PARAFAC) is very widely used to identify and quantify different fractions of DOM as proxies of DOM source, concentration and biogeochemical processing. A major limitation of the PARAFAC approach is the requirement for a large data set containing many variable samples in which the fractions vary independently. This severely curtails the possibilities to study fluorescence composition and behavior in small or unique datasets. Herein, we present a simple and inexpensive experimental procedure that makes it possible to mathematically decompose a small dataset containing only highly-correlated fluorescent fractions. The approach, which uses widely-available commercial extraction sorbents and previously established protocols to expand the original dataset and inject the missing chemical variability, can be widely implemented at low cost. A demonstration of the procedure shows how a robust six-component PARAFAC model can be extracted from even a river-water dataset with only five bulk samples. Widespread adoption of the procedure for analyzing small fluorescence datasets is needed to confirm the suspected ubiquity of certain DOM fluorescence fractions and to create a shared inventory of ubiquitous components. Such an inventory could greatly simplify and improve the use of fluorescence as a tool to investigate biogeochemical processing of DOM in diverse water sources.

Highlights

Dissolved organic matter (DOM) is an important component of the carbon cycle in both natural and engineered aquatic environments (Bianchi, 2011; Ridgwell and Arndt, 2014)
The whole-water samples from the drinking water treatment plant exhibited typical properties for water dominated by terrestrial DOM: for the two Dissolved organic carbon (DOC) samples, carbon concentrations were 4.3 and 3.7 mg / L and the specific ultraviolet absorbance was 2.8 and 2.9 L x mg x C−1 x m−1
The fluorescence properties were highly similar across all ten bulk samples once concentration effects were removed by unit-variance scaling (Fig 2A, C)

Summary

Introduction

Dissolved organic matter (DOM) is an important component of the carbon cycle in both natural and engineered aquatic environments (Bianchi, 2011; Ridgwell and Arndt, 2014). A variety of different approaches identify and quantify DOM with varying degrees of molecular insight and analytical complexity (McCallister et al, 2018; Mopper et al, 2007). Amongst the more rapid and affordable techniques, ultraviolet-visible spectroscopy gives insight into the optically-active fractions termed chromophoric DOM (CDOM, determined via absorbance measurements) and fluorescent DOM (FDOM, characterized through fluorescence measurements). The chemical origin of DOM’s optical properties and the chemical interpretation of the obtained signals remains poorly constrained (Aiken, 2014; Rosario-Ortiz and Korak, 2017).

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