Dissolved organic matter (DOM) is typically characterized by analysis of its bulk physical properties, such as absorbance, or mass spectrometry. While these methods allow inferences on the physical structure of DOM, they are of limited use in predicting DOM chemical reactivity. An important chemical quality of natural DOM is its ability to absorb light in the UV and visible range and take on excited triplet states (3DOM) that are capable of reacting with a wide variety of aquatic constituents. However, structurally dissimilar DOM may produce 3DOM that tends to interact with surrounding compounds through disparate mechanisms, such as energy transfer, charge transfer, or hydrogen abstraction. We present a method for characterizing bulk DOM reactivity by quantifying both the quantum yield of 3DOM and the relative tendency of produced 3DOM to react via specific mechanisms. We demonstrate that this method can distinguish between DOM populations that have been isolated by two size-fractionation techniques: dialysis and downflow ultrafiltration. While these techniques use similar membranes to fractionate DOM, we show that it is possible to distinguish each resultant DOM subpopulation by quantifying the relative rates of 3DOM reaction mechanisms . This 3DOM based technique represents an important step towards an interaction-based characterization scheme for DOM. We use four probe compounds that react with 3DOM by well-defined processes. 2,4,6-Trimethylphenol is used in conjugation with its singly deuterated isotopologue to probe the ability of 3DOM to abstract both electrons and hydrogen atoms from phenols, while 2,4-hexadienoic acid and O2 are used to probe the capability of higher and lower energy triplets to react through energy transfer mechanisms. Multiple photosensitization trials are conducted with each probe compound over a wide range of probe concentrations, allowing for the determination of probe-labile 3DOM production rate, steady state concentration, and quenching rate. Together, these series of measurements of probe-labile 3DOM production allow for both the characterization of size-fractionated DOM populations and insight into likely reactions that occur when photosensitized 3DOM reacts in a natural environment.

Comparing triplet reaction mechanisms for DOM characterization

249th National Meeting (2015)

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