AC gate ion filter method and apparatus
The present invention uses an AC voltage instead of DC voltage on an ion gate to filter/selectively pass ions. The ions that pass through the AC ion gate can be further separated in a spectrometric instrument. An ion mobility spectrometer using the AC ion gate can achieve better gating performance. For a time of flight ion mobility spectrometer with an AC ion gate, a narrow pulse of selected ions can be passed into a drift tube where they are separated based on their low field ion mobility. Moreover, when the AC voltage at the AC ion gate has a waveform as used for differential ion mobility spectrometry, the time of flight ion mobility spectrometer is converted into a two dimensional separation spectrometer, where ions are first separated based on their high field ion mobility and then further separated based on their low field ion mobility.
1. A method for operating an ion mobility spectrometer comprising:
(a) ionizing a sample to form sample ions;
(b) guiding some of the sample ions toward an ion gate, passing some of the sample ions through the ion gate, and continuously guiding the ions in a drift tube after the ion gate;
(c) applying at least one AC voltage to at least one of the grid elements of the ion gate to selectively pass a pulse of at least one kind of sample ions through the ion gate, wherein the pulse of ions is generated by applying a first frequency and a first amplitude of the AC voltage to at least one of the grid elements for a period time and subsequently applying a second frequency and/or a second amplitude of the AC voltage; and
(d) separating the sample ions in a drift tube.
2. The method of claim 1 , wherein guiding the ionized sample includes using a gas flow and/or an electric field.
3. The method of claim 1 , wherein the AC voltage has a constant or a varying frequency and/or amplitude during one ion mobility measurement.
4. The method of claim 1 , wherein selectively passing ions is realized by changing a waveform of the AC voltage and/or frequency.
5. The method of claim 4 , wherein the waveform is asymmetric or symmetric.
6. The method of claim 1 , wherein the AC voltage has a waveform that is same as the asymmetric waveform used in differential/field asymmetric ion mobility spectrometers.
7. The method of claim 1 , wherein the AC voltage is a controlled sweep through a range of voltages and/or frequencies to select different sample ions to pulse into the drift tube.
8. An apparatus of an ion mobility spectrometer comprising:
(a) an ionization source for ionizing a sample to form sample ions;
(b) an ion gate to pulse some of the sample ions into a drift tube that separates the sample ions; and
(a) an AC voltage that is applied to at least one of the grid elements of the ion gate to selectively pass a pulse of at least one kind of sample ions through the ion gate, wherein the pulse of ions is generated by applying a first frequency and a first amplitude of the AC voltage to at least one of the grid elements for a period time and subsequently applying a second frequency and/or a second amplitude of the AC voltage.
9. The apparatus of claim 8 , further comprise an electric field in the drift tube guiding the ionized sample ions.
10. The apparatus of claim 8 , wherein the AC voltage has a constant or a varying frequency and/or amplitude during one ion mobility measurement.
11. The apparatus of claim 8 , wherein selectively passing ions is achieved by changing a waveform of the AC voltage and/or frequency.
12. The apparatus of claim 11 , wherein the waveform is asymmetric or symmetric.
13. The apparatus of claim 8 , wherein the AC voltage has a waveform that is the same as the asymmetric waveform used in differential/field asymmetric ion mobility spectrometers.
14. The apparatus of claim 8 , wherein the AC voltage is a controlled sweep through a range of voltages and/or frequencies to select different sample ions to pulse into the drift tube.
15. The apparatus of claim 8 , wherein the grid elements have a large surface area in the direction that is parallel to the charged particle stream.