A method for analyzing a gas by mass spectrometry includes exciting ions of the gas to be analyzed in an FT ion trap, and recording a first frequency spectrum in a first measurement time interval during or after the excitation of the ions. The first frequency spectrum contains ion frequencies of the excited ions and interference frequencies. The method also includes recording a second frequency spectrum in a second measurement time interval. The second frequency spectrum contains the interference frequencies, but not the ion frequencies of the first frequency spectrum. The method further includes comparing the first frequency spectrum with the second frequency spectrum to identify the interference frequencies in the first frequency spectrum. The disclosure also relates to a mass spectrometer which is suitable for carrying out the method for analyzing the gas by mass spectrometry.