I have a His-tagged protein which I am coexpressing with it's binding partner to prevent proteolysis. Once on the Nickel column I can remove 80% of the partner by flushing 2l of 1.3M NaCl solution buffered at pH 8.5 overnight. However the last 20% is difficult to remove, even if I reload the Nickel column and flush a further 2l of salt solution. I am wondering if I can increase the pH to 9.0 or 9.5. It should not effect the binding of His for the Nickel as the His-tag has to be deprotonated to bind, though will it causing stripping of the Nickel?