A method of analysing a sample comprising: operating a mass spectrometer (1;2;3;4;5) in a state or mode wherein first ions are analysed; switching a state or mode of said mass spectrometer (1;2;3;4;5) so that second ions are analysed, wherein said step of switching a state or mode of said mass spectrometer (1;2;3;4;5) comprises switching or altering an operational parameter of said mass spectrometer (1;2;3;4;5); acquiring a stream of mass spectral data wherein the acquisition of said mass spectral data is substantially asynchronous with the switching of said mass spectrometer (1;2;3;4;5) between states or modes; and post-processing said stream of mass spectral data to produce: (i) mass spectral data relating to said first ions; and/or (ii) mass spectral data relating to said second ions; wherein: said stream of mass spectral data is acquired during a mass spectral data acquisition period; said acquisition period comprises a plurality of sample periods; said sample periods are substantially asynchronous with and/or are not synchronised with the switching of said mass spectrometer (1;2;3;4;5) between states or modes; and the state or mode of said mass spectrometer (1;2;3;4;5) is repeatedly switched with a frequency f1 and wherein said sample periods have a frequency f3, wherein f3 > f1; said method being characterised in that: said step of post-processing said stream of mass spectral data comprises: detecting ion peaks in said stream of mass spectral data and determining a plurality of ion peak times and a plurality of ion peak intensities associated with said stream of mass spectral data; determining which portions or sample periods of said stream of mass spectral data relate to said first ions or to said second ions; determining which portions or sample periods of said stream of mass spectral data relate to both said first ions and to said second ions, and rejecting those portions or sample periods; and producing a mass spectrum for (i) said first ions and/or (ii) said second ions by combining portions or sample periods of said stream of mass spectral data determined to relate to said first ions and/or by combining portions or sample periods of said stream of mass spectral data determined to relate to said second ions. operating a mass spectrometer (1;2;3;4;5) in a state or mode wherein first ions are analysed; switching a state or mode of said mass spectrometer (1;2;3;4;5) so that second ions are analysed, wherein said step of switching a state or mode of said mass spectrometer (1;2;3;4;5) comprises switching or altering an operational parameter of said mass spectrometer (1;2;3;4;5); acquiring a stream of mass spectral data wherein the acquisition of said mass spectral data is substantially asynchronous with the switching of said mass spectrometer (1;2;3;4;5) between states or modes; and post-processing said stream of mass spectral data to produce: (i) mass spectral data relating to said first ions; and/or (ii) mass spectral data relating to said second ions; wherein: said stream of mass spectral data is acquired during a mass spectral data acquisition period; said acquisition period comprises a plurality of sample periods; said sample periods are substantially asynchronous with and/or are not synchronised with the switching of said mass spectrometer (1;2;3;4;5) between states or modes; and the state or mode of said mass spectrometer (1;2;3;4;5) is repeatedly switched with a frequency f1 and wherein said sample periods have a frequency f3, wherein f3 > f1; said method being characterised in that: said step of post-processing said stream of mass spectral data comprises: detecting ion peaks in said stream of mass spectral data and determining a plurality of ion peak times and a plurality of ion peak intensities associated with said stream of mass spectral data; determining which portions or sample periods of said stream of mass spectral data relate to said first ions or to said second ions; determining which portions or sample periods of said stream of mass spectral data relate to both said first ions and to said second ions, and rejecting those portions or sample periods; and producing a mass spectrum for (i) said first ions and/or (ii) said second ions by combining portions or sample periods of said stream of mass spectral data determined to relate to said first ions and/or by combining portions or sample periods of said stream of mass spectral data determined to relate to said second ions. detecting ion peaks in said stream of mass spectral data and determining a plurality of ion peak times and a plurality of ion peak intensities associated with said stream of mass spectral data; determining which portions or sample periods of said stream of mass spectral data relate to said first ions or to said second ions; determining which portions or sample periods of said stream of mass spectral data relate to both said first ions and to said second ions, and rejecting those portions or sample periods; and producing a mass spectrum for (i) said first ions and/or (ii) said second ions by combining portions or sample periods of said stream of mass spectral data determined to relate to said first ions and/or by combining portions or sample periods of said stream of mass spectral data determined to relate to said second ions.