Two designs of microdevices for microanalysis by mass spectrometry have been developed. In both cases the electrospray is used for sample ionization and transfer of the analytes from the microdevice into the mass spectrometer. The first design incorporates a capillary electrophoresis separation channel and a micromachined pneumatic nebulizer for generation of a stable sample flow and electrospray. The second design is aimed at a high throughput infusion analysis in a format compatible with a standard microtiter well plate technology. The samples, deposited into the wells, can be analyzed in rapid sequence by consecutive application of the electrospray voltage and pressure in the respective channel. During operation, the microdevice is positioned on a motorized translation stage. This assures that each consecutive sample can be analyzed immediately after collection of sufficient amount of data from the previous sample.