Analysis of Surface-Inactive Ions

In the field of anion analysis, ion-pair chromatography is an alternative to ion-exchange chromatography. If a coelution of two anions is suspected with one of the two methods, it is often possible to solve the separation problem with the other method two different compounds rarely show the same retention behavior under completely different chromatographic conditions. 

A typical example is the analysis of nitrate and chlorate. In the past, it was not possible to separate the two analytes on nanobead-agglomerated anion exchangers of the first generation (e.g., lonPac AS4A-SC), which made it impossible to accurately interpret the chromatogram. The only way to distinguish between nitrate and chlorate was to make use of their different absorption characteristics chlorate is UV-transparent while nitrate can be detected at a wavelength of 215 nm. If both species were present in solution, determinations were possible only via differential detection. 

Tetraalkylammonium salts that are suitable ion-pair reagents for ion-pair chromatography of simple inorganic anions are predominantly employed in combination with polymer phases [24,25]. However, they may also be used when applying chemically bonded reversed phases [16,26]. The separation of inorganic 

LiChrosorb RP 18,10 eluent 2mmol/L TBAOH -I- 0.05 mol/L phosphate buffer (pH 6.7) flow rate 2 mL/min detection nonsuppressed 

20 pU peaks 25-100 mg/L each of acetate (1), acrylate (2), glycolate (3), formate (4), nitrite (5), bromide (6), nitrate (7), iodide (8), and dichloroacetate (9) (reproduced with permission from Ref. [31]. Copyright 1982, Elsevier). 

On the other hand, with ion-pair chromatography, nitrate and chlorate are resolved using tetrabutylammonium hydroxide as the ion-pair reagent, with nitrate eluting prior to chlorate (Fig. 6-11). Pursuing the hypothesis that the 

Tetraalkylammonium salts that are suitable ion-pair reagents for ion-pair chromatography of simple inorganic anions are predominantly employed in combination with polymer phases [24, 25]. However, they may also be used when applying chemically bonded reversed phases [16, 26]. The separation of inorganic anions on LiChrosorb RP18 shown in Fig. 6-13 is an example. In contrast to polymer phases, the fluoride determination is a problem with chemically bonded reversed phases because fluoride is poorly retained. If direct conductivity detection is applied, a quantitative evaluation of the fluoride signal is difficult 

To analyze bromide and nitrate in foodstuff, Leuenberger et al. [30] used a chemically bonded aminopropyl phase with a phosphate buffer eluant. Cortes [31] applied this method successfully to the separation of other inorganic anions. The chromatogram depicted in Fig. 6-16 confirms the extraordinary selectivity of ion-pair chromatography for monovalent anions. A final assessment of the 

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