Ion separation methods

An electro-osmotic ion separation method was patented by D. R. Dewey and E. R. Gilliland (37). 

The concept of mass spectrometry is relatively simple A compound is ionized (ionization method), the ions are separated on the basis of their mass/charge ratio (ion separation method), and the number of ions representing each mass/charge unit is recorded as a spectrum. For instance, in the commonly used electron-impact (El) mode, the mass spectrometer bombards molecules in the vapor phase with a high-energy electron beam and records the result as a spectrum of positive ions, which have been separated on the basis of mass/charge (mlz). ... 

An important adaptation of the FA technique comprises the implementation of ion separation methods, which allows for more advanced flow drift tubes and selected ion flow tubes (SIFT) [105, 106] (see below). More recently, flow drift tubes (see Sect. 4.6) and flowing atmospheric pressure afterglow (FAPA) devices [107] have been developed. 

Finally, micellar systems are useful in separation methods. Micelles may bind heavy-metal ions, or, through solubilization, organic impurities. Ultrafiltration, chromatography, or solvent extraction may then be used to separate out such contaminants [220-222]. 

The wastes from uranium and plutonium processing of the reactor fuel usually contain the neptunium. Precipitation, solvent extraction, ion exchange, and volatihty procedures (see Diffusion separation methods) can be used to isolate and purify the neptunium. 

These values, which match experience, suggest that distillation should be the preferred separation method for feed concentrations of 10—90%, but is probably a poor choice for feed concentrations of less than 1%. Techniques such as adsorption (qv), chemical reaction, and ion exchange (qv) ate chiefly used to remove impurity concentrations of <1%. 

An on-line concentration, isolation, and Hquid chromatographic separation method for the analysis of trace organics in natural waters has been described (63). Concentration and isolation are accompHshed with two precolumns connected in series the first acts as a filter for removal of interferences the second actually concentrates target solutes. The technique is appHcable even if no selective sorbent is available for the specific analyte of interest. Detection limits of less than 0.1 ppb were achieved for polar herbicides (qv) in the chlorotriazine and phenylurea classes. A novel method for deterrnination of tetracyclines in animal tissues and fluids was developed with sample extraction and cleanup based on tendency of tetracyclines to chelate with divalent metal ions (64). The metal chelate affinity precolumn was connected on-line to reversed-phase hplc column, and detection limits for several different tetracyclines in a variety of matrices were in the 10—50 ppb range. 

Nonionic surfactants, including EO-PO block copolymers, may be readily separated from anionic surfactants by a simple batch ion exchange method [21] analytical separation of EO-PO copolymers from other nonionic surfactants is possible by thin-layer chromatography (TLC) [22,23] and paper chromatography [24], and EO-PO copolymers may themselves be separated into narrow molecular weight fractions on a preparative scale by gel permeation chromatography (GPC) [25]. 

Separation techniques may have to be applied if the given sample contains substances which act as interferences (Section 21.10), or, as explained above, if the concentration of the element to be determined in the test solution is too low to give satisfactory absorbance readings. As already indicated (Section 21.10), the separation methods most commonly used in conjunction with flame spectrophotometric methods are solvent extraction (see Chapter 6) and ion exchange (Chapter 7). When a solvent extraction method is used, it may happen that the element to be determined is extracted into an organic solvent, and as discussed above it may be possible to use this solution directly for the flame photometric measurement. 

Research should continue on traditional separation methods. For example, there is a continuing need for more selective extraction agents for liquid-liquid and ion-exchange extractions. High-temperature processes that use liquid metals or molten salts as extraction agents should have potential in nuclear fuel reprocessing and... 

Peschanski, using the isotopic method ( ° Hg), has found complete exchange (0 °C) in methanol and various other non-aqueous media. The separation methods used were, (a) paper and column chromatography, (b) paper electrophoresis, and (c) precipitation of Hg(I) with chloride. In the presence of cyanide ions, however, less than complete exchange could be observed. Zero-time exchange was again found to vary in the same manner as for aqueous media. Similar effects were observed in the presence of chloride ions. 

Krishnamurty and Wahl tried a number of separation methods and eventually used a modified dipyridyl-ammonia separation to obtain kinetic data for this exchange. The zero-time exchange lay between 25 and 50 %, depending on the conditions the activity of the V(dipy)3 ion was measured. The rate law obtained for perchlorate media was... 

Anderson and Bonner made the first detailed kinetic study on the exchange using the isotopic method ( Cr) and a separation method based on the conversion of Cr(II) into Cr(IIl) oxalate and an ion-exchange treatment. To prevent oxidation of Cr(II) during exchange a hydrogen atmosphere was maintained over the reaction mixture. The rate law found to be obeyed for the concentration ratio range Cr(III)/Cr(II) of between 3.3 x 10 and 2.0 in perchlorate media was... 

The radioisotope Cr was used to tag the species and the separation of Cr from CrX " was achieved using an ion-exchange method, after oxidation of the Cr to Cr with Fe(III) or oxygen. The reactions were carried out in the absence of oxygen in perchlorate media. For the systems involving chloride, fluoride, and azide . King et al. have found a rate law... 

Separation of the products was achieved using an ion-exchange method, the reaction taking place in sealed vessels with the Cr(Vl) labelled ( Cr). Under various conditions only 40% of the Cr appears in the product Cr2(OH)2. This led these workers to propose a modified mechanism in which the step involving Cr(V) proceeds in two ways... 

The exchange of Mo between the anions Mo(CN)g and Mo(CN)g has been investigated by the isotopic method ( Mo) and the separation methods (a) precipitation of Mo(CN)g with either ethanol or cadmium ions, and (b) precipitation of Mo(CN) with tetraphenylarsonium chloride. Complete exchange was observed by Wolfgang even with reactant concentrations 5x10 M. An estimate of the rate coefficient at 2 °C of >10 l.mole . sec has been sug-geMd. 

Silverman and Dodson made the first detailed isotopic study of this exchange system using the separation afforded by the addition of 2,2 -dipyridyl at pH 5, followed by the precipitation of the ferric iron with either ammonia or 8-hydro-xyquinoline. Dodson , using this separation method, had previously obtained an overall rate coefficient of 16 l.mole" sec at 23 °C for 0.4 M perchloric acid media. The exchange in perchlorate and perchlorate-chloride media was found to conform to a rate law, first order with respect to both total ferrous and ferric ion concentrations, with an observed rate constant (k bs) dependent on the hydrogen-ion concentration, viz. 

Wahl and Deck were able to obtain an estimate of an assumed second-order rate coefficient ( 10 l.mole" .sec at 4°C) using a separation procedure based on the extraction of Fe(CN)e by a chloroform solution of Ph AsCl, in the presence of the ions Co(CN)g and Ru(CN)6, to reduce the exchange between the iron species in the two liquid phases. A similar estimate was obtained using a precipitation method in the presence of the carrier Ru(CN)6. A direct injection technique was used as short reaction times were necessary. Wahl has reviewed the large induced exchanges occurring in the chemical separation methods. The extraction procedure when the carriers Co(CN)6 and Ru(CN) are present provides the most satisfactory method of separation. ... 

The earliest investigation of the exchange reaction between the aquated ions of Co(III) and Co(II) was carried out by Hoshowsky et al., using the isotopic method ( Co). When sulphate salts ( 10 M) were employed, complete exchange was observed between the two oxidation states of cobalt, in a time of less than two min. Two separation methods were employed (a) adsorption on an alumina column, and (b) precipitation of the Co(III) as the cobaltinitrite. 

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