Impedance detection time

The concept of impedance microbiology is more than a century old however, it gained its popularity only in the mid-seventies. Impedance is based on the changes in conductance in a medium due to the microbial breakdown of inert substrates into electrically charged ionic compounds and acidic by-products. The detection time, that is, the time necessary for... 

In addition to comparing the sum of squares, the experimental and simulated data should be compared by using complex plane and Bode plots. The phase-angle Bode plot is particularly sensitive in detecting time constants. Boukamp proposed to study the residual sum of squares after subtracting the assumed model values from the total impedance data. If the model is valid, the residuals should behave randomly. If they display regular tendencies, it may mean that the model is not correct and further elements should be added. However, the variations of the residuals should be statistically important. 

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. 

Impedance Detection, Fig. 29 Schematic for transformation between frequency and time domain... 

A double IDpEs based flow cell was successfully used in an impedance biosensor for the detection of E. coli 0157 H7 in a range from 8.0 to 8.2 x 10 cfii/ml after an enrichment growth of 14.7 and 0.8 h, respectively. A logarithmic linear relationship between detection time (Td) in h and initial cell concentration (Aq) in cfu/ml was 7b = - 1.73 log Aq -I-14.62, with R = 0.93. Specificity of this impedance biosensor could be ensured by the use of a... 

There are important figures of merit (5) that describe the performance of a photodetector. These are responsivity, noise, noise equivalent power, detectivity, and response time (2,6). However, there are several related parameters of measurement, eg, temperature of operation, bias power, spectral response, background photon flux, noise spectra, impedance, and linearity. Operational concerns include detector-element size, uniformity of response, array density, reflabiUty, cooling time, radiation tolerance, vibration and shock resistance, shelf life, availabiUty of arrays, and cost. 

The chemical world is often divided into measurers and makers of molecules. This division has deep historic roots, but it artificially impedes taking advantage of both aspects of the chemical sciences. Of key importance to all forms of chemistry are instruments and techniques that allow examination, in space and in time, of the composition and characterization of a chemical system under study. To achieve this end in a practical manner, these instruments will need to multiplex several analytical methods. They will need to meet one or more of the requirements for characterization of the products of combinatorial chemical synthesis, correlation of molecular structure with dynamic processes, high-resolution definition of three-dimensional structures and the dynamics of then-formation, and remote detection and telemetry. 

It has been shown that radio frequency impedance (RFI) is an effective tool for moifitoring cell density and cell growth of bioprocesses. The fermentation process, quite complex, is oftentimes difficult to sample and monitor. The RFI measurement could detect cell viability of Escherichia coli during the fermentation, serving as a qualitative measure of the metabolic load of the cell, and thus provide an in situ indicator of the optimal harvesting times. 

Microbial metabolism results in an increase in both conductance and capacitance causing a decrease in impedance and a consequent increase in admittance. In the Rapid Automated Bacterial Impedance Technique (RABIT) system, the admittance was plotted against time to provide results (Bolton, 1990). The final electrical signal is frequency- and temperature dependent and it has a conductive and a capacitive component. At present, impedance instruments are able to detect 10 —10 bacteria/ml (Ivnitski et ah, 2000). Several commercially available systems are operated... 

Upadhyay, P., Patra, A. K., Mukhopadhyay, R., and Panda, A. K. (2001). Real time detection and quantification of inclusion bodies expressed in Escherichia coli by impedance measurements. Biotechnol. Lett. 23, 839-843. 

Application of Impedance. Initial use of impedance centered on the conductive measurement of microbial metabolic products. As microorganisms grow, their metabolic products increase the conductivity of a medium. For example, the conductivity of putrefying defibrinated blood increased over time (30). Clinical microbiologists used impedance to detect urinary tract infections in half the time of standard methods (31). 

Several objective methods are available to determine the freshness of shrimp however, many require a) raw products for analysis, b) complex chemistry and equipment for testing, or c) highly trained technicians. Additionally, some of these methods require extensive analysis time making results meaningless if product has already spoiled or has been released to consumers. Results for the impedance method discussed in the present paper demonstrate that spoilage of raw or thermally processed shrimp can be detect in 30 min with easy sample preparation. The limitations of this method revolve around the requirement for authentic fresh frozen standards as a basis for comparison. Further research is necessary to define the effect of seasonal and geographical variations, and species and size difference. The sum of these factors will likely affect the reproducibility of this method. 

The pH meter is a specialized voltmeter that measures the potential difference (in mV) between the sensing and reference electrode and converts it to a display of pH. To provide an accurate measurement of the voltage of an extremely high resistance electrode (108 Q) [5], this specialized voltmeter must be designed with high input resistance or impedance characteristics (100 times that of the electrode used). Since the measurement potential difference per pH change is very small (59.16 mV/pH unit at 25°C), a reliable amplifier in the pH meter is also essential. It should be sufficiently sensitive to detect changes of at least 0.05 pH unit (or 3 mV). 

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