Field Quantification of Hydroxyl Radicals by Flow-Injection Chemiluminescence Analysis with a Portable Device

Hydroxyl radical (^•OH) is a powerful oxidant abundantly found in nature and plays a central role in numerous environmental processes. On-site detection of ^•OH is highly desirable for real-time assessments of ^•OH-centered processes and yet is restrained by a lack of an analysis system suitable for field applications. Here, we report the development of a flow-injection chemiluminescence analysis (FIA-CL) system for the continuous field detection of ^•OH. The system is based on the reaction of ^•OH with phthalhydrazide to generate 5-hydroxy-2,3-dihydro-1,4-phthalazinedione, which emits chemiluminescence (CL) when oxidatively activated by H₂O₂ and Cu³⁺. The FIA-CL system was successfully validated using the Fenton reaction as a standard ^•OH source. Unlike traditional absorbance- or fluorescence-based methods, CL detection could minimize interference from an environmental medium (e.g., organic matter), therefore attaining highly sensitive ^•OH detection (limits of detection and quantification = 0.035 and 0.12 nM, respectively). The broad applications of FIA-CL were illustrated for on-site 24 h detection of ^•OH produced from photochemical processes in lake water and air, where the temporal variations on ^•OH productions (1.0-12.2 nM in water and 1.5-37.1 × 10⁷ cm^-3 in air) agreed well with sunlight photon flux. Further, the FIA-CL system enabled field 24 h field analysis of ^•OH productions from the oxidation of reduced substances triggered by tidal fluctuations in coastal soils. The superior analytical capability of the FIA-CL system opens new opportunities for monitoring ^•OH dynamics under field conditions.