Anion exchange techniques

Rubidium acid salts are usually prepared from rubidium carbonate or hydroxide and the appropriate acid in aqueous solution, followed by precipitation of the crystals or evaporation to dryness. Rubidium sulfate is also prepared by the addition of a hot solution of barium hydroxide to a boiling solution of rubidium alum until all the aluminum is precipitated. The pH of the solution is 7.6 when the reaction is complete. Aluminum hydroxide and barium sulfate are removed by filtration, and rubidium sulfate is obtained by concentration and crystallization from the filtrate. Rubidium aluminum sulfate dodecahydrate [7488-54-2] (alum), RbA SO 12H20, is formed by sulfuric acid leaching of lepidolite ore. Rubidium alum is more soluble than cesium alum and less soluble than the other alkali alums. Fractional crystallization of Rb alum removes K, Na, and Li values, but concentrates the cesium value. Rubidium hydroxide, RbOH, is prepared by the reaction of rubidium sulfate and barium hydroxide in solution. The insoluble barium sulfate is removed by filtration. The solution of rubidium hydroxide can be evaporated partially in pure nickel or silver containers. Rubidium hydroxide is usually supplied as a 50% aqueous solution. Rubidium carbonate, Rb2C03, is readily formed by bubbling carbon dioxide through a solution of rubidium hydroxide, followed by evaporation to dryness in a fluorocarbon container. Other rubidium compounds can be formed in the laboratory by means of anion-exchange techniques. Table 4 lists some properties of common rubidium compounds. 

Anion-exchange resins have been used (Wright and Schnitzer, 1960) in an attempt to fractionate soil humic substances. Some of the humic material is readily retained and a fractionation can be achieved by elution with a salt gradient and/or an alkaline reagent (usually NaCl and NaOH, respectively). In theory the anion-exchange technique should work well, but in practice the... 

Fig A impure DEQUES 2060 Pilot Scale Fractionation Linear Gradient Elution Anion Exchange Technique... 

Hplc techniques are used to routinely separate and quantify less volatile compounds. The hplc columns used to affect this separation are selected based on the constituents of interest. They are typically reverse phase or anion exchange in nature. The constituents routinely assayed in this type of analysis are those high in molecular weight or low in volatility. Specific compounds of interest include wood sugars, vanillin, and tannin complexes. The most common types of hplc detectors employed in the analysis of distilled spirits are the refractive index detector and the ultraviolet detector. Additionally, the recent introduction of the photodiode array detector is making a significant impact in the analysis of distilled spirits. 

For the analysis of americium in water, there is a broad array of sample preparation and detection methodologies that are available (see Table 7-2). Many of the common and standardized analytical methodologies typically include the minimization of sample volume, purification through co-precipitation, anion exchange column chromatography, and solvent extraction techniques followed by radiochemical detection of americium in the purified sample. Gross alpha analysis or liquid scintillation are common... 

Despite its potential importance, formic acid has proven difficult to quantify at submicromolar levels in non-saline water samples. Formidable analytical difficulties are associated with its detection in highly saline samples. Ion exclusion, anion exchange, and reversed-phase high performance liquid chromatography techniques based on the direct detection of formic acid in aqueous samples are prone to interferences (especially from inorganic salts) that ultimately limit the sensitivity of these methods. 

Falkner and Edmond [334] determined gold at femtomolar quantities in seawater by flow injection inductively coupled plasma quadrupole mass spectrometry. The technique involves preconcentration by anion exchange of gold as a cyanide complex, [AulCNjj], using 195Au radiotracer (ti/2 = 183 days) to monitor recoveries. Samples are then introduced by flow injection into an inductively coupled plasma quadrupole mass spectrometer for analysis. The method has a detection limit of 10 fM for 4 litres of seawater preconcentrated to 1 ml, and a relative precision of 15% at the 100 fM level. 

Various chromatographic techniques may be utilized to purify urokinase further. Commonly employed methods include anion-exchange (DEAE-based) chromatography, gel filtration on Sephadex G-100 and chromatography on hydroxyapatite columns. Urokinase is a relatively stable molecule. It remains active subsequent to incubation at 60 °C for several hours, or brief incubation at pHs as low as 1.0 or as high as 10.0. 

Several limitations on the synthetic techniques that can be employed are imposed by the need for rapidity and minimization of handling because of the radiation hazard, and the low concentration and small physical quantities of the compounds. Purification steps should be eliminated if possible by optimizing yields. Where purification is unavoidable, simple procedures are employed such as use of anion exchange columns to remove perrhenate (the most common contaminant in the final product). A variety of disposable sample preparation columns are well suited to this purpose and are available containing small quantities of anion or cation exchange materials (0.1 to 0.5 g typically) such as quaternary ammonium-, primary ammonium-, or sulfonate-derivatized silica. Reversed phase columns are also often used (C8 or C18-derivatized silica). The purification is often thus reduced to a simple filtration step which can be performed aseptically. 

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