A modular flow injection analysis system including a sample feed, a reagent feed and a media discharge line, formed of a plurality of functional modules stacked on top of one another, the functional modules comprising at least: a) a dosing module with a dosing channel having an inlet and an outlet, b) a mixing module with a mixing channel having an inlet and an outlet, c) an analysis module with a media chamber connected to the outlet of the mixing channel, d) a media control module; wherein the modules include passage openings that are aligned with one another and form a continuous media channel through which the modules are connected, the media channel being connected to a sample port; the modular flow injection analysis system being characterized in that - an injection quantity of a reagent is determined by a length and a cross section of the dosing channel; - the media control module selectively connects the sample feed or the reagent feed to the inlet of the dosing channel of the dosing module in a valve-controlled manner; - the media control module selectively connects the outlet of the dosing channel to the inlet of the mixing channel or to a reagent discharge line in a valve-controlled manner; - the media control module is adapted to measure a determined reagent quantity by controlling the reagent feed to the inlet of the dosing channel and connecting the outlet of the dosing channel to a reagent discharge line for a time period which ensures that the dosing channel is completely filled with reagent. a) a dosing module with a dosing channel having an inlet and an outlet, b) a mixing module with a mixing channel having an inlet and an outlet, c) an analysis module with a media chamber connected to the outlet of the mixing channel, d) a media control module; wherein the modules include passage openings that are aligned with one another and form a continuous media channel through which the modules are connected, the media channel being connected to a sample port; the modular flow injection analysis system being characterized in that - an injection quantity of a reagent is determined by a length and a cross section of the dosing channel; - the media control module selectively connects the sample feed or the reagent feed to the inlet of the dosing channel of the dosing module in a valve-controlled manner; - the media control module selectively connects the outlet of the dosing channel to the inlet of the mixing channel or to a reagent discharge line in a valve-controlled manner; - the media control module is adapted to measure a determined reagent quantity by controlling the reagent feed to the inlet of the dosing channel and connecting the outlet of the dosing channel to a reagent discharge line for a time period which ensures that the dosing channel is completely filled with reagent. - an injection quantity of a reagent is determined by a length and a cross section of the dosing channel; - the media control module selectively connects the sample feed or the reagent feed to the inlet of the dosing channel of the dosing module in a valve-controlled manner; - the media control module selectively connects the outlet of the dosing channel to the inlet of the mixing channel or to a reagent discharge line in a valve-controlled manner; - the media control module is adapted to measure a determined reagent quantity by controlling the reagent feed to the inlet of the dosing channel and connecting the outlet of the dosing channel to a reagent discharge line for a time period which ensures that the dosing channel is completely filled with reagent.