Electrolysis: processes – compositions used therein – and methods – Electrolytic analysis or testing – For oxygen or oxygen containing compound
Patent
1996-01-05
1998-02-24
Tung, T.
Electrolysis: processes, compositions used therein, and methods
Electrolytic analysis or testing
For oxygen or oxygen containing compound
204415, 205781, G01N 27404
Patent
active
057208700
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a method and device for determining the concentrations of one or more gases in a fluid. The fluid may be in the liquid or gas phase and is, for example, a body fluid such as serum. The invention is of primary interest for determining the concentrations of oxygen and carbon dioxide, as well as nitrous oxide and certain anaesthetic gases. But the principles of the invention are applicable to other gases.
2. Discussion of Prior Art
The continuous measurement of O.sub.2 and CO.sub.2 in clinical medicine has led to a whole industry of measurement devices. In the blood, oxygen is measured by the amperometric Clark PO.sub.2 electrode; and CO.sub.2 is measured by the potentiometric (glass electrode) Stow-Severinghaus electrode. Thus two sensors, working on entirely different principles, have to be employed whenever PO.sub.2 and PCO.sub.2 are measured. Blood-gas analyzers therefore use two separate sensors. Intravascular measurements can only be made for PO.sub.2, using Clark cells fabricated on the tip of a polymer catheter. It has proved impossible, so far, to miniaturize the glass electrode, and so measure intravascular PCO.sub.2, with the Stow-Severinghaus technique. Paediatric intravascular oxygen sensors have been successfully developed, first by D. G. Searle and then by Hoffman la Roche, for paediatric use, and these sensors are now manufactured by Biomedical Sensors Ltd. (High Wycombe).
In the gaseous phase, oxygen is measured with Clark-type sensors for steady-state analysis (e.g. for anaesthetic machines); and by fast paramagnetic analyzers for breath-by-breath analysis. Expired CO.sub.2 is almost inevitably measured with an infrared analyzer.
Outside medicine, as the control of CO.sub.2 increases in importance in various technologies, there is an ever growing need for inexpensive CO.sub.2 sensors with high sensitivity and selectivity. Such examples include the fermentation industry in general; brewing; on-line industrial monitoring; pollution measurement; CO.sub.2 level measurement in large auditoria; vehicle exhaust analysis, etc. In many instances it would be a great advantage to be able to measure O.sub.2 simultaneously, with the same sensor measuring both O.sub.2 and CO.sub.2, and using the same analysis principle.
At potentials of the order of -0.5 to -1.0 V, against Ag/AgCl reference electrode, in an aprotic solvent such as DMSO, oxygen in solution is reduced by the reaction:
This superoxide radical is stable for short periods in non-aqueous solvents. But it reacts rapidly with carbon dioxide, by a series of reactions which may be summarised as:
At potentials in the range -1.5 to -2.5 V, dissolved carbon dioxide is reduced, initially by virtue of the reaction:
European patent 162622 describes a gas sensor and method which used reactions (1) and (2) to provide a simultaneous determination of oxygen and carbon dioxide concentrations. There was described a pulsed CO.sub.2 titration technique, whereby the electrode surface was kept deliberately large in order to produce enough O.sub.2..sup.- to consume all the CO.sub.2 present. A pulsed voltage sufficiently negative to reduce the O.sub.2 molecule (but not sufficiently negative to reduce CO.sub.2) was first applied to the electrode surface, followed by an oxidizing pulse to oxidize those O.sub.2..sup.- ions which had remained after reacting with the CO.sub.2 molecules.
The problems with this technique were that a large cathode surface was needed, leading to high sample consumption; a complicated mathematical relationship was required to extract the CO.sub.2 concentration, and the measured O.sub.2 concentration was complicated by the enhancement of its signal from the chemical reactions (1) and (2) shown above.
SUMMARY OF THE INVENTION
It is an object of this invention to provide a device and method by which these problems may be overcome. This is achieved by using reactions (1) and (3) above under conditions to minimize interference from reaction (2).
Apart from oxygen a
REFERENCES:
patent: 4062750 (1977-12-01), Butler
patent: 4148305 (1979-04-01), Reichenberger
patent: 4851088 (1989-07-01), Chandrasekhar et al.
Compton et al, "Novel Amperometric Senser for the Detection of the Anaesthetic Gases Isoflurane and Nitrous Oxide", J. Chem. Soc. Faraday Trans., 1990 Month Unavailable, 86(7), pp. 1077-1081.
Clark David
Hahn Clive
Isis Innovation Limited
Tung T.
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