Abstract: The invention provides a device and a process for analysing the effect of a concentration of a gas on a sample, especially measurement of the gas concentration within a sample, and/or measurement of the effect of the absorption and/or desorption of a gas by the sample, using spectrophotometric detection of a gas-sensitive and optically detectable analyte of the sample while measuring the concentration of the gas in a gas composition that is continuously delivered to each sample well in order to contact the samples with a gas having a predetermined and/or measured concentration, respectively a pre-determined and/or measured partial pressure.