Properties, such as concentrations, of gas phase bases or acids of a gas sample are determined by providing a sample gas flow, which includes the bases or acids to be determined as sample constituents, as well as also interfering constituents, which are other constituents than the sample constituents. The controllers are coupled on an output side to the common signal output through the change-over unit. The insert is not readily visible when inserted into the vase structure and provides a watertight seal. The first or second unit is formed from a variable focal length optical element having at least two continuous rotationally asymmetric surfaces.