A method and system for preconcentrating analytes at a microvalve in a microfluidic device is disclosed. The system includes a sample channel loaded with a sample solution. The sample channel includes a semi-permeable membrane microvalve. An electric potential is applied at or across the microvalve to preconcentrate the sample solution when the microvalve is closed. The method includes pretreatments of the device or valve for preconcentration of the analytes. For preconcentration of anionic analytes, the device is baked. For preconcentration of the cationic analytes, the surface of the membrane microvalve is coated with a polycationic coating, and the device is baked.