What buffer conditions give the best resolution for agarose gel electrophoresis?
FAQ ID -2256

We recommend the use of 1x TBE buffer for small DNA fragments (<1000 bp) when DNA recovery is not necessary. Gels made using TBE buffer give sharper bands than gels made using TAE buffer.  For large DNA fragments of >15000 bp, we recommend the use of 1x TAE buffer.  However, since TAE buffer has a lower buffering capacity, it may be necessary to change the buffer when performing electrophoresis for an extended period of time.