Chemically bonded silica, chromatography

The TLC process is an off-line process. A number of samples are chromatographed simultaneously, side-by-side. HPTLC is fast (5 min), allows simultaneous separation and can be carried out with the same carrier materials as HPLC. Silica gel and chemically bonded silica gel sorbents are used predominantly in HPTLC other stationary phases are cellulose-based [393]. Separation mechanisms are either NPC (normal-phase chromatography), RPC (reversed-phase chromatography) or IEC (ion-exchange chromatography). RPC on hydrophobic layers is not as widely used in TLC as it is in column chromatography. The resolution capabilities of TLC using silica gel absorbent as compared to C S reversed-phase absorbent have been compared for 18 commercially available plasticisers, and 52 amine and 36 phenolic AOs [394]. 

Reversed-phase chromatography is the predominant technique in HPLC, and chemically bonded silica gel supports are made specifically for the nonpolar stationary phase. In the last decade, as many as 60% of the published LLPC techniques refer to RPC. The reasons for this involve the significantly lower cost of the mobile liquid phase and a favorable elution order that is easily predictable based on the hydrophobicity of the eluate. 

Most HPLC is based on the use of so-called normal-phase columns (useful for class separations), reverse-phase columns (useful for homolog separations), and polar columns (used in either the normal- or reverse-phase mode). Since reverse-phase HPLC columns are generally easier to work with, almost all authors use high-performance reverse-phase liquid chromatography with octade-cyl chemically bonded silica as the stationary phase and nonaqueous solvents as mobile phases (so-called NARP, or nonaqueous reverse-phase chromatography). 

Heavy and transition metals may also be separated via ion-pair chromatography on macroporous PS/DVB-resins or chemically bonded silica phases, respectively [154], The mobile phase contains complexing agents and a respective ion-pair reagent. If these columns are equilibrated with a surface-active acid such as octanesulfonic acid, metal ions such as Cu2+, Ni2+, Zn2+, and Co2+ elute in the same order as on surface-sulfon-... 

We compare in Figures 10.16a to 10.16d [70] the experimental band profiles in overloaded elution (symbols) and the profiles calculated for elution performed in normal and reversed phase chromatography (solid lines). Figure 10.16a corresponds to the elution of large bands of benzyl alcohol on silica with a THF/n-hexane solution. Figure 10.16b corresponds to the elution of acetophenone on silica with a (97.5 2.5) mixture of n-hexane and ethyl acetate. Figure 10.16c illustrates the profiles of bands of benzyl alcohol eluted on CIS silica by a methanol/water solution. Figure 10.16d corresponds to the elution of phenol on C18 chemically bonded silica with a (20 80) mixture of methanol and water. In all four cases. 

The last two examples show that, in a number of cases, cation-exchange chromatography can be regarded as an alternative to reversed-phase chromatography on chemically bonded silica phases, especially because the characteristic interactions between the basic compounds and the free silanol groups of silica are not observed with polymer-based cation exchangers. 

As can be seen from Figure 6.63, very small and symmetric peaks result when a salt gradient is applied. Separations of this kind are not possible with either the ion-suppression technique or with ion-pair chromatography. This is especially true for aromatic polycarboxylic acids, which elute from a mixed-mode phase such as OmniPac PAX-500 according to their valency (Figure 6.64b). When separating on chemically bonded silica under ion-suppression conditions (Figure 6.64a), a shorter analysis time is observed but not all components of the test mixture are separated. Moreover, penta- and... 

PTFE membranes loaded with particles of silica gel or chemically bonded silica gel are a relatively new type of flexible support developed for TLC (97-99). These Empore sheets, which are similar to Empore extraction disks used for solid phase extraction (100), are especially useful for recovery of separated samples in preparative layer chromatography or scintillation counting of radioisotopes by cutting out sections of the chromatogram containing the analytes of interest. Empore sheets have lower efficiency than conventional thin layers (29) and have not yet been applied widely for TLC. 

Tswett s initial column liquid chromatography method was developed, tested, and applied in two parallel modes, liquid-solid adsorption and liquid-liquid partition. Adsorption ehromatography, based on a purely physical principle of adsorption, eonsiderably outperformed its partition counterpart with mechanically coated stationary phases to become the most important liquid chromatographic method. This remains true today in thin-layer chromatography (TLC), for which silica gel is by far the major stationary phase. In column chromatography, however, reversed-phase liquid ehromatography using chemically bonded stationary phases is the most popular method. 

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