Drawback potentiometric

Many IC techniques are now available using single column or dual-column systems with various detection modes. Detection methods in IC are subdivided as follows [838] (i) electrochemical (conductometry, amper-ometry or potentiometry) (ii) spectroscopic (tJV/VIS, RI, AAS, AES, ICP) (iii) mass spectrometric and (iv) postcolumn reaction detection (AFS, CL). The mainstay of routine IC is still the nonspecific conductometric detector. A significant disadvantage of suppressed conductivity detection is the fact that weak to very weak acid anions (e.g. silicate, cyanide) yield poor sensitivity. IC combined with potentiometric detection techniques using ISEs allows quantification of selected analytes even in complex matrices. The main drawback... 

The obvious advantage of the symmetrical arrangement is that the processes at all internal interfaces can be well defined and that most nonidealities at the mem-brane/solution interface tend to cancel out. Because the volume of the internal reference compartment is typically a few milliliters, the electrode does not suffer from exposure to electrically neutral compounds that would penetrate the membrane and change the composition of this solution. This type of potentiometric ion sensor has been used in the majority of basic studies of ion-selective electrodes. Most commercial ion-selective electrodes are also of this type. The drawbacks of this arrangement are also related to the presence of the internal solution and to its volume. Mainly for this reason, it is not conveniently possible to miniaturize it and to integrate it into a multisensor package. 

Other recently developed probes, which consist of potentiometric sensors with solid-state internal contact (1, 26, 27), internal solution). But there are some drawbacks related to reproducibility of enzyme immobilization and light sensitivity (28). 

Urease (urea amidohydrolase, EC 3.5.1.5) has the advantage that in the presence of the pH indicator bromocresol purple it produces a sharp unequivocal endpoint with its substrate, urea (Chandler et al, 1982). Moreover, it is absent from mammalian cells. Though urease can also be used for potentiometric EIA (Section 14.6.5), other deaminating enzymes, in particular asparaginase (Gebauer and Rechnitz, 1982), are more promising. The major drawback of urease is the possibility of rapid loss of enzyme activity. Type VII or C-3 urease (Sigma) from Jack beans [Canavalia ensiformis) are the most frequently used enzyme preparations. The physicochemical properties of urease are compiled in Table 10.14. 

Potentiometric gas sensors for the reaction products, NH3 and CO2, have also been employed. Since these measurements are based on gas diffusion through a hydrophobic membrane, no direct disturbances by sample constituents occur. As early as 1969, Guilbault et al. coupled immobilized urease with a carbon dioxide sensor. Anfalt et al. (1973) applied an ammonia gas sensitive electrode to urea assay. A major drawback of these sensors is their long response time which is due to the slow diffusion of the gases. Since it takes several additional minutes to reach a new baseline after each measurement, only a few samples can be processed per hour. Guilbault et al. (1985) therefore tried an NH3 electrode, the interned buffer of which was exchanged after each measurement (double injection electrode). This approach led to a substantial decrease of the washing time. 

As in amperometric applications, many experiments can be conducted in the close proximity mode where the tip is moved very close to the substrate surface and a perturbation is applied to the sample. This perturbation may take several forms, typically potentiostatic or galvanostatic excursions if the sample is acting as an electrode, but also optical illumination with a laser beam, change of solution, etc. The tip response is then recorded as a function of time following the application of the perturbation. In these conditions potentiometric detection offers two advantages over amperometric detection (1) the range of ions detectable is extended to nonelectroactive species such as alkali metals, and (2) the tip response is selective. There are, however, some drawbacks. Because of the high impedance of the electrometer, the response time is worse in potentiometric applications where the t90 is rarely below 30 s. This must be compared to the millisecond time scale available with amperometric responses (89). Ohmic drop may also affect the tip potential. 

However, one should not forget that the potentiometric mode has several drawbacks. The fabrication of most potentiometric tips is much more involved than that of the conventional amperometric tips. It is necessary to have the electrode made by a very skilled technician. Despite this, even when following a proven recipe the success rate is relatively low. The response of potentiometric tips is not always Nemstian and a calibration is required before and after performing the experiment. The behavior typically varies from one microelectrode to another. Potentiometric tips cannot rely on positive and negative feedback diffusion, thus it is difficult to assess the tip-substrate distance from the tip response. Several approaches are available, but most are cumbersome. In the potentiometric mode the response... 

Most ligand pAa values can be determined via a potentiometric titration, where the pH is monitored after sequential additions of either acid or base. The requirement of millimolar ligand concentration is a major drawback of this method, especially for ligands with poor water solubility. Another common method for determination of protonation constants when a potentiometric titration is not possible is a spectrophotometric titration, where both pH and spectral changes are monitored upon the addition of acid or base. 

The drawback of the spectrophotometric technique is that it is bound to ionization induced changes in UV spectra. It is difficult to guess whether a particular ionization will induce an change in UV spectra a priory. Practically, one combines this approach with an in silico tool like ACD pKa database, which predicts the number of ionizations and gives estimates of the pKa values. If the number of theoretical ionizations is lower than the number of ionizations obtained from the experimental data, one must reanalyze the sample using the potentiometric titration approach. 

Potentiometric detectors typically measure the potential difference (A ) across a membrane, which originates from the difference in analyte concentration in the eluent versus an internal reference solution. The most common potentiometric measuring device is a pH electrode, in which a glass membrane responds to hydronium ion concentration in the test solution. Other ion-selective, or indicator, electrodes are also available commercially. The attribute of an indicator electrode to be highly selective for a particular species is also its drawback, in that a different electrode is needed for each type of ion. Halides and sulfates can be monitored using silver/silver salt and lead/lead salt electrodes, respectively [50]. 

Electrochemical methods (ISE). Ion-selective electrode measurements provide information about a number of anions/cations through potentiometric measurements against a suitable reference electrode in water solutions (requires polymer digestion/ash-ing). Detection limits vary for different ions but are usually in the upper parts-per-million range. Interference by similar ions (Br, F when measuring d ) is one of the major drawbacks of this techique, which limits its applicability. 

CsVCs, and T1+/T1 are also available [23]. They are compared in this table with values obtained by Inerowicz et al. [24] using the TATB assumption and with those obtained by Cox et al. [25] using the NUP assumption. The drawback of the polarographic method is that it is not directly applicable to the electrode potentials involving anions, say with an X /AgX, Ag electrode obtained potentiometrically. Such potentials for X=Cl and I obtained using the TATB assumption are shown in Table 8.3. Some additional values of standard electrode potentials are in the reports by Parker and coworkers and by Johnsson and Persson [27-29]. 

Measuring surfactant concentration by using surface tension suffers from the same drawback as the potentiometric measurements, i.e. the change in surface tension varies with the logarithm of the surfactant concentration, rendering a low accuracy in the calculated adsorbed amount. Another drawback is due to the fact that the method is very sensitive to the most surface-active species in the measured sample. In the adsorption of a polydispersed surfactant sample on a hydrophobic surface, the most hydrophobic surfactant species will adsorb and the more hydrophilic species will remain in the solution. In the calibration, however, the original sample is used and hence the calibration curve reflects a more hydrophobic system than is used... 

There are however serious drawbacks. As it is well known, the potentiometric electrode is a passive sensor. It just reports the concentration of the observed ion in... 

Among the drawbacks the high noise usually observed in potentiometric mode has to be also mentioned. The leads to the electrodes act as noise picking antennas. Therefore the possible shortest coimecting wires between the preamplifier and the... 

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