Sensing electrode resistance

This type of bulk property detector monitors the conductivity of the eluent. All ions from the analyte and from the buffer contribute to produce a signal. Detector response is linear over a wide range. Cell resistance is inversely proportional to electrolyte concentration. Since AC voltages must be used to avoid polarization of the sensing electrodes, the physical quantity measured is impedance, not resistance. 

It was hrst reported in 2001 [32] that the evaluation by impedance spectroscopy of a biased zirconia sensor with an attached LaFeOa-SE has shown that only the electrode resistance is a function of NO2 content. Consequently, impedance spectroscopy offers a method for directly probing the electrode reactions that are the basis for mixed-potential-type gas sensors [67]. As a result of further development by using impedance spectroscopy in zirconia-based gas sensors with oxide-SEs, a new type of YSZ-based sensor for detecting total NO and HCs at high temperatures has been proposed recently [2, 14, 21, 62, 74, 96-100]. In this case, the change in the complex impedance of the device attached with a specific oxide-SE was measured as a sensing signal. 

The electrical conductivity detector measures the conductivity of the mobile phase. Conductivity detectors are universal and nondestructive and can be used in either direct or indirect modes. The conductivity sensor is the simplest of all the detectors, consisting of only two electrodes situated in a suitable flow cell. The basis of conductivity is the forcing of ions in solution to move toward the electrode of opposite charge on the application of a potential. To prevent polarisation of the sensing electrodes, AC voltages must be used and so it is the impedance (not the resistance) of the electrode system that is actually... 

Impedance electrodes have also been investigated as detectors for microchip microdialysis applications. An on-chip microdialysis system with inline sensing electrodes for impedance detection was developed [41], Cr/Au electrodes were used to determine the electrical resistance of changes in the concentration of phosphate-buffered saline (PBS) solutions that were used to characterize the system. The system monitored concentration changes with a 210-s system response delay. The lag time was attributed to dead volume in the tubing between the syringe pumps and the microsystem. 

One of the key barriers to the more widespread adoption of voltammetric techniques was the limited range of media in which analysis could traditionally be performed, i.e., aqueous solutions containing a relatively high concentration of supporting electrolyte. This restriction arose because resistance between the working or sensing electrode and the reference electrode limited the precision with which the applied potential could be controlled. However, microelectrodes, also commonly known as ultramicroelectrodes, whose critical dimension is in the... 

Figure 13 (A) Faradaic impedance spectra of (a) the (17)-functionalized Au electrode, (b) after interaction of the sensing electrode with (18) (5 x ICf M, 15 min, 25°C), and (e) after interaction with the (19)-functionalized liposomes. Inset Faradaic impedance spectra of (a) the (17)-modifled electrode, (d) after interaction with (18a) (5x10 M), (e) after treatment with (19)-functionalized liposomes. All measurements were performed in a 0.1 M phosphate buffer (pH 7.2) in the presence of [Fe(CN)g] /[Fe(CN)g] (5 x lO"- M, 1 1) as a redox-probe. (B) Changes in the electron transfer resistance of the (17)-functionalized electrode upon treatment with the analyte DNA, (18), at different concentrations and upon the secondary amplification with the (19)-functionalized liposomes corresponding to the difference in the electron transfer resistance, AR, after amplification with the (19)-func-tionalized liposomes and the resistance of the (17)-functionalized electrode.

The aperture impedance principle of blood cell counting and sizing, also called the Coulter principle (5), exploits the high electrical resistivity of blood cell membranes. Red blood cells, white blood cells, and blood platelets can all be counted. In the aperture impedance method, blood cells are first diluted and suspended ia an electrolytic medium, then drawn through a narrow orifice (aperture) separating two electrodes (Fig. 1). In the simplest form of the method, a d-c current flows between the electrodes, which are held at different electrical potentials. The resistive cells reduce the current as the cells pass through the aperture, and the current drop is sensed as a change in the aperture resistance. 

The pH (or pI) term of the Nemst equation contains the electrode slope factor as a linear temperature relationship. This means that a pH determination requires the instantaneous input, either manual or automatic, of the prevailing temperature value into the potentiometer. In the manual procedure the temperature compensation knob is previously set on the actual value. In the automatic procedure the adjustment is permanently achieved in direct connection with a temperature probe immersed in the solution close to the indicator electrode the probe usually consists of a Pt or Ni resistance thermometer or a thermistor normally based on an NTC resistor. An interesting development in 1980 was the Orion Model 611 pH meter, in which the pH electrode itself is used to sense the solution temperature (see below). 

---