Direct Analysis of Free Aqueous and Organic Operational Solutions as a Tool for Understanding Fundamental Principles of Electromembrane Extraction

Aqueous and organic phases in microelectromembrane extraction (μ-EME) were formed as adjacent plugs of free immiscible solutions in narrow-bore polymeric tubing, and each single phase was recovered and analyzed after μ-EME. A three-phase μ-EME setup was employed for investigation of time-dependent distribution of model basic drugs among aqueous and organic phases. Exact concentrations of nortriptyline and papaverine in donor solution, acceptor solution, and free liquid membrane (FLM) were determined by capillary electrophoresis with ultraviolet detection (CE-UV). At typical μ-EME conditions (acceptor, 1 μL of 25 mM HCl; FLM, 1 μL of 4-nitrocumene; donor, 1 μL of basic drugs in 10 mM HCl; and extraction potential, 250 V), experimentally determined distribution of the drugs confirmed the kinetic model for electrically mediated transfer of charged analytes. Rapid depletion of the drugs from donor solution (0–180 s) and rapid saturation of FLM with the drugs (15–60 s) were followed by gradual transfer of the drugs across FLM and gradual liberation into acceptor solution (30–240 s). Exhaustive transfer of the drugs from donor to acceptor solution was obtained in 15 min. A good correlation between the analytes’ distribution and μ-EME electric currents was observed; the currents increased during drugs’ transfer across FLM, were concentration dependent, and demonstrated transfer of the drugs across FLM in their ionized forms. Proper understanding of the fundamental principles of μ-EME transfer enabled further fine-tuning of the μ-EME process. Transfer of the drugs across FLM was controlled by optimizing the composition and pH of acceptor solution, and quantitative fractionation of nortriptyline into aqueous acceptor (96%) and of papaverine into organic FLM (95%) was achieved based on their different pK_a values. μ-EME fractionation of the two drugs was compatible with raw human urine and excellent repeatability (RSD ≤ 3.9%), linearity (r² ≥ 0.9989), and limits of detection (≤ 0.15 μg/mL) were achieved for μ-EME-CE-UV of the drugs in standard solutions and urine samples.