Ammonium di(2-ethylhexyl)phosphate-based supramolecular solvent formation: Liquid-phase microextraction of fluoroquinolones from human urine followed by liquid chromatography determination

In this work, a phenomenon of a supramolecular solvent formation from an isotropic solution of ammonium di(2-ethylhexyl)phosphate under electrolyte-induced coacervation by salting-out effect was discovered for the first time. Ammonia was employed as a weak base to obtain the amphiphile in situ from di(2-ethylhexyl)phosphoric acid. The extraction and preconcentration ability of the supramolecular solvent towards amphoteric analytes (fluoroquinolones) was studied in detail. The maximum enrichment factors (19 and 42 for ofloxacin and moxifloxacin, respectively) were obtained in the presence of sodium chloride as a coacervation agent. The proposed extraction mechanism consists mainly of hydrophobic and electrostatic interactions between the analytes and the aggregates. In contrast to the previously reported supramolecular solvent based on di(2-ethylhexyl)phosphoric acid (molecular form), the novel one ensured efficient extraction of amphoteric analytes. The physicochemical properties (water content, pH, density, viscosity) of the supramolecular solvent phase were established. Moreover, microscopic images of coacervates were obtained, and the phase diagram of the ternary system ammonium di(2-ethylhexyl)phosphate-sodium chloride-water was acquired. The novel liquid-phase microextraction approach was successfully applied to the determination of ofloxacin and moxifloxacin in human urine samples by liquid chromatography with fluorescence detection. The limits of detection were established at 3 and 6 μg L⁻¹ for moxifloxacin and ofloxacin, respectively.