An apparatus for use in determining the presence and/or concentration of one or more selected materials in a matrix stream of materials within a process pipe, reactor or other process vessel, including (a) a support member having a first, internal end for inserting into the matrix stream when the apparatus is placed in service and a second end positioned externally of the pipe, reactor or other vessel in use, wherein the support member defines a groove therein of which at least a portion extends into the matrix stream in use, and (b) a conduit, for example a capillary membrane, positioned and supported substantially wholly within at least that portion of the groove in the support member which extends into the matrix stream in use.