Anions relative retention

To accomplish any separation of two cations (or two anions), one of these ions must be taken up by the resin in distinct preference to the other. This preference is expressed by the separation factor (or relative retention), using K+ and Na+ as the example ... 

A high-performance liquid chromatographic method for nalidixic acid on a strong anion-exchange resin column has been reported, using a mobile phase of 0.01 M sodium tetraborate at pH 9.2 and 0.003 M sodium sulfate. The relative retention time for nalidixic acid in the system reported by Sondach and Koch was 0.86 with sulfanilic acid as the standard at... 

Separations based on complex formation The relative retentions for closely similar metal ions often may be enhanced by use of a suitable reagent (such as EDTA) to take advantage of differences in formation constants of the complexes. A classic example is the separation of the lanthanides with a buffered dtrate solution as the eluting agent. Especially effective separations are possible when one metal is converted to an anionic complex while another is present as a cation. Teicher and... 

Fig. 4-18. Comparison of relative retentions of amino acids on anion and cation exchangers.

The retention times of 17 monovalent anions on resins with different functional groups but with almost identical exchange capacities (average 0.027 mequiv/g) were compared with the use of a solution of a monovalent anion (sodium benzoate) as the eluent [8]. Relative retention times were calculated by dividing the measured retention times by that of chloride. Data for resins with various trialkylammonium groups are presented in Table 3.1. 

Table 3.1. Relative retentions of anions on trialkylammonium resins [24].

The data show that the relative retentions of the weak-acid anions are almost independent of the size of the alkyl groups. However, as the size of the R groups increase, large changes occur with the more polarizable anions such as nitrate, iodide, chlorate and BF4 ions. 

Most of the monovalent anions exhibit only small changes in their relative retention on resins containing one, two, or three hydroxyethyl groups, compared to the tri-methylamine resins (Table 3.1). However, when a stronger eluent was used (phthalate,... 

Table 3 Relative retentions of ions on three anion exchangers of differing polaritv. Phthalatc eluent (0.4 mM), pH 5.0.

The TBP resin is quite stable and is suitable for anion chromatography. Warth, Cooper and Fritz [12] compared the retention times of several anions relative to chlo-ride using columns packed with quaternary ammonium anion exchangers of the conventional trimethyl type (TMA) and tributylamine (TBA). The selected results in Table 3.5 show that bromide, nitrate, chlorate and iodide are retained more strongly by the TBP resin. 

Table 3.5. Relative retention times (min) of selected anions on columns prepared from trimethylamine (TMA), tributylamine (TBA) and tributylphosphine (TBP). Eluent 2.2 mM sodium phthalate at pH 6.0.

Experiments with several different hydrophobic quaternary ammonium salts as coating reagents showed that the adjusted retention times of test anions relative to t for chloride varied somewhat with the chemical structure of the coating [20]. However, it was found that the relative adjusted retention times also varied considerably... 

Table 3.9. Adjusted retention times for anions relative to chloride. Capacity of trimethylamine XAD-1 was 0.027 mequiv/g theoretical capacity of cetylpyridinium coated resins was 0.050 mcquiv/g. Eluent was 0.2 raM tetrabutylammonium phthalate, pH 6.5. Conductivity detection.

Relative retention times of the sample anions in Table 6.8 show rather small differences from one acid eluent to another in a few cases. However, iodide has relative t that would make separations easier with some eluents than with others. 

Table 6.9 lists the relative retention time of the chloride sample anion with the acid eluents chosen for this study. The adjusted retention time of chloride decreases as the retention time for the eluent anion increases and it also decreases as the amount of ionization of the eluent increases. The most satisfactory separations were obtained with either nicotinic acid or succinic acid as the eluent acid. 

The relative response of an analyte is influenced by two factors (equations 10 and 11) the relative retention of the solute (aj and the fractional coverage of the adsorbent exerted by the probe (0 ). A system with naphthalene-2-sulfonate as the probe is shown in Figure 10, where the dotted lines give the estimated relative response for cationic and anionic solutes [19, 57]. There is an increase from zero to a maximum value in the range 0-1, and the relative response then decreases and approaches an almost constant level. In order to achieve high detection sensitivity, optimization of the retention is more important than a high absorptivity of the probe. 

Chromatographic retention times of a number of anions were compared, with 6.0 mM nicotinic acid and 2.0 mM phthalate (pH 6) as the eluents. In many cases, the length of the spacer arm had very little effect on the relative retention times (relative to Cl ). However, the selected data in Table 3.4 show that the relative retention times of bromide, nitrate, chlorate and iodide decreased, while that of sulfate increased slightly. 

A change in pFI or salt type in ion-exchange gradient elution will often affect the band spacing for peptide or protein samples. Examples of this are shown in Fig. 16 for a mixture of five proteins. The relative retention of bands 3 and 4 can be reversed by either a change in pH or a change in the salt anion from sulfate to chloride. 

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