Bonded-phase

Another approach to improving resolution is to use thin films of stationary phase. Capillary columns used in gas chromatography and the bonded phases commonly used in HPLC provide a significant decrease in plate height due to the reduction of the Hs term in equation 12.27. 

Bonded Phases. Substrate-bond hydrocarbon coatings for high pressure Hquid chromatography (hplc) and flash chromatography are prepared from octyltrialkoxysilanes and other long-chain alkyltrialkoxysilanes (see Chromatography). 

Terpene chemists use mainly gas chromatography in dealing with terpene mixtures in research and development as weU as in quahty control. Capillary gas chromatography with stable bonded-phase columns, the primary analytical method, is also being used more frequendy in the 1990s in product quahty control because its greater resolution is helpful in producing consistent products. 

Cyclodextrin stationary phases utilize cyclodextrins bound to a soHd support in such a way that the cyclodextrin is free to interact with solutes in solution. These bonded phases consist of cyclodextrin molecules linked to siUca gel by specific nonhydrolytic silane linkages (5,6). This stable cyclodextrin bonded phase is sold commercially under the trade name Cyclobond (Advanced Separation Technologies, Whippany, New Jersey). The vast majority of all reported hplc separations on CD-bonded phases utilize this media which was also the first chiral stationary phase (csp) developed for use in the reversed-phase mode. 

Appllca.tlons. The first widely appHcable Ic separation of enantiomeric metallocene compounds was demonstrated on P-CD bonded-phase columns. Thirteen enantiomeric derivatives of ferrocene, mthenocene, and osmocene were resolved (7). Retention data for several of these compounds are listed in Table 2, and Figure 2a shows the Ic separation of three metallocene enantiomeric pairs. P-Cyclodextrin bonded phases were used to resolve several racemic and diastereomeric 2,2-binaphthyldiyl crown ethers (9). These compounds do not contain a chiral carbon but stiU exist as enantiomers because of the staggered position of adjacent naphthyl rings, and a high degree of chiral recognition was attained for most of these compounds (9). 

The Liquid Phase. The stationary phase in an open tubular column is generally coated or chemically bonded to the wall of the capillary column in the same way the phase is attached to the support of a packed column. These are called nonbonded and bonded phases, respectively. In capillary columns there is no support material or column packing. 

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... 

Sedimentary rocks (like sandstone) have a microstructure rather like that of a vitreous ceramic. Sandstone is made of particles of silica, bonded together either by more silica or by calcium carbonate (CaCOj). Like pottery, it is porous. The difference lies in the way the bonding phase formed it is precipitated from solution in ground water, rather than formed by melting. 

The stationary phase can be apportioned in a similar manner. For example, with a bonded phase, due to the porous nature of the support, some of the pores will become blocked with stationary phase and so the total amount of stationary phase can be divided into that which is chromatographically available (Vs(A)) and that which is chromatographically unavailable (Vs(u)). 

R. P. W. Scott, "Silica gel and Bonded Phases", John Wiley and Sons, Chichester (1993)147. 

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. 

Silica gel, per se, is not so frequently used in LC as the reversed phases or the bonded phases, because silica separates substances largely by polar interactions with the silanol groups on the silica surface. In contrast, the reversed and bonded phases separate material largely by interactions with the dispersive components of the solute. As the dispersive character of substances, in general, vary more subtly than does their polar character, the reversed and bonded phases are usually preferred. In addition, silica has a significant solubility in many solvents, particularly aqueous solvents and, thus, silica columns can be less stable than those packed with bonded phases. The analytical procedure can be a little more complex and costly with silica gel columns as, in general, a wider variety of more expensive solvents are required. Reversed and bonded phases utilize blended solvents such as hexane/ethanol, methanol/water or acetonitrile/water mixtures as the mobile phase and, consequently, are considerably more economical. Nevertheless, silica gel has certain areas of application for which it is particularly useful and is very effective for separating polarizable substances such as the polynuclear aromatic hydrocarbons and substances... 

The full explanation of solute retention on silica, bonded phases or for that matter in liquid/liquid systems is still elusive and controversial. The thermodynamic approach... 

Pore size PSM 60 PSM 300 PSM 1000 PSM 3000 Bonded phase Silanized packing Unsilanized packing... 

The other restriction of SynChropak size exclusion columns is a general one for silica-based supports, that of pH. The most harmful pH is that above 7.5 due to silica dissolution. The bonded phase of SynChropak GPC has some polymeric properties therefore, it is not removed rapidly from the silica at pH 2-3. The bonded phase of SynChropak CATSEC is polymeric and stable at pH 2-7.5. 

Subsequent sections describe and illustrate in general terms how PDVB gels are produced as well as how some of the bonded phases have been developed at Jordi Associates. 

The next major bonded phase project was the development of the GBR resin, which stands for modified glucose bonded on both the backbone and the ring of basic PDVB gels. The manufacture of this product was ultimately achieved, as outlined later. The gel is first brominated, which places bromine atoms on both tertiary hydrogens of the PDVB. The brominated gel is then reacted with chlorosulfonic acid, and a specially treated reduced D-glucosamine is coupled to the gel. This process has the potential to covalently bond up to three sugar residues to each available divinylbenzene residue in the PDVB polymer. The exact reaction conditions used are proprietary however, the surface of the finished product is believed to look similar to Figs. 13.11 and 13.12. 

It has been found that both bromination and chlorosulfonation reactions are very useful in coupling a wide range of bonded phases to the gels. It is possible to couple virtually any primary amine to brominated and/or chlorosulfonated PDVB gels. This has allowed the development of many other phases such as polyamino, octadecyl, polyethyleneimine, and quaternary amine. Figures 13.13-13.16 show an assortment of various bonded phases developed since the mid-1980s. 

The Jordi Cl 8-DVB column has Cl 8 chains bonded to the DVB backbone. It is a nonpolar reversed-phase or GPC material recommended for applications that require a C18 bonded phase (Fig. 13.15). 

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. 

Scientific (Northbrook, IL) contain a silica support with a -y-glycidoxypropylsi-lane-bonded phase to minimize interaction with anionic and neutral polymers. The columns come in five different pore sizes ranging from 100 to 4000 A. The packing material has a diameter from 5 to 10 /cm and yields in excess of 10,000 plate counts. With a rigid silica packing material, the columns can withstand high pressure (maximum of 3000 psi) and can be used under a variety of salt and/or buffered conditions. A mobile phase above pH 8, however, will dissolve the silica support of the column (21). A summary of the experimental conditions used for Synchropak columns is described in Table 20.8. 

The use of bonded, silica column supports has also become a useful way to characterize cationic, water-soluble polymers. CATSEC SEC columns from Micra Scientific contain a silica support with a polymerized polyamine-bonded phase. This imparts a cationic surface charge on the packing that can be... 

A computer program was compiled to work out the ray-tracing of UV detector of high performance capillary electrophoresis at the investigation of 5 and 6 (98MI59). The capacity factor of 5 at different temperature and at different mobile phase compositions was experimentally determined in bonded-phase chromatography with ion suppression (98MI15). 

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). 

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