Phase ratio and eluotropic strength changes on retention variations in subcritical fluid chromatography (SubFC) using packed octadecyl columns
In SubFC, using packed octadecyl columns, different retention variations occur when a modifier (methanol or acetonitrile) is added to carbon dioxide. A study of these variations was carried out by using alkylbenzene homologues as probes. An experimental design has been developed to describe the retention behaviorvs modifier percentage, outlet pressure and temperature.
The retention in SubFC was found to be govemed mainly by the modifier percentage and the temperature, regardless of the modifiers used, and to follow a quadratic mathematical model.
For both tested modifiers, different retention behavior was observed depending on the alkyl chain length. Retention of the small homologues is ruled by the changes of the phase ratio. These changes are due to either adsorption/desorption of the mobile phase onto the stationary phase or to mobile phase density variations. Retention of the longest homologues does not depend on the phase ratio but is govemed by the eluotropic strength, i.e., by modification of the solute solubility in the mobile phase.
On the other hand, temperature induces mainly solubility modifications when pressure acts through density variations with low modifier percentages, or through eluotropic strength variation with high modifier percentages.
However, differences in retention behavior between this homologous series and high molecular mass compounds, as well as with methanol and acetonitrile, underline that a retention model based on a compound family used as probes may sometimes be, unable to describe the behavior of other compounds.
Finally, the additional calculation of efficiency and resolution enabled choice of different mobile phases to achieve a better separation of alkybenzenes.
For these compounds and for those differing in the hydrophobic chain length, resolution depends rather more on selectivity than on efficiency. Moreover, for identical separation levels, the use of acetonitrile is suggested to obtain lower retention times.
Key WordsColumn liquid chromatography Subcritical fluid chromatography Eluotropic strength changes Retention behaviors Homologous series
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