A Comparison of Silica C and Silica Gel in HILIC Mode: The Effect of Stationary Phase Surface Area

Silica Hydride: A Separation Material Every Analyst Should Know About

This review describes the development, special features and applications of silica hydride-based stationary phases for HPLC. The unique surface of this material is in contrast to ordinary, standard silica, which is the material most frequently used in modern HPLC stationary phases. The standard silica surface contains mainly silanol (Si-OH) groups, while the silica hydride surface is instead composed of silicon-hydrogen groups, which is much more stable, less reactive and delivers different chromatographic and chemical characteristics. Other aspects of this material are described for each of the different bonded moieties available commercially. Some applications for each of these column types are also presented as well as a generic model for method development on silica hydride-based stationary phases.

Mixed-Mode Hydrophilic Interactions/Reversed-Phase Retention Mechanism in Thin-Layer Chromatography

We investigated the dual retention mechanism in thin-layer chromatography taking place on three stationary phases of different polarity (C-18, plain silica gel and DIOL) and using binary mobile phases composed of acetonitrile as the main component and water, or methanol as a modifier. As the test analytes, we selected a set of 12 compounds of pharmaceutical importance and considerably different chemical structure, i.e. the imidazoline and serotonin receptor ligands, and their related compounds. Retention of each analyte in each investigated chromatographic system was determined in a wide enough range of the mobile phase composition, with volume fraction of the mobile phase modifier ranging from 0.10 to 0.90. Calculation of the exact turning point values as a proof of occurrence of the reversed-phase hydrophilic interaction chromatography (HILIC/RP) retention mechanism was based on the multimodal retention model. The dual retention mode was described with the use of the volume fraction of the mobile phase modifier, the total polarity and the total solubility models. For the DIOL, C-18 and silica gel stationary phase, the dual (HILIC/RP) retention mechanism was confirmed. In the case of the DIOL stationary phase and acetonitrile/methanol mobile phase, the observed retention mechanism was more complicated than the dual HILIC/RP one.

Exploring the Effect of Buffer Strength on the Retention Time of Weak Acids, Neutral and Weak Bases in Hydrophilic Interaction Liquid Chromatography (HILIC) Mode

Background:

Hydrophilic Interaction Liquid Chromatography (HILIC) orthogonal to conventional reversed phase High-Performance Liquid Chromatography (HPLC) mode allowing separation of polar compounds. HILIC has been reported to be an alternative to normal phase liquid chromatography, yet the separation mechanism reported in HILIC is much more complicated than that in normal phase liquid chromatography.

Objective:

To investigate the effect of water layer thickness on silica gel and the amount of ammonium ions present within the buffer on retention mechanism in hydrophilic interaction chromatography.

Methodology:

A test system was designed which used weak acids, neutrals and weak bases as probes with three different strengths (5, 10 and 20 mM) of ammonium acetate, ammonium formate and ammonium propionate as the counter-ions to compete with the test probes with ionised silanol groups and water present in the stationary phase. A Kromasil 60-5SIL column (150 mm×4.6 mm×4 μm, pore size 60Å) was used as the stationary phase to perform the study.

Results:

Retention times were examined for the test probes at 90% acetonitrile (ACN) with 10% of 5, 10 and 20 mM of ammonium acetate, ammonium formate and ammonium propionate. As the buffer strength increases, the thickness of the water layer on the surface of the silica gel increases and also the repulsion between ionized silanol groups and acidic test probes will decrease. On the other hand, such increase in buffer strength will increase the competition between the ammonium ions and basic test probes. In addition, the hydration energy of buffer’s counter ions and hydrophilicity may be important in retention mechanism in HILIC mode.

Conclusion:

At 20 mM buffer strength acidic probes with low log P values retain more due to reduced repulsion by silanol groups, while basic probes retention time will decrease due to increased competition from ammonium counter ions. However, in 5 mM buffer strength basic probes with low logP value will be retained longer, while acidic probes will be eluted earlier due to the repulsion between ionized acids and ionized silanol groups.