End cap voltage control of ion traps
An ion trap for a mass spectrometer has a conductive central electrode with an aperture extending from a first open end to a second open end. A conductive first electrode end cap is disposed proximate to the first open end thereby forming a first intrinsic capacitance between the first end cap and the central electrode. A conductive second electrode end cap is disposed proximate to the second open end thereby forming a second intrinsic capacitance between the second end cap and the central electrode. A first circuit couples the second end cap to a reference potential. A signal source generating an AC trap signal is coupled to the central electrode. An excitation signal is impressed on the second end cap in response to a voltage division of the trap signal by the first intrinsic capacitance and the first circuit.
1. An ion trap mass spectrometer comprising:
a conductive ring-shaped central electrode having a first aperture extending, from a first open end to a second open end;
a signal source generator configured to generate a trap signal having at least an alternating current (AC) component between a first and second terminal, wherein the first terminal is coupled to the central electrode and the second terminal is coupled to a reference voltage potential;
a conductive first undivided electrode end cap disposed adjacent to the first open end of the central electrode and coupled to a first DC reference voltage potential, wherein a first intrinsic capacitance is formed between a surface of the first undivided electrode end cap and a surface of the first open end of the central electrode; and
a conductive second undivided electrode end cap disposed adjacent to the second open end of the central electrode and coupled to a second DC reference voltage potential with a first electrical circuit, wherein a second intrinsic capacitance is formed between a surface of the second undivided electrode end cap and a surface of the second open end of the central electrode, wherein a fractional part of the trap signal is impressed on the second undivided electrode end cap in response to a voltage division of the trap signal by the second intrinsic capacitance and an impedance of the first electrical circuit; and
wherein the central electrode, the first undivided electrode end cap, and the second undivided electrode end cap together form a cylindrical ion trap; and
wherein the first electrical circuit comprises a resistor having, an impedance in the range of 1 MΩ to 10 MΩ.
2. The ion trap mass spectrometer of claim 1 , wherein the first electrical circuit comprises a capacitor in parallel with the resistor.
3. The ion trap mass spectrometer of claim 2 , wherein the impedance of the resistor is greater than one fourth of an impedance of the capacitor at a frequency of the trap signal.
4. The ion trap mass spectrometer of claim 1 , wherein the reference voltage potential is ground or zero volts.
5. The ion trap mass spectrometer of claim 1 , wherein the reference voltage potential is an adjustable DC voltage.
6. The ion trap mass spectrometer of claim 1 , wherein the capacitor is a variable capacitor adjustable to optimize an operating characteristic of the ion trap.
7. The ion trap mass spectrometer of claim 1 , wherein the ion trap is a mass analyzer, and wherein the first DC reference voltage potential, the second DC reference voltage potential, or both are an adjustable DC voltage.
8. The ion trap mass spectrometer of claim 1 , wherein the first and second DC reference voltage potentials are generated by corresponding DC voltage sources.
9. The ion trap mass spectrometer of claim 1 , wherein the ion trap is configured to impress the fractional part of the trap signal only on the second undivided electrode end cap.
10. The ion trap mass spectrometer of claim 1 , wherein the ion trap is configured to receive a resonance ejection signal.
11. The ion trap mass spectrometer of claim 1 , wherein the first electrical circuit includes a capacitor, the resistor having an impedance greater than an impedance of the capacitor at the frequency of the trap signal such that the amplitude of the fractional part of the trap signal is substantially independent of the frequency of the trap signal.
12. The ion trap mass spectrometer of claim 1 , wherein the first electrical circuit includes a capacitor, the resistor having an impedance greater than an impedance of the capacitor at the frequency of the trap signal such that the phase difference between the fractional part of the trap signal and the trap signal is substantially independent of the frequency of the trap signal.
13. The ion trap mass spectrometer of claim 1 , wherein the ion trap is configured to impress a fractional part of the trap signal on both the first undivided electrode end cap and the second undivided electrode end cap.
14. The ion trap mass spectrometer of claim 1 , further comprising a second electrical circuit coupled between the first undivided electrode end cap and the first DC reference voltage potential, wherein a fractional part of the trap signal is impressed on the first undivided electrode end cap in response to a voltage division of the trap signal by the first intrinsic capacitance and an impedance of the second electrical circuit.