Alternating current amperometry at a carbon paste electrode with charging current nulling

Gelbert, Mark B.; Curran, David J.

doi:10.1021/ac00157a013

Cited by 1 publication

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“…The phase angle adjustment of the reference square wave input to the lock-in amplifier was made as described elsewhere [7]. Alternating current amperometric experiments were done by setting the frequency to 10 Hz and the peak amplitude to the desired value, adjusting voltage drop (IR) compensation, and adjusting the dc potentia!…”

Section: Methodsmentioning

confidence: 99%

An instrument for alternating current voltammetry featuring a digital phase‐sensitive detector: Application to flow‐injection analysis using ac amperometry

1990

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An instrument, which incorporates a lock-in amplifier based on a digital phase sensitive detector, has been designed and constructed for ac electrochemical measurements of solutions containing low concentrations of analyte. The performance of the lock-in amplifier was characterized experimentally. Phase angle resolution is better than 0.01", and signal capture ratios up to about 80,000 can be attained. An up/down counter replaces the multiplier of the typical analog phase-sensitive detector. Provision was made to sum the net count over a preselected number of cycles of the reference signal to improve the sensitivity and signal-to-noise ratio of the phase-sensitive detector. The lock-in amplifier output can be read on a recorder via a digital-to-analog converter or sent to a computer. Overall instrument performance is demonstrated using ac amperometry at a carbon paste electrode and flow injection techniques to determine o-dianisidine. Peak currents corresponding to a solution concentration of 6 nM in the detector were successfully measured. MRODUCTIONThe advantages of phase-sensitive detection in the determination of the faradaic current by alternating current electrochemical techniques are well known. The lock-in amplifier discriminates against charging current because the output of its phase-sensitive detector (PSD) is proportional to the cosine of the phase angle between the measured current and the applied signal. In the case of charging current, the phase angle is 90" and the cosine function is zero. The ability to reject the quadrature component is often of little interest in spectroscopic applications of the lock-in amplifier, but it is critical in electroanalytical applications. The importance of quadrature signal rejection places unusual demands on some features of the lock-in amplifier, particularly when measurements are made on very dilute solutions of the sought constituent. For example, since the charging current is not usually dependent on the concentration of analyte while the faradaic current is, the ratio of faradaic current to charging current can be on the order of 0.00001 as nanomolar concentrations are approached. If considers this as the signal-to-noise ratio, it is indeed small.Under these circumstances, two key features of the lock-in amplifier are the phase angle resolution and the signal capture ratio. The former is important because the faradaic component of the current vector is so small relative to the charging current component, and the latter is a measure of the ability of the device to extract signal from noise. Both of these characteristics relate to the quality of the PSD. The major operations carried out by the PSD are multiplication followed by low-pass filtering. A choice of analog or digital implementation is available.In this work, we have elected to use digital implementation of the PSD because of the better resolution this offers over analog circuits. A choice then has to be made between software or hardware approaches. Software implementation has been demonstrat...

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Section: Methodsmentioning

confidence: 99%