Silica with Bonded Polar Functional Groups

4 Silica with Bonded Polar Functional Groups 

Due to the many good properties of silica as a chromatographic material, several polar organic functions have been bonded to the surface to avoid the unwanted 

Solvents similar to the solvents used on silica can be used, but the materials can also be combined with aqueous solvents, such as for hydrophilic interaction chromatography. 

Depending on the selected group and the choice of mobile phase, silica with bonded polar groups can be used in adsorption chromatography, in hydrophilic interaction chromatography, and in ion exchange chromatography. 

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Residual Silanol-Solute. Even after bonding hydrocarbon chains to the silica, some residual unreacted silanol groups remain. These polar groups can interact with polar functional groups of solutes, i.e., normal phase attraction. The precise role they play in retention and selectivity is not well understood, but the different selectivities exhibited by various packings probably result at least in part from their different amounts of unreacted silanol groups. 

Different polar functional groups bonded onto silica can also be used for normal phase. The most frequently used bonded phases are amino-, diol-, cyano-, and nitro-groups. These phases, when used in the normal mode, are similar to adsorption onto silica gel. The use of bonded-phase columns in the normal phase mode is shown in Figure 5-44. There is a different order of elution on the amino-bonded phase compared to the cyano phase and different elution than on silica (ortho < meta < para), which is not shown. This is due to interactions with the functional groups on the packing rather than solely with the hydroxyl groups on the silica gel. 

Amino acids bonded to silica and loaded with Cu ions can interact in a steroselective manner with amino acids in aqueous solution. The copper ion forms a complex with both the bound and the sample amino acids, as shown in Figure 22.1. Ligand-exchange phases are suited for the separation of amino acids as well as of some )3-amino alcohols and similar molecules because these compounds bear two polar functional groups in adequate spacing. This approach has found limited interest because the column efficiencies are rather low, the detectability of the nonderivatized sample compounds can be a problem and the mobile phase needs to contain copper. 

In contrast, reversed-phase sorbents have non-polar functional groups, e.g. octadecyl, octyl and methyl, and conversely are more likely to retain non-polar compounds, e.g. polycyclic aromatic hydrocarbons. Ion-exchange sorbents have either cationic or anionic functional groups and when in the ionized form attract compounds of the opposite charge. A cation-exchange phase, such as benzene-sulfonic acid, will extract analytes with positive charges (e.g. phenoxyacid herbicides) and vice versa. A summary of the commercially available silica-bonded sorbents is given in Table 8.1. 

The special phases with an embedded polar functional group are designed for reversed-phase chromatography. The polar function shields the silica surface, preventing the interaction of analytes with the acidic silanols on the silica surface. For many samples, they exhibit a significant difference in selectivity compared to simple hydrocarbon bonded phases. Also, the tailing of basic analytes is reduced. 

Before the development of reversed-phase bonded phases, normal-phase chromatography was the most popular separation technique. It relies on the interaction of analytes with polar functional groups on the surfooe of the stationary phase, which is strongest when nonpolar solvents are used as mobile phase. Previously, it was also called adsorption chromatography. However, the technique has expanded from the exclusive application of metal oxide adsorbents such as silica and alumina as stationary phases to the use of polar bonded phases. Thus the name adsorption chromatography has become too narrow. 

Similarly, other polar functional groups in the stationary phase can influence the selectivity of the separation as well. Thus one can find significant differences in selectivity between reversed-phase packings based on silica and those based on polymers. Also, reversed-phase bonded phases with polar functional groups incorporated in the ligand can exhibit significant differences in selectivity compared to their purely hydrophobic counterparts. 

Reversed-phase extractions are used to enrich nonpolar compounds from a polar sample matrix. The extraction is based on interactions between carbon-hydrogen bonds from the compound and the sorbent. These hydrophobic interactions are shown in Figure 9.1a, and have been discussed in Section 3.5.2. Most organic compounds contain carbon-hydrogen-rich groups and will, therefore, be retained. Exceptions are ionized compounds or compounds with relatively many polar functional groups. Silica-based materials modified with C2, C4, C8, C18, cyclohexyl, phenyl, and cyano groups are examples of reversed-phase sorbents (see Table 9.3 for their structures). 

AMINO PHASE. Chemically bonded silica gel with a NH2 functional group and short-chain -alkyl (e.g., n-propyl) spacer group. Aminopropyl silica gel is polar and can function as a weakly basic ion exchanger or in straight-or re-versed-phase modes, depending on the mobile phase. 

CYANO PHASE. Silica gel chemically bonded with a CN functional group through a hydrophobic spacer. The layer has polarity between amino and di-methylsiloxane reversed-phase plates and can function with normal- or re-versed-phase mechanisms, depending on the mobile phase. 

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