An electrophoretic method for purifying a nucleic acid sample is disclosed. The method generally comprises the steps of (1) providing a nucleic acid sample comprising a desired nucleic acid and one or more contaminants, (2) providing an electrophoresis matrix having a loading well and a recovery well formed therein, (3) placing the nucleic acid sample into the loading well, (4) performing a first electrophoresis comprising electrophoresing the nucleic acid sample for a first time effective to transport the desired nucleic acid out of the loading well and into the electrophoresis matrix; and (5) performing a second electrophoresis comprising electrophoresing the nucleic acid sample for a second time effective to transport the desired nucleic acid out of the electrophoresis matrix and into the recovery well. According to the method, the first and second electrophoresis steps are effective to substantially reduce the concentration of contaminants relative to the concentration of desired nucleic acid in the nucleic acid sample, thereby producing a purified nucleic acid. In the method, the loading and recovery wells may be the same or different, and the electric fields may be DC or alternating. Also disclosed is a preparative electrophoresis method employing an alternating electrical field.