Anion exchangers relative retention

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... 

Considering these reactions, it is apparent that an increase in the mobile phase counter ion concentration (Na+ or SO3 ) will proportionately decrease the retention of the sample ion. The pH of the mobile phase is very important as it can affect the relative ionisation of acids or bases, i.e. a higher pH leads to increased ionisation of acids on anion exchangers while a lower pH favours increased ionisation and retention of bases on cation exchangers. 

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 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. 

Bonded-phase silica materials are the most widely used particles for SPE. Since the technology of bonded-phase silica packings for LC columns was already well developed, it was relatively easy for suppliers to offer similar materials in a form suitable for SPE. The most common types are listed in Table 1. Octadecylsilane, or sometimes octylsilane, particles are used for retention of relatively hydro-phobic organic analytes from predominately aqueous samples. A more polar particle, such as cyano-propylsilane, is selected for SPE of less hydrophobic analytes. Cation- and anion-exchange resins are used in small precolumns to concentrate sample ions from very dilute aqueous samples prior to their... 

Polarizable anions are often found to give relative long retention times and tailed peaks when chromatographed on anion exchangers with a polydivinylben-zene polymeric matrix [2, 3]. This is most likely the result of hydrophobic interaction of the sample anions with the resin. Incorporation of about 10% methanol into the mobile phase has been used to alleviate this difficulty. 

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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