Selects against anions

While selective complexation makes the membrane more permeable to the analyte ion than the co-ions, selectivity against counter ions in neutral-ionophore-based membranes is achieved by ionic sites, not by the ionophore. In fact, the ionophore-analyte complexation decreases the free analyte activity in tlie membrane to enhance the salt extraction into the membrane phase, which may result in counter-ion interference due to Donnan exclusion failure (7). Although the PVC matrix has inherent negative sites as an impurity (11), the concentration is so low that the anionic sites were initially introduced in cation-selective ISEs based on neutral ionophores to suppress the counter-ion interference (12). 

Moreover, selectivity could be also enhaneed if a specific interaction between the immobilized catalyst and the substrate in solution were to occur. A significant example is that of the voltam-metric detection of NO in the rat brain from a carbon fiber microelectrode modified with a [(H20)Fe PWii039] -containing poly(N-methylpyrrole) film and a Nation outer layer [150]. The good selectivity of this sensor was attributed not only to the Nafion membrane, which constituted an efficient electrostatic barrier against anionic interferents, but also to the formation of a metal-nitrosyl complex between the heteropolyanion and NO. The in vivo NO measurements were validated by injecting the rat with an NO-synthase inhibitor, which led gradually to the disappearance of the NO oxidation peak (Fig. 7). 

Fig. 3.7 Plot of the D parameter of cations (DJ against the D parameter of anions (Da) for selected binary structures. Compounds with homoatomic bonds are excluded.

Partly fluorinated or perfluorinated sulfonic and carboxylic acids are compounds with excellent surface activity combined with an extreme stability against chemical or physico-chemical attacks as also described for non-ionic fluorine containing surfactant compounds. The anionic surfactants are shown with their structural formulae in Fig. 2.11.24(1)—(III). A selected ESI-FIA-MS(-) spectrum of a partly fluorinated surfactant (CF3-(CF2)4-(CH2)8-S03H, m/z 461) is presented in Fig. 2.11.25. The FIA spectrum also contains ions of the by-products CF3-(CF2)2-(CH2)8-S03H (m/z 361) and CF3-(CF2)3-(CH2)8-S03H (m/z 411). 

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