title: A closer study of methanol adsorption and its impact on solute retentions in supercritical fluid chromatography.
abstract: 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 (4Å) 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.
authors.affiliation: Department of Engineering and Chemical Sciences, INTERACT, Karlstad University, SE-651 88 Karlstad, Sweden.
authors.forename: Emelie
authors.initials: E
authors.lastname: Glenne
authors.affiliation: Respiratory, Inflammation and Autoimmunity, Innovative Medicines, AstraZeneca R&D, Molndal, 431 83, Sweden.
authors.forename: Kristina
authors.initials: K
authors.lastname: Öhlén
authors.affiliation: Respiratory, Inflammation and Autoimmunity, Innovative Medicines, AstraZeneca R&D, Molndal, 431 83, Sweden.
authors.forename: Hanna
authors.initials: H
authors.lastname: Leek
authors.affiliation: Respiratory, Inflammation and Autoimmunity, Innovative Medicines, AstraZeneca R&D, Molndal, 431 83, Sweden.
authors.forename: Magnus
authors.initials: M
authors.lastname: Klarqvist
authors.affiliation: Department of Engineering and Chemical Sciences, INTERACT, Karlstad University, SE-651 88 Karlstad, Sweden.
authors.forename: Jörgen
authors.initials: J
authors.lastname: Samuelsson
authors.affiliation: Department of Engineering and Chemical Sciences, INTERACT, Karlstad University, SE-651 88 Karlstad, Sweden. Electronic address: Torgny.Fornstedt@kau.se.
authors.forename: Torgny
authors.initials: T
authors.lastname: Fornstedt
mesh_terms: D000327:Adsorption; D000438:Alcohols / Q000737:chemistry; D002245:Carbon Dioxide / Q000737:chemistry; D025924:Chromatography, Supercritical Fluid / Q000592:standards*; D000432:Methanol / Q000737:chemistry / Q000378:metabolism*; D011312:Pressure; D012822:Silicon Dioxide / Q000737:chemistry
chemical_list: D000438:Alcohols; D002245:Carbon Dioxide; D012822:Silicon Dioxide; D000432:Methanol
keywords: Excess adsorption; SFC; Solute retention; Solvent adsorption; Supercritical fluid chromatography; Tracer-pulse method