Measurement of ion mobility spectra with analog modulation
A method is provided for measuring mobility spectrum of ions in an ion mobility spectrometer having an ion source, a modulator, an ion drift region and an ion detector disposed at the end of the ion drift region. The method includes the steps of modulating an ion current from the ion source, and measuring the mobility spectrum, where a predistortion of the continuous modulation function substantially compensates for a distortion created by the modulator. The ion current is modulated with the modulator by varying an instantaneous frequency of a continuous modulation function across a frequency range. The mobility spectrum is measured by correlating the ion current measured at the detector and the modulation function.
1. A method for measuring mobility spectrum of ions in an ion mobility spectrometer having an ion source, a modulator, an ion drift region and an ion detector disposed at the end of the ion drift region, the method comprising:
modulating an ion current of the ion source with the modulator by varying an instantaneous frequency of a continuous modulation function across a frequency range; and
measuring the mobility spectrum by correlating the ion current measured at the detector and the modulation function;
where a predistortion of the continuous modulation function substantially compensates for a distortion created by the modulator.
2. The method of claim 1 , where at least asymmetric portions of the distortion are compensated by the predistortion of the continuous modulation function.
3. The method of claim 1 , where the predistortion of the continuous modulation function includes superimpositions with contributions of higher powers of the continuous modulation function.
4. The method of claim 1 , where the continuous modulation function is a substantially linear chirp defined as f(t)=a+b sin(πv t 2 /T), and where the predistorted continuous modulation function is defined as f*(t)=A+B sin(πv t 2 /T)+C sin 2 (πv t 2 /T).
5. The method of claim 1 , where the continuous modulation function is digitally predistorted.
6. The method of claim 1 , where the continuous modulation function is analogly predistorted via an electric circuit.
7. The method of claim 1 , further comprising decoding the measured ion current by correlating ion current values of the measured ion current with the continuous modulation function.
8. The method of claim 1 , further comprising decoding the measured ion current by correlating ion current values of the measured ion current with the predistorted continuous modulation function.
9. The method of claim 1 , where the modulator comprises a bipolar gating grid.
10. The method of claim 1 , where the instantaneous frequency ranges between a lower frequency limit and an upper frequency limit selected according to resolution requirements.
11. The method of claim 1 , further comprising extending the predistortion of the continuous modulation function over a measuring period.