Development of an Automated Online System for Minute-Resolution Measurement of PM2.5 Oxidative Potential Using the DTT Assay

Oxidative potential (OP) is a critical metric for assessing the health effects of particulate matter (PM) pollution. Among the various OP assays, the dithiothreitol (DTT) assay is the most widely used and several online systems have been developed. However, existing online systems either lack sufficient reliability for field deployment or offer relatively low temporal resolution, typically around 1 hour. This study presents an automated online DTT measurement system designed for minute-scale monitoring of PM_2.5 OP with enhanced sensitivity. The system consists of three main components: a Particle-Into-Liquid Sampler (PILS) for PM_2.5 collection, a dual-path incubation module with precisely controlled reaction temperatures (37°C) and durations (~5 and ~10 minutes), and an absorbance-based detector utilizing a liquid waveguide capillary cell (LWCC). This dual-path design enables DTT consumption measurement at two time points within a 15-minute cycle. Assay optimization was conducted to improve the sensitivity of the system. System calibration demonstrated strong correlation (R² > 0.99) and repeatability for DTT detection (0-10 μM) with rapid response. Validation using standards 9,10-phenanthraquinone and Cu²⁺ solutions showed excellent correlations between species concentrations and DTT consumption (R² > 0.99), with strong agreement between online and offline methods. Field deployment further confirmed the system's capability for real-time monitoring and long-term atmospheric OP observation