A chemical signal-impermeable mask is positioned in the electrolyte flow
such that the mask is between a source of chemical signal and a working
electrode which senses the chemical signal transported from the source
(e.g., by diffusion). The configuration of the mask is such that the mask
prevents substantially all chemical signal transport from the chemical
signal source having a radial vector component relative to a plane of the
mask and the catalytic face of the working electrode, thus allowing
primarily only chemical signal transport that is substantially
perpendicular to the place of the mask and the catalytic surface of the
working electrode. By reducing or eliminating chemical signal radial
transport toward the working electrode, the mask thus significantly
reduces or eliminates edge effects. By substantially reducing edge effects
created by radial transport of chemical signal, it is possible to obtain a
more accurate measurement of the amount (e.g., concentration) of chemical
signal that is transported from a given area of source material.
