Determination of sildenafil, vardenafil and aildenafil in human plasma by dispersive liquid–liquid microextraction-back extraction based on ionic liquid and high performance liquid chromatography-ultraviolet detection

• A two-step extraction method was developed for sildenafil, vardenafil and aildenafil.
• DLLME based on IL was the first step to transfer the analytes from sample solution.
• Back extraction was performed to transfer the analytes from IL to aqueous phase.
• The final extractant was clean and compatible with HPLC analysis.
• It was applicable to human plasma with low LOD and acceptable recovery.

A novel method which involved dispersive liquid–liquid microextraction (DLLME)-back extraction based on ionic liquid (IL) was developed for the determination of three phosphodiesterase-5 (PDE-5) inhibitors, sildenafil (SD), vardenafil (VD) and aildenafil (AD), in human plasma. DLLME based on IL as the extractant solvent and methanol as the dispersive solvent was the first step to extract PDE-5 inhibitors from sample solution; the other step of back extraction was followed by transferring target analytes from the IL to acidified aqueous solution. This two-step extraction ensured the compatibility of the final extractant phase, acidified aqueous solution herein, with the reversed phase high performance liquid chromatography-UV detection, and afforded clean extractant phase. The optimal extraction condition was obtained after systematical optimization. The sample solution (960 μL) was extracted by 20 μL of 1-octyl-3-methylimidazolium hexafluorophosphate in the presence of 20 μL methanol and 300 mg mL−1 NaCl with the assistance of vortex; IL phase enriched with the target analytes was then extracted by 10% acetic acid aqueous solution. Good linearity ranges (SD 1–500 ng mL−1, VD 2–2000 ng mL−1 and AD 2–2000 ng mL−1) with suitable r2 (=0.9999) were achieved. Limits of detection (LODs) in pure water were 0.15 ng mL−1, 0.30 ng mL−1 and 0.43 ng mL−1 for VD, SD and AD, respectively. Intra-day and inter-day relative standard deviations were below 6.38%. Finally, this method was applied for the determination of PDE-5 inhibitors in human plasma with satisfactory LODs of 0.92 ng mL−1, 1.19 ng mL−1 and 2.69 ng mL−1 for VD, SD and AD, respectively. Acceptable absolute recoveries were obtained from 100.4% to 103.9%. The developed method afforded a convenient, fast and cost-saving operation with high extraction efficiency for the test analytes. It has potential to be applicable to biological samples.