Nontarget Approach to Identify Complexing Agents in Atmospheric Aerosol and Rainwater Samples

Atmospheric particles and droplets contain numerous organic substances, some of which form complexes with metal ions, significantly affecting bulk physicochemical properties and chemical reactivity. However, the detection and identification of complexing agents and their corresponding metal complexes remains an analytical challenge. In this study, we developed an LC/HRMS nontarget screening (NTS) approach which allows the selective detection of complexing agents in aerosol particle extracts and rainwater. To achieve this, a T-junction is installed between the LC outlet and the ion source, and a FeCl₃ solution is added for postcolumn complexation. The resulting mass spectra are screened for the three characteristic iron(III)-complexes [M - H + FeCl₃]^-, [M - 2H + FeCl₂]^-, and [M - 3H + FeCl]^- with mass differences (Δm/z) between the complexing agent and the iron complex of 160.8416, 124.8648, and 89.8959, respectively. Up to 29 di- or tricarboxylic acids were identified as complexing agents in aerosol particle samples from two different sites (Melpitz, Germany, and Wangdu, China) at concentrations as low as 50 nM. Thirteen complexing agents were detected even in measurements without postcolumn iron addition from complexation with background Fe³⁺ traces from the analytical system. At least for the highest concentrated complexing agents, the proposed screening approach can thus be exploited in a NTS approach without any device modification. Besides carboxylic acids, 4-nitrophenol and 4-nitrocatechol were identified as further complexing agents, demonstrating the applicability of the approach to other matrices and to a range of different complexing agents.