Amplifier spurious input current components in electrode-electrolyte interface impedance measurements.
BACKGROUND
In Impedance Microbiology, the time during which the measuring equipment is connected to the bipolar cells is rather long, usually between 6 to 24 hrs for microorganisms with duplication times in the order of less than one hour and concentrations ranging from 10(1) to 10(7) [CFU/ml]. Under these conditions, the electrode-electrolyte interface impedance may show a slow drift of about 2%/hr. By and large, growth curves superimposed on such drift do not stabilize, are less reproducible, and keep on distorting all over the measurement of the temporal reactive or resistive records due to interface changes, in turn originated in bacterial activity. This problem has been found when growth curves were obtained by means of impedance analyzers or with impedance bridges using different types of operational amplifiers.


METHODS
Suspecting that the input circuitry was the culprit of the deleterious effect, we used for that matter (a) ultra-low bias current amplifiers, (b) isolating relays for the selection of cells, and (c) a shorter connection time, so that the relays were maintained opened after the readings, to bring down such spurious drift to a negligible value. Bacterial growth curves were obtained in order to test their quality.


RESULTS
It was demonstrated that the drift decreases ten fold when the circuit remained connected to the cell for a short time between measurements, so that the distortion became truly negligible. Improvement due to better-input amplifiers was not as good as by reducing the connection time. Moreover, temperature effects were insignificant with a regulation of +/- 0.2 [ degrees C]. Frequency did not influence either.


CONCLUSIONS
The drift originated either at the dc input bias offset current (Ios) of the integrated circuits, or in discrete transistors connected directly to the electrodes immersed in the cells, depending on the particular circuit arrangement. Reduction of the connection time was the best countermeasure.
Felice
Carmelo J
CJ
Departamento de Bioingeniería, Facultad de Ciencias Exactas y Tecnología, Universidad Nacional de Tucumán, Argentina. cfelice@herrera.unt.edu.ar <cfelice@herrera.unt.edu.ar>
Madrid
Rossana E
RE
Valentinuzzi
Max E
ME
D000669:Amplifiers, Electronic; D000818:Animals; D016477:Artifacts; D002468:Cell Physiological Phenomena; D017097:Electric Impedance; D004566:Electrodes; D004573:Electrolytes / Q000737:chemistry; D004867:Equipment Design; D019544:Equipment Failure Analysis; D006801:Humans; D008564:Membrane Potentials / Q000502:physiology*; D010992:Plethysmography, Impedance / Q000295:instrumentation* / Q000379:methods; D013499:Surface Properties
D004573:Electrolytes