Amperometric measurement system

K. Brunt, Comparison Between the Performances of an Electrochemical Detector Flow Cell in a Potentiometric and an Amperometric Measuring System Using Glucose as a Test Compound. Analyst, 107 (1982) 1261. 

In the direct-reading instruments the emf of the cell is led through an (operational) amplifier across a standard high resistor yielding a current that is measured by a milliammeter calibrated to be read in pH units or millovolts. So, while the null-point system provides a truly potentiometric (non-faradaic) measurement where the off-balance adjustment remains limited to an interrupted temporary current draw-off, the direct-reading system represents an amperometric measurement where a continuous steady-state current draw-off takes place as long as the meter is switched on. In fact, the latter is a deflection method as a pointer indicates the pH units or millivolts by its deflection on the meter scale. 

Henderson or Plank formalisms. Mobilities for several ions can be seen in Table 18a. 1. Liquid junction potentials can become more problematic with voltammetric or amperometric measurements. For example, the redox potentials of a given analyte measured in different solvent systems cannot be directly compared, since the liquid junction potential will be different for each solvent system. However, the junction potential Ej can be constant and reproducible. It can also be very small (about 2-3 mV) if the anion and cation of the salt bridge have similar mobilities. As a result, for most practical measurements the liquid junction potential can be neglected [9]. 

Osakai et al. used a microcomputer-controlled system for the ion transfer voltammetry procedure [20]. The system used is based on a NEC PC-9801 microcomputer, which was designed by using a polarizable oil-water interface as an ion-selective electrode surface. The system was applied to the determination of acetylcholine ion by cyclic, differential pulse, and normal pulse voltammetry at the PVC-nitrobenzene gel electrode. The amperometric measurement was carried out with voltage pulses of short durations and constant amplitude. 

First ferrocene-mediated amperometric glucose biosensor by Cass et al. [135]. The work led to the development of the first electronic blood glucose measuring system which was commercialized by MediSense Inc. (later bought by Abbott Diagnostics) in 1987. 

Fig. 2. Measuring set-up (A) photograph of the piezoelectric device and flow system, the inset shows the cell holding the quartz sensor (B) sample QCM sensor with 10 MHz base frequency (as used throughout the described experiments) (C) cross-section through the piezo-cell showing the two rubber O-rings holding the quartz plate, only one side of the sensor is in contact with the fluid (D) cross-section of the cell used for combined piezoelectric and amperometric measurements, the lid also hold a titanium wire electrode and the Ag/AgCI reference electrode.

The preparation technique of the disposable sensing elements used in this flow-injection amperometric immunofiltration system demonstrated high reproducibility of the analytical parameters. The variation between the responses to a standard concentration of mortalized cells (100 cell/ml) obtained with membranes prepared from one and the same batch was 16 %. It should be noted that the data points in Figures 4, 5 and 6 (curves a) are obtained as an average of 4 independent measurements, eadi performed by an individual sensing element. The error bars in Figures 4,5 6 represent the standard deviation obtained from these 4 independent measurements. 

Naturally, in other areas with different technical requirements for the measuring system, preference will be given to other gas sensors with other sets of advantages, which depend not only on the sensor type but also on its version. This facet can be well illustrated using the example of electrochemical sensors, which can be realized either as conductometric, potentiometric, or amperometric (voltammetric) sensors (Brett and Brett 1998 Brett 2001). 

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