Sample impedance

The circuit equivalent taking into account parasitic effects consists of a contact resistance (Rs), a lead inductance Ls in series as well as a stray capacitance C() and the resistance of the substrate material between the leads 1/G0 parallel to the impedance of the sample. These parasitic effects, resulting from the compact arrangement of the electrodes can be eliminated by an offset adjustment. Capacitive parasitic effects are acquired by compensation measurements. The electrode array comprises some IDCs, which have almost negligible parasitic pathway effects, like position 5-5 (Fig. 11.1), where the conductor pathways are perpendicular to each other.22 These positions show the smallest capacities, which result only from the IDC and can be taken to determine the parasitic capacitive values of the other positions.24 Inductive parasitic effects Ls are acquired by data fitting and elimination from the sample impedance. Resistances of the conductor path (Rs generally <10 mQ) and conductivity of the substrate material (1/G0, >20mQ, exceeds measurement limit of the impedance analyser) are unaccounted. 

A fixed resistor R especially for low frequencies f suffers several limitations. Therefore, the resistor R is replaced by an amplifier with a variable gain according to Fig. 12.7. This results in a variable impedance Zx(ffl) which can be adjusted to the impedance of the sample Zs(m). For the sample impedance, then... 

To derive Eq. 12.40 ideal coaxial lines have to be assumed which is not the case in practice. Therefore, calibration procedures have to be applied. First, the influence of the measuring cell has to be obtained and considered during the calculation of the sample impedance. Second, the direction-dependent resistance of the line has to be measured by a second calibration procedure because it cannot be obtained by an equivalent circuit diagram. 

Z Cco) Zr( >) Sample impedance Reference impedance < .. > Correlation function Averaged quantities... 

In zone electrophoretic techniques described so far, even if all the care Is taken to layer the sample over the gel, the sample can never be loaded In a sharp band and this diffused layer of sample Impedes In a sharp resolution of the components. Disc gel electrophoresis (so called because of the discontinuous buffer employed and discoid appearance of the macromolecular zones) is a modification of conventional zone electrophoresis, which allows the sample to enter the gel as a sharp band, thereby helping further resolution. Here the macromolecule mixture to be analyzed Is subjected to an electric field in a retarding gel support that is separated Into two sections differing In porosity and buffered at different pHs. The macromolecular mixture migrates from the more porous into the less porous gel, a process accompanied by a change In pH. As a result, each macromolecular species becomes concentrated into a very thin, sheirp band, producing much higher resolution than can be achieved In a continuous buffer. 

A kinetic study of the hydrolysis of woods and purified plant polysaccharides in 75 % sulphuric acid was monitored by electrical conductivity measurements. The coefficients of resistance of all polysaccharide substrates increased with increasing hydrolysis time. From these measurements, the degrees of polymerization of cellulose, xylan, and inulin were estimated to be 1900, 66, and 16 respectively. The polysaccharide components of various woods and pulps can be hydrolysed with trifluoracetic acid which, in contrast to sulphuric acid hydrolysis, does not require a neutralization step since trifluoracetic acid is volatile. The presence of lignin in the wood samples impeded the hydrolysis of the polysaccharides, requiring longer reaction times and correction factors to compensate for loss of monosaccharide by degradation reactions. 

A circuit which avoids the need for a differential amplifier with high common-mode rejection is shown in Figure 2.14 (Schwan and Ferris, 1968). The null detector is a single-ended grounded amplifier which acts as a bridge null detector. Different parts of the total sample impedance are balanced against other parts of the sample in series with the variable RC network. Let the subscripts b and c represent the impedance values of the RC network which apply for the different balance equations when the detector is con-... 

Coaxial line reflectometry has to be employed at frequencies from 1 MHz to -1 GHz. In contrast to the low-frequency techniques already described, here the sample capacitor is used as the termination of a low-loss precision coaxial line. The complex reflection factor is measured with a microwave reflectometer at the analyzer end of the fine, depending on the sample impedance. For this purpose, the incoming and reflected waves are separated with two directional couplers and their amplitudes are measured. The BDS concept 70 is a commercially available system (Novocontrol Instruments) that uses the coaxial line reflectometry method. This system covers the frequency range between IMHz and 3 GHz with loss factor tan 5 resolution of less than 3 x 10 . The... 

Unfortunately, for samples with bulk resistance in excess of 1 Mohm accurate impedance characterization becomes difficult and requires a careful analysis of several contributions. Traditional "aqueous type" reversible reference electrodes such as Ag/AgCl cannot be used in nonaqueous environments, and metal "pseudoreference" electrodes have to be employed [25,27], That leads to experimental complications, represented in Figure 8-11. The frequency analysis of the input, reference, and sample impedances demonstrated that the difference between Z p and Z p is minimal at high frequencies, leading to a relatively high /,... 

Four-electrode cell impedance measurements are reliable if the sample impedance is sufficiently small with respect to the input impedance... 

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