I've developed a procedure to separate seven cyclic and aromatic amines on a LUNA SCX ion exchange column using a mobile phase of 90% 20mM KH2PO4 at pH 2.4 and 10%ACN. There are 6 compounds in all and separation of 2 of them is ultra pH dependant. If I change the pH by 0.1 upwards or downwards I loose resolution. Does this fail the robustness check? Also is there anything I can do to improve matters? Th3e compound which 'suffers' most from pH change is pyrimidine (like pyridine except its got 2 Ns )
Best regards
Hendrik

Use e.g. nitric acid plus ACN.