Salicylate-selective sensors

The most widely used sensor for chloride ions in clinical analyzers is based on an ion-exchanger, a quaternary alkylammonium chloride, dispersed in a plastic membrane. It is not an ideal sensor due to the interference of lipophilic anions (e.g., salicylates, bromides) and lip-ophylic cations (e.g., bacteriostatic agents, anesthetics) and a relatively poor selectivity towards hydrogen carbonates (bicarbonates). However, compared to charged anion- and neutral carrier-based membranes that have been tested, it is still the best-suited for automated analyzers. 

Although the ISEs based on cobyrinates have good selectivity for nitrite over several anions, they also respond to salicylate and thiocyanate. To eliminate this interference, the nitrite-selective electrode based on ionophore 2 was placed behind a microporous gas-permeable membrane (GPM) in a nitrogen oxide gas-sensor mode (75). NOx was generated from nitrite in the sample at pH 1.7 and, after crossing the GPM, was trapped as nitrite by an internal solution that was buffered at pH 5.5 (0.100 M MES-NaOH, pH 5.5, containing 0.100 M NaCl). The internal solution was "sandwiched" between the nitrite-selective electrode and the GPM. 

Figure 6 shows the selectivity behavior of this NOx gas sensor. The sensor had a sub-Nernstian response toward nitrite, with slopes in the range of -45 to -50 mV/decade. Further, the response observed with salicylate and thiocyanate was diminished substantially, as compared to that obtained with the original nitrite-selective electrode (Figure 3). In addition, the gas sensor described here does not suffer interferences from nitrate, bicarbonate, acetate, benzoate, or chloride. These excellent selectivity properties of the sensor are a combination of the selectivity characteristics of the nitrite-selective electrode and the additional discrimination provided by the GPM.

Figure 6. Selectivity pattern of the NOx gas sensor. The sensor was exposed to 0.010 M H2SO4 containing the following anions nitrite (1), salicylate (2), thiocyanate (3), benzoate (4), nitrate (5), chloride (6), bicarbonate (7), acetate (8). (Adapted from ref. 15.)...

The exceptional selectivity of this sensor allowed for the direct monitoring of salicylate in blood and serum with very good precision and accuracy. 

Fonong and Rechnitz (1984a) entrapped the enzyme physically with a dialysis membrane at the sensing tip of a carbon dioxide selective electrode. The sensor was suitable for salicylate concentrations in the range 0.04-2.2 mmol/l. 

Tetrakis(4-A, A -dimethylaminobenzene)porphyrinato-manganese(III) acetate was used as a novel carrier for a selective iodide ion electrode (Farhadi et al, 2004). The sensor exhibited not only excellent selectivity to iodide ion compared to Cl and lipophilic anions such as CIO4 and salicylate, but also a Nernstian response for iodide ion over a wide concentration range from 1.0 X 10 to 7.5 X 10 mol-l h The potentiometric response was independent of the pH of the solution in the pH range 2-8. The electrode could be used for at least 2 months without any considerable divergence in the potential. The electrode was applied to the determination of iodide in seawater samples and drug formulations. 

As described above, significant progress in the design of anion and gas selective membrane electrodes has been made. While further work is needed to understand fully the response mechanisms and to improve the performance of the new thiocyanate, salicylate, and sulfite selective membrane electrodes, each of these sensors appears to offer adequate selectivity for use in real sample measurements. In addition, by carefully... 

Most potentiometric anion-selective electrodes are based on anion exchangers such as quaternary ammonium salts. The selectivity pattern of these sensors correlates with anion lipophilicity. Highly hydrated anions such as fluoride, bicarbonate and chloride are difficult to monitor due to significant interference from more lipophilic anions like perchlorate, salicylate and nitrate which may be present in the analyzed sample. The anions can be classified according to their lipophilicity resulting in the classical Hofmeister series (ClOi > SCN > salicylate > I > NOJ > Br >... 

The salicylate sensor responds to pH changes and its detection limit deteriorates as the pH increases. The selectivity coefficient for the salicylate relative to OH , log iTfai, OH = 4.2 (at pH 7.2), limits practical applications of the salicylate sensor for in situ monitoring of salicylate ions in biological systems. 

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