Selective electrodes detection limits

As far as detection limits, selectivity, and Nemstian response ranges, the performance of CWEs is essentially analogous to that of conventional polymer membrane electrodes. However, one major drawback to their use appears to be the lack of reproducible potentials. Even within a single day, cell potentials can vary substantially for the same standard solution. As a result, CWEs must be recalibrated often, or better yet, standard addition or titration techniques must be utilized to ensure accurate results. Obviously, such analytical techniques could not be employed if in situ determinations are desired. 

A number of ion-selective electrodes are available from laboratory supply houses whilst not intended to be an exhaustive list, Table 15.3 serves to indicate the variety of determinations for which electrodes are available. An indication is also given of the lower limit of detection of the electrodes this figure may vary somewhat according to the source of the electrode but full details are furnished by the manufacturer of the effective range of use of each electrode and of likely interferences. 

The advantages of controlled-potential techniques include high sensitivity, selectivity towards electroactive species, a wide linear range, portable and low-cost instrumentation, speciation capability, and a wide range of electrodes that allow assays of unusual environments. Several properties of these techniques are summarized in Table 1-1. Extremely low (nanomolar) detection limits can be achieved with very small sample volumes (5-20 pi), thus allowing the determination of analyte amounts of 10 13 to 10 15 mol on a routine basis. Improved selectivity may be achieved via the coupling of controlled-potential schemes with chromatographic or optical procedures. 

FIGURE 5-5 Determination of die detection limit of an ion-selective electrode. (Reproduced with permission from reference 12.)... 

L-arginine and the excellent selectivity of the ammonia electrode for NH3. The working range of biosensors of this type is typically only two to three orders of magnitude with a detection limit of 10 to 10 M. 

Conventional ion-selective electrodes have been used as detectors for immunoassays. Antibody binding measurements can be made with hapten-selective electrodes such as the trimethylphenylammonium ion electrode Enzyme immunoassays in which the enzyme label catalyzes the production of a product that is detected by an ion-selective or gas-sensing electrode take advantage of the amplification effect of enzyme catalysis in order to reach lower detection limits. Systems for hepatitis B surface antigen and estradiol use horseradish peroxidase as the enzyme label and... 

Puacz et al. (1995) developed a catalytic method, based on the iodine-azide reaction, for the determination of hydrogen sulfide in human whole blood. The method involves the generation of hydrogen sulfide in an evolution-absorption apparatus. In addition, the method allows for the determination of sulfide in blood without interference from other sulfur compounds in blood. A detection limit of 4 g/dm3 and a percent recovery of 98-102% were achieved. Although the accuracy and precision of the catalytic method are comparable to those of the ion-selective electrode method, the catalytic method is simpler, faster, and would be advantageous in serial analysis. 

Alemu et al. [35] developed a very sensitive and selective procedure for the determination of niclosamide based on square-wave voltammetry at a glassy carbon electrode. Cyclic voltammetry was used to investigate the electrochemical reduction of niclosamide at a glassy carbon electrode. Niclosamide was first irreversibly reduced from N02 to NHOH at —0.659 V in aqueous buffer solution of pH 8.5. Following optimization of the voltammetric parameters, pH and reproducibility, a linear calibration curve over the range 5 x 10 x to 1 x 10-6 mol/dm3 was achieved, with a detection limit of 2.05 x 10-8 mol/dm3 niclosamide. The results of the analysis suggested that the proposed method has promise for the routine determination of niclosamide in the products examined [35]. 

Surface modified NO sensors incorporate an electrode surface that has been modified or treated in some way so as to increase the selectivity of the sensor for NO and promote catalytic oxidation of NO. An early example of such a sensor was presented by Malinski and Taha in 1992 [27], In this publication an —500nm diameter carbon fiber electrode was coated with tetrakis(3-methoxy-4-hydroxyphenyl)porphyrin, via oxidative polymerization, and Nation. This electrode was shown to have a detection limit of — lOnM for NO and great selectivity against common interferences. However, recently it has been shown that this electrode suffers severe interference from H202 [28],... 

This field is therefore at an exciting stage. Ion-selective electrodes have a proven track record in terms of clinical and biomedical analysis, with a well-developed theory and a solid history of fundamental research and practical applications. With novel directions in achieving extremely low detection limits and instrumental control of the ion extraction process this field has the opportunity to give rise to many new bioana-lytical measurement tools that may be truly useful in practical chemical analysis. 

T. Sokalski, A. Ceresa, T. Zwickl, and E. Pretsch, Large improvement of the lower detection limit of ion-selective polymer membrane electrodes. J. Am. Chem. Soc. 119, 11347-11348 (1997). 

T. Sokalski, T. Zwickl, E. Bakker, and E. Pretsch, Lowering the detection limit of solvent polymeric ion-selective membrane electrodes. 1. Steady-state ion flux considerations. Anal. Chem. 71,1204—1209 (1999). 

T. Zwickl, T. Sokalski, and E. Pretsch, Steady-state model calculations predicting the influence of key parameters on the lower detection limit and ruggedness of solvent polymeric membrane ion-selective electrodes. Electroanalysis 11, 673-680 (1999). 

U. Schefer, D. Ammann, E. Pretsch, U. Oesch, and W. Simon, Neutral carrier based Ca-2+-selective electrode with detection limit in the sub-nanomolar range. Anal. Chem. 58, 2282-2285 (1986). 

Electrochemical biosensors based on detection of hydrogen peroxide at platinized electrodes were found to be more versatile allowing a decrease in detection limit down to 1 i,mol L 1 [109]. However, all biological liquids contain a variety of electrochemically easily oxidizable reductants, e.g. ascorbate, urate, bilirubin, catecholamines, etc., which are oxidized at similar potentials and dramatically affect biosensor selectivity producing parasitic anodic current [110]. 

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