Response of an enzyme-modified pH-sensitive ion selective device; consideration of the influence of the buffering capacity of the analyte solution

Abstract

Equations describing the response of an enzyme-modified pH-sensitive ion selective device in buffered solution are derived. Approximate analytical solutions for the coupled mass transport, enzyme kinetics and buffer protonation kinetics are considered, corresponding with two limiting cases; on the one hand, where the concentration of pH buffer greatly exceeds that of the enzyme substrate and, on the other, where the concentration of substrate exceeds that of buffer. Qualitative consideration is given to the intermediate region between these two limits which is, perhaps, the region of greatest practical interest. It is concluded that the device response will be dependent upon solution pH and buffering capacity as well as substrate concentration, implying that the first two of these parameters must be carefully controlled if the device is to be employed for determination of the last of them. The device cannot be expected to show either a simple linear or logarithmic potential response to substrate over any extended concentration range, which would appear to represent a further limitation on its practical exploitation. In the absence of buffer and at around neutral pH, a detection limit in the substrate concentration range 10 −6 - 10 −5 M is predicted. In the presence of buffer the sensitivity is significantly diminished and, in around 10 −2 M buffer, for example, a sensitivity to substrate concentrations significantly below 19 −3 M is not to be expected. This predicted behaviour is generally in agreement with available experimental data.