Nanobead-Agglomerated Anion Exchangers

Nanobead-agglomerated anion exchangers are comprised of a surface-sulfonated poly(styrene-co-divinylbenzene) substrate with particle diameters between 5 and 10 pm and fully aminated, high-capacity porous polymer beads made of polyvinylbenzyl chloride or polymethacrylate, which are called nanobeads. The latter have a much smaller diameter (about 0.1 pm) and are 

Column Metrosep A Supp 4 Metrosep A Supp 5 Metrosep A Supp 7 Novosep A-2 Anion Shimpack IC-SA3 

Manu cturer Metrohm Metrohm Metrohm Grace Shimadzu 

The surface sulfonation of the substrate prevents the diffusion of inorganic species into the inner part of the stationary phase via Donnan exclusion (see Section 5.1). Therefore, the diffusion process is dominated by the functional groups bonded to the nanobeads. The size of these beads determines the length of the diffusion paths and thus the rate of the diffusion process [32]. 

This complex system offers several advantages compared to other column packings such as silica-based anion exchangers (see Section 3.4.2) and direcdy aminated resins developed by Fritz and Story [33]. The advantages are as follows  

---

With the exception of nanobead-agglomerated anion exchangers (see Section 3.4.1.4), where totally porous nanobeads act as ion-exchange material, organic polymers are functionalized directly at their surface. Surface-functionalized, the so-called pellicular substrates, show a much higher chromatographic efficiency than fully functionalized resins. 

Figure 3.27 Structure of a nanobead-agglomerated anion-exchange resin.

Carbonate-Selective Nanobead-Agglomerated Anion Exchangers At present. Thermo Fisher Scientific offers four different carbonate-selective, nanobead-agglomerated anion exchangers with diverse selectivities. The structural and technical characteristics of these separator coliunns are summarized in Table 3.6. Special columns for the separation of polyvalent anions and carbohydrates, amino acids, and oligonucleotides are also available. 

Table 3.6 Structural and technical properties of carbonate-selective nanobead-agglomerated anion exchangers.

When it was introduced, lonPac AS12A was clearly the most modern nanobead-agglomerated anion exchanger for the separation of fluoride, oxyhalides, and mineral acids. The latest development in the field of universal purpose anion exchangers for water analysis is lonPac AS22. It is based on a hyper-branched polymer and discussed in Section 3.4.I.5. 

A much higher resolution between fluoride and chloride is obtained by using a CarboPac PAl-type stationary phase. Compared to nanobead-agglomerated anion exchangers such as lonPac AS4A-SC, this separator column, which was initially developed for the analysis of carbohydrates, exhibits a signiflcantly higher capacity. As a result, chloride retention increases to more than 20 min. 

Endothall can be analyzed together with common inorganic anions under standard chromatographic conditions on nanobead-agglomerated anion exchangers. It typically elutes between nitrate and orthophosphate. 

Arogenic acid is not stable at acidic pH. Thus, it cannot be analyzed by cation-exchange chromatography. However, on a nanobead-agglomerated anion exchanger with alkaline eluents, the separation of this compound from the amino acids phenylalanine and tyrosine is accomplished without any problem. 

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. 

Heitkemper et al. [202] applied element-specific detection for the simultaneous determination of arsenic(III)/arsenic(V) in food additives. The separation shown in Figure 8.93 was obtained on a nanobead-agglomerated anion exchanger with a NaOH eluent. 

---