Our approach to use SIL as the only external standard for bothidentification and quantification of different PDE-5 inhibitors wasbased on the idea that an HPLC–UV system is capable of deliveringorthogonal data, which – when combined in an appropriate way– can be characteristic for a molecule within a certain group. Inthis case, relative retention of any of the chosen compounds withreference to SIL and comparison of the UV spectrum of the samecompound to a reference spectrum provide sufficient informationto identify the analyte. In order to render the method reliable itwas essential to ensure that (1) the robustness of the method iswell studied and (2) the acceptance criteria for a compound tobe declared as identified are carefully established. Relative reten-tion intervals are presented in Table 2. From the robustness datait appeared that for certain analytes (e.g. ACE, NDV, HV) relativeretention to SIL is more sensitive to small changes in experimen-tal conditions than for others (e.g. VAR, DMS, HS). However, it wasconsidered to be reasonable to use an interval of the same widthfor all analytes. The ±0.15 min range was found to be an accept-able compromise. Even though there are some overlaps betweenthe intervals, this leads to ambiguity only in the case of TDS and TSwhere UV spectra are similar.