Silica alternatives

Column temperature affects the relative retention of different compounds and elevated temperature permits high-speed chromatography to be conducted.25 Figure 25-28 suggests a systematic procedure for method development in which solvent composition and temperature are the two independent variables.1 For elevated temperature operation, pH should be below 6 to retard dissolution of silica. Alternatively, zirconia-based stationary phases work up to at least 200°C. 

Some materials tend to form granules when sieved. Coating the particles in order to reduce cohesiveness can often reduce this effect. Powders may, for example, be shaken in a container with 1% fatty acid (stearic acid is often used) or fumed silica. Alternatively, the powder may be sieved wet. The addition of 0.1% sub-sieve carbon black has been found useful (although rather messy) for eliminating electrostatic charge. 

Load the sample as a solution, in the same solvent as used for pre-elution, in an even layer onto the surface of the silica. Alternatively, if the sample is insoluble in the pre-elution solvent, it can be pre-adsorbed onto a small quantity of silica (see Section 11.5.1, loading a flash column)) which is then spread on the surfece of the silica in the funnel. 

Ion pairing agents in liquid-liquid systems in reversed-phase mode have included dihydrogenphos-phate for separation of tricyclic amines, octyl sulfate for catecholamines, and tetrabutylammonium for aromatic carboxylates and anions of sulfonamides, to exemplify some of the comparatively few applications. Liquid stationary phases coated on the alkyl-bonded phase include 1-pentanol, butyronitrile, and tributylphosphate. In normal-phase liquid-liquid ion pair chromatography aqueous perchlorate solution has been coated on to silica particles for ion pair separation of catecholamines and related compovmds and tetrabutylammonium ion at neutral pH for carboxylates and anions of sulfonamides. The organic mobile phase often contained dichloromethane and butanol. In the normal-phase mode on silica alternative separation systems have been described with aqueous perchloric acid in methanol added to dichloromethane as mobile phase for separation of amines such as drug substances. This is not an extensively utilized, but quite useful, kind of separation, which has been named ion pair adsorption chromatography. 

The furnace and thermostatic mortar. For heating the tube packing, a small electric furnace N has been found to be more satisfactory than a row of gas burners. The type used consists of a silica tube (I s cm. in diameter and 25 cm. long) wound with nichrome wire and contained in an asbestos cylinder, the annular space being lagged the ends of the asbestos cylinder being closed by asbestos semi-circles built round the porcelain furnace tube. The furnace is controlled by a Simmerstat that has been calibrated at 680 against a bimetal pyrometer, and the furnace temperature is checked by this method from time to time. The furnace is equipped with a small steel bar attached to the asbestos and is thus mounted on an ordinary laboratory stand the Simmerstat may then be placed immediately underneath it on the baseplate of this stand, or alternatively the furnace may be built on to the top of the Simmerstat box. 

The discrepancy between the pore area or the core area on the one hand and the BET area on the other is proportionately larger with silica than with alumina, particularly at the higher degrees of compaction. The fact that silica is a softer material than alumina, and the marked reduction In the BET area of the compact as compared with that of the loose material, indicates a considerable distortion of the particles, with consequent departure of the pore shape from the ideal of interstices between spheres. The factor R for cylinders (p. 171), used in the conversion to pore area in the absence of a better alternative, is therefore at best a crude approximation. 

Analyses of alloys or ores for hafnium by plasma emission atomic absorption spectroscopy, optical emission spectroscopy (qv), mass spectrometry (qv), x-ray spectroscopy (see X-ray technology), and neutron activation are possible without prior separation of hafnium (19). Alternatively, the combined hafnium and zirconium content can be separated from the sample by fusing the sample with sodium hydroxide, separating silica if present, and precipitating with mandelic acid from a dilute hydrochloric acid solution (20). The precipitate is ignited to oxide which is analy2ed by x-ray or emission spectroscopy to determine the relative proportion of each oxide. 

Filter aids should have low bulk density to minimize settling and aid good distribution on a filter-medium surface that may not be horizontal. They should also be porous and capable of forming a porous cake to minimize flow resistance, and they must be chemically inert to the filtrate. These characteristics are all found in the two most popular commercial filter aids diatomaceous silica (also called diatomite, or diatomaceous earth), which is an almost pure silica prepared from deposits of diatom skeletons and expanded perhte, particles of puffed lava that are principally aluminum alkali siheate. Cellulosic fibers (ground wood pulp) are sometimes used when siliceous materials cannot be used but are much more compressible. The use of other less effective aids (e.g., carbon and gypsum) may be justified in special cases. Sometimes a combination or carbon and diatomaceous silica permits adsorption in addition to filter-aid performance. Various other materials, such as salt, fine sand, starch, and precipitated calcium carbonate, are employed in specific industries where they represent either waste material or inexpensive alternatives to conventional filter aids. 

Anthrarufin [l,5-dihydroxy-9,10-anthraquinone] [117-12-4] M 240.1, m 280 (dec), pKj 9.90, pK 11.05. Purified by column chromatography on silica gel with CHCl3/Et20 as eluent, followed by recrystn from acetone. Alternatively recrystd from glacial acetic acid [Flom and Barbara J Phys Chem 89 4489 1985]. 

Diethyl sulfide [352-93-2] M 90.2, m 0 /15mm, 90.1 /760mm, d 0.837, n 1.443. Washed with aq 5% NaOH, then water, dried with CaCl2 and distd from sodium. Can also be dried with MgS04 or silica gel. Alternative purification is via the Hg(II) chloride complex [(Et)2S.2HgCl2] (see dimethyl sulfide). 

Dried with CaH2, then passed through a column of silica gel to remove oleFinic impurities and fractionally distd. Freed from peroxides and moisture by refluxing with sodium, then distilling from LiAlH4. Alternatively, peroxides can be removed by treatment with aqueous ferrous sulfate and sodium bisulfate, followed by solid KOH, and fractional distn from sodium. 

The progress of the reaction was monitored hy injecting after each 24-hour period an aliquot into a gas chromatograph and checking the peak corresponding to isophorone. Alternatively, thin-layer chromatography (E. Merck 0.25-mm. silica gel plates developed with ethyl acetate) can be used,... 

Thus, in this case the simple form of equation (34) is quite adequate. Alternatively, employing a wide pore silica base in LC for separating small molecular weight... 

The second type of interaction, displacement interaction, is depicted in Figure 10. This type of interaction occurs when a strongly polar solute, such as an alcohol, can interact directly with the strongly polar silanol group and displaces the adsorbed solvent layer. Depending on the strength of the interaction between the solute molecules and the silica gel, it may displace the more weakly adsorbed solvent and interact directly with the silica gel but interact with the other solvent layer by sorption. Alternatively, if solute-stationary phase interactions are sufficiently strong, then the solute may displace both solvents and interact directly with the stationary phase surface. 

Where there are multi-layers of solvent, the most polar is the solvent that interacts directly with the silica surface and, consequently, constitutes part of the first layer the second solvent covering the remainder of the surface. Depending on the concentration of the polar solvent, the next layer may be a second layer of the same polar solvent as in the case of ethyl acetate. If, however, the quantity of polar solvent is limited, then the second layer might consist of the less polar component of the solvent mixture. If the mobile phase consists of a ternary mixture of solvents, then the nature of the surface and the solute interactions with the surface can become very complex indeed. In general, the stronger the forces between the solute and the stationary phase itself, the more likely it is to interact by displacement even to the extent of displacing both layers of solvent (one of the alternative processes that is not depicted in Figure 11). Solutes that exhibit weaker forces with the stationary phase are more likely to interact with the surface by sorption. 

Note Rhodamine B is a universal reagent that can be used on silica gel, talc, starch [5] and cellulose layers, just as on urea [1] or silver nitrate-impregnated [7] phases. Liquid paraffin-impregnated silica gel and RP layers are less suitable, since the background to the chromatographic zones is also intensely colored. It is often possible to increase the detection sensitivity by placing the plate in an atmosphere of ammonia after it has been sprayed or dipped, alternatively it can be oversprayed with sodium or potassium hydroxide solution. 

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