The effect of membrane thickness on supported liquid membrane extractions in-line coupled to capillary electrophoresis for analyses of complex samples.

The effect of membrane thickness on extraction performance was systematically examined in extractions through supported liquid membranes (SLM), which were in-line coupled to capillary electrophoresis (CE). Three porous polypropylene membranes with different thickness (25, 100 and 170 μm) were used as supports for SLM extractions of model basic drugs (nortriptyline, papaverine, haloperidol and loperamide) from complex samples. The analytes were transferred through the SLMs by a pH gradient and were in-line injected, separated and quantified using a commercial CE instrument with ultraviolet (UV) detection. Transfers of the model drugs through SLM decreased with the increased membrane thickness (in the order: 25 > 100 > 170 μm) and highest transfers were achieved for the thinnest membrane. Interferences from complex sample matrices were efficiently eliminated, moreover, impregnation of the 25 μm membrane required significantly reduced volume of organic solvent. Mechanical stability of the impregnated 25 μm membrane was excellent during in-line injections, which necessitated direct contact of CE separation capillary with the SLM. Repeatability of the hyphenated SLM-CE-UV method (using the 25 μm membrane) was lower than 11% (RSD values of peak areas) and calibration curves were strictly linear in 0.5-30 μg/mL concentration range (coefficients of determination ≥ 0.997). Transfers of the basic drugs from donor solutions (standard and undiluted human urine/plasma) through the SLMs ranged from 45 to 231% and limits of detection were between 0.02 and 0.15 μg/mL.