Advances in Chromatography, Volume 57

Nelu Grinberg, Peter W. Carr, Nelu Grinberg, Peter W. Carr

• Publisher: CRC Press
• Year: 2020
• ISBN:
  9781000062014
• Edition: 1
• Topic: Sciences physiques
• Subtopic: Chimie analytique

For more than five decades, scientists and researchers have relied on the Advances in Chromatography series for the most up-to date information on a wide range of developments in chromatographic methods and applications. The clear presentation of topics and vivid illustrations for which this series has become known makes the material accessible and engaging to analytical, biochemical, organic, polymer, and pharmaceutical chemists at all levels of technical skill. This volume considers the achievements and legacy of Lloyd R. Snider in separation science and analytical chemistry.

Key Features:

• Provides a historical perspective of the evolution of SMB technology together with a theoretical analysis of the most relevant underlying phenomena

• Presents a brief survey of the polar columns suitable for HILIC separations and pays special attention to the role of the mobile phase in RP and HILIC modes

• Describes recent strategies of method development in Kosmotropic chromatography

• Surveys the many approaches to avert the effects of temperature in reversed-phase liquid chromatographic separations

• Reviews separation of isotopic compounds by HPLC in relation to the advances of columns and stationary phases

1 Mobile Phase Effects in Reversed-Phase and Hydrophilic Interaction Liquid Chromatography

Pavel Jandera and Tomáš Hájek

Contents

1.1 Introduction

1.2 Stationary Phases in Liquid Chromatography

1.2.1 Sample Distribution in Liquid Chromatography

1.2.2 Stationary and Mobile Phase Volumes in Column Liquid Chromatography

1.2.2.1 Fully Porous Packing Particles

1.2.2.2 Core-Shell Columns

1.2.2.3 Monolithic Columns

1.2.2.4 Micro-Pillar Array Columns

1.3 Phase Systems and Separation Modes in HPLC

1.3.1 Organic Solvent Normal-Phase (Adsorption) Chromatography

1.3.2 Reversed-Phase Chromatography

1.3.3 Hydrophilic Interaction Liquid Chromatography (HILIC)

1.4 Mobile Phase Effects on Retention

1.4.1 Solvent Adsorption in the Stationary Phase

1.4.2 The Mobile Phase-Retention Models

1.4.3 Modeling Gradient-Elution Liquid Chromatography

1.4.4 Structural Retention Correlations

1.4.5 Combined-Retention-Mechanism LC Systems

1.4.5.1 Mixed-Mode and Zwitterionic Stationary Phases

1.4.5.2 Serially Coupled HILIC/RP Columns

1.4.5.3 Dual-Mode Retention Mechanism on a Single Column

1.5 Conclusions, Further Perspectives

References

1.1 Introduction

The ultimate target of high-performance liquid chromatography (HPLC), like of other analytical separation techniques, is the resolution and determination of sample components. Retention, separation selectivity and chromatographic efficiency control the resolution and separation quality in liquid chromatography (LC). The development of efficient chromatographic columns has experienced remarkable success in the last two decades. Some of the modern types of efficient stationary phases, including the sub-2 µm particle, core-shell, monolithic and micropillar-array columns, challenge the correct definition of the volumes of the stationary and mobile phases, between which the solute distributes during the migration along the column. Adopting a phase volume convention can mitigate the problems caused by the possible mixing of the adsorption and partition mechanism and by the selective preferential adsorption of mobile phase components in the stationary phase.

The retention and resolution on even the most efficient LC column results from the interactions between the solute, the stationary phase and the mobile phase, the cocktail of which characterizes different LC separation modes. Non-polar interactions are the predominant (but not the only) forces controlling the separation mechanism in reversed-phase (RP) systems employing low-polarity columns and polar aqueous-organic mobile phases. In contemporary HPLC, most frequently used are the reversed-phase separation systems. However, many polar compounds elute too early in RP LC. Hydrophilic interaction liquid chromatography (HILIC) is becoming increasingly popular, as it often provides significant improvement in the retention and separation efficiency for the separation of polar and weakly ionic compounds [1]. HILIC is essentially a normal-phase (NP) mode employing a polar column, like classical adsorption chromatography with a mixed organic solvent mobile phase. However, HILIC employs an aqueous-organic mobile phase with a high concentration of the organic solvent; it is therefore also known as aqueous normal-phase (ANP) liquid chromatography. There are a plethora of polar columns suitable for HILIC separations, including silica gel, bare or with various bonded polar ligands, and polar organic polymers [2], showing different properties such as chromatographic selectivity and water adsorption [3]. The adsorbed water forms a part of a HILIC stationary phase, and therefore the appropriate convention defining the volumes of the stationary and mobile phases is especially important for the quantitative description of retention.

In both reversed-phase and HILIC systems, the mobile phase is a very active – but often underestimated – player affecting the retention, separation selectivity and ultimately the sample resolution in HPLC. The present work focuses on the role of the mobile phase in RP LC and in HILIC. It compares several two- and three-parameter models describing the effects of the mobile phase on the separation, dating from the early days of HPLC, with the ABM three-parameter model, which does not presume either adsorption or partition retention mechanisms [4]. In gradient elution, increasing (in RP LC) or decreasing (in HILIC) the concentration of the organic solvent in water accelerates the elution of strongly retained compounds and improves the resolution of complex samples. The retention models allow a prediction of the retention data in gradient LC from the isocratic experiments [5].

Several theoretical models correlate the contributions of various interactions characterized by structural parameters to the retention. The linear solvation energy relationships (LSER) model applies in HILIC, like in RP LC [6]. The mixed-mode columns improve separation of ionic (ionizable) compounds combining ion exchange with either RP or HILIC retention mechanisms. A single polar column often shows a dual HILIC/RP retention mechanism, depending on the mobile phase. In the organic solvent-rich mobile phase, polar interactions control the retention (HILIC or ANP mode), whereas in more aqueous mobile phases the column shows essentially reversed-phase behavior with hydrophobic interactions playing a major role. Alternating RP and HILIC runs may provide different (even orthogonal) separation selectivity on a single column and complementary information on the sample composition [7].

1.2 Stationary Phases in Liquid Chromatography

1.2.1 Sample Distribution in Liquid Chromatography

In high-performance liquid chromatography, the stationary phase is usually a bed of fine solid particles with narrow size distribution, densely packed in a metal, glass or plastic tube – a chromatographic column. The particles may be either fully or only partially porous, such as core-shell columns with a layer of the stationary phase chemically bonded to a support material. On the contrary, monolithic columns do not contain particles; instead, a continuous chromatographic bed fills the full inner column volume. The mobile phase (eluent) is a liquid, usually a mixture of two or more solvents (often containing suitable additives) forced through the column by applying elevated pressure in HPLC. The sample compounds move at different velocities along the column, together with – but more slowly than – the mobile phase. The elution process ideally leads to the eventual sample separation. The separated compounds appear at different times at the outlet from the column as the elution waves (peaks) monitored by a detector attached to the outlet of the column. The elution (retention) time, t _R, of the peak maximum is a characteristic property of each sample compound, depending on the distribution constant between the stationary and the mobile phases in the chromatographic column. Hence, the t _R, or the retention volume V _R, is a useful tool for solute identification.

HPLC has become one of the most powerful tools for the separation and determination of even very complex samples containing non-polar, moderately or strongly polar, and ionic compounds, either simple species or high-molecular synthetic...