Inter-droplet gas diffusion in segmented-flow analysis for ammonium determination

Gas diffusion enables ammonium determination by converting it to ammonia gas under alkaline conditions. Ammonia is typically transferred across membranes or air gaps and absorbed by an acceptor phase, inducing measurable changes. This study presents a novel donor-oil-acceptor system for ammonium determination. Immiscible fluorinated oils, such as FC-40, which dissolve ammonia, are used to efficiently segment donor and acceptor droplets within a single flow stream. These oils act as liquid membranes, facilitating ammonia transfer from the donor phase to the acceptor phase through both diffusion and inner droplet circulation. An automated flow-through analyzer, incorporating peristaltic micro-pumps, was developed to generate multiple paired donor-acceptor droplets, enabling concurrent gas diffusion and enhancing sample throughput. Bromothymol blue (BTB) was used as a pH indicator for absorbance-based detection, while o-phthalaldehyde (OPA)-sulfite reagent served as a selective reagent for fluorescence-based detection. The limits of detection were determined to be 14 μmol L -1 for the absorbance method and 2 μmol L -1 for the fluorescence method. Interference studies revealed that precipitation from alkaline earth metals had minimal effect on recovery, while amines exhibited significantly higher transfer efficiency through the oil phase. To address this, the OPA-sulfite reagent in the acceptor phase effectively mitigated interference from amines. The proposed droplet-based system provides an innovative and versatile platform for analyzing volatile compounds in complex sample matrices.