Simultaneous Separation of Anions and Cations

There is no need for the additional hardware (second pump, mixing tee or reaction chamber) commonly used in post-column reaction systems. Consequently, the problems inherent in a typical post-column reaction system are avoided. 

Chromatograms for determination of small amounts of water in several aldehydes are shown in Fig. 8.6. This same two-column method is also useful for other difficult samples. For example, peroxides are highly oxidizing and interfere with the Karl Fischer titrimetric determination of water. Water has been determined by the chromatographic method in several organic peroxides [17]. 

The inability to obtain really dry methanol limits the ability of the liquid chromatographic (LC) method to determine very low concentrations of water. Attempts to remove water from methanol by treatment with molecular sieves or distillation from calcium hydride still gave a product with at least 50 to 150 ppm water. 

Virtually all the water can be removed from methanol by adding an ortho ester, trimethy-lorthoformate (TMOF), and a small amount of sulfuric acid to catalyze the reaction [17]. 

Removal of almost all the water from the methanol reduces the eluent baseline considerably and also increases the height of the water peak for a sample. With this treatment the detection limit is estimated to be 5 ppm water. 

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Baryla, N.E. Lucy, C.A. Simultaneous separation of cationic and anionic proteins using zwitterionic surfactants in capillary electrophoresis. Anal. Chem. 2000, 72 (10), 2280-2289. 

Multi-crown dendrimers (Fig. 3.7-12) were found to exhibit good solubihty in solvents of low polarity, making them attractive as extractants for separation processes. The combination of protonated tertiary amine groups as anion binding sites together with cation-active crown ether moieties represents an approach for the simultaneous binding of cations and anions. Extraction studies performed with sodium pertechnetate and mercury(II) chloride have shown that the guest molecules are mainly bound in the interior of the protonated polyamine skeleton [18]. 

A number of investigators have demonstrated simultaneous detection of cationic and anionic surfactants by CE. Such analyses require compromises in conditions so that cat-ionics are resolved while anionics elute in a reasonable time. Since anionics and cationics are not formulated together, there is rarely a need for their simultaneous determination and these separations are not discussed here. 

Shantsi, S. A., N. D. Danielson, Individual and simultaneous class separations of cationic and anionic surfactants using CE with indirect photometric detection. Anal. Chem., 1995, 67, 4210-4216. 

Simultaneous determination of both cations and anions in acid rain has been achieved using a portable conductimetric ion-exclusion cation-exchange chromatographic analyzer.14 This system utilized the poly(meth-ylmethacrylate)-based weak acid cation exchange resin TSK-Gel OA-PAK-A, (Tosoh , Tokyo, Japan) with an eluent of tartaric acid-methanol-water. All of the desired species, 3 anions and 5 cations, were separated in less than 30 minutes detection limits were on the order of 10 ppb. Simultaneous determination of nitrate, phosphate, and ammonium ions in wastewater has been reported utilizing isocratic IEC followed by sequential flow injection analysis.9 The ammonium cations were detected by colorimetry, while the anions were measured by conductivity. These determinations could be done with a single injection and the run time was under 9 minutes. 

Isotachophoresis. In isotachophoresis (ITP), or displacement electrophoresis or multizonal electrophoresis, the sample is inserted between two different buffers (electrolytes) without electroosmotic flow. The electrolytes are chosen so that one (the leading electrolyte) has a higher mobility and the other (the trailing electrolyte) has a lower mobility than the sample ions. An electric field is applied and the ions start to migrate towards the anode (anions) or cathode (cations). The ions separate into zones (bands) determined by their mobilities, after which each band migrates at a steady-state velocity and steady-state stacking of bands is achieved. Note that in ITP, unlike ZE, there is no electroosmotic flow and cations and anions cannot be separated simultaneously. Reference 26 provides a recent example of capillary isotachophoresis/zone electrophoresis coupled with nanoflow ESI-MS. 

Complexation reactions are assumed to proceed by a mechanism that involves initial formation of a species in which the cation and the ligand (anion) are separated by one or more intervening molecules of water. The expulsion of this water leads to the formation of the inner sphere complex, in which the anion and cation are in direct contact. Some ligands cannot displace the water and complexation terminates with the formation of the outer sphere species, in which the cation and anion are separated by a molecule of water. Metal cations have been found to form stable inner and outer sphere complexes and for some ligands both forms of complexes may be present simultaneously. 

Figure 13.6 Schematic diagram of the dual-end injection CE microchip system with the movable conductivity detector for simultaneous measurements of explosive-related anions and cations, (a) injection mode and (b) separation mode. (a,e) Running buffer reservoirs, (b,d) unused reservoirs, (c, f) sample reservoirs, (g) injected cation plug, (h) injected anion plug, (i) movable contactless conductivity detector, (j—1) cations 1-3, (m-o) anions 1-3. (Reprinted in part with permission from [33]. Copyright 2003 Wiley Interscience.)...

Figure 5. Schematic diagram of the instruments used for simultaneous separation of anions and cations [10].

Separation selectivify is one of the most important characteristics of any chromatographic sfationary phase. The functionality of the cation and anion and their unique combinations result in ILs with not only tunable physicochemical properties (i.e., viscosity, thermal stability, and surface tension), but also unique separation selectivities. Although the selectivity for different analytes is dominated by the solvation interactions imparted by the cation and anion, all ILs exhibit an apparent and xmique dual-nature selectivity that is uncharacteristic of other popular nonionic stationary phases. Dual-nature selectivity provides the stationary phases the ability to separate nonpolar molecules like a nonpolar stationary phase but yet separate polar molecules like a polar stationary phase [7,8]. Typically, GC stationary phases are classified in terms of their polarity (see Section 4.2.2) and the polarity of the employed stationary phase should closely match that of the analytes being separated. ILs possess a multitude of different but simultaneous solvation interactions that give rise to unique interactions with solute molecules. This is illustrated by Figure 4.2 in which a mixture of polar and nonpolar analytes are subjected to separation on a 1-benzyl-3-methylimidazolium triflate ([BeQlm][TfO] IL 6 in Table 4.1) column [21]. 

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