Measurement arrangement for determining a constituent substance or quality parameter of water by thermal decomposition in a reaction module, delivery of the reaction product to a detector in a carrier gas flow, and evaluation of a detector signal for deriving a value of the constituent substance or quality parameter, wherein the reaction module is a vessel of vertical orientation during operation having an internal resistance heating or infrared heating, and has a head section into which the sample is introduced, a reaction zone, where the thermal decomposition is performed, as well as a foot section, from which the reaction product is output in the carrier gas flow, wherein the head section of the reaction module has an injection port for temporarily introducing an injection needle or permanently supporting a small supply tube, which injection port comprises an O-ring which is internally spring-loaded by a silicon padding or an inserted spring.