Heterogeneous membrane electrodes

#The invention relates to electrochemical sensors for the analysis of aqueous solutions. In particular, the invention relates to the construction of unit-use reference electrodes for such sensors.

#Prior-art electrochemical sensors typically consist of an electrochemical cell with two, sometimes three electrodes. The first electrode is responsive to a chemical species in the test solution and is called the indicator electrode. The second electrode called the reference electrode is typically non-responsive to changes in the composition of the test solution. In polarography a third, current-injecting counter electrode is sometimes used.

#As is appreciated by those in the art, the performance of an electrochemical sensor as part of a chemistry analyzer for quantitative measurement of chemicals in aqueous solutions is determined by its dose-response curve. For a linear sensor this can be uniquely determined by two coefficients: a slope and an intercept. For a dose-response curve that is non-linear, three or more coefficients may be required. As is also known in the art, a sensor's coefficients vary over time if it is used more than once. The coefficients also vary to some extent from sensor to sensor because no two sensors can be manufactured identically. Therefore, a calibration is generally required to uniquely determine a sensor's dose-response curve. In an automated chemistry analyzer the calibration is provided by fluidic elements (calibration fluids, pumps, valves, conduits etc.) contained within the analyzer. If a sensor is deployed as a reusable device, the chemistry analyzer's calibration fluidics often provides for at least two calibration points and a wash solution. This is because the slope and intercept of the dose-response curve can change through repeated uses. For a unit-use device no calibration would be required if the slope and intercept were sufficiently reproducible from sensor to sensor during manufacture and storage. A single calibrator would be required if either one of the coefficients was reproducible, the other not, and two calibrators if neither coefficient was reproducible (more calibrators could be required for devices with non-linear dose-response curves).

#Preferred embodiments of the invention will now be further described by way of example only and with reference to the attached drawings, wherein

#FIG. 1A is a principle schematic cross-section through an electrode in accordance with the invention;

#FIGS. 1B and 1C are horizontal cross-sections of preferred embodiments of the device according to this invention;

#In the most general construction as shown in FIG. 1A, electrodes in accordance with the invention include a planar electric conductor 1. The planar conductor is coplanar with, and contacted by an insulator 2 with a thoroughgoing aperture 3 defining an electrode region. A heterogeneous membrane 4 is constructed for direct contact with an aqueous sample fluid, is located in the electrode region and extends through the aperture 3 for electric contact with the conductor 1. The heterogeneous membrane comprises a hydrophobic gas permeable compartment and a hydrophilic electrolyte permeable compartment.

#Heterogeneous Membrane Electrodes

#The heterogeneous membranes according to this invention are materials consisting of an intimate admixture of two components. The first is a hydrophobic gas permeable material component, the second is a hydrophilic electrolyte conducting component. In a preferred composition, the hydrophobic component is in excess by volume over the hydrophilic component. The essential transport property of the heterogeneous membrane of the invention is that the membrane diffusion coefficient for gas through the hydrophobic compartment (water vapor for wet-up) is significantly larger than the membrane diffusion coefficient of species dissolved in the water (ions and neutral non-volatile molecules) contained within the hydrophilic compartment. We have found that sensors can be made with adequate performance attributes when the ratio of these diffusion coefficients is about 10, but preferably the ratio should be at least 50 and better still greater than 100.