Characterisation of pi-pi interactions which determine retention of aromatic compounds in reversed-phase liquid chromatography

This paper presents the influence of pi-pi interactions on the retention be haviour of aromate-rich and aromate-poor analytes separated on various chro matographic sorbents. The presence of pi-pi interactions between the analyt es and a polymer based sorbent can be very useful for separating compounds with similar retention behaviour on silica based reversed-phase sorbents (w here this type of interaction is absent). Various C-4 and C-18 modified sil ica sorbents (5 mu m and 15 mu m) are compared with some poly(styrene-divin yl benzene) (PS-DVB) sorbents, For this purpose five aromates and four non- aromates were selected. The retention times of the nine uncharged test comp ounds (hydrocortisone, hydrocortisone acetate, testosterone, testosterone p ropionate, nitrobenzene, anisole, toluene, valerophenone, cumene) increase with increasing hydrophobicity (log P). On silica based reversed-phase sorb ents this relationship is the same for aromates and non-aromates. On PS-DVB sorbents this relationship is different: the aromatic compounds need more acetonitrile to elute from the column than the non-aromatic compounds with a similar hydrophobicity: at log P 2.7 the difference in acetonitrile conce ntration is approximately 18-20% for all tested polymer matrices which have no groups covalently bound to their PS-DVB moieties (Vydac(TM), Polymer La boratories, Amersham Pharmacia Biotech). This difference is probably due to the presence of an pi-pi interaction between the sorbent and the analyte. The importance of the pi-pi interaction decreases when more acetonitrile is needed to elute an aromatic analyte from the column. The retention of thes e mono-aromates largely depends on hydrophobic interactions and pi-pi inter actions. The higher the log P of the analyte (containing an aromatic group) , the more important the hydrophobic interaction becomes. Although covalent coupling of functional groups to the PS-DVB sorbent (with a phenyl, isopro pyl or ether linked via a -CH2CHOHCH2O- spacer) leads to a more hydrophilic sorbent, it does not lead to a decrease in pi-pi interaction since the sur face still accessible for the compounds tested. A comparison of the retenti on behaviour of the aromatic test compounds with the non-aromatic test comp ounds on silica based reversed-phase sorbents with that on PS-DVB based sor bents shows the importance of determining pi-pi interactions. (C) 1999 Else vier Science B.V. All rights reserved.