Data-processing method to reduce error coefficients for membrane base analytical systems. I. Amperometric-based sensor evaluated for quantification of oxygen.

Abstract

This paper describes the use of a predictive, curve-fitting method to reduce the effects of experimental variables on results obtained with membrane-based devices. Multipoint data from the transient regions of responses are used with suitable models and curve-fitting methods to predict the signal that would be measured for the system at equilibrium. The resulting equilibrium response usually is much less dependent on experimental variables than the transient responses used to predict it. The approach is evaluated for the membrane-based amperometric electrode for oxygen. Current vs time data are used to predict the equilibrium current expected when oxygen concentrations are the same on both sides of the membrane. Predicted equilibrium currents vary linearly with oxygen concentration. Relative to the more common steady-state method, the sensitivity of the predictive method is about 5-fold higher, the measurement time is about 17-fold shorter and the dependencies on membrane thickness and stirring rate are 125- and 8-fold lower, respectively. Pooled standard deviations (n = 40) correspond to uncertainties in oxygen concentration of about 0.009 mmol L-1.

Improved Ruggedness for Membrane-Based Amperometric Sensors Using a Pulsed Amperometric Method

“…In the conventional applications of the oxygen electrode, in which a fixed voltage is applied to the electrodes, we have observed experimentally that the rate of electrochemical reaction inside the membrane is approximately 10-fold faster than diffusion of oxygen across a single layer of the membrane. For example, in an early study, the pseudo-first-order rate constant for equilibration of oxygen across a single layer of the isolation membrane was determined to be 0.2 s -1 , corresponding to a half-life of about 3.5 s. In a subsequent study, it was determined that the pseudo-first-order rate constant for electrochemical depletion of oxygen inside the isolation membrane is 2.3 s -1 , corresponding to a half-life of about 0.3 s. Accordingly, for the usual configuration of the oxygen electrode, it follows that the rate of electrochemical depletion of oxygen inside the membrane is approximately 10-fold faster than the rate of mass transport of oxygen across a single layer of the isolation membrane.…”

“…Earlier studies in this , and other 4,5 laboratories have shown that alternative measurement/data-processing approaches can be used to improve the ruggedness of the membrane-based oxygen sensor significantly relative to the more conventional steady-state option. All of these alternative options can be grouped into one general category, identified herein as a “pseudoequilibrium” approach.…”

“…All of these alternative options can be grouped into one general category, identified herein as a “pseudoequilibrium” approach. In all these pseudoequilibrium options described to date, − the solution inside the membrane must be equilibrated with the sample solution prior to application of the polarizing voltage. After the polarizing voltage is applied to the preequilibrated sensor, some feature of the resulting current is measured and related to analyte concentration.…”

“…In a subsequent study, the measurement objective was the Faradaic current following application of a voltage pulse of a few milliseconds, with an off-time between pulses to permit restoration of oxygen depleted by the previous pulse. In a later study, the measurement objective was the Faradaic component of the current at the instant voltage was applied after preequilibration of the oxygen electrode with the sample solution; the initial current was obtained by nonlinear extrapolation of current vs time data to the point at which polarizing voltage was applied . In the most recent study, the measurement objective was the charge corresponding to electrolysis of all the oxygen in a fixed volume of solution inside the membrane, which was obtained by nonlinear extrapolation of charge vs time data to t = 0 (the point at which polarizing voltage was applied) and t → ∞ (the point at which electrolysis current would reach zero in the absence of mass transport from the sample solution).…”

“…All of these options had desirable performance characteristics, including linear dependence on analyte concentration and reduced dependence on variables that affect rates of mass transport. However, all the approaches have one or more limitations, including the need to equilibrate the electrode and sample prior to application of the polarizing voltage, − potential interference from charging currents, − and/or inability to compensate for variables that affect electrode response. − …”