Ion mobility mass spectrometer and method of performing ion mobility mass spectrometry

A method of ion mobility spectrometry comprising:

• transmitting a plurality of ions to an ion mobility separator;
• modulating the introduction of the ions into the ion mobility separator at a first modulation frequency;
• separating the ions that enter the ion mobility separator according to ion mobility;
• detecting ions that have exited by the ion mobility separator with a detector of a time of flight mass analyser;
• varying the first modulation frequency with time;
• recording the intensity of the ion signal output from the detector at an acquisition rate that is greater than the first modulation frequency;
• and
• determining, from the variation in intensity of the ion signal as a function of acquisition time, the ion mobilities of the ions that have been detected.

An ion mobility spectrometer comprising:

• an ion mobility separator for separating ions according to ion mobility;
• an ion entrance modulator for modulating the introduction of the ions into the ion mobility separator;
• a time of flight mass analyser having a detector for detecting ions that have exited by the ion mobility separator;
• a recording device for recording the intensity of the ion signal output from the detector to produce recorded data;
• and
• a controller configured and set up to:
• control the ion entrance modulator so as to modulate the introduction of the ions into the ion mobility separator with a first modulation frequency, wherein first modulation frequency varies with time;
• modulate the recorded data as a function of the time that the data was recorded and at a second modulation frequency, wherein the second modulation frequency is varied as a function of the time that the data was recorded;
• and
• determine, from the variation in intensity of the ion signal in the modulated data as a function of the second modulation frequency, the ion mobilities of the ions that have been detected:
• wherein said controller is configured and set up to modulate the recorded data by:
• defining the recorded data as having been recorded over a plurality of sequential time periods;
• and either:
• (i) retaining the recorded data that was recorded during alternate ones of said time periods, and discarding the data that was recorded during the time periods between said alternate time periods;
• or
• (ii) retaining, without attenuation, the recorded data that was recorded during alternate ones of said time periods, and attenuating and retaining the data that was recorded during the time periods between said alternate time periods;
• or
• (iii) attenuating to a first level the recorded data that was recorded during alternate ones of said time periods and retaining the attenuated data, and attenuating to a second greater level the recorded data that was recorded during the time periods between said alternate time periods;
• wherein the time periods differ in duration as a function of the time that the data was recorded such that the second modulation frequency varies as a function of the time that the data was recorded.