Pathway of Diethyl Phthalate Photolysis in Sea-water Determined by Gas Chromatography-mass Spectrometry and Compound-specific Isotope Analysis

The degradation mechanism of diethyl phthalate (DEP) in natural seawater under UV irradiation was investigated using a combination of intermediates detection and determination of stable carbon isotopic fractionation. Typical intermediates identified with gas chromatography-mass spectrometry (GC-MS) were mono-ethyl phthalate (MEP) and phthalic anhydride. Stable carbon isotope signature was determined by gas chromatography coupled with isotope ratio mass spectrometry through a combustion interface (GC-C-IRMS). A profound (13)C enrichment, with a δ(13)C isotope shift of 12.3±0.3‰ (f=0.09) in residual DEP molecule, was clearly an indicator to its photolysis. The reactive position isotope enrichment factor (ε(reactive position)) and apparent kinetic isotope effects (AKIE) were -35.25±2.26‰ and 1.075, respectively, indicating that the initial reaction step was cleavage of a CO bond in DEP photolysis. Based on these observations, a degradation pathway was proposed. First, a CO bond in DEP molecule was broken to form MEP. Then, MEP was further degraded to phthalic anhydride. Our work demonstrates that compound-specific isotope analysis (CSIA), when combined with intermediates analysis, is a reliable measure to deduce the mechanism of DEP photolysis. This approach might be extended as a reference for mechanism investigation in complicated environment systems.