Extent of the influence of phosphate buffer and ionic liquids on the reduction of the silanol effect in a C18 stationary phase

The presence of anionic free silanols in silica-based stationary phases leads to broad and asymmetrical peaks when cationic basic compounds are chromatographed with hydro-organic mobile phases. Adding an ionic reagent to the mobile phase helps prevent analytes from accessing free silanols, improving peak shape. However, the activity of silanols can be influenced by buffer concentration and mobile phase pH, factors often overlooked in literature. This study examined the chromatographic behaviour of three basic β-adrenoceptor antagonists (acebutolol, nadolol, and timolol) using acetonitrile-containing mobile phases at varying phosphate buffer concentrations (5 to 50 mM) and pH levels (2 to 8), both with and without three imidazolium-based ionic liquids (1-ethyl-, 1-butyl-, and 1-hexyl-3-methylimidazolium chloride). The effectiveness of the imidazolium cations in blocking cationic analytes arises from ionic interactions with anionic silanols or associations with alkyl chains on the stationary phase. Previous reports indicated that this protective mechanism correlates with cation size. Findings show that the efficacy of the mobile phase additive as a silanol blocker also depends on buffer anion concentration and the protonation degree of the silanols. Increasing phosphate concentrations at low pH result in longer retention times and improved peak shapes, suggesting phosphate's role in blocking free silanol activity. However, the advantages gained from buffering the mobile phase at low pH and using a bulky additive diminish at pH levels above 6.