Polymer nanobeads

As in the case of bulk optodes, plasticizers can be added to modify the properties of polymers (e.g., gas permeability). Plasticizers are mainly used to design ion-sensitive nanobeads. 

Dual lifetime referencing (DLR) is another powerful technique that enables referenced measurements in case of fluorescent indicators [23]. In this method, the analyte-dependent signal from an indicator is referenced against the signal from an inert luminophore. This can be realized in both the time domain [24] and in the frequency domain [25]. Often, a luminescent reference dye is embedded into gas blocking nanobeads to avoid oxygen quenching. Polymers with very low gas permeability such as poly(acrylonitrile) [24] or poly(vinylidene chloride-co-acry-lonitrile) [26] are the best choice here. 

Monodisperse polystyrene (latex) spheres having a radius value in the range 30-100 run with a standard deviation of 8% or less characterizing the size uniformity. Such samples can be obtained from Duke Scientific Corp. as 3000 Series Nanospheres (www. dukescientific.com), Polymer Laboratories as PL-Latex Plain White (www.polymerlabs. com), and Polysciences, Inc., as Nanobead Traceable Size Standards (www.polysciences. com). 

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. 

Carbonate/Bicarbonate-Selective Grafted Polymers The most common surface-functionalized, carbonate/bicarbonate-selective grafted polymer is available under the trade name Dionex lonPac AS14. This universal anion exchanger for the analysis of inorganic anions was developed as an advanced successor product of the nanobead-agglomerated lonPac AS12A [23] (see Section 3.4.1.4). 

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... 

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. 

These development goals were realized with lonPac ASH. The diameter of the nanobeads has been increased to 85 nm, yielding an ion-exchange capacity of 45 pequiv/column. As with all solvent-compatible columns, a highly cross-linked EVB/DVB polymer serves as the substrate material. The ASH particle diameter of 13 pm is large enough to ensiue a flow rate of 2 mL/min at moderate back pressure. 

---