chemistry

Once the LC mode and the type of column packing (porous, superficially porous) are selected, the choice of the stationary phase is determined by the requirements and the nature of the sample . In general, each stationary phase can have a unique selectivity towards sample components. Selection of the Liquid-Solid Chromatography ( LSC ) Stationary Phase Silica and alumina are the two most popular stationary phases ( adsorbents ). Silica is the preferred stationary phase mainly because of its availability, known performance and low cost. Silica has a lower reactivity, yields columns of better efficiency, and offers a higher linear capacity than alumina. In general, silica and alumina are both used for the separation of the same type of compounds, although certain compounds tend to favor one over the other. The following semiempirical relationships have been found by comparing the two adsorbents with regard to their selectivity for various functional groups: Moderately strong ba...

gradient elution in liquid chromatography LC Gradient Elution in Liquid Chromatography   Gradient elution is used in chromatography (mainly in reversed-phase but with other modes such as ion-exchange) to modify the separations achieved in a column. It involves a continuous change in the composition of the mobile phase to achieve separation of sample components of widely varying affinities for the stationary phase. The separation in a chromatographic column can be changed by changing the polarity of either the column (stationary phase) or the mobile phase. Generally, it is easier and cheaper to change the composition of the mobile phase (solvent) . The method used more often to modify the separation is to change the difference in polarity between the stationary phase and the mobile phase. The following is observed for the polarity difference of these phases: Increasing the difference in polarities between the stationary and the mobile phase makes...

In normal-phase chromatography a polar stationary phase is used in conjunction with a less polar mobile phase for elution of the analytes (in contrast to reversed phase chromatography where a nonpolar stationary phase is used with a more polar mobile phase). Neutral solutes in the mobile phase are separated on the basis of their polarity. The more polar the solute  the more it is retained on the stationary phase. The mobile phase that it is used is normally less polar than the stationary phase. If the polarity of the mobile phase continuously increases (in a gradient elution scheme) the solute retention is decreased. The stationary phases used in normal phase chromatography are usually silica or alumina and they are polar as a result of hydroxyl groups (-OH) (Fig. 1). The surface hydroxyl groups interact with the functional groups on the solute molecules and depending on the strength of this interaction preferentially absorb one solute relative to another (absorb the more pol...