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 YO,
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
ŷEGHP,GOD and ŷAA,GOD, respectively.