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Fundamental Investigations of Adsorption in SFC
Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Engineering and Chemical Sciences (from 2013).ORCID iD: 0000-0002-6970-0182
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In supercritical fluid chromatography (SFC), the eluent is composed by carbon dioxide, often with additional components, in a condition between gas and liquid. This thesis aims to reach a deeper understanding of SFC by revealing the function of the additional eluent components through systematic adsorption studies.

In Paper I, investigation of surface excess adsorption isotherms of methanol revealed that a monolayer of methanol was formed. In Paper II, severe peak deformation effects due to this adsorption were shown. The findings in these papers revealed that a competitive additive model best predicts the solute retention at low methanol fractions whereas at higher fractions, methanol acts just as a modifier. In Paper III, the generality of the effects was proven by investigation of several co-solvent and stationary phase combinations. In Paper IV it was investigated how the robustness of SFC separations depend on the co-solvent adsorption, pressure, and temperature. In Paper V, the impact of the addition of amine additives was investigated. Two different mechanisms for solute peak deformations were observed.

The knowledge achieved about SFC in this theses provides guidelines for development of more robust SFC methods where peak deformations/distortions can be avoided.

Abstract [en]

In supercritical fluid chromatography (SFC) the mobile phase is composed by carbon dioxide as the main weak solvent, in a condition between a gas and a liquid. The interest in SFC has recently increased due to several advantages compared to traditional liquid chromatography (LC) such as faster sample throughput and lower environmental impact. However, there is still a lack of fundamental knowledge about SFC, among others, due to the compressible mobile phase. This thesis work aims at a deeper understanding of the functions of the mobile phase components used in SFC through systematic adsorption studies. 

In Paper I, surface excess adsorption isotherms of the co-solvent methanol on a diol silica adsorbent was investigated. It was revealed that a monolayer of methanol was formed. In Paper II, severe peak deformation effects due to this adsorption were revealed, and it was demonstrated under which conditions these deformations appear and how the co-solvent fraction can tune the shape of the eluted peak. The findings in these papers revealed that a competitive additive model best predicts the solute retention at low methanol fractions whereas at higher fractions, when a solvent layer has formed, methanol acts just as a modifier. In Paper III, the generality of the effects was proven by investigations of other co-solvent/stationary phase combinations. In Paper IV it was investigated how the robustness of SFC separations depend on the co-solvent adsorption, pressure, and temperature. In Paper V, the impact of the addition of amine additives on separation performance was investigated. Two different underlying mechanisms for solute peak distortions were revealed: (i) deformations generated by the perturbation peak and (ii) deformation due to multilayer formation promoted by the additive.

The deeper knowledge about SFC obtained in this thesis provides guidelines for development of more robust SFC methods for analysis and preparative separations where peak distortions can be avoided.

Place, publisher, year, edition, pages
Karlstad: Karlstads universitet, 2019. , p. 68
Series
Karlstad University Studies, ISSN 1403-8099 ; 2020:3
Keywords [en]
Supercritical fluid chromatography, SFC, Adsorption isotherm, Excess adsorption isotherm, Solvent adsorption, Peak distortion, Solute retention, Overloaded peaks
National Category
Analytical Chemistry
Research subject
Chemistry
Identifiers
URN: urn:nbn:se:kau:diva-75766ISBN: 978-91-7867-070-3 (print)ISBN: 978-91-7867-080-2 (electronic)OAI: oai:DiVA.org:kau-75766DiVA, id: diva2:1371010
Public defence
2020-03-20, Nyquistsalen, 9C 203, Karlstads universitet, Karlstad, 10:00 (English)
Opponent
Supervisors
Available from: 2020-02-20 Created: 2019-11-18 Last updated: 2020-02-20Bibliographically approved
List of papers
1. A closer study of methanol adsorption and its impact on solute retentions in supercritical fluid chromatography
Open this publication in new window or tab >>A closer study of methanol adsorption and its impact on solute retentions in supercritical fluid chromatography
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2016 (English)In: Journal of Chromatography A, ISSN 0021-9673, E-ISSN 1873-3778, Vol. 1442, p. 129-139Article in journal (Refereed) Published
Abstract [en]

Surface excess adsorption isotherms of methanol on a diol silica adsorbent were measured in supercritical fluid chromatography (SFC) using a mixture of methanol and carbon dioxide as mobile phase. The tracer pulse method was used with deuterium labeled methanol as solute and the tracer peaks were detected using APCI-MS over the whole composition range from neat carbon dioxide to neat methanol. The results indicate that a monolayer (4 angstrom) of methanol is formed on the stationary phase. Moreover, the importance of using the set or the actual methanol fractions and volumetric flows in SFC was investigated by measuring the mass flow respective pressure and by calculations of the actual volume fraction of methanol. The result revealed a significant difference between the value set and the actually delivered volumetric methanol flow rate, especially at low modifier fractions. If relying only on the set methanol fraction in the calculations, the Methanol layer thickness should in this system be highly overestimated. Finally, retention times for a set of solutes were measured and related to the findings summarized above concerning methanol adsorption. A strongly non-linear relationship between the logarithms of the retention factors and the modifier fraction in the mobile phase was revealed, prior to the established monolayer. At modifier fractions above that required for establishment of the methanol monolayer, this relationship turns linear which explains why the solute retention factors are less sensitive to changes in modifier content in this region.

Keywords
Supercritical fluid chromatography, SFC, Excess adsorption, Solvent adsorption, Tracer-pulse method, Solute retention
National Category
Chemical Engineering
Research subject
Chemical Engineering
Identifiers
urn:nbn:se:kau:diva-42028 (URN)10.1016/j.chroma.2016.03.006 (DOI)000374083100014 ()26979267 (PubMedID)
Available from: 2016-05-13 Created: 2016-05-13 Last updated: 2019-12-09Bibliographically approved
2. Peak deformations in preparative supercritical fluid chromatography due to co-solvent adsorption
Open this publication in new window or tab >>Peak deformations in preparative supercritical fluid chromatography due to co-solvent adsorption
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2016 (English)In: Journal of Chromatography A, ISSN 0021-9673, E-ISSN 1873-3778, Vol. 1468, p. 200-208Article in journal (Refereed) Published
Abstract [en]

In supercritical fluid chromatography (SFC) the mobile phase comprises of carbon dioxide (CO2) as main solvent and smaller amounts of an organic polar solvent (often an alcohol) as co-solvent. The co-solvent is considered to function by changing the overall polarity of the eluent, i.e. by acting as a "modifier". However, recent studies indicate that the co-solvent methanol can also adsorb to some common SFC stationary phases. Hence, the co-solvent should also be able to function as an "adsorbing additive", i.e. an eluent component that competes with the injected solutes about the stationary phase surface. In this study it was found by fitting different mechanistic models to systematic experimental data, that the co-solvent methanol can have both functions: at low co-solvent fractions, methanol acts as an additive whereas at larger fractions it acts as a modifier. Moreover, it was found that when the co-solvent adsorbs more strongly to the stationary phase than the solute, "bizarre" deformations of the preparative band shapes can occur. This is illustrated by a solute that converts from a normal "Langmuirian" band shape to an "anti-Langmuirian" shape when changing from neat carbon dioxide (CO2) to an eluent containing co-solvent. This peak shape transition is dependent on both (i) the relative retention of the solute and co-solvent to the stationary phase in eluent containing neat CO2 and on (ii) the relative retention of the additive perturbation peak and the solute peak in eluent containing also co-solvent. 

Keywords
Supercritical fluid chromatography, SFC, Solvent adsorption, Adsorption strength, Langmuir band shape, Anti-Langmuir band shape
National Category
Chemical Sciences
Research subject
Chemistry
Identifiers
urn:nbn:se:kau:diva-47585 (URN)10.1016/j.chroma.2016.09.019 (DOI)000385326500025 ()27641721 (PubMedID)
Available from: 2017-01-05 Created: 2017-01-05 Last updated: 2019-11-18Bibliographically approved
3. Systematic investigations of peak deformations due to co-solvent adsorption in preparative supercritical fluid chromatography
Open this publication in new window or tab >>Systematic investigations of peak deformations due to co-solvent adsorption in preparative supercritical fluid chromatography
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2017 (English)In: Journal of Chromatography A, ISSN 0021-9673, E-ISSN 1873-3778, Vol. 1496, p. 141-149Article in journal (Refereed) Published
Abstract [en]

Strangely shaped overloaded bands were recently reported using a standard supercritical fluid chromatographic system comprising a diol column as the stationary phase and carbon dioxide with methanol as the mobile phase, Some of these overloaded elution profiles appeared strongly deformed and even had "anti-Langmuirian" shapes although their solute compounds had "Langmuirian" adsorption. To obtain a more complete understanding of the generality of these effects, the investigation was expanded to cover also other common co-solvents, such as ethanol, 2-propanol, and acetonitrile, as well as various stationary phase materials, such as silica, and 2-ethylpyridine. From this expanded study it could be confirmed that the effects of deformed overloaded solute band shapes, due to co-solvent adsorption, is general phenomena in supercritical fluid chromatographic. It could also be concluded that these effects as well as previously observed "solvent effects" or "plug effects" are entirely due to competition between the solute and solvent molecules for the adsorption sites on the stationary phase surface. Finally, guidelines were given for how to evaluate the risk of deformations occurring for a given solvent-column combination, based simply on testing retention times of solutes and co-solvent. (C) 2017 Elsevier B.V. All rights reserved.

Place, publisher, year, edition, pages
Elsevier, 2017
Keywords
Supercritical fluid chromatography, Co-solvent adsorption, Adsorption strength, Langmuir band shape, Anti-Langmuir band shape
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kau:diva-65479 (URN)10.1016/j.chroma.2017.03.053 (DOI)000400222200016 ()28366564 (PubMedID)
Conference
16th International Symposium on Preparative and Industrial Chromatography and Allied Techniques, OCT 09-12, 2016, Vienna, AUSTRIA
Available from: 2017-12-29 Created: 2017-12-29 Last updated: 2019-12-09Bibliographically approved
4. Impact of Methanol Adsorption on Robustness in Supercritical Fluid Chromatography
Open this publication in new window or tab >>Impact of Methanol Adsorption on Robustness in Supercritical Fluid Chromatography
(English)Manuscript (preprint) (Other academic)
National Category
Analytical Chemistry
Identifiers
urn:nbn:se:kau:diva-75765 (URN)
Available from: 2019-11-18 Created: 2019-11-18 Last updated: 2019-11-18
5. Systematic investigations of peak distortions due to additives in supercritical fluid chromatography
Open this publication in new window or tab >>Systematic investigations of peak distortions due to additives in supercritical fluid chromatography
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2020 (English)In: Journal of Chromatography & Separation Techniques, E-ISSN 2157-7064, article id 461048Article in journal (Refereed) In press
Abstract [en]

The impact of eluent components added to improve separation performance in supercritical fluid chromatography was systematically, and fundamentally, investigated. The model system comprised basic pharmaceuticals as solutes and eluents containing three amines (i.e., triethylamine, diethylamine, and isopropylamine) as additives with MeOH as the co-solvent. First, an analytical-scale study was performed, systematically investigating the impact of the additives/co-solvent on solute peak shapes and retentions, using a design of experiments approach; here, the total additive concentration in the eluent ranged between 0.021 and 0.105 % (v/v) and the total MeOH fraction in the eluent between 16 and 26 % (v/v). The co-solvent fraction was found to be the most efficient tool for adjusting retentions, whereas the additive fraction was the prime tool for improving column efficiency and peak analytical performance. Next, the impacts of the amine additives on the shapes of the so-called overloaded solute elution profiles were investigated. Two principal types of preparative peak deformations appeared and were investigated in depth, analyzed using computer simulation with mechanistic modeling. The first type of deformation was due to the solute eluting too close to the additive perturbation peak, resulting in severe peak deformation caused by co-elution. The second type of deformation was also due to additive–solute interactions, but here the amine additives acted as kosmotropic agents, promoting the multilayer adsorption to the stationary phase of solutes with bulkier aryl groups.

Place, publisher, year, edition, pages
Elsevier, 2020
Keywords
Supercritical fluid chromatography, Peak performance Peak distortions, Additives, Basic components, Overloaded peaks
National Category
Analytical Chemistry
Research subject
Chemical Engineering
Identifiers
urn:nbn:se:kau:diva-75764 (URN)10.1016/j.chroma.2020.461048 (DOI)
Note

This article was published as manuscript in Emelie Glennes PhD dissertation.

Available from: 2019-11-18 Created: 2019-11-18 Last updated: 2020-03-19

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