Simultaneous Determination of Multiple Photochemically Produced Reactive Intermediates Under Solar Illumination

Photochemically produced reactive intermediates (PPRIs), including HO^•, ¹O₂, triplet chromophoric dissolved organic matter (³CDOM*), CO₃^•-, and long-lived oxidative organic radicals (LOORs), play crucial roles in photochemical transformations in sunlit waters. Despite the existence of well-established probe methods for HO^•, ¹O₂, and ³CDOM*, reliable techniques for quantification of the CO₃^•- and LOORs remain lacking. Here, we developed a multiple-probe chemical method capable of quantifying all five key PPRIs under solar irradiation. The method integrates two complementary probe sets: (i) a high-level probe set (5 μM terephthalic acid (TA), furfuryl alcohol (FFA), 2,4,6-trimethylphenol (TMP), and 3,4-dimethoxyphenol (DMOP)) to determine ³CDOM* while suppressing the CO₃^•- and LOOR contribution, and (ii) a low-level probe set (0.1 μM 4-methylphenol, 0.05 μM TMP and DMOP, 5 μM TA and FFA) to quantify CO₃^•- and LOORs with minimal probe-probe interference. Using this framework, the five PPRIs were determined in both simulated and natural waters. Kinetic modeling revealed that at Br^- < 10 μM, halogen radicals minimally affected the method accuracy, confirming its applicability to most freshwaters and wastewater effluents. Overall, this multiple PC method provides a practical, sensitive, and accessible tool for the simultaneous determination of five major PPRIs in sunlit waters.