The reaction of superoxide with carbon dioxide is studied using voltammetry and potential step chronoamperometry at polycrystalline gold disk microelectrodes in a DMSO electrolyte. In agreement with prior work, it is found that a reaction occurs between the superoxide anion radical and carbon dioxide, effectively precluding their simultaneous detection at low levels of carbon dioxide. The reaction rate is found to be first-order with respect to both carbon dioxide and superoxide, consistent with an ECE or DISP1 type process. A rate constant is determined for this reaction based upon two independent methods: fast scan cyclic voltammetric measurements and steady-state voltammetric signals. These methods yield a consistent rate constant of 3.7 ( 1.6 × 10 5 M -1 s -1 . Potential step chronoamperometric measurements reveal that oxygen adsorbs onto a gold electrode surface, to form a monolayer both in the presence and absence of carbon dioxide. A rate constant for the reduction of surface-bound oxygen to superoxide is reported.

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This tutorial review charts the development of electrochemical sensors for the analysis of blood-gases, gases and vapours in clinical medicine over the past four decades. The development of each sensor is set in its historical and clinical context, and the first part of the review concentrates on aqueous electrolyte electrochemistry and on those sensors which have made a major impact on the clinical measurement of the partial pressures of oxygen and carbon dioxide in the blood. The electrochemical interference effects of anaesthetic agents on these measurements are also described. Those electrochemical sensors which have failed, in the past, to make a clear impact in this area are not considered, but the few attempts to devise aqueous electrolyte electrochemical sensors for anaesthetic agent measurement are reviewed. The second part of the review describes the chequered history of the development of non-aqueous solvent electrochemical sensors to measure the partial pressures of oxygen and carbon dioxide, in both the presence and absence of each other, in the gas phase. The last part of the review examines various attempts, using non-aqueous solvent electrochemistry, to measure the concentration of inhalational anaesthetic vapours in the gas phase. These sensors have yet to make an impact on clinical practice. Throughout this tutorial review, theoretical models of membrane-covered electrochemical sensors are described where appropriate. This review represents a personal view of the development of electrochemical sensors for clinical measurement, and it is therefore necessarily selective in its approach and emphasis.

The equations governing oxygen transport from blood to tissue are presented for a cylindrical tissue compartment, with blood flowing along a co-axial cylindrical capillary inside the tissue. These governing equations take account of: (i) the non-linear reactions between oxygen and haemoglobin in blood and between oxygen and myoglobin in tissue; (ii) diffusion of oxygen in both the axial and radial directions; and (iii) convection of haemoglobin and plasma in the capillary. A non-dimensional analysis is carried out to assess some assumptions made in previous studies. It is predicted that: (i) there is a boundary layer for oxygen partial pressure but not for haemoglobin or myoglobin oxygen saturation close to the inflow boundary in the capillary; (ii) axial diffusion may not be neglected everywhere in the model; (iii) the reaction between oxygen and both haemoglobin and myoglobin may be assumed to be instantaneous in nearly all cases; and (iv) the effect of myoglobin is only significant for tissue with a low oxygen partial pressure. These predictions are validated by solving the full equations numerically and are then interpreted physically.

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