A matrix-independent method for direct nanomolar dissolved aluminium analysis in seawater using programmable flow injection (pFI)

Existing shipboard methods for dissolved Aluminium (Al) analysis in seawater require complex instrumentation, highly trained analysts, and calibration with ultra-clean seawater standards to achieve sub-nanomolar sensitivity at sea. To address these challenges, we developed a compact, user-friendly, and low-maintenance method for the determination of dissolved Al in seawater based on programmable Flow Injection (pFI) and the lumogallion fluorescence assay. We optimized reaction conditions and the pFI mixing protocol to minimize matrix effects, enabling accurate quantification of seawater samples using standards prepared in ultrapure water. The resulting Brij-enhanced fluorometric method yields a detection limit of 0.5 nM and <3 % precision across its working range of 1.5-400 nM, without any prior handling such as a preconcentration step. These figures are achieved through direct injection of 600 μL of sample, 300 μL of reagents, with a total analysis time of 2 min. Validation against reference materials showed relative biases between -10 % and +4 % for open-ocean Al concentrations. For higher concentrations (>400 nM), omitting the Brij step via software control extends the linear range to 1.5 μM Al. A 12-day shipboard deployment confirmed the method's robustness, reproducibility and matrix independence, producing vertical Al profiles in the Arctic Ocean consistent with historical observations. This work extends the pFI platform to fluorescence assays and positions it as a robust method for seawater dissolved Al analysis in challenging operational settings. Its application will help expand high quality trace metal datasets and advance our understanding of the biogeochemical cycling of Al in the ocean.