Short alkyl silica

On the other hand, the results of several systematic studies of the parameters which govern retention in this type of chromatography were consistent with the predictions of the model for dynamic ion-exchange. Knox and Laird (53) examined the effect of hetaeron concentration on the retention of sulfonic acids and related dyestuffs on a short alkyl silica (SAS) stationary phase. The hetaeron used was cetrimide (cetyltrimeth-... 

Fig. 1. (A) Separation of a mixture of dimethylphthalate, di-n-butylphthalate, and pyrene by reverse-phase HPLC using methanol water (85 15) as eluant comparison of HPLC profiles on ODS-silicas from different manufacturers. (B) Separation of dimethyl- and di-n-butylphthalate on short alkyl silicas (methanol water (85 15) as eluant). Trademarks Spherisorb Phase Separations Limited, U.K. Bondapak Waters Associates Inc., Massachusetts, U.S.A. Hypersil Shandon Southern Products Limited, U.K. Zorbax E. I. DuPont de Nemours, Wilmington, Delaware, U.S.A. LiChrosorb E. Merck, Darmstadt, West Germany Partisil Whatman Inc., New Jersey, U.S.A. These figures are used by kind permission of Hichrom, manufacturers of HPLC columns (% Bennett and Co., Brimpton Common, Reading, Berkshire, U.K.).

In recent years, a small number of new phases have become available that are more suited to CEC. Phase Separations (Deeside. UK) have produced a SCX stationary phase. This is a strong cation-exchange material which contains aminopropyl-derivatised silica that has sulphonic acid groups covalently attached to the amino end of a short alkyl chain. The sulphonic acid groups are effectively ionised at all working pHs due to their low p/faS. Fig. 4.5 shows the dependence of EOF on pH of the mobile phase for a capillary packed with Phase Separations SCX stationary phase. The EOF is almost the same over the whole range. It increases beyond pH 7, presumably due to the added ionisation of the surface silanols. 

Supports for HIC have short alkyl chains or phenyl functionalities, the length of which is related to retention. The matrix can be either silica or polymer, as it is totally covered by the bonded phase and, thus, not ex-... 

Some of the more popular ones are phases with shorter alkyl chains such as Cs (8 carbons) or C4 (4 carbons). Specialized columns with chiral stationary phase are also available for the separation of enantiomers (mirror image isomers). Bonded stationary phases, which are polar, also exist common ones are cyano, phenyl, and amino columns, which have respectively GN, phenyl, and NH2 groups bonded to the silica support via short alkyl linker chains. These latter columns as well as HPLG columns packed with unbonded silica all have specialty uses but it is estimated approximately 80% of all HPLG separations are done using some form of reversed-phase, most commonly Gig because of its versatility and reproducibility. 

One of the most popular types of HILIC columns today is based on a grafted polymeric layer with sulfoalkylbetaine zwitterionic moieties on wide-pore silica, containing both strongly acidic sulfonate groups and strongly basic quaternary ammonium groups separated by a short alkyl spacer. 

Similar reactions form the basis of the separation of cations. An example of the separation of inorganic anions at the ppm level is shown in Figure 3(b). Size exclusion chromatography (SEC). This is suitable for mixtures of solutes with relative molecular masses (J(MM) in the range lOMO Da. Stationary phases are either microparticulate cross-linked co-polymers of styrene and divinyl benzene with a narrow distribution of pore sizes, or controlled-porosity silica gels, usually end-capped with a short alkyl chain reagent to prevent adsorptive interactions with solutes. Exclusion is not a true sorption mechanism because solutes do not interact with the stationary phase (Topic D2). They can be divided into three groups ... 

The main components of FCC catalysts are Zeolite Y, e.g., REY orUSY as the major active component (10 to 50%), and a binder that is typically an amorphous alumina, silica-alumina, or clay material. In addition to these main components, other zeolite components, e.g., ZSM-5, and other oxide or salt components are quite frequently used additives in the various FCC catalysts available on the market. The addition of 1 to 5% ZSM-5 increases the octane number of the gasoline. ZSM-5 eliminates feed compounds with low octane numbers because it preferentially center-cracks n-paraffins producing butene and propene [14], These short-chain olefins are then used as alkylation feedstocks... 

Reversed-phase chromatography employs a nonpolar stationary phase and a polar aqueous-organic mobile phase. The stationary phase may be a nonpolar ligand, such as an alkyl hydrocarbon, bonded to a support matrix such as microparticulate silica, or it may be a microparticulate polymeric resin such as cross-linked polystyrene-divinylbenzene. The mobile phase is typically a binary mixture of a weak solvent, such as water or an aqueous buffer, and a strong solvent such as acetonitrile or a short-chain alcohol. Retention is modulated by changing the relative proportion of the weak and strong solvents. Additives may be incorporated into the mobile phase to modulate chromatographic selectivity, to suppress undesirable interactions of the analyte with the matrix, or to promote analyte solubility or stability. 

A solution of 1 equiv of (S)- or (/ )-2-methoxymetliyl-1-[(2,2-dimethyl-l,3-dioxan-5-ylidene)amino]pyrro-lidine in THF (4 mL/mmol) is cooled to — 78 °C. 1.1 Equiv of tert-butyllithium in hexane (1.7 M) are added dropwise and the mixture is stirred for 2 h at — 78 °C. The solution of the metalated hydrazone is cooled to — 100 CC, 1.2 equiv of the alkyl halide (neat or as a solution in anhyd THF) are added dropwise, and the mixture is stirred for 1 h at —100 °C and then warmed slowly to r.t. (about 15 h). Finally, diethyl ether (30 mL/mmol) is added and the mixture is washed with pH 7 buffer (3 mL/mmol) and two 3-mL portions of brine, dried over MgSO and evaporated under reduced pressure. The Crude product is heated to 50 C for a short time if necessary (about 15 min for isomerization from the Z- to the L-isotiler monitored by TLC) and purified by silica gel column chromatography (diethyl ether/ pentane, 1 1 -2 5 Rf - > RfZ-iso-mer) to give a colorless or pale yellow product. See Table 2 for physical data. 

An aldehyde was mixed with solid supported triphenylphosphine oxide (3 equiv.), alkyl halide (4 equiv.), and potassium carbonate (4 equiv.) in methanol (2 ml). The mixture was heated at 150° for 5 min. The residue was filtered through a short plug of silica gel and washed. The solution was concentrated and purified by reverse-phase high-performance liquid chromatography (RP-HPLC). 

A high-yielding (84—90%) method for the N-alkylation of fused bicyclic /3-lactams uses cesium carbonate supported on silica gel in the absence of a solvent at room temperature for relatively short times (1-8h). The procedure gives higher yields than conventional techniques and in a shorter time <2005MI367>. 

Apolar stationary phases suffer from hydrolytic instability at pH extremes. The use of mixed phases of long (Cg, Clg) and short (C, C3) chain alkyls produces stationary phases with increased hydrolytic stability.7,8 Crowding of the long alkyl chains does not allow the alkylsilane molecules to deposit in close packing on a smooth or flat surface. Silane molecules polymerize in vertical direction, loosing contact with the silica surface. The insertion of short chain alkyls allows horizontal polymerization of the silane molecules. Thus, alkyl chains are aligned in a parallel way. The stability of the silane layer is increased consequently (figure 8.1). 

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