Chromatography Cation

A number of experimental variables can be manipulated in the quest to obtain a good analysis for an analytical sample. Each of these can exert a major effect 

Column. Various functional groups affect selectivity. 

Exchange capacity and column length affect the time required for a separation. Compatibility with organic solvents and pH stabUily also need to be considered. 

Eluent and Detector. When selecting the eluent and the detector for a particular separation, these need to be considered together and not separately. The chemical type and concentration of eluent cation must be able to separate the analyte cations within a reasonable time, but the eluent must also be compatible with the detector. Monovalent separation can be obtained. Gradient elution with a prc ammed change in eluent composition may be needed for more complex samples. 

Colurim Temperature. This is often a neglected parameter. A higher column temperature may improve chromatographic 

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Figure 26-4 Schematic illustrations of (a) suppressed-ion anion chromatography and (b) suppressed-ion cation chromatography. 

Benzene-1,4-diammonium cation is a stronger eluent that can be used instead of H for suppressed-ion cation chromatography. After suppression, a neutral product is formed ... 

Ion-Exchange and Ion Chromatography 26-1. State the purpose of the separator and suppressor in suppressed-ion chromatography. For cation chromatography, why is the suppressor an anion-exchange membrane ... 

Jensen, D. and Joppert, G. (1998) Advances in Analytical Cation Chromatography, LaborPraxis 22, 60-67. 

Figure 7.16 Schematic diagram of HPLC anion and cation chromatography system used with ICP-MS...

J. S. Fritz, D. T. Gjerde and R. M. Becker, Cation chromatography with a conductivity detector,... 

Modem suppressors for cation chromatography are both efficient and self-regenerating. The principles are similar to the suppressors for anion chromatography, described in Chapter 6. The mechanism of suppression for a cation self-regenerating suppressor is illustrated in Fig. 7.1 and described in some detail by Rabin et al. [3]. Suppressors for cation chromatography are limited to those cations that do not form precipitates with the hydroxide ions from the suppressor. 

Riviello et al. made a careful comparison of conductivity changes in cation chromatography between direct- and suppressed conductivity detection [7]. The calculation example is outlined in Fig. 7.5. The change in conductivity, AG, is actually slightly greater with non-suppressed conductivity. Flowever, the noise is much higher in the non-suppressed detection mode. Noise may be defined as the random signal that... 

Separations of metal cations with ionic eluents has been limited mostly to the alkali metals, ammonium, magnesium(II), calcium(Il), strontium(ll) and barium(ll). Separations of other metal cations are usually performed with eluents that complex the sample cations to varying degrees (see Section 7.4). Some organic cations have also been separated with ionic eluents, although this appears to be an under-utilized area of cation chromatography. 

Dumont, Fritz and Schmidt studied cation chromatography in organic solvents containing little if any water [11]. Under these conditions solvation of the lipophilic part of the cation should be sufficient to virtually eliminate the hydrophobic interaction between the sample cations and the ion-exchange resin. In this way the true ion-exchange selectivity could be measured. 

The very weak complexing of iron(ll) by tartrate suggested that iron(II) might be determined quantitatively by cation chromatography. This was proved to be true by Fritz and Sevenich [14] who determined iron(II) in the presence of iron(III) and several other metal ions. Total iron in solution was determined after a preliminary reduction to iron(II) with ascorbic add. 

Neutral organic compounds that cannot exist as cations may be retained by physical adsorption but can be washed off the cation exchange colunm by a brief rinse with an organic solvent. The amine cation can then be eluted from the column with a 1 M solution of trimethylamine in methanol. The trimethylamine converts the amine cation to the free amine which is no longer retained by the cation exchanger. Because of its volatility, trimethylamine is easily removed from the eluate. After acidification, the sample amines can be separated by cation chromatography. 

A method for determination of cations and anions in a single run was described in Section 8.6. The column was packed with a polyacrylate gel cation exchanger. It appears that sample cations were separated by cation chromatography and anions by ion-exclusion chromatography. 

For cation chromatography there are basically two eluent systems used. In the case of monovalent cations, the normal eluent is 0.005M HC1. However, the concentrations of hydrogen ion required for divalent cations are such that hydrogen ion is impractical eluent for divalent cations. Instead, the preferred cation for divalent cations like the alkaline earth cations is m-phenylenediamine dihydrochloride. The divalent nature of m-phenylenediamine makes it an efficient eluent for other divalent cations, while its weakly basic character results in very little conductivity when it is converted to the free base form in the suppressor. 

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