Bonded phase silica

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... 

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). 

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). 

As the capacity factors of p-CD complexes of disubstituted benzene derivatives are comparatively low and their differentation is also rather small - thus the final observed selectivity is mainly determined by the differences in the stability constants between p-CD complexes of ortho, meta and para isomers. The sequence of elution of ortho, meta and para isomers from RP column should be therefore reverse to the stabilities of their (3-CD complexes. This sequence should be opposite to that observed on the columns filled with P>-CD silica bonded phases (for the same compounds and under similar conditions). 

In fact, the results concerning J-CD activity in mobile phase solutions, concerning positional isomers of disubstituted benzene derivatives (23,28) seem to be consistent with those obtained by other authors using f-CD silica bonded phases (4-8). It should be mentioned that all of the above considerations concern mainly neutral molecules for the compounds undergoing dissociation, the equilibria are more complicated and the sequence of elution of isomers more variable. 

In a study presented by Jinno et al. [124], packed column capillary electrochromatography, open-tubular CEC, and microcolumn liquid chromatography using a cholesteryl silica bonded phase have been studied to compare the retention behavior for benzodiazepines. The results indicated that CEC was a promising method, as it yielded better resolution and faster analysis than microcolumn LC for benzodiazepines. Similar selectivity to HPLC was noted, except for a few solutes that were charged under the separation conditions. Columns packed with the ODS and cholesteryl phases were compared and showed totally different migration orders of the analytes. The retention on the cholesteryl silica sta-... 

The choice of the sorbent is dictated by the characteristics of both the analytes and their potential interferences. The sorbents most frequently employed here are silica, alkylsilane-modified silica (bonded phases), alumina, porous polymers (with and without ion-exchange groups) and carbon-based materials. One typical application is a method for the determination of hexavalent chromium in soils [10] using the on-line system depicted in Fig. 4.9. After USAL, the analytes in the leachate were directly determined or preconcentrated depending on their concentration. Concentration was performed by on-line solid-phase extraction using a laboratory-made minicolumn packed with a strong anion-exchange resin. The absolute limits of detection were 4.52 and 1.23 ng without and with preconcentration, respectively. 

Reversed phase HPLC Is the most popular chromatographic method for purification of polar conjugates. The reversed phase technique is commonly performed on silica bonded phases, such as RP 8 and RP 18. 

Silica bonded phases with functional groups (e.g. CN, NHj,... 

Figure 3.4. Three examples of the retention of water in C-18 silica bonded-phase sorbent.

Normal-phase SPE methods development is a straightforward process, and for this reason, many methods have been developed on silica sorbents, especially with the broad and comprehensive literature available in thin layer chromatography (TLC). The development of the silica bonded phases has introduced a new and important aspect to methods development with normal-phase SPE. 

Select columns packed with 3- or 5-pm high-purity silica-bonded phases from a reputable manufacturer. 

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. 

There are numerous modern extraction techniques that improve the reliability of the sampling process a silica bonded phase should be used to carry out a solid-phase microextraction of aromatic hydrocarbons such as benzene, toluene, m-xylene, and o-xylene 58 A derivatization step has been proposed59 to improve the separation process. The derivatization step assures the best reliability of fatty acids sampling (in water/air). Head space solid-phase microextraction (HS-SPME) has been employed for sampling of volatile components. This separation process can be successfully used for quality control of herbal medicines and other formulations containing herbal extracts.60... 

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