Understanding Dissolved Organic Matter Reactivity and Composition in Lakes and Streams Using Proton-Transfer-Reaction Mass Spectrometry (PTR-MS)

Here, we present a novel approach for investigating dissolved organic matter (DOM) composition using thermal desorption proton-transfer-reaction mass spectrometry (PTR-MS), a technique that provides insight into the molecular composition of DOM < m/z 500 (termed PTR-DOC). The applicability of PTR-MS for understanding the relationship between DOM composition and reactivity has yet to be explored. We present results from a synoptic sampling campaign of streams and lakes in a Swedish forest catchment where we measured DOM composition using PTR-MS and traditional optical methods and conducted DOM biodegradability assays. PTR-DOC comprised <= 12% of the total DOC pool. We found significant relationships between PTR-DOC and DOM degradability; reduced chemodiversity and low concentrations of PTR-DOC were both associated with the total DOM pool being more susceptible to microbial degradation. Furthermore, molecular differences were apparent among headwater lakes, headwater streams, and lakes further down the catchment. Direct linkages between PTR-DOC and optical methods were observed. Using the quantitative data that PTR-MS generates, it could become possible to identify the fluorescing components of DOM, and the method may be particularly informative in low-DOC waters such as marine environments where PTR-DOC may dominate the total DOM pool.