A novel micro-injector for capillary electrophoresis for the handling of samples with volumes down to as little as 300 nL was designed and built in our laboratory for analyses in which the available volume is a limitation. The sample is placed into a small cavity located directly in front of the separation capillary, and the injection is then carried out automatically by controlled pressurization of the chamber with compressed air. The system also allows automated flushing of the injection chamber as well as of the capillary. In a trial with a capillary electrophoresis system with contactless conductivity detector, employing a capillary of 25 m diameter, the results showed good stability of migration times and peak areas. To illustrate the technique, the fast separation of five inorganic cations (Na+, K+, NH4+, Ca2+, and Mg2+) was set up. This could be achieved in less than 3 min, with good limits of detection (10 M) and linear ranges (between about 10 and 1000 M). The system was demonstrated for the determination of the inorganic cations in porewater samples of a lake sediment core.