Impedance artifacts

Polar Cell Systems for Membrane Transport Studies Direct current electrical measurement in epithelia steady-state and transient analysis, 171, 607 impedance analysis in tight epithelia, 171, 628 electrical impedance analysis of leaky epithelia theory, techniques, and leak artifact problems, 171, 642 patch-clamp experiments in epithelia activation by hormones or neurotransmitters, 171, 663 ionic permeation mechanisms in epithelia biionic potentials, dilution potentials, conductances, and streaming potentials, 171, 678 use of ionophores in epithelia characterizing membrane properties, 171, 715 cultures as epithelial models porous-bottom culture dishes for studying transport and differentiation, 171, 736 volume regulation in epithelia experimental approaches, 171, 744 scanning electrode localization of transport pathways in epithelial tissues, 171, 792. 

In contrast to the case for copper, for silver a reduction of the tarnish rate by circa 100-fold has been observed as tarnish layers increase (Graedel et al., 1985), thus indicating that to a degree, the tarnish layers themselves impede further tarnishing. It is known that major silver tarnish can occur at concentrations of circa 100pptv and it has been shown that in effect it is the sum of OCS and H2S along with artifact reactivity, which determines the tarnish rate (Ankersmit, Tennent and Watts, 2005). 

Low impedance connectors for electrochemical measurements taken with equipment that is located outside the glove box. High impedance or bad shielding of these electrical lines may introduce artifacts. 

Finally, it should be mentioned that frequently, as in the case of Ti02, a frequency dispersion of the slope of the Mott-Schottky curves has been observed (see e.g. [67,68]), although the Hatband potential was not affected. Modern methods, such as impedance spectroscopy, have shown, however, that this frequency dispersion is an artifact [59]. 

A distinction is drawn in equation (21.1) between stochastic errors that are randomly distributed about a mean value of zero, errors caused by the lack of fit of a model, and experimental bias errors that are propagated through the model. The problem of interpretation of impedance data is therefore defined to consist of two parts one of identification of experimental errors, which includes assessment of consistency with the Kramers-Kronig relations (see Chapter 22), and one of fitting (see Chapter 19), which entails model identification, selection of weighting strategies, and examination of residual errors. The error analysis provides information that can be incorporated into regression of process models. The experimental bias errors, as referred to here, may be caused by nonstationary processes or by instrumental artifacts. 

Bias errors are systematic errors that do not have a mean value of zero and that cannot be attributed to an inadequate descriptive model of the system. Bias errors can arise from instrument artifacts, parts of the measured system that are not part of the system under investigation, and nonstationary behavior of the system. Some types of bias errors lead the data to be inconsistent with the Kramers-Kronig relations. In those cases, bias errors can be identified by checking the impedance data for inconsistencies with the Kramers-Kronig relations. As some bias errors are themselves consistent with the Kramers-Kronig relations, the Kramers-Kronig relations cannot be viewed as providing a definitive tool for identification of bias errors. 

The impedance response of low-impedance systems may include the finite impedance behavior of wires and connectors. These may be considered, from the perspective of model identification, as yielding artifacts in the mezisured response. Such artifacts may be simply resistive, but may also exhibit a capacitive or even an inductive frequency dependence. Such artifacts will be generally consistent with the Kramers-Kronig relations. 

Most electrochemical systems show some nonstationary behavior due, for example, to growth of surface films, changes in concentrations of reactants or products in the electrolyte, or changes in surface reactivity. As discussed in Section 21.3.4, the issue is not whether a system is perfectly stationary, but, rather, whether the system has chamged substantially during the course of the impedance measurement. The Kramers-Kronig relations are particularly useful for identification of artifacts introduced by nonstationary behavior. These artifacts are most visible at low frequencies, but can be seen at all frequencies if the system change is sufficiently rapid. 

Remember 21.2 Bias errors in impedance measurements can arise from instrument artifacts, parts of the measured system that are not part of the system under investigation, and nonstationary behavior of the system. 

It should be noted that the error analysis methods using measurement models are sensitive to data outliers. Occasionally, outliers can be attributed to external influences. Most often, outliers appear near the line frequency and at the beginning of an impedance measurement. Data collected within 5 Hz of the line frequency and its first harmonic (e.g., 50 and 100 Hz in Europe or 60 and 120 Hz in the United States) should be deleted. Startup transients cause some systems to exhibit a detectable artifact at the first frequency measured. This point, too, should be deleted. 

In principle, the Kramers-Kronig relations can be used to determine whether the impedance spectrum of a given system has been influenced by bias errors caused, for example, by instrumental artifacts or time-dependent phenomena. Although this information is critical to the analysis of impedance data, the Kramers-Kronig relations have not found widespread use in the analysis and interpretation of electrochemical impedance spectroscopy data due to difficulties with their application. The integral relations require data for frequencies ranging from zero to infinity, but the experimental frequency range is necessarily constrained by instrumental limitations or by noise attributable to the instability of the electrode. 

Ionic Conductivity. The electrical conductivity measurements were performed using a Hewlett Packard model 4192 impedance analyzer under computer control, using a conductance cell similar to that described by Pauly and Schwan (5). The conductivity measurements were essentially constant between 1-100 kHz, ruling out electrode polarization or other artifacts. In 0/W microemulsions, no appreciable dielectric relaxation effects are expected or observed below 1 GHz (U. 

To avoid artifacts in impedance measurements when modulating the input voltage of the potentiostat at high frequencies (>10 kHz), the reference electrode should be short-circuited via a capacitance of 10 nF and a Pt wire dipped into the solution in the main part of the cell (Fig. 4.2). The surface of the counter electrode should be sufficiently large so that its interface with the electrolyte does not influence the current-potential curve. Usually a platinized Pt sheet is used as a counter electrode. The electrolyte is made conductive by adding an inert salt of a concentration in the range of 10-3-10- M. 

Localized impedance measurements with an SVE have been described [202] and limitations and artifacts observed when using an SVE under specific corrosion conditions were discussed in detail [203]. 

The silver-silver chloride electrode has characteristics similar to a perfectly nonpolarizable electrode and is practical for use in many biomedical applications. The electrode (Figure 4.1a) consists of a silver base structure that is coated with a layer of the ionic compound silver chloride. Some of the silver chloride when exposed to light is reduced to metallic silver hence, a typical silver-silver chloride electrode has finely divided metallic silver within a matrix of silver chloride on its surface. Because silver chloride is relatively insoluble in aqueous solutions, this surface remains stable. Moreover, because there is minimal polarization associated with this electrode, motion artifact is reduced compared to polarizable electrodes such as the platinum electrode. Furthermore, owing to the reduction in polarization, there is also a smaller effect of frequency on electrode impedance, especially at low frequencies. 

Such a method was used in the literature for the determination of impedances [99-101], and a commercial apparatus [102] applying a current step was described. Taking the FT of the derivative of the potential and current versus time gives the impedance as a function of frequency. However, some authors [100, 101] tried to extrapolate the obtained results to low frequencies beyond the experimental values. If the data are acquired during time T = NAt, the information in the measured signal is obtained for frequencies from l/T up to the Nyquist frequency l/2At, where At is the sampling time. It has been shown [103] that extrapolating impedances to frequencies lower than l/T introduces artifacts. In addition, the measured... 

One of the most often found artifacts is related to the reference electrode, its impedance, and the distance of the Luggin capillary from the electrode. Its impedance must be low, and if the distance from the working electrode is changed, only the solution resistance should change without affecting the shape of the impedances. 

Other tests that are of great value are to check the stability and linearity of the cell under the imposed test conditions. The stability may easily be checked by repeating the impedance tests on the same cell within a short period of time and in the same environmental conditions to check if the impedance results are repeatable and stable. If this is not done, time can be wasted in trying to interpret artifacts that are simply due to an unstable cell. In addition, it is very important to test for cell linearity by referring to the tests using harmonic or FPT analysis that were suggested in section 3.2.1.5. 

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