The protein concentration of the samples for a electrophoretic separation must be within a certain region.
Taking too less means nothing can be detected afterwards - and too much of protein overloads the separation leading too unsharp bands.
The protein concentration range depends on the sensitivity of the visualization process following the electrophoretic procedure:
The Coomassie Staining
This staining is less sensitive but can be quantificated.
Concentration range per slot with 15 ul sample volume: 1 - 5 ug
The Silver Staining
Very sensitive but no quantification possible.
Concentration per slot with 6 ul sample volume: 50 - 500 ng