预电泳，是除去
凝胶中没有聚合的单体和双体和聚合引发剂，提高分辨率的一种做法，不加任何物质，一般30－60分钟后再加样电泳。
将聚合好的凝胶安置于电泳槽中，小心拔去梳子，加入
电泳缓冲液后低电压10-20V，预电泳20-30min，目的是清除凝胶内的杂质， 疏通凝胶孔径以保证电泳过程中电泳的畅通。
预电泳后依次加入标准品和待分析样品。（加样时间要尽量短，以免样品扩散，可在未加样的孔中加入等量的样品缓冲液避免边缘效应。样品一般在1.0mm厚的胶 加样50-100ug/lane）。
It is really dependent on what gel you are talking about, pro-run for different gels has different purpose. In some case, for instance, SDS PAGE, there may be some residue chemicals in your gel, such as TEMED or ammonium persulfate. Pre-electrophoresis will move those ahead of your samples. Some times, people put some chemicals in the gel, then the pre-run makes the chemical react with something else. At these day, very few people would do pre-electrophoresis to SDS PAGE, but some other electrophoresis may require the pre-run. It really depends.