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

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.