Response characteristics selectivity and detection limits

Electrochemical Sensors, Biosensors and Their Biomedical Applications 

There are two main factors that influence the selectivity of a sensor limits in discrimination of an interfering ion and upper limits in stability constant of an analyte-ionophore complex. While an ideal ionophore does not form complexes with interfering ions, too strong complexation with the primary ion leads to a massive extraction of analyte into membrane phase coupled with a coextraction of sample counter-ions, known as Donnan exclusion failure. In such cases, at high activities and hpophilicities of sample electrolytes, fli(org) increases and a breakdown of membrane permselectivity prevents the Nemst equation to hold. 

however, the PBP model can be used to describe the influence of key membrane parameters on the selectivity with a simphfied equation after making certain assumptions. It is here given for cation-selective electrodes based on neutral carriers [30]  

The selectivity here is directly proportional to complex formation constants and can be estimated, once the latter are known. Several methods are now available for determination of the complex formation constants and stoichiometry factors in solvent polymeric membranes, and probably the most elegant one is the so-called sandwich membrane method [31]. Two membrane segments of different known compositions are placed into contact, which leads to a concentration polarized sensing membrane, which is measured by means of potentiometry. The power of this method is not limited to complex formation studies, but also allows one to quantify ion pairing, diffusion, and coextraction processes as well as estimation of ionic membrane impurity concentrations. 

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Meyerhoff et al have reported the use of a non-metallated porphyrin film as an ion-selective electrode. They have shown that the electropolymerized films offer significant selectivity for the detection of iodide over a wide range of anions. The lifetime of these electrodes was estimated by analyzing some of their characteristics such as sensitivity and detection limits. The effect of pH on the detection of iodide by the film-supported electrodes was also established. For pH values from 3 to 8, there is relatively little effect of the pH on the electrochemical response of the electrodes. At pH values higher than 8, the electrode response is affected by the interference by hydroxide ions. 

Characteristics of the fluoride ion-selective electrode The LaFj ISE exhibits Nernstian response to the activity of fluoride ions in the concentration range 1 M to 10" M [31, 53,325]. When the solution is buffered for fluoride ions using ZiO and Th " salts, Baumann [28] has demonstrated that Nernstian response can be obtained down to a free fluoride ion concentration of 10" ° moldm". In solutions with an ionic strength of 2 moldm", the detection limit is 0.2 ppm, i.e. about 10" moldm", while in pure NaF solutions it may be as low as 10" moldm ... 

The magnitude of the dissociation constant A plays an important role in the response characteristics of the sensor. For a weakly dissociated gas (e.g., CO2, K = 4.4 x 10-7), the sensor can reach its equilibrium value in less than 100 s and no accumulation of CO2 takes place in the interior layer. On the other hand, SO2, which is a much stronger acid (K = 1.3 x 10-2), accumulates inside the sensor and its rep-sonse time is in minutes. The detection limit and sensitivity of the conductometric gas sensors also depend on the value of the dissociation constant, on the solubility of the gas in the internal filling solution, and, to some extent, on the equivalent ionic conductances of the ions involved. Although an aqueous filling solution has been used in all conductometric gas sensors described to date, it is possible, in principle, to use any liquid for that purpose. The choice of the dielectric constant and solubility would then provide additional experimental parameters that could be optimized in order to obtain higher selectivity and/or a lower detection limit. 

M, the pH glass electrode being an exception with a very broad linear range (pH 0-14). However, recently the detection limit of several ISEs has been dramatically improved towards picomolar (10-12 M) concentrations, which is promising concerning potentiometric trace-level analysis. For characteristics of ion-selective electrodes see also response time, - response volume, and - selectivity coefficient. 

The validation of ion-selective membrane electrodes is based on the reproducibility of their development, the simplicity and rapidity of their construction, and their response characteristics. Typically, these response characteristics include minimum 50 mV/decade for the slope, 10 6 mol/L for limit of detection, large working range, and low response time, which can be assured only by the best counter ion and matrix (PVC and plasticizer for solid membrane electrodes and solvent for liquid membrane electrodes). 

The performance characteristics for ISEs are evaluated in terms of ion selectivity, detection limit, dynamic range [measurable concentration (activity) range], and response time. 

A NOx gas sensor constructed from a molecularly-imprinted nitrate-selective electrode has also been developed in our laboratory (30). This gas sensor is produced by placing the nitrate-selective electrode behind a gas-permeable membrane. Also, there is a buffer compartment present between the gas-permeable membrane and the ISE. As before, NOx species that pass through the gas-permeable membrane are trapped in the buffer compartment as NO2 and NOs . In this case, the electrode responds proportionately to the amount of nitrate in the buffer, rather than nitrite as in the NOx sensor described above. The gas sensor based on the N03 -selective electrode offers advantages over the Severinghaus arrangement similarly to the other aforementioned gas sensors (SO2 and NOx). Detection limits for this gas sensor were on the order of 1x10 M, with response characteristics being retained for over 80 days. This lifetime is consistent with lifetimes of the molecularly-imprinted nitrate-selective electrodes. 

Ion-selective electrodes are widely used in pharmaceutical analysis. Numerous ion-selective electrodes based on poly(vinyl)chloride (PVC) have been constructed in order to determine various pharmacologically active substances in pharmaceutical preparations. The inherent advantages of ISE-s are simple design, construction and manipulation, reasonable selectivity, fast response time, low cost, adequate detection limit and adequate precision and accuracy. Therefore, the ISE potentiometric method is very attractive for pharmaceutical analysis. The first part of this chapter describes the properties and uses of PVC, as well as the general characteristics and preparation of the polymer membranes of ion-selective electrodes. The second part deals with the development of solid contact electrodes formed on non-crystal electrodes with a microporous matrix (e.g., vinyl polychloride dissolved in an appropriate modifier) selective for various nonsteroidal anti-inflammatory drugs (NSAlD-s) and their applications in pharmaceutical research. 

The linear range, detection limit, sensitivity, selectivity, reproducibility, response time, and stability are the major characteristics of the biosensor. 

High field H NMR spectroscopy has shown itself to be a very powerful tool with which to diagnose inborn errors of metabolism and to monitor the clinical response of patients to therapeutic interventions. Characteristic patterns of abnormal metabolites are observed in the patient s urine for each disease. The main advantages of using H NMR spectroscopy in this area are its speed, lack of sample preparation and the provision of non-selective detection for all the abnormal metabolites in the biofluid regardless of their structural type, providing only that they are present above the detection limit of the NMR experiment. 

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