On-line capillary electrophoresis-mass spectrometry (CE-MS) is finding increased use in the analysis of a wide variety of chemically diverse complex mixtures. It is characterized by minimal sample loss and enhanced separation efficiencies as compared to conventional techniques such as high-performance liquid chromatography-mass spectrometry (HPLC-MS). However, the major limitation of both CE and CE-MS is the limited sample loading capacity of conventional CE capillaries. "pica1 loading volumes are approximately 1-100 nL, which afford optimal C E performance, and this is in stark contrast to the 1-100 pL commonly injected onto capillary HPLC columns. The limited loading of CE leads to relatively poor concentration limits of detection. In this work a unique method for analyte preconcentration with CE is described. A cartridge containing an impregnated membrane is installed at the inlet of the CE capillary, and we term this approach membrane piwoncentration-CE (mPC-CE), and in conjunction with mass spectrometry, mPC-CE-MS. It allows both online sample concentration and cleanup and concentration limits of detection of fg/mL are possible using this approach. The analysis of in viva derived metabolites, peptides, and proteins is described. This demonstrates the wide applicability of the technolow in the analysis of any class of compounds, ranging in molecular weight from 100 to 70,000 Da with loading capacities of approximately 1->100 CI.L sample volumes.