Abstract: A spectroscopic sensor assembly for detecting at least one predefined analyte constituent of a measurement fluid, the sensor assembly including a sensor housing; a radiation source; a detector device; a barrier assembly, which is transmissive for measurement radiation and is impermeable to the analyte constituent; a polymer matrix, which absorbs and releases the analyte constituent; a reflector assembly, which has a signal side facing the polymer matrix and facing the barrier assembly; wherein the reflector assembly has at least one passage extending through the reflector assembly, through which passage the analyte constituent is exchanged between a measurement environment and the polymer matrix, which polymer matrix is located on the signal side of the reflector assembly; and wherein the reflector assembly reflects incident measurement radiation back toward the apparatus portion; the sensor assembly having a spacing means different from the polymer matrix, the spacing means being designed to prevent the refl