Measurement of ion mobility spectra
The invention relates to measuring the mobility spectra of ions with ion mobility spectrometers (IMS). The invention provides an analog modulation of the ion current of an IMS ion source with a continuous modulation function, the instantaneous frequency of which temporally varies across a large frequency range, and a generation of the mobility spectrum from the measured ion current by a correlation analysis with the modulation pattern. The modulation can, for example, be produced by the gating grid, which is usually present. The analog modulation removes many of the difficulties which occur with square-wave modulation and leads to a surprisingly stable evaluation which is relatively insensitive to noisy signals and produces a high mobility resolution at very low noise.
1. A method for the measurement of the mobility spectrum of ions in an ion mobility spectrometer with an ion source, an ion drift region and an ion detector at the end of the ion drift region, comprising:
a modulation device modulating ion current of the ion source with a continuous analog modulation function whose instantaneous frequency varies across a range of frequencies and whose amplitude is simultaneously modulated;
and correlating in a controller the ion current signal measured at the detector with the modulation function to obtain the mobility spectrum.
2. The method according of claim 1 , where the modulation function is a chirp with an amplitude modulation.
3. The method of claim 1 , where the instantaneous frequency of the modulation stretches from a lower frequency limit to an upper frequency limit which is selected according to the resolution required.
4. The method of claim 3 , wherein the instantaneous frequency starts at the lower frequency limit and increases linearly to the upper frequency limit.
5. The method of claim 3 , where the instantaneous frequency is nonlinearly changed between the lower and upper frequency limits.
6. The method of claim 1 , where the change of the modulation frequency is extended in such a way that it extends over the measuring period required.
7. The method of claim 1 , wherein the modulation device comprises a gating grid located at the start of the ion drift region.
8. A method for the measurement of the mobility spectrum of ions in an ion mobility spectrometer with an ion source, an ion drift region and an ion detector at the end of the ion drift region, comprising:
a modulation device modulating ion current of the ion source, with a continuous analog modulation function being a nonlinear chirp or a wavelet;
and correlating in a controller the ion current signal measured at the detector with the modulation function to obtain the mobility spectrum.
9. A method for the measurement of the mobility spectrum of ions in an ion mobility spectrometer with an ion source, an ion drift region and an ion detector at the end of the ion drift region, comprising:
a modulation device modulating ion current of the ion source, with a continuous analog modulation function whose instantaneous frequency is modulated in a non-continuous way by switching phases during zero crossovers according to a coding;
and correlating in a controller the ion current signal measured at the detector with the modulation function to obtain the mobility spectrum.