Stationary phases hydrocarbon

A low value of k indicates that the solute is less retained in the column. Figure 8.6 illustrates a chromatogram in which the reference compound (unretained) elutes at 1.86 min (to). The first peak after that elutes at 2.16 min (Ir), thus the capacity factor is 0.3/1.86 = 0.16. The largest peak elutes at 2.60 min and thus its capacity factor equals 0.40. Different solutes will have different retention times depending on their polarity and the polarity of the stationary phase. Hydrocarbons (in the same family) will have increasing affinity with the stationary phase with increasing carbon number but permanent gases may have more or less affinity than methane, for example. 

Water is the most common component of liquid partition systems. It forms the basis of one of the phases, the other being an organic solvent saturated with water. Multicomponent mixtures are common. In some cases, both phases are organic solvents but with differences in polarity. Formamide, dimethylformamide or propylene glycol are examples of organic solvents used as the main components of polar stationary phases. Hydrocarbons or silicone polymers are the most commonly used non-polar stationary phase components. 

The resolution of capillary columns enables the separation of all principal components of a straight-run gasoline. The most frequently used stationary phases are silicone-based, giving an order of hydrocarbon elution times close to the order of increasing boiling point. 

In reverse-phase chromatography, which is the more commonly encountered form of HPLC, the stationary phase is nonpolar and the mobile phase is polar. The most common nonpolar stationary phases use an organochlorosilane for which the R group is an -octyl (Cg) or -octyldecyl (Cig) hydrocarbon chain. Most reverse-phase separations are carried out using a buffered aqueous solution as a polar mobile phase. Because the silica substrate is subject to hydrolysis in basic solutions, the pH of the mobile phase must be less than 7.5. 

The Diacel columns can be used for the separation of a wide variety of compounds, including aromatic hydrocarbons having hydroxyl groups, carbonyls and sulfoxides, barbiturates, and P-blockers (35,36). There are presendy nine different cellulose derivative-based columns produced by Diacel Chemical Industries. The different columns each demonstrate unique selectivities so that a choice of stationary phases is available to accomplish a separation. 

In classical column chromatography the usual system consisted of a polar adsorbent, or stationary phase, and a nonpolar solvent, mobile phase, such as a hydrocarbon. In practice, the situation is often reversed, in which case the technique is known as reversed-phase Ic. 

Alhedai et al also examined the exclusion properties of a reversed phase material The stationary phase chosen was a Cg hydrocarbon bonded to the silica, and the mobile phase chosen was 2-octane. As the solutes, solvent and stationary phase were all dispersive (hydrophobic in character) and both the stationary phase and the mobile phase contained Cg interacting moieties, the solute would experience the same interactions in both phases. Thus, any differential retention would be solely due to exclusion and not due to molecular interactions. This could be confirmed by carrying out the experiments at two different temperatures. If any interactive mechanism was present that caused retention, then different retention volumes would be obtained for the same solute at different temperatures. Solutes ranging from n-hexane to n hexatriacontane were chromatographed at 30°C and 50°C respectively. The results obtained are shown in Figure 8. 

The induced counter-dipole can act in a similar manner to a permanent dipole and the electric forces between the two dipoles (permanent and induced) result in strong polar interactions. Typically, polarizable compounds are the aromatic hydrocarbons examples of their separation using induced dipole interactions to affect retention and selectivity will be given later. Dipole-induced dipole interaction is depicted in Figure 12. Just as dipole-dipole interactions occur coincidentally with dispersive interactions, so are dipole-induced dipole interactions accompanied by dispersive interactions. It follows that using an n-alkane stationary phase, aromatic... 

Alternatively, using a polyethylene glycol stationary phase, aromatic hydrocarbons can also be retained and separated primarily by dipole-induced dipole interactions combined with some dispersive interactions. Molecules can exhibit multiple interactive properties. For example, phenyl ethanol possesses both a dipole as a result of the hydroxyl group and is polarizable due to the aromatic ring. Complex molecules such as biopolymers can contain many different interactive groups. 

On the other hand, if the stationary phase consists of bonded silica containing hydrocarbon chains, then the stationary phase interactions with the solute would be... 

The high thermal and chemical stability of fluorocarbons, combined with their very weak intermolecular interactions, makes them ideal stationary phases for the separation of a wide variety of organic compounds, including both hydrocarbons and fluorine-containing molecules Fluonnated stationary phases include per-fluoroalkanes, fluorocarbon surfactants, poly(chlorotrifluoroethylene), polyfper-fluoroalkyl) ethers, and other functionalized perfluoro compounds The applications of fluonnated compounds as stationary phases in gas-liquid chroma... 

To retain solutes selectively by dispersive interactions, the stationary phase must contain no polar or ionic substances, but only hydrocarbon-type materials such as the reverse-bonded phases, now so popular in LC. Reiterating the previous argument, to ensure that dispersive selectivity dominates in the stationary phase, and dispersive interactions in the mobile phase are minimized, the mobile phase must now be strongly polar. Hence the use of methanol-water and acetonitrile-water mixtures as mobile phases in reverse-phase chromatography systems. An example of the separation of some antimicrobial agents on Partisil ODS 3, particle diameter 5p is shown in figure 5. 

In summary, examples of the successful use of silica gel as a conventional stationary phase are in the analysis of mixtures containing polarizable and relatively low polarity solutes typified by mixtures of aromatic hydrocarbons, polynuclear aromatics, nitro compounds, carotenes and vitamin A formulas. 

The analysis demonstrates the elegant use of a very specific type of column packing. As a result, there is no sample preparation, so after the serum has been filtered or centrifuged, which is a precautionary measure to protect the apparatus, 10 p.1 of serum is injected directly on to the column. The separation obtained is shown in figure 13. The stationary phase, as described by Supelco, was a silica based material with a polymeric surface containing dispersive areas surrounded by a polar network. Small molecules can penetrate the polar network and interact with the dispersive areas and be retained, whereas the larger molecules, such as proteins, cannot reach the interactive surface and are thus rapidly eluted from the column. The chemical nature of the material is not clear, but it can be assumed that the dispersive surface where interaction with the small molecules can take place probably contains hydrocarbon chains like a reversed phase. 

Prus and Kowalska [75] dealt with the optimization of separation quality in adsorption TLC with binary mobile phases of alcohol and hydrocarbons. They used the window diagrams to show the relationships between separation selectivity a and the mobile phase eomposition (volume fraction Xj of 2-propanol) that were caleulated on the basis of equations derived using Soezewiriski and Kowalska approaehes for three solute pairs. At the same time, they eompared the efficiency of the three different approaehes for the optimization of separation selectivity in reversed-phase TLC systems, using RP-2 stationary phase and methanol and water as the binary mobile phase. The window diagrams were performed presenting plots of a vs. volume fraetion Xj derived from the retention models of Snyder, Schoen-makers, and Kowalska [76]. 

The most common technique used for agrochemicals is reversed-phase SPE. Here, the bonded stationary phase is silica gel derivatized with a long-chain hydrocarbon (e.g. C4-C18) or styrene-divinylbenzene copolymer. This technique operates in the reverse of normal-phase chromatography since the mobile phase is polar in nature (e.g., water or aqueous buffers serve as one of the solvents), while the stationary phase has nonpolar properties. 

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