Chemically bonded phase

Most used reversed-phase stationary phases are based on covalently bound phenyl, 

and C18 to silica. Interactions between the stationary phase and the analytes are based on hydrophobic interactions (Section 3.5.2). The nature of these hydro-phobic phases can contribute to insufficient wetting by both mobile phase and sample solvent. 

Examples of hydrophilic bonded phases are NH2, diol, and CN. Using these stationary phases, separations are achieved on the basis of normal-phase interactions. These bonded phases can also be used in the reversed-phase mode, depending on the mobile phase composition. 

In contrast to H PLC, there are little limitations when it comes to choice of the mobile phase. The mobile phase components must however be volatile such that they can 

For reversed-phase development, mixtures of water, buffer components, acetonitrile, methanol, and THF are common. 

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Chemically bonded phases are usually more resiHent than nonbonded phases, tending not to wash out as large amounts of solvent pass through the column, and having much better thermal stabiHty than do the nonbonded phases. Frequendy a chemically bonded phase can be identified to effect a given separation at the same efficiency as a nonbonded one, thus the bonded phases are generally preferred... 

D. E. Martire and R. E. Boehm, A unified theory of retention and selectivity in liquid chromatography. 2. Reversed-phase liquid clrromatography with chemically bonded phases , J. Phys. Chem. 87 1045-1062 (1983). 

Recently, the use of chemically bonded phases has become of great interest. In the typical case an organochlorosilane is reacted with dry porous silica to produce a surface that consists of an organic matrix (10). 

Column Activity Tests for Chemically Bonded Phases... 

Residual silanol groups in chemically bonded phases have been associated with a number of undesirable interactions with polar solutes such as excessive peak tailing, irreproducible retention times, and excessively long retention times. These problems are particularly prevalent for amines and other strong bases. A large number of test systems have been proposed to characterize the concentration of residual silanol groups on bonded phase packings, and some representative examples are... 

The PRISMA model was developed by Nyiredy for solvent optimization in TLC and HPLC [142,168-171]. The PRISMA model consists of three parts the selection of the chromatographic system, optimization of the selected mobile phases, and the selection of the development method. Since silica is the most widely used stationary phase in TLC, the optimization procedure always starts with this phase, although the method is equally applicable to all chemically bonded phases in the normal or reversed-phase mode. For the selection of suitable solvents the first experiments are carried out on TLC plates in unsaturated... 

Berendsen, GE. and de Galan, L., Preparation and chromatographic properties of some chemically bonded phases for reversed-phase liquid chromatography, J. Liq. Chromatogr, 1, 561, 1978. 

Another way to increase sample capacity is to increase the surface area for conventional chemically bonded phases. Two methods have been reported for increasing surface area (a) laying down a thin layer of porous material on the surface and (b) etching the surface. The precursors and catalyst dictate the characteristics of the final sol-gel. Manipulation of the components and procedures in the sol-gel process can control the phase ratio and the retention properties of the sol-gel-derived phase. 

The influence of the bonded organic moiety on solute retention has not yet been elucidated and only a very small number of papers discuss the properties and use of such phases so far. The numerous advantages of chemically bonded phases make the application of polar chemically bonded phases with nonpolar eluents quite attractive even if the standardization of these phases may pose problems 106) similar to those encountered in the standardization of aidsorbents as well as of polymeric liquid phases in gas chromatography. A detailed discussion of the properties and chromatographic use of bonded stationary phases is given by Melander and Horvath (this volume). 

It is generally accepted that the only important polar adsoiption sites on the silica surface are the silanol functions, i.e., hydroxyl groups, that are attached to silicon atoms (2). They can interact with the sample molecules by hydrogen bonding and various physical observations ctAi be used to prove this statement. Complete dehydration of silica by beating, i.e., removal of all surface hydroxyl groups, yields a hydrophobia silica which no longer shows adsorption for unsaturated and polar molecules and is no more wetted by water (15). Chemical modification of the surface hydroxyls such as used in the preparation of chemically bonded phases also eliminates the selective adsorption properties of the silica. ... 

Alumina and silica columns, formerly used for PAH separation, were later replaced with chemically bonded phases [685]. RP-HPLC based on the use of C18 columns is nowadays the most popular mode for PAH separation, and specially designed colnmns from different vendors are commercially available. Colnmns of 100-250cm length (3.0-4.6mm i.d.), packed with 3-5pm particles. 

When the analysis began, the chemically bonded phase was not available. An SE-30 WCOT capillary column of the same physical dimensions as the column just described was used instead. The change was made because the chemically bonded phase was found to deteriorate more slowly than a conventional coated SE-30 column (16). This claim was substantiated during this study. 

Microporous particles are available in two sizes 20 to 40 jam diameter with longer pores and 5 to 10 fim with short pores (see Figure 3.14B). These are now more widely used than the porous layer beads because they offer greater resolution and faster separations with lower pressures. The micro-porous beads are prepared from alumina, silica, ion-exchanger resins, and chemically bonded phases (see next section). 

Miller, N.T. and Shieh, C.H., Preparative hydrophobic interaction chromatography of proteins using ether based chemically bonded phases, J. Liq. Chromatogr., 9, 3269, 1986. 

C. A. T. Brinkman and G. de Vries, Thin-layer chromatography on chemically bonded phases a comparison of precoated plates, J. Chromatogr., 258 43 (1983). 

S. H. Hansen, P. Helboe, and M. Thomsen, Dynamically modified silica—an alternative to reversed-phase high-performance liquid chromatography on chemically bonded phases, J. Pharm. Biomed. Anal., 2 167 (1984). 

The area available for the stationary phase will directly affect the phase ratio. If a solid material is used as the stationary phase in a packed column, if a liquid phase is deposited on a solid adsorbent with a constant film thickness, or if chemically bonded phases are employed, the phase ratio (through VJ will be directly proportional to the available surface area. The surface area of an adsorbent is usually given per unit weight (i.e. the specific surface area in m2/g). However, it should be noted that the relevant quantity is the surface area per unit volume (m2/ml) in the packed column. 

Clearly, with all other factors constant, Vs will increase linearly with the film thickness (this is also true for the phase ratio V/ Vm as long as Vs < V. For solid adsorbents this effect does not occur. For chemically bonded phases the (mono-)layer thickness is not as well defined as the film thickness of a bulk liquid, and neither is the description of variation in the layer thickness as straightforward as it is for liquids (see section 3.2.2). 

Chemically bonded phases (CBP s) are very commonly used in LC, and occasionally also in GC. Such phases cannot be seen as either a solid or a liquid. The common term [201] used for LC involving such phases is bonded phase chromatography (BPC). To be consistent, the stationary phase identification should follow that of the mobile phase in defining the chromatographic system. Hence, LBPC should be used for liquid chromatography using chemically bonded stationary phases. 

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