Silica bonding

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 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... 

Zirconium, titanium, and hafnium hydrides can activate the C-H bonds of several alkanes at low temperatures (even at room temperature) because they are very electrophilic and reactive. Moreover, the surface complex is immobilized by the strong metal-silica bonds, and this immobilization can prevent the coupling reactions leading to the deactivation of the complex. 

A variety of preconcentration procedures has been used, including solvent extraction of metal chelates, coprecipitation, chelating ion exchange, adsorption onto other solids such as silica-bonded organic complexing agents, and liquid-liquid extraction. 

The herbicides are extracted from soil samples with methanol. They are chromatographed on microparticulate silica bonded with octadecyltrichlorosilane using a mixture of methanol, water and ammonia as mobile phase and an ultraviolet spectrophotometric detector. 

The effect of pore size on the retentive capacity of various octadecyl silica bonded phases is illustrated in Fig. 10. Exclusion effects become appreciable when bulky octadecyl groups (see Table V) are attached to the pore wall in 6 nm pore diameter silica (Si-60). As a result, the stationary... 

Fio. 12. Graph illustrating the dependence of the logarithm of retention factor for aromatic hydrocarbons on the carbon load of octadecyl silica bonded phases prepared from Par-tisil with octadecyhrichlorosilane. Mobile phase methanol-water (70 30) eluitest A, benzene A, naphthalene , phenanthrene , anthracene O, pyrene. Reprinted with permission from Herndon t al. (70). 

Similar to other coupled methods of polymer HPLC, for example, LC CC (Section 16.5.2), the choice of the column packing and the mobile phase components for EG-LC depends on the retention mechanism to be used. Adsorption is preferred for polar polymers applying polar column packings, usually bare silica or silica bonded with the polar groups. The eluent strength controls polymer retention (Sections 16.3.2 and 16.3.5). The enthalpic partition is the retention mechanism of choice for the non polar polymers or polymers of low polarity. In this case, similar to the phase separation mechanism, mainly the solvent quality governs the extent of retention (Sections 16.2.2, 16.3.3, and 16.3.7). It is to be reminded that even the nonpolar polymers such as poly(butadiene) may adsorb on the surface of bare silica gel from the very weak mobile phases and vice versa, the polymers of medium polarity such as poly(methyl methacrylate) can be retained from their poor solvents (eluents) due to enthalpic partition within the nonpolar alkyl-bonded phases. 

This may suggest that the CT-complexes in solution are different from those formed during resolution of helicenes by HPLC on TAPA modified columns. Interestingly, when the three alternating thiahelicenes [3], [5] and [7] were subjected to HPLC on silica bonded to TAPA, the order of elution ([3]>[7]>[5]) was equal to that of the complexation constants with another n-acceptor (TCNQ)91> (see Sect. 4.4). 

Bakerbond Cig, Baker Silica Bond-Elut Cg, Bond-Elut Silica, Bond-Elut NHg Bond-Elut Ci8... 

Derivatized-silica bonded phases are available in disposable 1-, 3-, and 5-mL columns as follows nonpolar phases (ethyl, octyl, cyclohexyl,... 

Goewie et al. (19) developed an organometallic-silica bonded phase for the selective retention of phenylurea herbicides and anilines from water. Seven-micrometer diameter silica was derivatized with 2-amino-1-cyclopentene-l-dithiocarboxylic acid (ACDA), resulting in Structure I. Capacity factors of phenylurea herbicides and corresponding anilines were measured on the ACDA-silica and ACDA-metal-loaded silica. The platinum-loaded material was found to selectively retain the anilines. Anilines could be eluted with acetonitrile, but not with methanol or tetrahydrofuran, because of the strength with which acetonitrile forms complexes with platinum and thus displaces the anilines. Application of the ACDA-Pt precolumn in series with an ODS-silica precolumn for... 

The feasibility of extracting substituted phenols from an aqueous solution with supercritical CO2 is reported A special extraction vessel was used in order to overcome the mechanical difficulty in retaining the liquid matrix in the extraction vessel. Solid phase trapping was utilized with a diol silica bonded phase. Methanol was used to rinse the trap. Below 300 atm extraction recovery paralleled CO2 pressure at fixed temperature. Phenol was least extractable while, 2,4-dichlorophenol yielded the greatest percent recovery. Above 300 atm extraction yield declined with pressure. It is theorized that at high CO2 density there is less mixing with the aqueous phase because of increased fluid-fluid interaction. 

Bagnati, R., E. Benfenati, E. Davoli, and R. Fanelli (1988). Screening of 21 pesticides in water by single extraction with C18 silica bonded phase columns and HRGC-MS. Chemosphere, 17(1) 59-65. 

Applications of HPLC for Hg speciation studies have been reviewed by Harrington.83 Practically all HPLC methods for Hg speciation reported in the literature are based on reversed-phase separations, involving the use of a silica-bonded phase column and a mobile phase containing an organic modifier, a chelating or an ion pair reagent, and in some cases, a pH buffer. 

A general strategy for relatively polar analytes has been developed [38]. In this approach, the cyanopropyl-silica-bonded phase remains the... 

M. Moors, B. Steenssens, I. Tielemans, and D. L. Massart, Solid-phase extraction of small drugs on apolar and ino-exchanging silica bonded phases towards the development of a general strategy, J. Pharm. Biomed. Anal., 12 463 (1994). 

In order to study these compounds further, it was necessary to obtain them enantiomerically pure. This was done bypassing a solution of the racemic compound through a column of silica bonded to an amino-acid-derived chiral stationary phase. The ( )-(-)-enantiomer showed a lower affinity for the stationary phase, and therefore was eluted from the column first, followed by the (S)-(+)-enantiomer. 

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