Abstract: A fluorimetric method for the quantitative determination of SO.sub.2 in gas provides a stable measuring base. It consists in lowering the background values in terms of SO.sub.2 concentrations to about 2/10 of one ppm by increasing the oxygen concentration in a reference gas to at least 2%. This is effected by adding to this reference gas an O.sub.2 -containing stream or an ozone-containing stream. When the ozone-oxygen mixture is used the increase in O.sub.2 concentration need not be higher than 0.5-1.0% (instead of at least 2%) to achieve the same background SO.sub.2 reduction (see FIG. 4).

Abstract: A continuous sampling flow consisting of a plurality of gases is provided. The sampling of a plurality of gases according to the present invention comprises separately forming continuous gas streams of different gaseous phases, discharging said gas streams simultaneously and continuously into a common definite space in such a manner as to prevent them from mixing together, and sequentially positioning an inlet of open-sampling means successively in said discharged gas streams so that the gases in the individual gas streams are successively sampled and that a single stream formed by the sampled gases from the individual gas streams being in series one after another is taken out. In a preferred embodiment of the invention, the amount of gas sampled from each of said discharged gas streams is such that it will not disturb the discharged gas streams, e.g.