Sensor electrodes calibration

In a limited number of experiments, in which six different hydrogen peroxide concentrations were examined with four different pH values, the validity of the procedure used was checked by comparing the results obtained with the sensor electrode to those obtained by means of titration. The results obtained with the sensor electrode have a maximum divergence of 3% compared with the concentrations obtained by titration. The final aspect of the amperometrical detection method that was examined at laboratory scale is the stability in time of a calibrated sensor electrode. 

To avoid this problem, the total current was not used to measure the solids concentration. Instead the voltage was measured across the two sensor electrodes located between the field electrodes, as shown in Figure 22. Because the impedance of the sensor circuit is virtually infinite, practically no current flows into the sensors. Consequently, no polarization occurs on their surfaces. Thus, the calibration curve obtained should be independent of velocity, as shown in Table II. 

Figures 29 and 30 show that particle size has no significant effect on the probe calibration curve for particles of diameter comparable to the sensor electrode spacing and smaller. For coarser particles, particle size has an effect on the calibration curve. As particle size is increased, the sensor voltage decreases, and it becomes lower than Maxwell s predictions. These results indicate that there is a limitation on the use of a probe with a fixed geometry for measuring solids concentration. This limitation would have to be considered when selecting sensor electrode spacings. The effect is probably due to packing, because as the particle diameter is increased, the mean concentration at the probe surface falls.

Errors in the signal produced by microelectrodes can come from both chemical and physical sources. Contamination of the sensor element by reaction with the environment should be avoided. Investigators doing this type of work are careful to calibrate the electrode ahead of time for the species to be measured and then make sure the electrode will recalibrate after the measurement. One should consult the literature cited for specific procedures and cautions for using each type of electrode. Physical errors come from electrodes that are inappropriately sized for the variability of the environment. Problems may also come from vibration of the electrode tip or flow of bulk water along the outer sheath of the electrode into the measurement volume [31,36]. 

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. 

FIGURE 4.11 Amplitude-time behavior of potential during calibration in 0.1 M Nad solution containing 50 mM TRIS (pH 7.40) with a pulsed galvanostatic sensor (a) and a classical potentiometric protamine-selective electrode (b) [54], Logarithmic protamine concentrations (mg l-1) are indicated on the traces. The first and last samples did not contain protamine. 

Although a few amperometric pH sensors are reported [32], most pH electrodes are potentiometric sensors. Among various potentiometric pH sensors, conventional glass pH electrodes are widely used and the pH value measured using a glass electrode is often considered as a gold standard in the development and calibration of other novel pH sensors in vivo and in vitro [33], Other pH electrodes, such as metal/metal oxide and ISFETs have received more and more attention in recent years due to their robustness, fast response, all-solid format and capability for miniaturization. Potentiometric microelectrodes for pH measurements will be the focus of this chapter. 

The error from pH measurement systems depends on the slope of the calibration line. For a pH sensor with a slope of 59mV/pH, an error of lmV in measuring the electrode potential will cause a 0.017 pH change in the concentration. The lower the slope, the higher the errors are on the sample measurements [39],... 

An electron transfer type of enzyme sensor was thus fabricated by a electrochemical process. Although no appreciable leakage of ADH and MB from the membrane matrix was detected, NAD leaked slightly. To prevent this leakage, the ADH-MB-NAD/polypyrrole electrode was coated with Nation. A calibration curve is presented in Fig.25 for ethanol determination in an aquous solution with the enzyme sensor. Ethanol is selectively and sensitively determined in the concentration range from 0.1 nM to 10 mM. 

Figure 13.10 shows the calibration curve of the LED-based optical oxygen sensor compared with the calibration curve of a commercially available Clark-type sensor (Ingold Electrodes, Wilmington, Massachusetts). While the Clark-type shows a linear calibration, the optical sensor allows a hyperbolic response as predicted by the Stem-Volmer-type equation 72 ... 

Figure 13.10. Calibration curves for optical sensor and Clark-type oxygen electrode. The phase response of optical sensor and the voltage response of the Clark-type electrode is plotted against percent oxygen in the gas mixture (oxygen and nitrogen) sparged. While the Clark-type electrode shows a linear calibration the optical sensor shows a Siem-Volmer lype relationship (see Section 13.10.2).

This chapter deals with ISE construction, their characteristic properties such as selectivity coefficient, response time, temperature coefficient and drift, as well as electrode calibration and composite sensors containing ISEs. 

The electroreduction of some typically inorganic compoimds such as nitrogen oxides is catalysed by the presence of polymeric osmium complexes such as [Os(bipy)2(PVP)2oCl]Cl, where bipy denotes 2,2 -bipyridyl and PVP poly(4-vinylpyridine). This polymer modifies the reduction kinetics of nitrite relative to the reaction at a bare carbon electrode, and provides calibration graphs of slope 0.197 nA with detection limits of 0.1 pg/mL and excellent short-term reproducibility (RSD = 2.15% for n = 20). The sensor performance was found to scarcely change after 3 weeks of use in a flow system into which 240 standards and 30 meat extracts were injected [195]. 

The ISFET-based integrated coulometric sensor-actuator system was introduced in 1985 [154] in order to facilitate in situ calibration of ISFETs. The essential components of a prototype sensor based on this operational principle are shown in Fig. 4.20.B. The system was built by integrating a large noble-metal actuator electrode and a counter-electrode in a piece of silicon. A window in the actuator electrode was etched to receive the gate of the ISFET, which functioned as a pH indicator. The flow-through cell was constructed by sealing a silicon cover with an etched cavity of the chip. The system operation resembles that of a conventional coulometric titration system very closely. The sample was first injected into the cavity and the... 

Here is an example from my research group s work on biosensors. The sensor has an electrode modified with a peptide that binds a target metal ion, which is then electrochemically reduced, the current being proportional to the concentration of metal. The electrode is calibrated with solutions of known concentration. Two experimental designs were used. The first was a two-level factorial design to find the effects of temperature, pH, added salt, and accumulation time. Spreadsheet 3.1 shows the design. 

The five-layered modified electrode was calibrated for 8 using various concentrations of the analyte. The calibration curve is approximately linear with respect to [8], demonstrating that the sensor is not easily saturated. The detection limit for 8 is as low as 1 x 10-6M. For a modified electrode using /V,/V -diaminoethyl-4,4 -bipyridinium... 

---