Ion-molecule-reaction - mass spectrometry (IMR-MS) device, comprising (i) an ion source (ii) an adjacent reaction chamber and (iii) a mass spectrometer subsequent to the reaction chamber, wherein the reaction chamber comprises an RF device for creating a temporally changing electromagnetic field and wherein an adjustable reduced electric field strength (E/N) can be applied to the reaction chamber, characterized by an input device for entering a desired reduced electric field strength (E/N) by an operator when operating said IMR-MS device for analysing a sample, and a controlling device that operates the IMR-MS device by adjusting the settings of the IMR-MS device relating to a defined data set of a pseudo reduced electric field strength (PE/N 1,2) for the entered reduced electric field strength (E/N), wherein the pseudo reduced electric field strength (PE/N 1,2) has been determined by analysing a first analyte (A 1) in the IMR-MS device, wherein intensity signals (RS 1) of at least two product ions of the analyte (A 1) are recorded and wherein the settings of the IMR-MS device are changed until the measured branching ratios of the at least two product ions match reference branching ratios within a given tolerance level of the at least two product ions determined in an IMR-MS device comprising an ion source, an adjacent reaction chamber with a DC-drift tube and a mass spectrometer subsequent to the reaction chamber, wherein the reaction chamber is operated only with an activated DC-drift tube at a certain actual reduced electric field strength (E a1 /N), wherein these settings of the IMR-MS device relating to the pseudo reduced electric field strength (PE/N 1) are stored in the controlling device, wherein the controlling device controls said IMR-MS device by performing analysis of the sample with the settings corresponding to the pseudo reduced electric field strengths (PE/N 1). (i) an ion source (ii) an adjacent reaction chamber and (iii) a mass spectrometer subsequent to the reaction chamber, wherein the reaction chamber comprises an RF device for creating a temporally changing electromagnetic field and wherein an adjustable reduced electric field strength (E/N) can be applied to the reaction chamber, characterized by an input device for entering a desired reduced electric field strength (E/N) by an operator when operating said IMR-MS device for analysing a sample, and a controlling device that operates the IMR-MS device by adjusting the settings of the IMR-MS device relating to a defined data set of a pseudo reduced electric field strength (PE/N 1,2) for the entered reduced electric field strength (E/N), wherein the pseudo reduced electric field strength (PE/N 1,2) has been determined by analysing a first analyte (A 1) in the IMR-MS device, wherein intensity signals (RS 1) of at least two product ions of the analyte (A 1) are recorded and wherein the settings of the IMR-MS device are changed until the measured branching ratios of the at least two product ions match reference branching ratios within a given tolerance level of the at least two product ions determined in an IMR-MS device comprising an ion source, an adjacent reaction chamber with a DC-drift tube and a mass spectrometer subsequent to the reaction chamber, wherein the reaction chamber is operated only with an activated DC-drift tube at a certain actual reduced electric field strength (E a1 /N), wherein these settings of the IMR-MS device relating to the pseudo reduced electric field strength (PE/N 1) are stored in the controlling device, wherein the controlling device controls said IMR-MS device by performing analysis of the sample with the settings corresponding to the pseudo reduced electric field strengths (PE/N 1). wherein intensity signals (RS 1) of at least two product ions of the analyte (A 1) are recorded and wherein the settings of the IMR-MS device are changed until the measured branching ratios of the at least two product ions match reference branching ratios within a given tolerance level of the at least two product ions determined in an IMR-MS device comprising an ion source, an adjacent reaction chamber with a DC-drift tube and a mass spectrometer subsequent to the reaction chamber, wherein the reaction chamber is operated only with an activated DC-drift tube at a certain actual reduced electric field strength (E a1 /N), wherein these settings of the IMR-MS device relating to the pseudo reduced electric field strength (PE/N 1) are stored in the controlling device,