High-efficiency electron ionizer for a mass spectrometer array

Details High-efficiency electron ionizer for a mass spectrometer array High-efficiency electron ionizer for a mass spectrometer array

2001-07-10

2003-02-18

Nguyen, Kiet T. (Department: 2881)

Radiant energy

Ion generation

Electron bombardment type

C250S42300F, C250S288000

Reexamination Certificate

active

06521898

ABSTRACT:

ORIGIN OF INVENTION
The invention described herein was made in performance of work under a NASA contract, and is subject to the provisions of Public Law 96-517 (35 U.S.C. 202) in which the Contractor has elected to retain title.
TECHNICAL FIELD
The invention relates to an improved electron ionizer for a mass spectrometer array for the separation of ions with different masses.
BACKGROUND
A quadrupole mass spectrometer separates ions with different masses by applying a DC voltage and an rf voltage on four rods having circular or hyperbolic cross sections and an axis equidistant from each rod. Sample ions enter this cross sectional area through an aperture at the ends of the rods. The variation of the applied rf voltages on the four rods selects sample ions of a certain mass-to-charge ratio (m/e) to exit the quadrupole mass spectrometer to be detected. Sample ions with different m/e values either impact the rods and are neutralized or deflected away from the axis of the quadrupole.
A miniature quadrupole mass spectrometer array is described in U.S. Pat. No. 5,596,193, the disclosure of which is herein incorporated by reference.
FIG. 1
shows a block diagram of a typical prior art quadrupole mass spectrometer
100
constructed of 16-rod electrodes
106
in a 4×4 array to form nine separate quadrupole regions. Ionization of a gas sample begins in an ionizer chamber within an ionizer
102
. Sample atoms or molecules are injected into this chamber where they are intercepted by electron beams and are ionized to positive ions. These are then extracted through the entrance apertures
104
of the quadrupole mass spectrometer
100
and are detected.
Electron ionizers, as used in mass spectrometers, have applications in environmental monitoring, semiconductor etching, residual gas analysis in laboratory vacuum chambers, monitoring of manufacturing plants against toxic substances, protection of buildings, harbors, embassies, airports, military sites, and power plants against terrorist attacks.
SUMMARY
The inventors noticed that the existing electron ionizers are relatively inefficient. They found that the electron beams are not passing to a proper area, near enough to the entrance apertures
104
. Hence, those apertures are “starved” for ions. Proportionately more electrons escape out the exit than are extracted as ions through the entrance apertures
104
. Even those apertures that have coverage lack efficient ion transport means to optimally focus ions onto the quadrupolar regions.
The system disclosed herein meets these drawbacks by using an electron beam collimator, preferably, at least one shim plate
310
, to collimate an electron beam
306
emitted from a cathode
302
. The electron beam intercepts sample atoms and molecules ejected from a repeller plate
312
and ionizes them to positive ions. The ions are then extracted by static fields formed by a repeller plate
312
and a first lens element
316
. Three lens elements
316
,
408
and
410
extract and focus these ions onto entrance apertures
412
.


REFERENCES:
patent: 3247373 (1966-04-01), Herzog et al.
patent: 4313911 (1982-02-01), Moran et al.
patent: 4943718 (1990-07-01), Haines et al.
patent: 5756996 (1998-05-01), Bier et al.
patent: 6072182 (2000-06-01), Chutjian et al.
patent: 6271527 (2001-08-01), Chutjian et al.

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