Eluents hydroxide-based

Other eluent systems in suppressed ion chromatography are typically chosen based on specific separation requirements. For routine analysis of monovalent and divalent anions, carbonate-based eluents represent a reasonable alternative to hydroxide-based eluent systems. Carbonate eluents are simple to prepare and can be useful in cases where anion analysis is only occasionally performed. It must be kept in mind, however, that carbonate lowers the detection sensitivity for anionic species and introduces significant nonlinearity into the analysis. ... 

In preparing sodium hydroxide-based eluents, care must be taken to ensure that they are carbonate free. Because the carbonate ion exhibits a much higher elution strength than the hydroxide ion, the presence of even small traces of carbonate reduces the resolution. The eluents should, therefore, be prepared from a 50% NaOH concentrate which only contains minute amounts of carbonate. The de-ionized water being used to... 

Figure 2. Ion chromatographic separation of a series of anions on polymer-based column with adsorbed decyl-2.2.2 using gradient capacity from sodium hydroxide to lithium hydroxide aqueous eluent. Anions 1) fluoride 2) acetate 3) chloride 4) nitrite 5) bromide 6) nitrate 7) sulfate 8) oxalate 9) chromate 10) iodide 11) phosphate 12) phthalate 13) citrate 14) thiocyanate (from refs. 13,14)...

Ion exchange chromatography is based upon the differential affinity of ions for the stationary phase. The rate of migration of the ion through the column is directly dependent upon the type and concentration of ions that constitute the eluent. Ions with low or moderate affinities for the packing generally prove to be the best eluents. Examples are hydroxide and carbonate eluents for anion separations. 

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 second direct method depends on the ability of aqueous potassium chloride, adjusted to pH 4 with carbon dioxide, to selectively elute hydrogen ions from sulfonic acid groups in pulps that first have been converted to their hydrogen form with 0.1 M hydrochloric acid (Cappelen and Schoon 1966). Carbon dioxide is removed from the eluent by sparging with nitrogen and the remaining acid is titrated with 0.1 M sodium hydroxide. Again, a correction factor for interference from carboxylic acids is required. This factor, as before, is based on the protons eluted from bleached pulps by the eluent. As the results depend on the concentration of potassium chloride used, the letter is adjusted so that the sulfonate content corresponds to the sulfur content of pulps assumed to contain only acidic sulfur. 

Eluent pH is limited to a maximum of 7 to 8 due to the reduced chemical stability of a chromatographic bed in an alkaline medium. The nucleophilic attack of Si-0 bonds by hydroxide ions leads to the erosion of the silica surface as shown by back pressure increases caused by the formation of Si(OH)4. With polystyrene-divinyl-benzene-based stationary phases, pH stability is not an issue and a very wide mobile phase pH range can be used, thereby providing additional selectivity [1]. Several silica-based and polymeric columns claimed to be stable in pH ranges from 1 to 13 are commercially available, however, they are not commonly used. 

The principle of conductivity suppression is the reduction of background conductivity by converting the eluent to a less conductive medium (H2O) through acid-base neutralization while the analyte ions conductivity is increased, by converting them to a more conductive medium Anions are converted to their acid forms and cations to their hydroxide forms. These reactions lead to higher S/N ratios, thus significantly improving baseline stability and detection limits. 

For the detection of mineral acids in the presence of an excessive amount of nitrate, the IonPac AS2 separator column was developed from which bromide and nitrate elute after sulfate. The selectivity of this stationary phase is based on the hydrophobic properties of the exchange groups bound to the latex beads (see Section 3.3.1.2). As shown in Fig. 3-47, small quantities of chloride, orthophosphate, and sulfate can be determined in the presence of high amounts of nitrate. The best separation is obtained with an eluent mixture of sodium carbonate and sodium hydroxide. 

Using anion exchange chromatography with pulsed amperometric detection, polymers up to DP70 may be analyzed. The necessary gradient elution technique is based on the combination of sodium hydroxide and sodium acetate eluents described above. 

Most emulsion polymerization is based on free-radical reactions, involving monomers (e.g., styrene, butadiene, vinyl acetate, vinyl chloride, methacrylic acid, methyl methacrylate, acrylic acid, etc.), surfactant (sodium dodecyl diphenyloxide disulfonate), initiator (potassium persulfate), water (18.2MQ/cm), and other chemicals and reagents such as sodium hydrogen carbonate, toluene, eluent solution, sodium chloride, and sodium hydroxide. 

In suppressed ion chromatography, anions are separated on a separator column that contains a low-capacity anion-exchange resin. A dilute solution of a base, such as sodium carbonate/sodium bicarbonate or sodium hydroxide is used as the eluent. Immediately following the anion-exchange separator column, a cation-exchange unit (called the suppressor) is used to convert the eluent to molecular carbonic acid. 

Organic amines require a basic eluent, such as dilute aqueous sodium hydroxide, to ensure that the amines are in the molecular form and are not ionized. Water alone can be used to elute weak molecular bases. 

Haddad et al. measured retention volumes for a variety of bases on a quaternary ammonium functionalized PS-DVB stationary phase using dilute aqueous sodium hydroxide as the eluent [11]. Values for the retention volumes and distribution coefficients of selected bases are given in Table 8.3. Strong bases, which are fully ionized a the eluent pH, elute at the column void volume and have a value of 1.0. Solutes intermediate between these two extremes are partly ionized and generally can be separated by an ion-exclusion mechanism. 

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