Buffer Mixing Acceptance Criteria

I am looking for some guidance regarding setting acceptance criteria limits for buffer mixing (within a single batch). We have looked into f and t tests, however our sample results are often identical. Therefore the f and t test cannot be applied. The only mixing information I could find was in regards to mixing batches of API. Any information on mixing buffers, or any advice on comparing a buffer against itself, is appreciated. Top, Middle, Bottom, vs. 3 side port samples or something of the like.