Although it is realized that this approach is not suitable for iden-tification of molecules other than those of the target group, it couldbe used to tentatively predict the general structure of any new PDE-5 inhibitor based on its relative retention to SIL and its UV spectrum.Of course, in such cases as well as in case of ambiguity, other meth-ods should be used to confirm the structure. Because the mobilephase used in this method is MS compatible, direct coupling of anMS to the HPLC–UV system may easily solve this problem.