Method for the
mass spectrometric analysis of a gas (4), comprising: Ionizing the gas (4) to generate ions (4a, 4b), storing, exciting and detecting at least a portion of the generated ions (4a, 4b) in an FT
ion trap (2), wherein the generation and storage of the ions (4a, 4b) in the FT
ion trap (2) and / or the excitation of the ions (4a, 4b) prior to the detection of the ions (4a, 4b) in the FT
ion trap (2) comprises at least one selective IFT excitation, in particular a SWIFT excitation (10), dependent on the
mass-to-charge ratio m / z of the ions (4a, 4b), characterized by the steps: Determining a starting phase position α 0 a trajectory B of ions (4a, 4b) at a given ion
resonance frequency f ion after an IFT excitation (10) based on a time-dependent ion
signal u recorded during the detection of the ions (4a, 4b) ion (t), and Determining a charge polarity of the ions (4a, 4b) based on the starting phase position α 0 of the ions (4a, 4b) after IFT excitation (10).