Eluant selection

Depending on the chemistry adopted by the column manufacturer, eluant selection may be limited with respect to pH and type and level of organic solvent that can be tolerated. For example, the choice and level of cross-linking agent in polyvinyl alcohol (PVA)-based packings (e.g., Asahipak) influences both the pH stability and the organic solvent compatibility. In all cases the manufacturer s literature should specify eluant compatibility. 

During the solvent elution of compoimds from the isolated bands of adsorbent, the elution solvent effectively displaces the components from the adsorbent so that all of the component is contained in the first liquid emerging from the sintered glass thimble. Loss of the component by its failure to elute is not usually serious, provided that the eluant selected is one which would elute the particular compounds to the solvent front (ie. R f near unity) on a thin-layer plate. In fact, the behaviour of the component on the plate with plate elution solvents is a good guide to the selection of an appropriate solvent for desorbing the same component from the isolated gel in the extraction thimble. If the elution solvent used is less effective than this, then losses can be expected of the compound at the adsorbent extraction stage. 

To accomplish any separation of two cations (or two anions) of the same net charge, the stationary phase must show a preference for one more than the other. No variation in the eluant concentration will improve the separation. However, if the exchange involves ions of different net charges, the separation factor does depend on the eluant concentration. The more dilute the counterion concentration in the eluant, the more selective the exchange becomes for polyvalent ions. 

The more a deviates from unity for a given pair of ions, the easier it will be to separate them. If the selectivity coefficient is unfavorable for the separation of two ions of the same charge, no variation in the concentration of H+ (the eluant) will improve the separation. 

Isolation procedures for many biochemicals are based on chromatography. Practically any substance can be selected from a crude mixture and eluted at relatively high purity from a chromatographic column with the right combination of adsorbent, conditions, and eluant. For bench scale or for a small pilot plant, such chromatography has rendered alternate procedures such as electrophoresis nearly obsolete. Unfortunately, as size increases, dispersion in the column ruins resolution. To produce small amounts or up to tens of kilograms per year, chromatography is an excellent choice. When the scale-up problem is solved, these procedures should displace some of the conventional steps in the chemical process industries. 

Ion exchange (IX) is a very useful technique for the concentration, the purification and the separation of chemically similar metallic elements present in an aqueous solution. In its most popular form of application, the metal-bearing aqueous solution is passed through a bed of solid organic resin in a particulate form wherein the sorption of the metal ions on the resin takes place by ion-exchange reactions. The pregnant resin is washed free of the entrapped feed solution and then brought into contact with an eluant of suitable composition and volume so that the resin releases the metal ions back to the eluant. The ratio of the volume of the feed and that of the eluant determines the extent of concentration that can be achieved. Purification and separation are achievable if the resin is selective or specific with respect to the metal ions of interest in comparison to impurity ions. 

By gel chromatography on Sephadex G-200, with an eluant consisting of 0.15 M sodium chloride mixed with 0.12 M sodium phosphate buffer solution, pH 7.4 (9 1, v/v), heparin of porcine mucosal origin has been shown to be highly polymolecular.127 No increase in blood anticoagulant activity with increasing molecular size, indicated by the results of an earlier study,128 was observed when the activity of fractions selected from different parts of the elution curve was determined. The anticoagulant potency of the sample studied was,... 

For small molecule analytes (see Note 6) for which a radiotracer form is available, sequentially load a known quantity of tracer dissolved in buffer and determine the amount of analyte in the eluant. When the radioactivity not retained by the immunoaffinity column plateaus, the column binding sites are saturated. Wash the column, and elute the retained radioactivity. The mass of analyte in the eluted volume is the apparent column capacity. In many instances a radio-labeled analyte may not be available. In such cases, high-performance liquid chromatography, UV spectroscopy, or any other analytical tool capable of selectively quantifying the analyte may be used to determine column capacity. 

Although many columns and eluants can be used for gradient elution, the conditions used in Figs. 2.16 and 2.17 are recommended as a good starting point. The HPIC-ASSA (5 i) separator with sodium hydroxide eluant provides the optimum combination of efficiency, selectivity, and speed without an unacceptable baseline slope. If fewer ions than the 36 shown in Fig. 2.16 need to be separated, the gradient steepness can be increased to reduce the run time. 

Ion chromatography is not restricted to the separate analysis of only anions or cations. With the proper selection of eluant and separation columns, the technique can be used for the simultaneous analysis of both anions and cations. 

From these results it can be seen that there is a rapid deactivation of the freshly reduced catalyst, accompanied by extensive carbon laydown in the veiy early stages of the reaction. Analysis after 10 sec. on stream showed that there was a carbon laydown of 48.4% of the propane feed and the only product in the reactor eluant was methane. In all subsequent analyses, methane, ethane, ethene and dihydrogen were detected along with propene and unreacted propane, the catalyst approaching a steady state with respect to conversion and selectivity after ca. 30 min. on stream. 

The first phase selectively separates the n-butenes from a feedstock previously vapo< rized around lOO C. at 0. 10 Pa absolute, by circulation in an upflow stream through the adsorbent The isobutene-rich fraction of the residual C4 cut also contains a heavier hydrocarbon such as hexaDe.-an eluant used in subsequent operations, which is displaced from the pores it initially occupied. This eifruent is cooled to 4CPC and partly condensed. The liquid and gaseous fracuons are separated, and then pumped and compressed respectively at 0.6.10 Pa absolute, to allow the production, by simple distillation, of isobutene at the top and the eluant at the bottom. 

How is selectivity of detection achieved when using a fluorescence detector to detect analytes in an HPLC eluant ... 

Solubility of Exocellular Nickel Complexes in the Soil. To evaluate the effect of microbial metabolites on Ni solubility in soil, the soluble exocellular solution, separated after growth of the 20 selected bacterial isolates and all fungal isolates, was eluted through a column of soil. The concentrations and forms of Ni in the eluates were compared to the concentrations and forms of Ni in water and in the growth medium before and after microbial growth. A column (0.8 x 4 cm) was packed (1.5 g/cc) with sieved (<2 mm), air-dry Ritzville silt loam (1.0 g) and an aliquot of the filtered (<0.4 M<) exocellular medium (2 ml) was eluted through the soil. After 1 ml of eluant was collected, the Ni concentration was determined from the concentration of radio-tracer. Metal form was characterized in soil eluates by TLC and TLE as previously described. 

The net retention of analyte is a function of all the solute-stationary phase, solute-mobile phase and mobile phase-stationary phase interactions that contribute to retention. It follows that if the interactions with the stationary phase are constant then retention and selectivity are only functions of the composition of the mobile phase. The simplest mobile phase will generally be composed of a mixture of two solvents a weak solvent A and a strong solvent B. For example a typical reversed-phase eluant will consist of a mixture of water, the weakest solvent in reversed-phase systems, and a stronger organic modifier such as methanol or acetonitrile. Similarly, a normal-phase eluant will typically comprise a mixture of a weak solvent such as a -heptane and a stronger modifier such as chloroform or ethyl acetate. 

Early separation methods, based on the selective partition of crude taxine between an organic solvent and water solutions of different pH values [31] have been abandoned, and column chromatography on silica gel using mixtures of CHClj-EtOH as eluant is today the method of choice. Further purification can be achieved on TLC or with HPLC. Unlike crude taxine, taxane alkaloids are often crystalline compounds, especially those of the taxine A-type. 

Tetrasubstituted cyclopropenes (e.g. 6) can be selectively cw-hydrogenated with 5% palladium on charcoal if no additional double bonds are present in the molecule. Rapid absorption of the stoichiometric amount of hydrogen leads to stereospecific formation of the cu-configurated cyclopropanecarboxylate derivative 7 which is isolated in high yield (>94%) after chromatography on silica gel with hexane/diethyl ether (2 1) as eluant. 

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