Investigating the use of unconventional temperatures in supercritical fluid chromatography.

The use of unorthodox temperatures, ranging from -5 °C up to 80 °C, have been thoroughly investigated in supercritical fluid chromatography. To this purpose, an initial evaluation of the kinetic and thermodynamic performance has been made with a set of 4 analytes eluting at different percentages of organic co-solvent in the mobile phase (3%-10% - 45%-80%). The van Deemter plots have demonstrated how, at low organic modifier presence, the use of low temperatures did not necessarily translate into worse performance, while high temperatures could pose more issues due to the poor handling of the super/subcritical mobile phase by the chromatographic system. With important percentages of co-solvent, however, high temperatures were fundamental in ensuring better profiles of the van Deemter plots, compared to low temperatures. Pressure plots have demonstrated that gradients reaching elevated percentages of organic modifiers can also be used on stationary phases packed with sub 2 μm silica particles if high temperatures are employed. The thermodynamic evaluation, made via the analysis of van't Hoff plots, indicates the presence of three retention behaviors happening in UHPSFC when switching from high to low temperatures, depending on the co-solvent percentage needed to elute one analyte. Finally, an assessment of the stationary phase stability at high temperatures was performed: the retention times variabilities recorded were minimal (RSD < 2.5%), as well as the peak widths and inlet column pressures were somewhat constant throughout the analyses. In the second part of this study, a focus on potential applications benefiting from such unconventional temperatures has been made. A series of challenging analytes have experienced better chromatographic resolution at either high or low temperatures, providing therefore a potentially interesting tool to analysts during the chromatographic method development process. In conclusion, the UV sensitivity at different temperatures was also taken into consideration, with no significant impact on the quality of the UV signal under any condition.

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Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.