Chemically bonded silica phases

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-... 

Figures 13.4a and 13.4b compare chromatograms published by Heme et al. [23] and chromatograms calculated from the data formd in their paper, assuming Langmuir competitive isotherms (Eq. 13.3) and numerical values of the other parameters that permit the best approximation of the experimental results. In this experiment, a sample containing acetonitrile, N,N-dimethylformamide, ethyl formate, fso-butanol, ethyl acetate and 1-pentanol is eluted on a C18 chemically bonded silica phase (Nucleosil C18), using a water-methanol solution containing 0.00025 M salicylamide, the UV-absorbing additive. The agreement between experimental and calculated chromatograms is excellent. It demonstrates the validity of the theoretical approach. The system peaks associated with the first five components, those that are eluted before the additive system peak (peak 6), are...

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. 

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. 

Chemically bonded stahonary phases, e.g. alkylamide silica reversed phases, were also developed. Despite a generally good stability and good quality of resolution and less interachons with free silanol groups, correlahons between log Pod and log kw are relahvely poor compared to a number of other stahonary phases [26]. Finally, monolithic silica stahonary phases have also been applied for Upo-philicity determinahon of a series of P-blockers [27]. 

Buszewski, B., Gadza-la-Kopciuch, R. M., Markuszewski, M. L, Kaliszan, R. Chemically bonded silica stationary phases synthesis, physicochemical characterization, and molecular mechanism of reversed-phase HPLC retention. Anal. Chem. 1997, 69, 3277-3284. 

Stationary phase Chemically bonded silica, alumina, polymeric resins Polysiloxanes... 

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. 

This problem was remedied by the discovery of methods for chemically bonding the active stationary phase to the inert support. Most chemically bonded stationary phases are produced by covalent modification of the surface silica. Three modification processes are shown in Equations 3.6-3.8. 

The use of nonpolar chemically bonded stationary phases with a polar mobile phase is referred to as reverse-phase HPLC. This technique separates sample components according to hydrophobicity. It is widely used for the separation of all types of biomolecules, including peptides, nucleotides, carbohydrates, and derivatives of amino acids. Typical solvent systems are water-methanol, water-acetonitrile, and water-tetrahydrofiiran mixtures. Figure 3.15 shows the results of protein separation on a silica-based reverse-phase column. 

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). 

For the samples that will be subjected to other (so-called interactive) LC techniques, the next question involves the nature of the solvent in which the sample has been or can be dissolved. If this is a non-polar solvent, such as n-hexane, then the sample solution is compatible with Normal Phase LC (NPLC), in which mobile phases with a relatively low polarity are used in combination with more polar stationary phases (see section 3.2.3). In this form of chromatography solid adsorbents (such as silica or alumina) may be used as stationary phases (LSC). Alternatively, polar chemically bonded stationary phases may be used (see section 3.2.2). 

Reversed phase ion-pair systems (RP-IPC) could be of the LLC type, but the use of chemically bonded (alkyl) phases has become increasingly popular, because of the increased stability and flexibility of the system. Even if an LLC system is used for RP-IPC, then a chemical modification of the surface is still required to coat an organic liquid on the particles of (for example) silica. 

The columns used for the GC separation of phytosterols are currently almost exclusively capillary columns with 0.1-0.3 mm internal diameter, and fused-silica capillary columns with chemically bonded stationary phases are commonly used (Abidi, 2001). The best separation of structurally very similar sterols, such as sitosterol and its saturated counterpart sitostanol, is obtained with slightly polar stationary phases like 5% diphenyl-95% dimethylpolysiloxane, and they are currently the most used columns for the separation of phytosterols (Lagarda et al., 2006). For detailed lists of different columns used in sterol analysis, see the papers by Abidi (2001) and Lagarda (2006). 

We now have a fairly adequate understanding of the different properties, including the particle diameter i/p, the pore size, the degree of permeability, and the chemical composition of the surface of the support matrix, to know which type of stationary phase can be successfully used with a particular class of peptides. Most of the HPLC packing materials now in use for peptide separations are based on the wide pore microparticulate silica gels with polar or nonpolar carbonaceous phases chemically bonded to the surface of the matrix. Methods for the preparation of these chemically bonded stationary phases, their available sources of supply. 

In the present paper the pore textures of several porous silicas have been analyzed on the basis of liquid nitrogen data and compared with the results obtained by SAXS and PALS. Silica gels with chemically bonded hydrocarbon phases C-2, C-8 and C-18 were used in experiment. 

Figure4.3 Formation of chemically bonded silica stationary phases.

Chemically bonded silica gels with cyanopropyl, aminopropyl, and diol functional groups are also available and each has been used for normal-phase as well as reversed-phase separations. Polymeric-based packings are available and can be used for ion-exchange or reversed-phase applications. Cross-linked polymeric based packings have been combined with ion-exchange materials to create a more pH-stable stationary phase. Graphitized carbon... 

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