Bonded phases chemical stability

The binary systems we have discussed so far have mainly included phases that are solid or liquid solutions of the two components or end members constituting the binary system. Intermediate phases, which generally have a chemical composition corresponding to stoichiometric combinations of the end members of the system, are evidently formed in a large number of real systems. Intermediate phases are in most cases formed due to an enthalpic stabilization with respect to the end members. Here the chemical and physical properties of the components are different, and the new intermediate phases are formed due to the more optimal conditions for bonding found for some specific ratios of the components. The stability of a ternary compound like BaCC>3 from the binary ones (BaO and CC>2(g)) may for example be interpreted in terms of factors related to electron transfer between the two binary oxides see Chapter 7. Entropy-stabilized intermediate phases are also frequently reported, although they are far less common than enthalpy-stabilized phases. Entropy-stabilized phases are only stable above a certain temperature,... 

These separations can be carried out using a silica-based bonded phase however, the important advantage of organic polymer stationary phase materials is their chemical stability. The columns can be washed by using an alkaline solution after a certain number of injections. According to the chromatograms, the proteins in serum are completely eluted and nothing remains inside the column. However, the pressure drop in this type of analysis... 

The great interest in the possibility of replacing silici based bonded phases by carbon in RPC is understandable because the carbon is expected to be more stable toward aqueous eluents than the iflica-suppoited hydrocarbonaceous phases that are used almost exclusively today. Even if a carbonaceous sorbent with uniform surface and fovdrable porosity would be avaflable its stability may not live up to this expectation, however. The carbon surfoce is readily oxidized and can undergo other chemical transformations with concomitant changes in its retention properties. 

The major advantage of a bonded stationary phase is stability. Since it is chemically bonded, there is very little loss of stationary phase with column use. The siloxanes are the most widely used silica supports. Functional groups that can be attached as siloxanes are alkylnitriles (—Si—CH2CH2—CN),... 

Zirconia is an amphoteric metal oxide. Zirconia has the desirable physical and mechanical properties of silica, and its chemical stability is high. Zirconia supports have been pioneered by Carr and co-workers (19). It can be coated with butadiene or reacts with silanes to produce alkyl-bonded phases. 

Hybrid bonded-phase columns are being produced with carbon chains cross-linked chemically to the silica surface to reduce the amounts of free silanols and to increase the stability of the surface in the presence of high pH media. Hybrid silica column with a bridged organo-silica coating (Fig. 5.2)... 

In the liquid phase method, the C-terminal N-protected amino acid of the desired peptide sequence is esterified to the terminal hydroxyl group of PEG174). By attaching a suitable anchoring group to the chain ends of PEG, prior to the attachment of the first amino add, the chemical stability of the polymer-peptide bond... 

Some of the chemical concepts with little or no quantum-mechanical meaning outside the Bohmian formulation but, well explained in terms of the new interpretation, include electronegativity, the valence state, chemical potential, metallization, chemical bonding, isomerism, chemical equilibrium, orbital angular momentum, bond strength, molecular shape, phase transformation, chirality and barriers to rotation. In addition, atomic stability is explained in terms of a simple physical model. The central new concepts in Bohmian mechanics are quantum potential and quantum torque. 

Eluent pH is limited to a maximum of 7 to 8 due to the reduced chemical stability of a chromatographic bed in an alkaline medium. The nucleophilic attack of Si-0 bonds by hydroxide ions leads to the erosion of the silica surface as shown by back pressure increases caused by the formation of Si(OH)4. With polystyrene-divinyl-benzene-based stationary phases, pH stability is not an issue and a very wide mobile phase pH range can be used, thereby providing additional selectivity [1]. Several silica-based and polymeric columns claimed to be stable in pH ranges from 1 to 13 are commercially available, however, they are not commonly used. 

Base material provides mechanically stable rigid porous particles (mostly spherical) for reversed-phase HPLC adsorbents. Particle porosity on the mesoporous level (30 to 500-A diameter) is necessary to provide high specific surface area for the analyte retention. Surface of the base material should have specific chemical reactivity for further modification with selected ligands to form the reversed-phase bonded layer. Base material determines the mechanical and chemical stability—the most important parameters of future (modified) reversed-phase adsorbent. 

Chemical stability of carbon over the entire pH range has led to considerable interest in the development of carbon-based stationary phases for RPC. Porous graphitised carbon with sufficient hardness, well-defined and stable pore structure without micropores, which ensures sufficient retention and fast mass transfer can be prepared by a complex approach consisting of impregnation of the silica gel with a mixture of phenol and formaldehyde followed by formation of phenol-formaldehyde resin in the pores of the silica gel, then thermal carbonisation and dissolution of the silica gel by hydrofluoric acid or a hot potassium hydroxide. solution [48. The retention and selectivity behaviour of carbon phases significantly differs from that of chemically bonded pha.ses for RPC. Carbon adsorbents have greater affinity for aromatic and polar substances so that compounds can be separated that are too hydrophilic for adequate retention on a Cix column. Fixed adsorption sites make these materials more selective for the separation of geometric isomers [49]. 

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