Reversed-phase high-performance liquid chromatography behavior of chaotropic counteranions

An unexpected observation concerning the effect of anionic additives on the retention behavior of basic drugs and peptides in reversed-phase liquid chromatography

“…When pure water is used as the mobile phase (i.e. ϕ = 0), eq 5 reduces to: (6) where w represents the pure water phase. Clearly, ln k' w values are related to the free energy of transferring the solute from the pure aqueous phase to the stationary phase.…”

“…More hydrophobic anions such as PF 6 -cause the largest retention increase, followed by ClO 4 -, CF 3 COO -, Cl -, HCOO -and H 2 PO 4 -. Roberts vaguely referred to the "Hofmeister effect" to explain this observation [9].…”

“…Roberts vaguely referred to the "Hofmeister effect" to explain this observation [9]. Kazakevich and coworkers attributed the retention increase due to addition of inorganic anions to a "chaotropic effect" [6]. Gritti and Guiochon studied the role of buffer (e.g.…”

Selective separation and purification of highly polar basic compounds using a silica-based strong cation exchange stationary phase

“…We believe that two competitive processes are involved in altering the retention of amines: a reduction in the interaction with sulfonic groups, and an increase in retention by enhancing hydrophobic interaction. According to previous discussions [27,29,32], chaotropic anions such as ClO 4 − and CF 3 COO − , can create ion-associated complexes with protonated basic molecules [11,26]. Formed ion-pairs underwent longer retention as a more hydrophobic species [33].…”

“…A number of studies [11,[26][27][28][29][30][31] indicated that anion modifiers play an important role in RP mode, to adjust retention, and to improve efficiency and sample loading capacity of basic analytes. However, only a few of studies have described the role of anion modifiers in SCX mode [17].…”