Phase composition Dotaz Zobrazit nápovědu
Correct adjustment of the mobile phase is equally important as the selection of the appropriate column for the separation of polar compounds in LC. Both solvophobic and selective polar interactions control the retention in the Reversed Phase and Hydrophilic Interaction modes. The retention models describing the effects of the volume fraction of the strong eluent component in binary mobile phases on the sample retention factors apply in a limited mobile phase composition range. We introduced a three-parameter retention model, which provides improved prediction of retention over a broad mobile phase range, under isocratic and gradient elution conditions. The model does not imply any assumptions concerning either adsorption or partition distribution mechanism, but allows estimating retention in pure strong and in pure weak mobile phase components. The experimental retention data for phenolic acids and flavones on several core-shell columns with different types of stationary phases agree with the theory. Many polar columns with important structural hydrophobic moieties show dual retention mechanism, (Reversed Phase in water rich mobile phases and Hydrophilic Interaction at high acetonitrile concentrations). It is possible to select the mobile phase compositions in each of the two modes for separations of samples containing compounds largely differing in polarity. The three-parameter model describes the retention in each mode, with separately determined best-fit parameters. We applied the two-mode model to the retention data of sulfonamides and benzoic acid related compounds on a new polymethacrylate zwitterionic monolithic micro-column.
The effects of mobile phase composition and of temperature on the retention behavior of phenolic acids were studied on 4 hydrosilated (type C silica) based columns in buffered aqueous acetonitrile, both in the aqueous normal phase (HILIC) and in the reversed-phase mobile phase range. The UDC cholesterol and the C₁₈ bidentate columns show significant reversed phase and normal-phase retention mechanisms, whereas very weak retention in the reversed-phase mode was observed on the silica hydride and the Diamond hydride columns. The concentration effects of the aqueous acetate buffer over the full mobile phase (HILIC and RP) composition range can be described by a simple four-parameter equation. At increasing temperature, the retention times and peak widths decrease both in the aqueous normal phase and in the reversed phase mobile phase range. Linear van't Hoff log k versus 1/T plots were observed, indicating a single retention mechanism predominating in the highly organic (HILIC), like in highly aqueous (RP) mobile phase ranges. Besides the type of the stationary phase, the separation selectivity of phenolic acids strongly depends on temperature and on the mobile phase composition. From among the 4 hydrosilated columns compared in this work, the UDC cholesterol column has high temperature stability (up to 100 °C) and is most suitable for selective and efficient separations of phenolic acids both in the HILIC and in the RP modes.
- MeSH
- acetonitrily chemie MeSH
- cholesterol chemie MeSH
- chromatografie kapalinová přístrojové vybavení metody MeSH
- chromatografie s reverzní fází MeSH
- hydrofobní a hydrofilní interakce MeSH
- hydroxybenzoáty izolace a purifikace MeSH
- oxid křemičitý chemie MeSH
- teplota MeSH
- Publikační typ
- časopisecké články MeSH
- práce podpořená grantem MeSH
A systematic study of the retention behavior of isomeric triacylglycerols (TGs) in silver-ion HPLC on a ChromSpher Lipids column has been performed between 10 to 40°C using the most widespread hexane- and dichloromethane-based mobile phases. The randomization of mono-acyl TG standards and the random esterification of glycerol with fatty acids are employed to produce mixtures of TG isomers. The mobile phase composition has no influence on the general retention pattern, but significant differences in the retention order of double bond (DB) positional isomers in hexane and dichloromethane mobile phases are described and compared with the previous literature data. Saturated TGs with fatty acyl chain length from C7:0 to C22:0 are partially separated using the hexane mobile phase but not at all with the dichloromethane mobile phase. The hexane mobile phase enables at least partial resolution of TG regioisomers with up to seven DBs, while the resolution of only ALA/AAL and ALnA/AALn isomers is achieved with the dichloromethane mobile phase. The effect of temperature differs significantly depending on the mobile phase composition. Retention times of TGs increase with increasing temperature in the hexane mobile phase, while an opposite effect is observed for the dichloromethane mobile phase.
We investigated the effects of mobile phase composition on the retention of flavones on four different hydrosilated C silica-based columns in buffered aqueous acetonitrile. Cogent UDC cholesterol™ and Cogent bidentate C18™ columns show significant dual reversed-phase/normal-phase retention behavior, while Cogent Diamond hydride™ and Cogent Silica hydride™ columns show negligible retention in the reversed-phase mode. The effect of the aqueous acetate buffer concentration on retention factors of flavones over the full mobile phase composition range, including both aqueous normal-phase (ANP) and reversed-phase mechanisms, can be described by a four-parameter equation for dual-retention mechanism. At increasing temperature, the retention factors and peak widths decrease both in the aqueous normal phase and in the reversed phase mobile phase range. In agreement with van't Hoff model, linear lnk versus 1/T plots were observed, showing a single retention mechanism in the highly organic normal-phase and in highly aqueous reversed-phase mobile phase ranges. From among the stationary phases tested, Cogent UDC cholesterol™ column has high temperature stability (up to 100 °C) and provides most selective and efficient separations of flavones both in the ANP and in the RP modes with almost reversed elution order.
- MeSH
- antropometrie metody MeSH
- distribuce tělesného tuku MeSH
- interpretace statistických dat MeSH
- lidé MeSH
- průřezové studie MeSH
- reprodukční lékařství metody MeSH
- složení těla genetika MeSH
- tělesná hmotnost fyziologie MeSH
- tloušťka kožní řasy MeSH
- Check Tag
- lidé MeSH
- ženské pohlaví MeSH
- Publikační typ
- srovnávací studie MeSH
- Geografické názvy
- Česká republika MeSH
Combined effects of temperature and mobile-phase composition on retention and separation selectivity of phenolic acids and flavonoid compounds were studied in liquid chromatography on a polydentate Blaze C8 silica based column. The temperature effects on the retention can be described by van't Hoff equation. Good linearity of lnk versus 1/T graphs indicates that the retention is controlled by a single mechanism in the mobile phase and temperature range studied. Enthalpic and entropic contributions to the retention were calculated from the regression lines. Generally, enthalpic contributions control the retention at lower temperatures and in mobile phases with lower concentrations of methanol in water. Semi-empirical retention models describe the simultaneous effects of temperature and the volume fraction of the organic solvent in the mobile phase. Using the linear free energy-retention model, selective dipolarity/polarizability, hydrogen-bond donor, hydrogen-bond acceptor and molecular size contributions to retention were estimated at various mobile phase compositions and temperatures. In addition to mobile phase gradients, temperature programming can be used to reduce separation times.
- MeSH
- antioxidancia analýza chemie MeSH
- chemické modely MeSH
- chromatografie kapalinová přístrojové vybavení metody MeSH
- flavonoidy analýza chemie MeSH
- hydroxybenzoáty analýza chemie MeSH
- oxid křemičitý chemie MeSH
- termodynamika MeSH
- vodíková vazba MeSH
- Publikační typ
- časopisecké články MeSH
- práce podpořená grantem MeSH
