Zero volume cell

DETAILED DESCRIPTION Apparatus and methods are provided for measuring, in an electrochemical cell, an analyte in a fluid medium.
The apparatus employs a cell having a working and a controlling electrode and, preferably, a reference electrode.
The cell also includes means for varying the volume of the cell, means for introducing and removing fluid from the cell, and, where the working electrode which is used is a photo-responsive electrode, illumination means for irradiating a photo-responsive surface of the working electrode.
In the preferred embodiment of the invention, the working and controlling electrodes comprise or are mounted to or coated on first and second elements in displaceable relationship.
The volume of the cell can be varied from a relatively large volume for convenient introduction and removal of the fluid medium, as well as for reaction between relatively large amounts of one or more dilute reactants to a small volume during measurement in which the fluid medium is distributed in a reproducibly thin layer at the surface of the working electrode.
The layer of fluid is, during the measurement, sufficiently thin that the interference effects of bulk solution on the electrode response are substantially reduced or eliminated and the small volume of fluid used is utilized as efficiently as possible.
The methods find potential application with specific binding pairs of species, where one of the pair is bound to a solid support, and is concentrated in the vicinity of the working electrode surface during measurement.
The other member of the pair will normally react with its homologous member and becomes bound to the solid support.
The reactions between the bound and unbound members can be performed in a relatively large volume thereafter, most of the fluid can be evacuated from the cell and the electrochemical measurement made in the absence of most of the assay medium.
The method finds particular application where the bulk medium tends to react with the agent for surface modification or unduly dilute the agent