Anion chromatography eluents

The general requirement for an anion chromatography eluent is that the eluting anion must have useful affinity for anion-exchange resins. Since it is important to minimize the conductivity of the eluent, the preferred anion is one which has a pKa... 

Yamamoto, A., Inoue, Y., Kodama, S., and Matsunaga, A., Capacity gradient anion chromatography with a borate complex as eluent, ]. Chromatogr. A, 850,... 

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. 

In nonsuppressed anion chromatography, the conductivity of the analyte anion is higher than that of the eluent, so conductivity increases when analyte emerges from the column. Detection limits are normally in the mid-ppb to low-ppm range but can be lowered by a factor of 10 by using carboxylic acid eluents instead of carboxylate salts. 

Lamb, J. D., Smith, R. G., and Jagodzinski, J. (1993) Anion Chromatography With a Crown Ether-based Stationary Phase and an Organic Modifier in the Eluent, J. Chromatogr. 640, 33-40,. 

Shintani and Dasgupta [32] have reported that post-suppression membrane-based ion exchange chromatography with fluorescence detection permits detection limits superior to those obtained by conductivity detection in hydroxide eluent suppressed anion chromatography... 

The eluent used in anion chromatography contains an eluent anion, E". Usually Na" or will be the cation associated with E". The eluent anion must be compatible with the detection method used. For conductivity, the detection E should have either a significantly lower conductivity than the sample ions or be capable of being converted to a non-ionic form by a chemical suppression system. When spectrophoto-metric detection is employed, E will often be chosen for its ability to absorb strongly in the UV or visible spectral region. The concentration of E in the eluent will depend on the properties of the ion exchanger used and on the types of anions to be separated. Factors involved in the selection of a suitable eluent are discussed later. 

Two important conclusions may be drawn from these tables. One is that the hydroxide ion is a much weaker eluent for anion chromatography than carbonate. The second is that the eluting power of sodium hydroxide is enhanced considerably by using latexes with one or two hydroxyethyl groups instead of those containing only alkyl groups. 

Gjerde and Benson discovered that post-column addition of a suspension of sulfo-nated polystyrene particles may be used to reduce the background conductance of basic eluents used in anion chromatography [7]. The eluent cation (typically Na" ) is also replaced in the analyte ion bands by the more highly conducting as the counterion to a sample anion. Since the added reagent is a solid, it is invisible to detectors that respond only to the liquid phase, for example, conductivity and potentio-metric detectors. 

The major eluents that have been used for suppressed anion chromatography are listed in Table 6.6. These are all basic eluents that produce a suppressor product that is a very weak acid and therefore one that has a very low conductivity. The ultimate eluent in terms of suppressed conductivity detection is the hydroxide ion. which gives water as the suppressor product. 

Both inorganic and organic anions may be determined by anion chromatography with direct UV detection. Figure 6.14 shows a separation of nucleotides with 25 mM citric acid buffered at pH 5.4 as the eluent. A gradient elution with increasing amounts of acetonitrile was used to speed up elution of the later peaks. 

Miura and Fritz investigated the use of polycarboxylic acid salts as eluents in anion chromatography [19]. At alkaline pH values, 13,5-benzenetricarboxylic acid (BTA)... 

A simple but useful example of pulsed-amperometric detection is shown in Fig. 6.21 where glucose, fructose and a trace of sucrose are determined in honey by anion chromatography. Much more complex samples can be resolved using gradient elution. This is demonstrated in Fig. 6.22 where 18 carbohydrates were separated. Elution of the later peaks is speeded up by gradually reducing the eluent pH to inhibit ionization of the carbohydrates. However, post-column addition of 0.4 M sodium hydroxide was needed to restore the effluent to a pH sufficiently alkaline for effective pulsed amperometric detection. 

D. T. Gjerde, J. S. Fritz and G. Schmuckler, Anion chromatography with low-conductivity eluents,... 

As in anion chromatography, the IC separation of organic cations has long been known, or at least suspected, that its mechanism involved more than simple ion exchange. Hoffman and co-workers [9,10] have shown that two mechanisms occur in such cases ion exchange and hydrophobic interaction between the sample cations and the resin matrix. For example, these authors showed that the slopes of the linear plot of log k vs. carbon number for protonated amine cations decrease going from 30 % acetonitrile (70 % water) to 70 % acetonitrile in the eluent. This is due to lower hydro-phobic interaction in the 70 % acetonitrile. 

Risner, C. H., Quantitation of some tobacco anions by eluent suppressed anion exchange chromatography using conventional liquid chromatographic equipment. Tobacco Set, 1986, 85-90,... 

Jardy, A., Caude, M., Diop, A., Curvale, C., and Rosset, R., Single-column anion chromatography with indirect UV detection using pyromelhtate buffers as eluents, J. Chromatogr., 439, 137-149, 1988. 

Humic acids, which often accumulate on the column in anion chromatography, can be removed by a combination of organic solvents and an acidic medium. However, these eluents can be used only with the newer generation columns that are compatible with organic solvents. Check the manufacturer s recommendations ... 

Commercial ion chromatograph instruments have become available since early 1976. Ion chromatography (IC) is a combination of ion exchange chromatography, eluent suppression and conductimetric detection. For anion analysis, a low capacity anion exchange resin is used in the separator column and a strong cation exchange resin in the H+ form is used in the suppressor column. 

Another suitable eluent for anion chromatography is a solution containing boric acid and a polyol, gluconic acid. These substances combine to give acids that are stronger than boric acid. At pH 8.5, the usual pH for borate-gluconate eluents, most of the acids are ionized and the eluting power is adequate. The conductivity of this eluent is low, and rises when other anions are eluted. 

Figure 23 7 Electrolytic suppressor for anion chromatography replaces KOH eluent with H2O.

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