To prevent microbial growth and contamination, buffers must be refrigerated when not in use. Using a buffer at refrigerator temperature also improves band resolution and lessens evaporation during electrophoresis.
It is recommended that electrophoresis using a small volume of buffer use a fresh buffer for each run. After a large volume buffer electrophoresis, the buffer can be reused if the buffer from each reservoir is combined, mixed well, and refrigerated.