Impact of Methanol Adsorption on the Robustness of Analytical Supercritical Fluid Chromatography in Transfer from SFC to UHPSFC.

Emelie Glenne,Jörgen Samuelsson,Torgny Fornstedt,Marek Leśko

doi:10.1021/acs.analchem.0c03106

Emelie Glenne, Jörgen Samuelsson + Show 2 more

Open Access

https://doi.org/10.1021/acs.analchem.0c03106

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Journal: Analytical chemistry	Publication Date: Nov 10, 2020
Citations: 14	License type: cc-by

Affiliation: Karlstad University

Abstract

In supercritical fluid chromatography (SFC), the retention of a solute depends on the temperature, density, pressure, and cosolvent fraction. Here, we investigate how the adsorption of the cosolvent MeOH changes with pressure and temperature and how this affects the retention of several solutes. The lower the pressure, the stronger the MeOH adsorption to the stationary phase; in addition, at low pressure, perturbing the pressure results in significant changes in the amounts of MeOH adsorbed to the stationary phase. The robustness of the solute retention was lowest when operating the systems at low pressures, high temperatures, and low cosolvent fractions in the eluent. Here, we found a clear relationship between the sensitivity of MeOH adsorption to the stationary phase and the robustness of the separation system. Finally, we show that going from classical SFC to ultrahigh-performance SFC (UHPSFC), that is, separations conducted with much smaller packing diameters, results in retention factors that are more sensitive to fluctuations in the flow rate than with traditional SFC. The calculated density profiles indicate only a slight density drop over the traditional SFC column (3%, visualized as lighter → darker blue in the TOC), whereas the drop for the UHPSFC one was considerably larger (20%, visualized as dark red → light green in the TOC). The corresponding temperature drops were calculated to be 0.8 and 6.5 °C for the SFC and UHPSFC systems, respectively. These increased density and temperature drops are the underlying reasons for the decreased robustness of UHPSFC.

Highlights

A trend seen in supercritical fluid chromatography (SFC), as in the transition from high-performance liquid chromatography (HPLC) to ultrahigh-performance liquid chromatography (UHPLC), is for the use of smaller particles to improve the efficiency and achieve better separation performance.[1]
Because of the compressibility of the fluid used in SFC, this additional pressure drop over the column could result in substantially larger density and viscosity gradients over the columns than those that are generally observed in UHPLC.[1−3] Poe et al reported that these gradients are more pronounced in columns packed with 3 μm-diameter particles than in those packed with 5 μm-diameter particles.[4]
In a series of studies, we have investigated several different cosolvents in several different columns, exploring how they adsorb and how their adsorption affects the separation process in SFC.[5−7] In these studies, we demonstrated that commonly used cosolvents adsorb to polar stationary phases, competing with the solute for available adsorption sites and affecting the solute retention and peak shape.[6,7]

Summary

Introduction

A trend seen in supercritical fluid chromatography (SFC), as in the transition from high-performance liquid chromatography (HPLC) to ultrahigh-performance liquid chromatography (UHPLC), is for the use of smaller particles to improve the efficiency and achieve better separation performance.[1] The use of sub-2 μm particles is often referred to as ultrahighperformance SFC or UHPSFC. Compared with UHPLC, the pressure drop over the column is much smaller in UHPSFC. Because of the compressibility of the fluid used in SFC, this additional pressure drop over the column could result in substantially larger density and viscosity gradients over the columns than those that are generally observed in UHPLC.[1−3] Poe et al reported that these gradients are more pronounced in columns packed with 3 μm-diameter particles than in those packed with 5 μm-diameter particles.[4]. In SFC, the fraction of the cosolvent in the eluent is often the most important factor controlling the retention.[2,3,5] In a series of studies, we have investigated several different cosolvents in several different columns, exploring how they adsorb and how their adsorption affects the separation process in SFC.[5−7] In these studies, we demonstrated that commonly used cosolvents adsorb to polar stationary phases, competing with the solute for available adsorption sites and affecting the solute retention and peak shape.[6,7]

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