Phasor transform to extract glucose and ascorbic acid data in an amperometric sensor

Abstract

A method for separating the signals from glucose and ascorbic acid on a single recognition surface using an ac immittance technique is presented. It is proposed that each oxidation process can be represented by a unique vector based on ψ and Y_O, and that the concentration of each analyte can be determined by monitoring the change in the admittance magnitude in the direction of the characteristic angle for that particular species. The total Faradaic admittance (_F,total) for all electroactive species present is given by a linear combination of the independent vectors from the different species. In the system tested, the analytes are glucose and ascorbic acid, the former being estimated via the measurand, hydrogen peroxide. Thus, one of the electroactive species (hydrogen peroxide) is not a bulk solution species, but is ‘generated’ in the enzyme matrix. The admittance measurements from ascorbic acid and the enzyme-generated hydrogen peroxide showed the characteristic phase angles of each oxidation signal, allowing for good spatial resolution. The behaviour of each of these analytes is presented and calibration curves tested. Based on the calibration curves and the basis vectors, samples containing both glucose and ascorbic acid were measured by transforming the measured total admittance from the complex Cartesian space into ‘analyte space’, where the X–Y axes are given by the basis vectors ŷ_EGHP,GOD and ŷ_AA,GOD, respectively.