Terephthalate Probe for Hydroxyl Radicals: Yield of 2-Hydroxyterephthalic Acid and Transition Metal Interference

Hydroxyl radicals (.OH) are key players in chemistry in surface waters, clouds, and aerosols. Additionally, .OH may contribute to the inflammation underlying adverse health outcomes associated with particulate matter exposure. Terephthalate is a particularly sensitive probe for hydroxyl radicals, with a detection limit as low as 2 nM. However, there is uncertainty in .OH quantification using this method, and potential for interferences from some transition metals. Terephthalate reacts with .OH to form a fluorescent product, 2-hydroxyterephthalic acid (hTA), with a moderate dependence on pH and temperature. However, there is disagreement in the literature on the yield of the fluorescent product (Y_hTA), which introduces a large uncertainty in the quantification of OH. Additionally, TA and similar organic probes are known to complex Cu(II) at high concentrations; thus, if this reaction is important at lower concentrations, Cu(II) could reduce apparent hTA formation, and reduce activity of Cu(II) in target samples. Using a pH 3.5 dark ferrous Fenton system to generate .OH radicals, we find that Y_hTA = 31.5 ± 7%. This is about double the recent literature value measured, but in excellent agreement with earlier measurements. Additionally, we find that interactions between Cu(II) and hTA are small enough to be ignored at Cu(II) concentrations below ∼50 µM.