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Title Solvent adsorption in SFC : Adsorption of methanol under supercritical conditions
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Discipline/Department Engineering and Chemical Sciences
University/Publisher Karlstad University
Abstract Chromatography is a widely used separation technique including many different modes, for example supercritical fluid chromatography (SFC) which uses a supercritical fluid as mobile phase. A supercritical fluid is achieved when a substance is subjected to a temperature and pressure above the critical point and the boundary between the liquid phase and gas phase is erased. The interest for SFC has increased in recent years, mainly for separation of chiral molecules in the pharmaceutical industry. What makes SFC interesting is that it is a quick, cost-efficient and green method. This is in part due to less organic solvent used in the mobile phase in SFC compared with liquid chromatography and that the carbon dioxide that represents the major part of the mobile phase is a by-product from other processes. In SFC modifiers, often small alcohols, are added to carbon dioxide based mobile phase in order to increase the solubility of polar compounds. In this study the adsorption of methanol to two different stationary phases; Kromasil-Diol and chiral Lux Cellulose-4 were studied. Adsorption is a phenomenon where surface interactions crate a higher density of molecules at the surface than in the bulk. The aim of this work has been to study the adsorption of modifier (methanol) to the stationary phase both to determine the extent of adsorption and the kinetics for system equilibration. These findings were then put into perspective of normal use of SFC for separation of molecules. There are a number of techniques for measuring adsorption; in this study the tracer pulse method is used. This is a pulse method where a concentration plateau is created and an isotope labelled molecule is injected. This was performed in the mobile phase composition from pure carbon dioxide to pure methanol. In addition to the tracer pulse experiments the isotope effect, the eluent flow, equilibration times for the column and retention times for a set of analytes were measured. For the Diol column no large isotope effect was observed, the method was also proved to be highly reproducible since several runs gave consistent results. Calculations based on the experimental data showed that a 6.3 Å thick layer was built up at a methanol fraction of 13% (v/v), corresponding to a monolayer. Changes of the methanol fraction below the saturation level has has greater effect on the retention factor for the analytes than at higher methanol fractions, when the monolayer was saturated. The conclusion of this is that SFC is more stable in the area where the layer has been built up. A preliminary study has been made for the chiral Lux Cellulose-4 column which was not as conclusive as for the Kromasil-Diol column. This type of column needs further studies to confirm the deviating observations and to investigate the cause for these.
Subjects/Keywords SFC; excess adsorption isotherm; methanol; tracer pulse
Language en
Country of Publication se
Record ID oai:DiVA.org:kau-35185
Repository diva
Date Indexed 2020-01-03

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…system without adsorption. b-e) with increasing bulk density, light blue, the adsorbed amount, including reference molecules, dark blue, and excess molecules, dark grey, increase. f) as the bulk density reaches the same density as the adsorbed…

…layer there is no longer any excess adsorption Adsorption measurements (see 1.3) near or above the critical point do not follow the Langmuir model of adsorption, see 1.2.2; increasing adsorption with increasing pressure [13]. Instead…

…molecules in the adsorbed phase. This maximum is called Gibbs excess maximum. As there is no more room on the surface, an increase in the bulk density will lead to a decrease of excess adsorption, Figure 1.3 e). As the bulk density reaches the same…

…density as the adsorbed layer there is no longer any excess adsorption, Figure 1.3 f). 1.3 Different methods to determine adsorption 1.3.1 Pulse methods The pulse methods; the tracer pulse (TP) and the perturbation peak (PP)…

…the physical properties of the supercritical mobile phase. Furthermore is the fundamental understanding of SFC limited compared with LC [23]. The aim of this study is to, by excess adsorption studies, provide deeper understanding of how and…

…can be expressed as: where VS is the volume of the eluent in the stationary phase. This gives that Equation 2.2 can be rearranged as: From the left hand side of this equation it is now possible to create a new definition of adsorption, excess

adsorption, which only consist of experimentally measurable quantities: where 𝑛𝑛 𝑖𝑖𝑥𝑥𝑥𝑥 is the excess of component i adsorbed, which can be defined as the total amount of component i minus the amount that would be present in the same system assuming…

…no adsorption (see Figure 1.3). If Equation 2.1 and 2.5 is combined it is possible to express the retention time for the tracer pulse in terms of excess adsorption as: ∗ 𝑉𝑉𝑅𝑅,𝑖𝑖 = 𝑉𝑉 0 + 𝑛𝑛 𝑖𝑖𝑥𝑥𝑥𝑥 𝜃𝜃𝑖𝑖𝑀𝑀 This can be…

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