The invention can be used to purify and extract a target oligonucleotide from a gel substrate. The current invention extracts the target oligonucleotide so quickly that burn-off against the positive electrode is greatly reduced, yielding high optical density and purity. The need for an osmotic membrane or heavy salt/light salt barrier to capture the oligonucleotide is eliminated. The invention does not require a high salt concentration and therefore does not require a desalting column that is time-consuming and reduces yield.