Suitability of different C-18 silica-based stationary phases for the transferability of an ion-interaction HPLC method

A critical chromatographic separation of a mixture containing inorganic ani ons (nitrate and nitrite) and organic cations (the three isomeric forms of phenylendiamine) was chosen as the System Suitability Test (SST) to compare the performance, under Ion-Interaction condition, of five different C-18 c olumns characterised by the same specifics. The ruggedness (ie. the capacity to yield good results also when there are variations in the "experimental circumstances") of the Ion Interaction Reag ent (IIR) HPLC method previously developed and optimised was evaluated with robustness tests for each stationary phase considered. Variations of +/-10 % to the nominal values of the chromatographic conditions were imposed and the effects on the chromatographic response were evaluated. For the different stationary phases the results showed performances and beh aviours unexpectedly different, concerning not only the resolution among th e components of the mixture but also their elution sequence order. The studies identified the pH value of the mobile phase as the most critica l parameter of the method, which must be very strictly controlled. For one of the stationary phases considered a pH variation of only 5 % with respect to the nominal value not only gave unsatisfactory resolutions, but also a different elution order of the analytes. Another of the tested columns was proved not suitable to work under these I IR conditions.