Silica as stationary phases

There is no doubt that even small modulator concentrations influence both iihsoliite ami lelalive leteiilions and that selectivity do s change with the type of modifier used. With silica as stationary phase dhd by using a... 

Hanson, M., Unger, K.K., Schomburg, G. Non-poros polybutadiene-coated silicas as stationary phases in reversed-phase chromatography, J. Chromatogr., 1990, 517, 269. 

The protected (S - S) (1-10) salmon calcitonin is purified by preparative chromatography on silica as stationary phase with a solvent mixture (dichoromethane / methanol / acetic acid (93 / 7 / 2 v / v / v)) as eluent phase. After precipitation with water the pure product is analysed by HPLC on Lichrospher 100 RP 18 (125 x 4 mm) 5 micron as stationary phase and with a mobile phase (methanol / water / N,N-dimethylformamide / trifluoracetic acid 70 / 30 / 5 / 0.4 v / v / v / v). The DMF is introduced in the eluent phase to solubilise the protected ( S - S) (1-10) salmon calcitonin. The HPLC profile of the pure protected (S - S) (1-10) salmon calcitonin is shown in Figure 8. 

The more volatile silicon tetrachloride and silicon tetrabromide may be effectively separated on silicone oils coated on a polytetrafluoroethylene supports. Less volatile metal bromides generally require high-temperature stationary phases. Thus, the use of alkali bromide salts coated on silica as stationary phases with the use of bromine/nitrogen and boron tribromide/bromine/nitrogen mobile... 

Figure 6 LC - ICP-MS chromatogram obtained from a 24-h mixture of cisplatin Rt = i. 9 min), carbopiatin (Rt = 3. i min), and oxaliplatin (Rt = 5.1 min) in subboiled water using pentafluorophenylpropyl-functionalized silica as stationary phase. In addition to the parent drugs, the chromatogram shows the major degradation products of cisplatin (monoaquacisplatin, Rt = 2.5 min diaquacisplatin Rt = 3.6 min)), the product of carboplatin (CP2, Rt = 2.1 min) and the product of oxaliplatin (OPl, Rt = 4.5 min). (Reproduced with permission Haim, Stefanka, Lenz and Stingeder 2005, Springer Science Business Media)...

P-06 - Evaluation of mesoporous silicas as stationary phases for high performance liquid chromatography (HPLC)... 

The retention mechanism and solvent selectivity have been studied most carefully with alumina or silica as stationary phases. The knowledge of both for bonded phases used in normal-phase chromatography is much more limited. Nevertheless, it is safe to assume that similar selectivity rules for solvent strength and selectivity can be applied, especially since the results obtained for alumina and silica correlate well with each other. 

Aboul-Enein, H.Y. El-Awady, M.I. Heard, C.H. Enantiomeric resolution of some 2-arylpropionic acids using L-(-)serine impregnated silica as stationary phase by thin layer chromatography. J. Pharm. Biomed. Anal. 2003, 32,1055-1059. 

Ultrathin TLC (UTLC) is one of the later developments in TLC leading to better detection limits and faster analyses. UTLC is based on a thin monolithic layer ( 10p,m) of silica as stationary phase and allows reduction of the sample size. 

The second example relates to the separation and determination of steroid isomers. The separation of R- and S-budesonide, syn- and antinorgestimate, -norgestreloxime, and 6-hydroxy-norgesti-mate have been done on silica, and amino-bonded silica stationary phases by conventional and overpressurized layer chromatography (OPLC) (35). Application of amino-bonded silica as stationary phase, and chloroform-diethyl ether-methyl ethyl-ketone (4 3 3, v/v) as mobile phase results in a satisfactory separation of the isomer pairs, especially, if OPLC technique is used. 

Although sample cleanup on open silica columns or Sep-Pak silica cartridges (and so on) removes parts of the lipids that show different polarity in comparison to vitamin K, many equipolar lipids are collected together with the vitamin fraction. Using silica as stationary phase, only little variability is possible in the choice of solvent, due to the lipophilic character of the vitamin K molecules. Hexane with a content of 3% to 5% diethylether or minimal amounts of acetonitrile, ethyl acetate, or diisopropylether (up to 1%) can be used in these systems, with the consequence of a relative poor separation from interfering lipids. That is why in the last few years adsorption chromatography (inclusive semipreparative silica HPLC) is used only as a cleanup step in sample preparation for re-versed-phase HPLC (see Table 1). 

An important publication by Kost et al. (63JGU525) on thin-layer chromatography (TLC) of pyrazoles contains a large collection of Rf values for 1 1 mixtures of petroleum ether-chloroform or benzene-chloroform as eluents and alumina as stationary phase. 1,3- and 1,5-disubstituted pyrazoles can be separated and identified by TLC (Rf l,3>i y 1,5). For another publication by the same authors on the chromatographic separation of the aminopyrazoles, see (63JGU2519). A-Unsubstituted pyrazoles move with difficulty and it is necessary to add acetone or methanol to the eluent mixture. Other convenient conditions for AH pyrazoles utilize silica gel and ethyl acetate saturated with water (a pentacyanoamine ferroate ammonium disodium salt solution can be used to visualize the pyrazoles). 

Other examples of irreversible derivatization on treatment with iodine have been described for phenohc steroids (estrone derivatives [256]), morphine [257] and 23 other pharmaceuticals [258]. These reactions are probably favored by the presence of silica gel as stationary phase and by the influence of light. 

Note Silica gel, kieselguhr and polyamide layers can be used as stationary phases. Not all acids are stained on RP layers. Amino layers yield a pale blue background. The detection limits are in the pg range for carboxylic acids [1], thioglycolic and dithioglycolic acids [2] and for antithyroid pharmaceuticals [4] they are about 5 ng per chromatogram zone for sterols and steryl esters [6]. 

Silica gel, kieselguhr. Si 50000 and cellulose layers, amongst others, can be used as stationary phases. 

The materials originally used as stationary phases for GPC were the xerogels of the polyacrylamide (Bio-Gel) and cross-linked dextran (Sephadex) type. However, these semi-rigid gels are unable to withstand the high pressures used in HPLC, and modern stationary phases consist of microparticles of styrene-divinylbenzene copolymers (Ultrastyragel, manufactured by Waters Associates), silica, or porous glass. 

Adsorption and ion exchange chromatography are well-known methods of LC. In adsorption, one frequently selects either silica or alumina as stationary phase for separation of nonionic, moderately polar substances (e.g. alcohols, aromatic heterocycles, etc.). This mode works best in the fractionation of classes of compounds and the resolution of isomeric substances (J). Ion exchange, on the other hand, is applicable to the separation of ionic substances. As to be discussed later, this mode has been well developed as a tool for analysis of urine constituents (8). 

The choice of the stationary phase depends on the type of solutes. Silica gel is usually used if there are no special requirements as far as stationary phase is concerned. 

Fig. 3.1a shows the different sizes and shapes of particles that have been used as stationary phases in hplc. The particles are usually silica, although in ion exchange and exclusion chromatography polymeric gels or resins are common. 

Liquid chromatography is carried out in columns. The common columns are packed using reversed-phase ( jx-bonded silica gel as stationary phase. Elution systems are... 

Silica stationary phases display some ion exchange properties, which may also influence the separation characteristics of silica. One of the main disadvantages of the use of silica and silica-based stationary phases is their instability even at slightly alkaline pH, such as 8.0. HPLC stationary phases can be characterized with the average particle diameter and the distribution of particle size. Smaller average diameter and narrow particle size distribution generally enhances the efficacy of separation. The average particle diameter can be calculated with different methods ... 

Only the silica-based stationary phases with covalently bonded alkyl chain, cyano and propylamino ligands have found practical applications in HPLC. Besides these common ligands, the experimental use of naphthalene, pyrene and nitroaromatic as ligands has also been reported. Silica-based stationary phases with covalently bonded cyclodextrins or cyclodextrin derivatives have been frequently employed in the separation and quantitative determination of isomer pairs. 

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