title: A case of Z/E-isomers elution order inversion caused by cosolvent percentage change in supercritical fluid chromatography.
abstract: A case of elution order inversion caused by cosolvent percentage change in supercritical fluid chromatography was observed and investigated in some detail. Z- and E-isomers of phenylisobutylketone oxime experience an elution order reversal on most columns if the mobile phase consists of CO2and alcohol. At lower percentages of alcohol Z-oxime is retained less, somewhere at 2-5% coelution occurs and at larger cosolvent volume elution order reverses - Z-oxime is eluted later than E-oxime. We suppose inversion with CO2-ROH phases happens due to a shift in balance between two main interactions governing retention. At low ROH percentages stationary phase surface is only slightly covered by ROH molecules so oximes primarily interact with adsorption sites via hydrogen bond formation. Due to intramolecular sterical hindrance Z-oxime is less able to form hydrogen bonds and consequently is eluted first. At higher percentages alcohols occupy most of strong hydrogen bonding sites on silica surface thus leaving non-specific electrostatic interactions predominantly responsible for Z/E selectivity. Z-oxime has a much larger dipole moment than E-oxime and at these conditions it is eluted later. Additional experimental data with CO2-CH3CN, hexane-iPrOH and CHF3-ROH mobile phases supporting this explanation are presented.
authors.affiliation: Kurnakov Institute of General and Inorganic Chemistry of Russian Academy of Sciences, Moscow, Russia. Electronic address: oleg.pokrovskiy@supercritical.ru.
authors.forename: Oleg I
authors.initials: OI
authors.lastname: Pokrovskiy
authors.affiliation: Kurnakov Institute of General and Inorganic Chemistry of Russian Academy of Sciences, Moscow, Russia.
authors.forename: Konstantin B
authors.initials: KB
authors.lastname: Ustinovich
authors.affiliation: Kurnakov Institute of General and Inorganic Chemistry of Russian Academy of Sciences, Moscow, Russia.
authors.forename: Oleg I
authors.initials: OI
authors.lastname: Usovich
authors.affiliation: Kurnakov Institute of General and Inorganic Chemistry of Russian Academy of Sciences, Moscow, Russia; Lomonosov Moscow State University, Chemical Department, Moscow, Russia.
authors.forename: Olga O
authors.initials: OO
authors.lastname: Parenago
authors.affiliation: Kurnakov Institute of General and Inorganic Chemistry of Russian Academy of Sciences, Moscow, Russia; Lomonosov Moscow State University, Chemical Department, Moscow, Russia.
authors.forename: Valeriy V
authors.initials: VV
authors.lastname: Lunin
authors.affiliation: Northern (Arctic) Federal University, Core Facility Center "Arktika", Arkhangelsk, Russia.
authors.forename: Denis V
authors.initials: DV
authors.lastname: Ovchinnikov
authors.affiliation: Northern (Arctic) Federal University, Core Facility Center "Arktika", Arkhangelsk, Russia.
authors.forename: Dmitry S
authors.initials: DS
authors.lastname: Kosyakov
mesh_terms: D000438:Alcohols; D002245:Carbon Dioxide / Q000737:chemistry; D002851:Chromatography, High Pressure Liquid; D025924:Chromatography, Supercritical Fluid; D006860:Hydrogen Bonding; D010091:Oximes / Q000737:chemistry*; D011789:Quantum Theory; D012997:Solvents / Q000737:chemistry*; D013237:Stereoisomerism
chemical_list: D000438:Alcohols; D010091:Oximes; D012997:Solvents; D002245:Carbon Dioxide
keywords: Elution order inversion; Modifier adsorption; Retention mechanisms; Supercritical fluid chromatography; Z/E-isomers