Retention behaviour of imidazolium ionic liquid cations on 1.7 μm ethylene bridged hybrid silica column using acetonitrile-rich and water-rich mobile phases

The potential of 1.7 μm ethylene bridged hybrid silica phase was investigated for the separation of twelve imidazolium-based ionic liquid cations. U-shaped retention profile was observed for all solutes with an increase in retention at both low and high acetonitrile content. Chromatographic behaviour of imidazolium cations in both hydrophilic interaction chromatography (HILIC) and per aqueous liquid chromatography (PALC) modes was studied by varying key parameters such as buffer concentration and pH, acid additive, organic modifier and column temperature. Experimental data provided some evidences that under PALC conditions cationic solutes are retained predominantly by mixed hydrophobic/ion-exchange interactions. In the HILIC mode, both partitioning and ion-exchange interactions are responsible for the retention of solutes. Compared to PALC, HILIC provided significantly higher efficiencies with less or even no peak tailing, better separation selectivity and greater resistance to overload. In PALC mode gradient elution was required to achieve adequate retentivity of all solutes but selectivity was not sufficient to distinguish between solutes with very similar hydrophobicity. In contrast, under HILIC conditions twelve solutes were almost completely resolved in less than 4 min by using isocratic elution. Summarizing, it could be concluded that ethylene bridged hybrid silica column providing a dual retention mechanism offers the possibility of selecting between the two retention modes with opposite separation selectivity, just by changing the composition of the mobile phase.     

Covalently bonded polysaccharide-modified stationary phase for per aqueous liquid chromatography and hydrophilic interaction chromatography

The mixed sulfated/methacryloyl polysaccharide derivative was prepared and successfully immobilized onto the surface of porous silica particles by polymerization. Polysaccharide derivative was calculated as 10.33% in the stationary phase prepared. The new stationary phase (PMSP) showed both hydrophilic interaction (HILIC) and per aqueous liquid chromatography (PALC) characteristics. The effects of column temperature, the water content, pH and ion strength of mobile phase on the retention time of test compounds in highly aqueous eluents were investigated to evaluate the PALC features of PMSP. The column efficiency is about 31,000 plates/m for benzoic acid in water/ACN (97/3, v/v) mobile phase at a flow rate of 1.0 mL/min. Compared with C18 column, the PMSP had shorter retention time for weak polar and non-polar compounds, but also showed stronger retention for strong polar compounds. It indicated that PALC was a suitable mode of chromatography as replacement of HILIC and complementarity of reversed-phase liquid chromatography (RPLC).     

Efficiency of the same neat silica column in hydrophilic interaction chromatography and per aqueous liquid chromatography

The dependencies on the mobile phase flow velocity of the efficiency of a column packed with shell particles of neat porous silica (Halo) was measured under two different sets of experimental conditions. These conditions corresponded to the retention mechanisms of per   aqueous liquid chromatography (PALC) at low acetonitrile concentrations and of hydrophilic interaction chromatography (HILIC) at high acetonitrile concentrations. The results are compared. Small amounts of a diluted solution of caffeine were injected in order to record the chromatograms under strictly linear conditions. These efficiencies were measured in both water-rich (PALC retention mechanism) and acetonitrile-rich (HILIC mechanism) mobile phases for the same retention factors, between 0.25 and 2.5. The mobile phases were mixtures of acetonitrile and water containing neither supporting salt nor buffer component. At low retention factors, the efficiency of caffeine is better in the PALC than in the HILIC mode. For k^′=0.5$k^{\prime }=0.5$, the minimum reduced height equivalent to a theoretical plate (HETP) is close to 2.5 in PALC while it exceeds 5 in HILIC. The converse is true for high retention factors. For k^′>2.5$k^{\prime }>2.5$, the HETP is lower in HILIC than in PALC, because the major contribution to band broadening and peak tailing in this latter mode originates from the heterogeneous thermodynamics of retention and eventually restricts column performance in PALC. Most interestingly, the reduced HETP measured in HILIC for caffeine never falls below 4. This suggests that the mass transfer of caffeine between the multilayer adsorbed phase (due to the interactions of the strong solvent and the silanol groups) and the acetonitrile-rich bulk eluent is slow.     

Comparison of the adsorption mechanisms of pyridine in hydrophilic interaction chromatography and in reversed-phase aqueous liquid chromatography

The adsorption isotherms of pyridine were measured by frontal analysis (FA) on a column packed with shell particles of neat porous silica (Halo), using water–acetonitrile mixtures as the mobile phase at 295 K. The isotherm data were measured for pyridine concentrations covering a dynamic range of four millions. The degree of heterogeneity of the surface was characterized by the adsorption energy distribution (AED) function calculated from the raw adsorption data, using the expectation-maximization (EM) procedure. The results showed that two different retention mechanisms dominate in Per aqueous liquid chromatography (PALC) at low acetonitrile concentrations and in hydrophilic interaction chromatography (HILIC) at high acetonitrile concentrations. In the PALC mode, the adsorption mechanism of pyridine on the silica surface is controlled by hydrophobic interactions that take place on very few and ultra-active adsorption sites, which might be pores on the irregular and rugose surface of the porous silica particles. The surface is seriously heterogeneous, with up to five distinct adsorption sites and five different energy peaks on the AED of the packing material. In contrast, in the HILIC mode, the adsorption behavior is quasi-homogeneous and pyridine retention is governed by its adsorption onto free silanol groups. For intermediate mobile phase compositions, the siloxane and the silanol groups are both significantly saturated with acetonitrile and water, respectively, causing a minimum of the retention factor of pyridine on the Halo column.