Ion transfer at micro-interfaces formed between two immiscible electrolyte solutions (µITIES) is a powerful platform for the detection of charged species. Ametryn is used as a model target analyte to investigate the detection at µITIES of lipophilic and ionisable herbicides derived from triazine. A twostep detection method is proposed, which is taking advantage of the spontaneous partition of triazine derivatives at open-circuit potential, and of their ability to assist proton transfer across the interface. First, the neutral form of ametryn is accumulated in a small volume of organic phase (Vorg = 50 µL) at open-circuit potential, which leads to its preconcentration (Vaq >> Vorg). In a second step, AC voltammetry was run to allow the limit of detection to reach sub-micromolar levels of ametryn in 3 river samples. Phosphate buffer was added to the real samples to provide a constant supply of proton for ametryn detection. The excess of buffer ensured that the analytical response was proportional to the ametryn concentration.