Potentiometric sensors calibration curve

Potentiometric pH Sensors at Ambient Temperature, Fig. 7 (a) Potentiometric pH calibration curve, (b) disposable microelectrode lab chip sensor, and... 

The behavior of potentiometric and pulsed galvanostatic polyion sensors can be directly compared. Figure 4.11 shows the time trace for the resulting protamine calibration curve in 0.1 M NaCl, obtained with this method (a) and with a potentiometric protamine membrane electrode (b) analogous to that described in [42, 43], Because of the effective renewal of the electrode surface between measuring pulses, the polyion response in (a) is free of any potential drift, and the signal fully returns to baseline after the calibration run. In contrast, the response of the potentiometric protamine electrode (b) exhibits very strong potential drifts. 

Fig. 18a.7. Typical calibration curve of a potentiometric sensor for measuring monovalent cations. From Ref. [70] with permission.

Because each enzyme sensor has its own unique response, it is necessary to construct the calibration curve for each sensor separately. In other words, there is no general theoretical response relationship, in the same sense as the Nernst equation is. As always, the best way to reduce interferences is to use two sensors and measure them differentially. Thus, it is possible to prepare two identical enzyme sensors and either omit or deactivate the enzyme in one of them. This sensor then acts as a reference. If the calibration curve is constructed by plotting the difference of the two outputs as the function of concentration of the substrate, the effects of variations in the composition of the sample as well as temperature and light variations can be substantially reduced. Examples of potentiometric enzyme electrodes are listed in Table 6.5. 

The stability of enzyme electrodes is difficult to define because an enzyme can lose some of its activity. Deterioration of immobilized enzyme in the potentiometric electrodes can be seen by three changes in the response characteristics (a) with age the upper limit will decrease (e.g., from 10-2 to 10 3 moll-1), (b) the slope of the analytical (calibration) curve of potential vs. log [analyte] decrease from 59.2 mV per decade (Nernstian response) to lower value, and (c) the response time of the biosensor will become longer as the enzyme ages [59]. The overall lifetime of the biosensor depends on the frequency with which the biosensor is used and the stability depends on the type of entrapment used, the concentration of enzyme in the tissue or crude extract, the optimum conditions of enzyme, the leaching out of loosely bound cofactor from the active site, a cofactor that is needed for the enzymatic activity and the stability of the base sensor. 

Potentiometric pH Sensors at Ambient Tempera- beverages [18] (Reprinted from Analytica Chunica Acta, ture. Fig. 2 (Left) Potentiometric calibration curve of 1997. 351(1-3) p. 143-149 with permission from screen-printed RUO2 in universal buffer and (right) com- Elsevier) parison to a commercial glass electrode in some drink and... 

Organophosphate pesticides studied in this work were the model low-toxic OPC trichlorfon, and some common organophosphate pesticides malathion, parathion, dichlorvos, and diazinon (Table I). Calibration curves for these pesticides (dependences of the sensor inhibition response on the analyte concentration) were obtained for all of these OPCs. These calibration curves were obtained under conditions (time of inhibition, pH and temperature) optimize with the model analyte trichlorfon. All of the pesticide calibration curves are similar and Fig. 4 illustrate the method by the example of malathion. The lowest concentration of pesticide samples assayed with 10 min. of incubation of the electrode in inhibitor containing solution was 5 ppb. This resulted in approximately 10 % of the relative inhibition signal. Fig. 4 predicts much better performance of our system compared with the literature data. For example, trichlorfon detection by means of ISFET had a reported limit of detection of ca 250 ppb (5), while conductometric sensor assay registered trichlorfon at ca. 25 ppb (5), still an order of magnitude higher than the described sensor. An amperometric sensor was used to detect dichlorvos with a limit of detection of 350 ppb (2J) and a potentiometric (pH-sensitive) sensor was shown to detect parathion at 39 ppm and diazinon at 35 ppb (9). 

A conductometric P(ANi)/glucose-oxidase sensor was described in Sec. 17.2 above. The same sensor construction could in principle also be used for potentiometric detection of glucose, since the variation of open circuit potential with pH is substantial such work has been described recently [191, 807]. The sensing range is claimed to be higher, with an acceptable calibration curve claimed to be obtained over three orders of magnitude [191]. 

The potentiometric SECM experiment yields the potential of the tip electrode, E, as a function of the tip position. To establish the correspondence between these data and the aforementioned theory, one needs to use a calibration curve, that is, a Nemstian E vs. c plot. Using such a calibration, one can transform the experimental results to the c vs. (z, r) dependence and fit them to the theory in order to find Js and establish the distance scale. The earlier discussion implies that the products generated on the substrate do not participate in any chemical reaction in solution. Otherwise, a more complicated treatment may be necessary (e.g., when the tip is a pH sensor used to monitor proton concentration in a buffered aqueous solution [43]). 

These sensors are usually used in one of two operational modes, direct potentiometry or potentiometric titrations. Direct potentio-metry (similar to a pH determination with a pH meter), which is based on the correlation of the electrode emf to a standard or calibration curve, is normally limited to a precision of, at best, 0.5 %. It is, nonetheless, a technique that is very simple, rapid, and convenient to use. 

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