Bonded phase types

Bonded phase type Surface coverage Elemental analysis... 

Bonded phase type, length (L), column diameter (dc), particle size (dp)... 

The above data were obtained on a polymeric bonded phase and not a brush phase. The so-called brush phases are made from monochloro-sxlants, (or other active group) and, thus, the derivative takes the form of chains attached to the silica surface [2]. The bulk phases are synthesized from polyfunctional silanes in the presence of water and, thus, are cross linked and form a rigid polymeric structure covering the silica surface. These two types of phases behave very differently at low concentrations of moderator. 

In contrast, the alkane chains on the polymeric phase cannot collapse in an environment of water as they are rigidly held in the polymer matrix. Thus, the retention of the solute now continuously falls as the methanol concentration increases as shown in Figure 4. It should be pointed out that if the nature of the solutestationary phase interactions on the surface of a bonded phase is to be examined in a systematic manner with solvents having very high water contents, then a polymeric phase should be used and brush type reversed phases avoided if possible. 

Even with mobile-phase modifiers, however, certain polymer types cannot be run due to their lack of solubility in organic solvents. In order to run aqueous or mixed aqueous/organic mobile phases, Jordi Associates has developed several polar-bonded phase versions of the PDVB gels as discussed earlier. Figures 13.60 thru 13.99 detail examples of some polar and ionic polymers that we have been able to run SEC analysis of using the newer bonded PDVB resins. 

The development of bonded phases (Section 8.2) for liquid-liquid chromatography on silica-gel columns is of major importance. For example, the widely used C-18 type permits the separation of moderately polar mixtures and is used for the analysis of pharmaceuticals, drugs and pesticides. 

To retain solutes selectively by dispersive interactions, the stationary phase must contain no polar or ionic substances, but only hydrocarbon-type materials such as the reverse-bonded phases, now so popular in LC. Reiterating the previous argument, to ensure that dispersive selectivity dominates in the stationary phase, and dispersive interactions in the mobile phase are minimized, the mobile phase must now be strongly polar. Hence the use of methanol-water and acetonitrile-water mixtures as mobile phases in reverse-phase chromatography systems. An example of the separation of some antimicrobial agents on Partisil ODS 3, particle diameter 5p is shown in figure 5. 

The most popular bonded phases are, without doubt, the reverse phases which consist solely of aliphatic hydrocarbon chains bonded to the silica. Reverse phases interact dispersively with solvent and solute molecules and, as a consequence, are employed with very polar solvents or aqueous solvent mixtures such as methanol/water and acetonitrile/water mixtures. The most commonly used reverse phase appears to be the brush type phase with aliphatic chains having four, eight or eighteen carbon atom chains attached. These types of reverse phase have been termed C4, C8 and Cl8 phases respectively. The C8... 

Bonded phases are the most useful types of stationary phase in LC and have a very broad range of application. Of the bonded phases, the reverse phase is by far the most widely used and has been applied successfully to an extensive range of solute types. The reverse phases are commonly used with mobile phases consisting of acetonitrile and water, methanol and water, mixtures of both acetonitrile and methanol and water, and finally under very special circumstances tetrahydrofuran may also be added. Nevertheless, the majority of separations can be accomplished using simple binary mixtures. 

The more useful types of chirally active bonded phases are those based on the cyclodextrins. There are a number of different types available, some of which have both dispersive or polar groups bonded close to the chirally active sites to permit mixed interactions to occur. This emphasizes the basic entropic differences between the two isomers being separated. A range of such products is available from ASTEC Inc. and a separation of the d and / isomers of scopolamine and phenylephrine are shown in figure 4. The separations were carried out on a cyclodextrin bonded phase (CYCLOBOND 1 Ac) that had been acetylated to provide semi-polar interacting groups in close proximity to the chiral centers of the cyclodextrin. The column was 25 cm long, 4.6 mm in diameter and packed with silica based spherical bonded phase particles 5pm in diameter. Most of the columns supplied by ASTEC Inc. have these dimensions and, consequently, provide a... 

A large range of stationary phases is available, and according to their polarity they can be divided into normal phase and reversed phase types. Silica gel, aluminium oxide, and a nitrile-bonded-phase are normal adsorbents used to separate carotenoids... 

Surface Adsorption Hater sample is passed through a column of the adsorbent and the adsorbed organic constituents subsequently eluted with a smaller volume of organic solvent. All sample types Adsorbents Include charcoal, macroretlcular resins, polyurethane foams, bonded phases and ion-exchangers. Generally have high capacity but sample discrimination may be a -problem. Sample modification and Incomplete recovery are further possible problems. 

Figure 4.27 Flow chart for coluwi selection based on sample type (m - molecular weight). PLC precipitation-liquid chromatography SEC = size-exclusion chromatography lEC - ion-exchange chromatography HIC hydrophobic interaction chromatography LSC liquid-solid chromatography RPC - reversed-phase liquid chromatography BPC (polar) bonded-phase chromatography and IPC - ion-pair chromatography.

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