Stationary phase chemical stability

Chemical surface modifications The first surface modification for the purpose of eliminating EOF and protein adsorption was recommended by Hjerten.28 The attachment of vinyl silanes allowed the polymerization of a variety of molecules to the surface. Most of the chemical modifications used for preparing capillaries for electrophoresis originated from the experience acquired over the years preparing GC and LC stationary phases. Chemical modification should conform to certain requirements, including the prevention of adsorption, the provision of stable and constant EOF over a wide pH range, chemical stability, ease of preparation, and reproduciblity of preparation. The effects of silanization of the inner surface of capillaries on electrophoretic separations have been extensively studied.26-29... 

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),... 

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. 

In WC OTcolumns, the stationary phase, which is a silicone or polyethylene glycol, is either mechanically distributed on the wall or immobilized, either by binding the stationary phase chemically to the wall or by polymerization of the stationary phase into a nonextractable rubber (or both). Columns with immobilized stationary phase have increased stability and lifetime and are now most common in use. 

BINDER. Any chemical added to a sorbent to improve the stability or hardness of the layer. BONDED PHASE. A stationary phase chemically bonded to (as opposed to mechanically deposited on) a support material. 

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... 

The great versatility of HPLC lies in the fact that the stability of the chemically bonded stationary phases used in partition chromatography allows the use of a wide range of liquids as a mobile phase without the stationary phase being lost or destroyed. This means that there is less need for a large number of different stationary phases as is the case in gas chromatography. The mobile phase must be available in a pure form and usually requires degassing before use. The choice of mobile phase (Table 3.6) is influenced by several factors. 

Some new benzanthrone dyes were synthesized and applied for the one-step colouration and stabilization of polystyrene. The chemical structures of monomeric benzanthrone dye (formula 1), the stabilizer TTMP 2,2-(2,2,6,6-tetramethylpiperidine-l-yl)-4,6-dichloro-l,3,5-triazine (formula 2) and the new synthetic product showing both colouration and stabillizer capacity (formula 3) are shown in Fig. 3.135. The synthesis process was controlled by TLC using a silica stationary phase and an n-heptane-acetone (1 1, v/v)... 

Stationary phase materials are synthesized from different raw materials. Those stationary phase materials synthesized from inorganic materials, such as silica and alumina, are physically strong but chemically unstable. Conversely, stationary phase materials synthesized from organic materials, such as polystyrene or poly(vinyl alcohol), are chemically stable but physically weaker. Improvements in the chemical stability of inorganic stationary phase materials and in the physical strength of organic stationary phase materials are required the marketed products do not have both and have to be used under restricted conditions in liquid chromatography. 

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... 

Mechanical and chemical stability of novel stationary phases are basic requirements concerning their application. A lack in stability generally causes a loss in resolution and thus reduces column efficiency. In addition, the reproducibility of retention times, being important for qualitative analysis, may be affected. Evaluation of the mechanical stability of polymeric stationary phases is usually accomplished by the determination of the pressure drop across the column, when employing solvents of different polarity within a wide range of flow rates. A stationary phase can be considered as mechanically stable if a linear relationship between applied flow rate and resulting back pressure is obtained. 

Heteroboranes, compounds where one or more of the cage borons are replaced by a main group element (33), are not themselves commercially available. However, carborane siloxanes containing w-carborane [16986-21-6], C2H12B10, are available under the trade name of Dexsil for the stationary phase in gas—liquid chromotography (qv) (34). The carborane, l,7-dicarba- / o-dodecaborane(10) (35), contributes enhanced chemical and thermal stability to the siloxane polymer. 

The primary requirements of a stationary phase are to provide separation of the sample with reasonable column life. Therefore, in addition to having suitable selectivity, the phase should have reasonable chemical and thermal stability. Many catalogs list upper temperature limits for stationary phases, but these should be used only as approximations because the true limit depends upon the type of detector used and the amount of column bleed one can tolerate to get the job done. Even if a phase is stable to 250°C, the column will last much longer if the temperature is limited to 200°C. Excessive temperatures result not only in shorter column life, but also in more rapid fouling of the detector. 

---