An apparatus for use in detecting analytes in a liquid sample, the apparatus comprising: a primary channel extending in a first direction through a primary channel member wherein the primary channel member is a plate or sheet; a first block comprising a first compartment defining a first regenerant channel extending in the first direction adjacent to the primary channel; a first charged barrier having exchangeable ions capable of passing ions of only one charge, positive or negative, and of blocking bulk liquid flow, the first charged barrier disposed between the primary channel member and the first block for separating the primary channel from the first regenerant channel; and a first sealing member disposed against the first charged barrier, wherein: the apparatus is arranged such that: either: a flow pathway configured to receive a regenerant is defined by the regenerant channel formed in the first block, the first sealing member and the first charge barrier; and a flow pathway configured to receive an ionic species is defined by the primary channel extending through the primary channel member and the first charged barrier; wherein the first sealing member directly provides the seal around the flow pathway in the first regenerant channel and indirectly provides the seal around the flow pathway in the primary channel by urging the first charged barrier against the primary channel member, or: a flow pathway configured to receive a regenerant is defined by the regenerant channel formed in the first block and the first charge barrier; and a flow pathway configured to receive an ionic species is defined by the primary channel extending through the primary channel member, the first sealing member and the first charged barrier; wherein the first sealing member directly provides the seal around the flow pathway in the primary channel and indirectly provides the seal around the flow pathway in the first regenerant channel by urging the first charged barrier against the first regenerant channel. a primary channel extending in a first direction through a primary channel member wherein the primary channel member is a plate or sheet; a first block comprising a first compartment defining a first regenerant channel extending in the first direction adjacent to the primary channel; a first charged barrier having exchangeable ions capable of passing ions of only one charge, positive or negative, and of blocking bulk liquid flow, the first charged barrier disposed between the primary channel member and the first block for separating the primary channel from the first regenerant channel; and a first sealing member disposed against the first charged barrier, wherein: the apparatus is arranged such that: either: a flow pathway configured to receive a regenerant is defined by the regenerant channel formed in the first block, the first sealing member and the first charge barrier; and a flow pathway configured to receive an ionic species is defined by the primary channel extending through the primary channel member and the first charged barrier; wherein the first sealing member directly provides the seal around the flow pathway in the first regenerant channel and indirectly provides the seal around the flow pathway in the primary channel by urging the first charged barrier against the primary channel member, a flow pathway configured to receive a regenerant is defined by the regenerant channel formed in the first block, the first sealing member and the first charge barrier; and a flow pathway configured to receive an ionic species is defined by the primary channel extending through the primary channel member and the first charged barrier; wherein or: a flow pathway configured to receive a regenerant is defined by the regenerant channel formed in the first block and the first charge barrier; and a flow pathway configured to receive an ionic species is defined by the primary channel extending through the primary channel member, the first sealing member and the first charged barrier; wherein the first sealing member directly provides the seal around the flow pathway in the primary channel and indirectly provides the seal around the flow pathway in the first regenerant channel by urging the first charged barrier against the first regenerant channel. a flow pathway configured to receive a regenerant is defined by the regenerant channel formed in the first block and the first charge barrier; and a flow pathway configured to receive an ionic species is defined by the primary channel extending through the primary channel member, the first sealing member and the first charged barrier; wherein