Hydrophilic Partitioning or Surface Adsorption? A Quantitative Assessment of Retention Mechanisms for Hydrophilic Interaction Chromatography (HILIC)

Insights into the selectivity of polar stationary phases based on quantitative retention mechanism assessment in hydrophilic interaction chromatography

Hydrophilic interaction chromatography (HILIC) offers different selectivity than reversed-phase liquid chromatography (RPLC). However, our knowledge of the driving force for selectivity is limited and there is a need for a better understanding of the selectivity in HILIC. Quantitative assessment of retention mechanisms makes it possible to investigate selectivity based on understanding the underlying retention mechanisms. In this study, selected model compounds from the Ikegami selectivity tests were evaluated on different polar stationary phases. The study results revealed significant insights into the selectivity in HILIC. First, hydroxy and methylene selectivity is driven by hydrophilic partitioning; but surface adsorption for 2-deoxyuridine or 5-methyluridine reduces the selectivity factor. Furthermore, the retention of 2-deoxyuridine or 5-methyluridine by surface adsorption in combination with the phase ratio explain the difference in hydroxy or methylene selectivity observed among different stationary phases. Investigations on xanthine positional isomers (1-methylxanthine/3-methylxanthine, theophylline/theobromine) indicate that isomeric selectivity is controlled by surface adsorption; however, hydrophilic partitioning may contribute to resolution by enhancing overall retention. In addition, two pairs of nucleoside isomers (adenosine/vidarabine, 2'-deoxy and 3'-deoxyguanosine) provide an example that isomeric selectivity can also be controlled by hydrophilic partitioning if their partitioning coefficients are significantly different in HILIC. Although more data is needed, the current study provides a mechanistic based understanding of the selectivity in HILIC and potentially a new way to design selectivity tests.

The potential of polyaniline-coated stationary phase in hydrophilic interaction liquid chromatography-based solid-phase extraction for glycopeptide enrichment

Glycopeptide enrichment is a crucial step in glycoproteomic analysis, often achieved through solid-phase extraction (SPE) on polar stationary phases in hydrophilic interaction liquid chromatography (HILIC). This study explores the potential of polyaniline (PANI)-coated silica gel for enriching human immunoglobulin G (IgG). Experimental conditions were varied to assess their impact on glycopeptide enrichment efficiency, comparing PANI-cotton wool SPE with conventional cotton wool as SPE sorbents. Two formic acid concentrations (0.1% and 1%) in elution solvent were tested, revealing that higher concentrations led to earlier elution of studied glycopeptides, especially for sialylated glycopeptides. Substituting formic acid with acetic acid increased the interaction of neutral glycopeptides with the PANI-modified sorbent, while sialylated glycopeptides showed no significant change in enrichment efficiency. Acetonitrile concentration in the elution solvent (5%, 10%, and 20%) affected the enrichment efficiency with most glycopeptides eluting at the lowest acetonitrile concentration. The acetonitrile concentration in conditioning and washing solutions (65%, 75%, and 85%) played a crucial role; at 65% acetonitrile, glycopeptides were least retained on the stationary phase, and neutral glycopeptides were even detected in the flow-through fraction. This study shows the potential of in-house-prepared PANI-modified sorbents for SPE-HILIC glycopeptide enrichment, highlighting the crucial role of tuning experimental conditions in sample preparation to enhance enrichment efficiency and selectivity.

Practical examination of flow rate effects and influence of the stationary phase water layer on peak shape and retention in hydrophilic interaction liquid chromatography

The effect of flow velocity on retention and peak shape of neutral, acidic and basic probe compounds was studied using seven different UHPLC hydrophilic interaction chromatography (HILIC) columns. Surprisingly on some columns, the retention factor k was found to vary somewhat with flow velocity, due to the combined effects of pressure and of frictional heating on retention. The selectivity of different HILIC columns was much greater than typically found with RP columns. The volume of the water layer on the HILIC columns was measured using the toluene exclusion procedure. For the neutral solute uridine, a good correlation was found between the volume of the water layer and retention, indicating the likely domination of a partition mechanism. For the ionogenic solutes, the correlation was generally poor, due to the presence of strong additional mechanisms such as ionic retention and repulsion. Reduced Van Deemter plots for uridine showed a negative correlation between the reduced b coefficient and the volume of the water layer, which can be attributed to reduced surface diffusion in this viscous layer. Once again, the behaviour of ionic solutes was complex on some columns making detailed interpretation difficult.