Multi-extraction system with identical supported semi-liquid membrane: Enhanced stability for coextraction of acidic and basic drugs.

Coextraction of different groups of analytes is vital for saving sample volumes and simplifying analytical procedures in bioanalysis. Conventionally, coextraction was achieved by using multi-extraction systems with different supported liquid membranes (SLMs). However, the different membrane solvents tended to diffuse into the aqueous solutions and the other SLM to reach distribution equilibrium during extraction process, causing the system instability. In this work, a stable multi-extraction system (integration of liquid-phase microextraction and electromembrane extraction, LPME/EME) based on the identical supported semi-liquid membrane (SsLM) was developed. Principally, the state of distribution equilibrium of the membrane solvent (polypropylene glycol with molecular weight 4000) in SsLM could be reached at the beginning of extraction, which enhanced the coextraction stability. With this multi-extraction system, acidic and basic analytes were simultaneously extracted from practical biological samples. The extraction recoveries of the six model drugs in undiluted urine samples were over 70%. Followed by LC-MS/MS, the limits of quantification (LOQs) were in the range of 5-10 ng mL. The multi-extraction system using the identical SsLM in this study shows promising potential in construction of other stable multi-extraction systems (e.g., LPME/LPME and EME/EME) in the future, which will greatly benefit the group separation of analytes in complicated biological samples.