Separation of anions

Figure 3 Gradient separation of anions using suppressed conductivity detection. Column 0.4 x 15 cm AS5A, 5 p latex-coated resin (Dionex). Eluent 750 pM NaOH, 0-5 min., then to 85 mM NaOH in 30 min. Flow 1 ml/min. 1 fluoride, 2 a-hydrox-ybutyrate, 3 acetate, 4 glycolate, 5 butyrate, 6 gluconate, 7 a-hydroxyvalerate, 8 formate, 9 valerate, 10 pyruvate, 11 monochloroacetate, 12 bromate, 13 chloride, 14 galacturonate, 15 nitrite, 16 glucuronate, 17 dichloroacetate, 18 trifluoroacetate, 19 phosphite, 20 selenite, 21 bromide, 22 nitrate, 23 sulfate, 24 oxalate, 25 selenate, 26 a-ketoglutarate, 27 fumarate, 28 phthalate, 29 oxalacetate, 30 phosphate, 31 arsenate, 32 chromate, 33 citrate, 34 isocitrate, 35 ds-aconitate, 36 trans-aconitate. (Reproduced with permission of Elsevier Science from Rocklin, R. D., Pohl, C. A., and Schibler, J. A., /. Chromatogr., 411, 107, 1987.)...

Green, E. D. and Baenziger, J. U., Separation of anionic oligosaccharides by high-performance liquid chromatography, Anal. Biochem., 158, 42, 1986. 

Madden, J. E. and Haddad, P. R., Critical comparison of retention models for the optimization of the separation of anions in ion chromatography II. Suppressed anion chromatography using carbonate eluents, /. Chromatogr. A, 850, 29, 1999. 

Counter-ions which are frequently used include tetrabutylammonium phosphate for the separation of anions and hexane sulphonic acid for cations. The appropriate counter-ions are incorporated in the solvent, usually at a concentration of about 5 mmol 1" and the separation performed on the usual reverse phase media. This ability to separate ionic species as well as non-polar molecules considerably enhances the value of reverse-phase chromatography. 

The use of surface-enhanced resonance Raman spectroscopy (SERRS) as an identification tool in TLC and HPLC has been investigated in detail. The chemical structures and common names of anionic dyes employed as model compounds are depicted in Fig. 3.88. RP-HPLC separations were performed in an ODS column (100 X 3 mm i.d. particla size 5 pm). The flow rate was 0.7 ml/min and dyes were detected at 500 nm. A heated nitrogen flow (200°C, 3 bar) was employed for spraying the effluent and for evaporating the solvent. Silica and alumina TLC plates were applied as deposition substrates they were moved at a speed of 2 mm/min. Solvents A and B were ammonium acetate-acetic acid buffer (pH = 4.7) containing 25 mM tributylammonium nitrate (TBAN03) and methanol, respectively. The baseline separation of anionic dyes is illustrated in Fig. 3.89. It was established that the limits of identification of the deposited dyes were 10 - 20 ng corresponding to the injected concentrations of 5 - 10 /ig/ml. It was further stated that the combined HPLC-(TLC)-SERRS technique makes possible the safe identification of anionic dyes [150],... 

Chiari, M., and Kenndler, E. (1995). Capillary zone electrophoresis in organic solvents-separation of anions in methanolic buffer solutions./. Chromatogr. A 716, 303-309. 

The use of IC for anion wastewater characterization is depicted in Figure 2. The separation of anions in wastewater from a process plant was performed using a standard eluent, 0.003 M NaHC03/0.0024 M Na2C03. Total analysis time was approximately 24 minutes. Two unidentified peaks are present, one elutes between F and Cl and the second is partially resolved from the large Cl peak. The second unknown may be C03 2, as exposure of the sample to air significantly reduced the response. N02 may also be present and would be unresolved from the C03 2 under these conditions. 

Analysis of non-steroidal anti-inflammatory drugs (NSAIDs) by CE and separation of anions on the basis of ionic radius... 

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

The separation of anions X can be used as another example. If the mobile phase contains diluted sodium hydroxide, a membrane allowing the diffusion of cations is used. The passage of hydronium ions towards the electrolyte will neutralise OH-ions. At the anode, Na+ ions will migrate and react with OH- ions. 

A step forward in simplification of multi-column systems involves the use of anion exchange and cation exchange columns connected in series. Such systems can be readily organized inside a standard isocratic ion chromatograph. In this approach a single eluent is used for separation of anions and cations and ideally the eluted analytes are detected with a single detection unit. In this way Takeuchi et al. [31 separated a mixture of Na+,... 

Fig. 3. IEC/CEC separation of anions and cations. Column TSK-Gel OA-Pak A (300 X 7.8 mm, 5 jam) eluent 5 mM malic acid-methanol (95 5) flow rate 1.2 ml/min sample volume 25 jal detection conductivity. Peaks (1) sulfate, (2) chloride, (3) nitrate, (4) fluoride, (5) sodium, (6) ammonium, (7) potassium, (8) magnesium, (9) calcium. Reprinted with permission from [19].

Capacity gradients can be achieved in another way in IC by changing the temperature of the macrocycle-based column. Since the reaction of cryptands with metal cations is typically exothermic, raising the temperature reduces the degree of complexation. Based on this concept, gradient separations of anions can be achieved using a 2.2.2 column when the temperature is raised to 80 °C during the separation. [19]... 

Lamb, J. D. and Smith, R. G. (1994) Use of Step Gradients on Different Polymetric Substrates in the Separation of Anions by Macrocycle-based Ion Chromatography, J. Chromatogr. A 671, 89-94. 

Figure 2.10 Separation of anions in a carbonated apple juice using suppressed ion chromatography. Chromatography conditions column, AS11 with AG11 guard detector, CD20 conductivity detector with the ASRS self-regenerating suppressor in the recycle mode. (Courtesy of Dionex Corporation.)...

The retardation of chloride and sulphate through complex formation with cadmium enabling the separation of anions of interest [28] to be carried out, was found to be unsuitable, as the high concentration of cadmium ions necessary to achieve the desired effect led to the loss of fluoride and phosphate (probably owing to precipitation). 

Although, the main emphasis of this chapter lies on the fundamental aspects of calixpyrrole-fluoride complexation reactions, with the major part being devoted to the thermodynamic properties of these systems, calixpyrrole receptors seem to be promising for the development of chemical sensors and for the removal of fluorides from water. They also show promise for the separation of anion substrates. 

As in 1EC, the counterion concentration has a considerable effect on the retention in IPC. In IPC the counterion is charged similar to the solute molecules, but opposite to the pairing ion. For example, for the separation of anionic solutes, the pairing agent may be a sodium sulfonate, in which the sulfonate is the pairing ion and sodium the counterion. The addition of a buffer salt (e.g. sodium phosphate) and a neutral salt (e.g. sodium bromide) may also contribute to the concentration of the counterion. Because of the similar retention mechanism, the counterion concentration has a similar effect on retention and selectivity in RP-IPC as in IEC. 

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