Chemically bonded polymer phases

Commercially successful methods for the production of chemically bonded polymer phases suitable for HPLC have not been widely advertised and can only be referred to in general terms. Nevertheless, there are two principle methods for their production. Firstly, modification of a pre-formed support is possible providing that suitable sites are available. Alternatively, a method which has proved popular with polystyrene-type supports is to prepare the required support by copolymerisation of appropriate monomer units. 

Although tetraalkylammonium salts, suitable as ion-pair reagents for ion-pair chromatography of simple inorganic anions, are predominantly employed in combination with polymer phases [24,25], they may equally be used when applying chemically bonded reversed phases [16,26]. The separation of inorganic anions on LiChrosorb RP18 shown in Fig. 5-13 serves as an example. In contrast to polymer phases, the fluoride determination is a problem because fluoride is poorly retained on chemically bonded reversed phases. If direct conductivity detection is applied, a quantitative evaluation of the fluor-... 

The difficulties encountered in LLC can be overcome by the use of chemically bonded stationary phases or bonded-phases. Most bonded phases consist of organochlorosilanes or organoalkoxysilanes reacted with micro-particulate silica gel to form a stable siloxane bond. The conditions can be controlled to yield monomeric phases or polymeric phases. The former provides better efficiency because of rapid mass transfer of solute, whereas the polymeric phases provides higher sample capacity. BPC can be used in solvent gradient mode since the stationary phase is bonded and will not strip. Both normal-phase BPC (polar stationary, non-polar mobile) and reversed-phase BPC (non-polar stationary, polar mobile) can be performed. The latter is ideal for substances which are insoluble or sparingly soluble in water, but soluble in alcohols. Since many compounds exhibit this behaviour, reversed phase BPC accounts for about 60% of published applications. The main disadvantage of silica bonded phases is that the pH must be kept between 2 to 7.5. However, bonded phases with polymer bases (polystyrene-divinylbenzene) can be used in the pH range of 0 to 14. 

Instabilities that manifest themselves in surfactant and polymeric systems have been considered in an attempted to elucidate the myelin instability. In polymer-like micelles (or wormlike micelles) instabilities have been observed in the directional growth of hexagonal phases in a temperature gradient (30), These instabilities are an example of the Mullins and Sekerloi type (31), In the case of polymer gels, instabilities appear during growth, which resemble a raspberry like texture at the surface. This instability is due to the elastic properties of the gel which is a network of chemically bonded polymers. As the gel swells at the surface it remains anchored to the rest of the unswollen gel and the surface buckles (50). 

Inorganic and organic anions can also be separated on cross-hnked polymers modified with cyclic ethers. Pioneering work in the field of adsorbing crown ethers onto chemically bonded reversed phases and PS-DVB polymers has been carried out by Kimura et al. The number of applicable crown ethers is limited because of their limited solubility and high price. Moreover, crown ether resins are mechanically unstable and are therefore operated with low flow rates, thus resulting in long analysis times. 

Engelhard , H. Mathes, D. High-performance exclusion chromatography of water-soluble polymers with chemically bonded stationary phases. J. Chromatogr. 1979, 755, 305. 

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. 

The same applies to the ion-pair chromatographic separation of cations, which is performed either with long-chain alkanesulfonic acids or, in the simplest manner, with mineral acids. A survey of the most commonly used reagents is listed in Table 6.1 in the order of increasing hydrophobicity. A comparison by Cassidy and Elchuk [18] of chemically bonded reversed phases with organic divinylben-zene-based (Dionex lonPac NSl) versus phases with poly(styrene-co-divinylben-zene)-based polymers (Hamilton PRP-1) revealed that the latter have a lower... 

Tetraalkylammonium salts that are suitable ion-pair reagents for ion-pair chromatography of simple inorganic anions are predominantly employed in combination with polymer phases [24,25]. However, they may also be used when applying chemically bonded reversed phases [16,26]. The separation of inorganic... 

Depending on their degree of ethoxylation, fatty alcohol ether sulfates are extremely complex mixtures, for which the separation efficiency of a polymer phase is not sufficient. Good separations are obtained with silica-based, chemically bonded reversed phases. The chromatographic conditions have to be adjusted accordingly. The free base ammonium hydroxide cannot be used as the ion-pair reagent because of the pH limitation of modified silica, so suppressor systems cannot be used for subsequent conductivity detection. Sodium acetate has proved to be a suitable ion-pair reagent for nonsuppressed conductivity... 

Chemically bonded stationary phases are covalently bonded to the fused silica surface, are highly temperature stable, and can be solvent rinsed for cleaning from matrix contaminations. In contrast, cross-linked stationary phases are not bonded to the surface, only use chemical links between the polymer chains. 

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