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. An apparatus of an ion gate for a charged particle stream in a spectrometer comprising:
(a) at least one pair of adjacent grid elements that can be set at the same or different potentials;
(b) at least one AC voltage is applied to the grid elements to pass a pulse of selected ions into the spectrometer; wherein the pulse of selected 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 to at least one of the grid elements.
2. The apparatus of claim 1 , wherein the grid elements have a large surface area in the direction that is parallel to the charged particle stream.
3. The apparatus of claim 1 , wherein the grid elements are 2 or more that are aligned in phase.
4. The apparatus of claim 1 , wherein the grid elements are 2 or more that are aligned out of phase.
5. A method for operating an AC ion gate for a charged particle stream comprising: Applying at least one AC voltage to at least one of the grid elements of the AC ion gate to pass a pulse of selected ions into the spectrometer; wherein the pulse of selected 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 to at least one of the grid elements.
6. The method of claim 5 , wherein the pulse of selected ions is generated by partially opening the ion gate using a selected AC voltage.
7. The method of claim 6 , further comprising:
filtering a percentage of some components of the charged particle stream from the other components of the particle stream by neutralizing such components on the grid elements.
8. The method of claim 7 , wherein the percentage of some components of the charged particle stream being filtered from the other components is 0 to 100%: in particular, 100, >75, >50, >25, >10, and >0.
9. The method of claim 7 , wherein some components of the charged particle stream have a smaller size and/or shape than the other components of the charged particle stream.
10. The method of claim 7 , wherein the other components of the charged particle stream have a larger size and/or shape than some components of the charged particle stream.
11. The method of claim 5 , wherein the amplitude can be a constant and/or controlled in a range during a period of closing the gate.
12. The method of claim 5 , wherein the AC voltage is a symmetric or asymmetric waveform.
13. The method of claim 5 , wherein the frequency can be a constant and/or controlled in a range during a period of closing the gate.
14. A time of flight ion mobility spectrometer comprising:
(a) an ionization source that ionize samples
(b) a first electric field that guides the ionized sample toward an ion gate
(c) at least one AC voltage is applied to the grid elements of the ion gate for a period of time, by which at least one pulse of the ionized sample is passed into a drift tube
(d) a second electric field that guides the pulse(s) of ions toward an ion detector.
15. The time of flight ion mobility spectrometer of claim 14 , wherein the AC voltage has a waveform that is same as the asymmetric waveform used in differential/field asymmetric ion mobility spectrometers.
16. The time of flight ion mobility spectrometer of claim 14 , 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.