Methods and apparatus for detecting negative ions from a mass spectrometer
First Claim
1. An improved mass spectrometry system for analyzing the composition of a sample, including an ion source and focusing means for generating an ion beam having ions representative of the sample, a mass analyzer for scanning selected ions from the beam either at a specific atomic mass unit or over a range of atomic mass units, and an ion detector having an electron multiplier with an anode and cathode for receiving ions from the beam discharged from the mass analyzer and for providing a detector output signal indicative of the intensity of ions at a specific atomic mass unit and representative of the composition of the sample, the system further comprising:
- power supply means for charging the cathode at a high negative potential to attract positive ions or at a high positive potential to attract negative ions while maintaining the anode at a positive potential relative to the cathode;
means for modulating the ion beam to the mass analyzer for producing a modulated ion beam to the detector and a modulated high-voltage output signal from the detector;
circuitry means for capacitively or inductively coupling the modulated high-voltage output signal to ground and providing a coupled output signal functionally related to the modulated high-voltage output signal;
sensing means for measuring distortion of the coupled output signal introduced by the capacitive or inductive coupling of the modulated high-voltage output signal to ground; and
signal modification means for correcting the coupled output signal in response to the measured coupled output signal distortion to provide a modified coupled output signal representative of the modulated high-voltage output signal and thus the composition of the sample.
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Abstract
Improved methods and apparatus are disclosed for detecting negative ions and, more particularly, for detecting negative ions produced from a quadrupole mass spectrometer. By modulating the ion beam either at the ion source or within the ion focusing system, the output current from the electron multiplier detector is a pulsating current which is then capacitively or inductively coupled from a high direct current potential to ground level. Electronics operating at ground level are employed to correct the current signal distortion due to the capacitive or inductive coupling of the detector output current. The present invention enables substantially increased detector sensitivity to negative ions compared to prior art equipment, and does not require expensive and complex preamplifier circuitry. The techniques of the present invention allow both positive and negative ions to be detected utilizing the same basic equipment, thereby increasing equipment versatility and reducing costs.
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Citations
21 Claims
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1. An improved mass spectrometry system for analyzing the composition of a sample, including an ion source and focusing means for generating an ion beam having ions representative of the sample, a mass analyzer for scanning selected ions from the beam either at a specific atomic mass unit or over a range of atomic mass units, and an ion detector having an electron multiplier with an anode and cathode for receiving ions from the beam discharged from the mass analyzer and for providing a detector output signal indicative of the intensity of ions at a specific atomic mass unit and representative of the composition of the sample, the system further comprising:
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power supply means for charging the cathode at a high negative potential to attract positive ions or at a high positive potential to attract negative ions while maintaining the anode at a positive potential relative to the cathode; means for modulating the ion beam to the mass analyzer for producing a modulated ion beam to the detector and a modulated high-voltage output signal from the detector; circuitry means for capacitively or inductively coupling the modulated high-voltage output signal to ground and providing a coupled output signal functionally related to the modulated high-voltage output signal; sensing means for measuring distortion of the coupled output signal introduced by the capacitive or inductive coupling of the modulated high-voltage output signal to ground; and signal modification means for correcting the coupled output signal in response to the measured coupled output signal distortion to provide a modified coupled output signal representative of the modulated high-voltage output signal and thus the composition of the sample. - View Dependent Claims (2, 3, 4, 5, 6, 7)
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- 8. An improved negative ion detector means for a scanning mass spectrometry system for analyzing the composition of a sample, the system including an ion source for producing an ion beam having ions representative of the sample, a mass analyzer for scanning selected ions from the beam over a range of atomic mass units, and power supply means for maintaining a cathode of an electron multiplier of the detector at a high positive potential and an anode of the electron multiplier at a higher positive potential for attracting negative ions, the detector means being charged by the power supply means for receiving negative ions discharged from the mass analyzer and providing a high-voltage output signal over the range of atomic mass units for analyzing the composition of the sample, the improved negative ion detector means further comprising
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9. circuitry means for capacitively or inductively coupling the high-voltage output signal to ground potential and providing a coupled output signal functionally related to the high-voltage output signal;
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sensing means for measuring distortion of the coupled output signal introduced by the capacitive or inductive coupling; and correcting means for altering the coupled output signal in response to the measured coupled output signal distor- tion to provide a modified essentially ground-voltage coupled output signal representative of the high-voltage output signal.
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14. In a mass spectrometry system for analyzing the composition of a sample, the system including an ion source for producing an ion beam having ions representative of the sample, a mass analyzer for scanning selected ions from the beam either at specific atomic mass unit or over a range of atomic mass units, and an ion detector having an electron multiplier with an anode and cathode for receiving ions from the beam discharged from the mass analyzer, an improved method of obtaining an output signal indicative of the intensity, of ions at a specific atomic mass unit and representative of the composition of the sample, the method comprising:
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charging the cathode at a high negative potential to attract positive ions or at a high positive potential to attract negative ions while maintaining the anode at a positive potential relative to the cathode; modulating the ion beam to the mass analyzer to produce a modulated ion beam to the detector and a modulated high-voltage output signal from the detector; capacitively or inductively coupling the modulated high-voltage output signal to ground and providing a coupled output signal functionally related to the modulated high-voltage output signal; measuring distortion of the coupled output signal introduced by the capacitive or inductive coupling of the modulated high-voltage output signal to ground; and correcting the coupled output signal in response to the measured coupled output signal distortion to provide an essentially ground-voltage modified coupled output signal representative of the modulated high-voltage output signal.
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Specification