High-speed low-noise ion current detection circuit and mass spectrometer using the same

Patent metadata

Field	Value
Publication number	US-10224192-B2
Application number	US-201715656909-A
Country	US
Kind code	B2
Filing date	Jul 21, 2017
Priority date	Jul 21, 2017
Publication date	Mar 5, 2019
Grant date	Mar 5, 2019

Abstract

Methods and circuits for detecting an ion current in a mass spectrometer are described. A circuit and a method may involve converting, over a length of integration time, the ion current to a voltage ramp by an integrating circuit having a gain setting. The circuit and the method may also involve determining a slope of the voltage ramp. The circuit and the method may also involve determining a magnitude of the ion current based on the slope of the voltage ramp and the gain setting. The circuit and the method may further involves determining an out-of-range state based on the voltage ramp and adjusting the gain setting of the integrating circuit, or the length of integration time or both, in response to the determining of the out-of-range state.

First claim

What is claimed is:

1. A method of detecting an ion current in a mass spectrometer, comprising:
converting, over a length of integration time, the ion current to a voltage ramp by an integrating circuit having a gain setting;
determining a slope of the voltage ramp by:
digitizing, by an analog-to-digital converter (ADC), the voltage ramp into a plurality of voltage samples, the plurality of voltage samples representing the voltage ramp; and

analyzing, by a processor, the plurality of voltage samples to determine the slope of the voltage ramp;
determining a magnitude of the ion current based on the slope of the voltage ramp and the gain setting; and
determining an out-of-range (OOR) state based on the voltage ramp and a predetermined detectable range; and
adjusting the gain setting of the integrating circuit, the length of integration time, or both, in response to the determining of the OOR state such that the voltage ramp is within the predetermined detectable range at an end time of the length of integration time.

2. The method of claim 1 , wherein the analyzing of the plurality of voltage samples to determine the slope of the voltage ramp comprises:
determining a first-order fitting line based on the plurality of voltage samples; and
designating a slope of the first-order fitting line as the slope of the voltage ramp.

3. The method of claim 1 , further comprising:
reducing, by one or more digital filters coupled in series, a noise component of the plurality of voltage samples before analyzing the plurality of voltage samples.

4. The method of claim 1 , further comprising:
repeating the converting of the ion current to the voltage ramp for multiple times, wherein:
the plurality of voltage samples comprise multiple sets of voltage samples resulted from the repeating, and
the analyzing of the plurality of voltage samples to determine the slope of the voltage ramp comprises averaging over the multiple sets of voltage samples.

5. The method of claim 1 , further comprising:
calibrating the gain setting of the integrating circuit by sending a calibrating current of a known value to the integrating circuit and recording the slope of the voltage ramp resulted from the calibrating current.

6. A circuit of detecting an ion current and implementable to a mass spectrometer, the circuit comprising:
an integrating circuit having a gain setting and configured to convert the ion current to a voltage ramp over a length of integration time;
an analog-to-digital converter (ADC) configured to digitize the voltage ramp into a plurality of voltage samples; and
a processor configured to determine a slope of the voltage ramp based on one or more voltage samples of the plurality of voltage samples and further configured to determine a magnitude of the ion current based on the slope of the voltage ramp and the gain setting;
wherein the integrating circuit comprises:
an operational amplifier (op-amp) having an inverting terminal as an input terminal, a non-inverting terminal connected to a reference voltage as a ground terminal, and an output terminal, the input terminal configured to receive the ion current;
a reset switch connected between the input terminal and the output terminal of the op-amp, the reset switch configured to short-circuit the output terminal of the op-amp to the input terminal of the op-amp when the reset switch is turned on; and
a variable relay connected between the input terminal and the output terminal of the op-amp, the variable relay configured to provide the gain setting of the integrating circuit; and
wherein the processor is further configured to determine an out-of-range (OOR) state based on the voltage ramp and a predetermined detectable range, and wherein the processor is further configured to adjust the gain setting of the integrating circuit and reset the voltage ramp via the reset switch according to the OOR state.

7. The circuit of claim 6 , further comprising:
one or more digital filters configured to reduce a noise component of the plurality of voltage samples and generate the one or more voltage samples of the plurality of voltage samples.

8. The circuit of claim 6 , wherein the variable relay comprises: a plurality of capacitors; and
a plurality of range switches, each of the plurality of range switches connected to at least one of the plurality of capacitors,
wherein the plurality of range switches are configured to connect one or more capacitors of the plurality of capacitors to provide the gain setting of the integrating circuit, and
wherein the plurality of range switches are further configured to connect one or more capacitors of the plurality of capacitors in series, in parallel, or both in series and in parallel, to adjust the gain setting of the integrating circuit.

9. The circuit of claim 6 , wherein the processor is further configured to adjust the length of integration time according to the OOR state.

10. The circuit of claim 6 , wherein the integrating circuit further comprises:
an input switch configured to pass the ion current while the ion current is converted to the voltage ramp, and further configured to block the ion current while the reset switch is turned on to reset the voltage ramp.

11. A miniaturized mass spectrometer for analyzing gas molecules, comprising:
an ion drive configured to ionize the gas molecules into an ion flow comprising a plurality of gas ions having a plurality of values of atomic mass unit (AMU);
a quadrupole mass filter (QMF) configured to selectively pass a first part of the plurality of gas ions, each gas ion of the first part of the plurality of gas ions having a first value of AMU;
an ion sensing device configured to sense the first part of the plurality of gas ions and generate a first ion current; and
an ion current detection circuit configured to detect the first ion current, the ion current detection circuit comprising:
an integrating circuit having a gain setting and configured to convert the first ion current to a voltage ramp over a length of integration time;
an analog-to-digital converter (ADC) configured to digitize the voltage ramp into a plurality of voltage samples; and
a processor configured to determine a slope of the voltage ramp based on one or more voltage samples of the plurality of voltage samples and further configured to determine a magnitude of the first ion current based on the slope of the voltage ramp and the gain setting;
wherein the processor is further configured to determine an out-of-range (OOR) state based on the voltage ramp and a predetermined detectable range, and wherein the processor is further configured to adjust the gain setting of the integrating circuit, the length of integration time, or both, according to the OOR state such that the voltage ramp is within the predetermined detectable range at an end time of the length of integration time.

12. The miniaturized mass spectrometer of claim 11 , wherein the ion drive comprises:
a filament heater configured to generate a plurality of electrons; and
one or more acceleration electrodes configured to accelerate the plurality of electrons to form a high velocity electron flow that ionize the gas molecules into the ion flow.

13. The miniaturized mass spectrometer of claim 11 , wherein the ion current detection circuit further comprises:
one or more digital filters configured to reduce a noise component of the plurality of voltage samples and generate the one or more voltage samples of the plurality of voltage samples.

14. The miniaturized mass spectrometer of claim 11 , wherein the integrating circuit comprises:
an operational amplifier

Assignees

• Atonarp Inc

Inventors

• Pandurangan Anand
• Selvaraj Siva
• Hegde Anoop

Classifications

• G01R19/0061

  Measuring currents of particle-beams, currents from electron multipliers, photocurrents, ion currents; Measuring in plasmas · CPC title

• H01J49/4215

  Quadrupole mass filters (H01J49/4225 takes precedence) · CPC title

• H01J49/08

  Electron sources, e.g. for generating photo-electrons, secondary electrons or Auger electrons · CPC title

• H01J49/025

  Detectors specially adapted to particle spectrometers (data acquisition H01J49/0036; detectors per se G01T, e.g. G01T1/28, G01T1/29) · CPC title

• H01J49/022 Primary

  Circuit arrangements, e.g. for generating deviation currents or voltages (regulating electric or magnetic variables in general, e.g. current, magnetic field G05F); Components associated with high voltage supply (high voltage supply per se H02M) · CPC title