U.S. patent application number 10/706329 was filed with the patent office on 2005-05-12 for carbon nanotube electron ionization sources.
Invention is credited to Traynor, Peter John, Wright, Robert George.
Application Number | 20050098720 10/706329 |
Document ID | / |
Family ID | 34435628 |
Filed Date | 2005-05-12 |
United States Patent
Application |
20050098720 |
Kind Code |
A1 |
Traynor, Peter John ; et
al. |
May 12, 2005 |
CARBON NANOTUBE ELECTRON IONIZATION SOURCES
Abstract
An ion source for use in a mass spectrometer includes an
electron emitter assembly configured to emit electron beams,
wherein the electron emitter assembly comprises carbon nanotube
bundles fixed to a substrate for emitting the electron beams, a
first control grid configured to control emission of the electron
beams, and a second control grid configured to control energies of
the electron beams; an ionization chamber having an electron-beam
inlet to allow the electron beams to enter the ionization chamber,
a sample inlet for sample introduction, and an ion-beam outlet to
provide an exit for ionized sample molecules; an electron lens
disposed between the electron emitter assembly and the ionization
chamber to focus the electron beams; and at least one electrode
disposed proximate the ion-beam outlet to focus the ionized sample
molecules exiting the ionization chamber.
Inventors: |
Traynor, Peter John;
(Scottsdale, AZ) ; Wright, Robert George; (Helsby
Cheshire, GB) |
Correspondence
Address: |
ROSENTHAL & OSHA L.L.P.
Suite 2800
1221 McKinney Street
Houston
TX
77010
US
|
Family ID: |
34435628 |
Appl. No.: |
10/706329 |
Filed: |
November 12, 2003 |
Current U.S.
Class: |
250/288 |
Current CPC
Class: |
H01J 49/147 20130101;
H01J 2201/30434 20130101; B82Y 30/00 20130101; B82Y 10/00 20130101;
Y10S 977/939 20130101 |
Class at
Publication: |
250/288 |
International
Class: |
H01J 049/00 |
Claims
What is claimed is:
1. An ion source for use in a mass spectrometer, comprising: an
electron emitter assembly configured to emit electron beams,
wherein the electron emitter assembly comprises carbon nanotube
bundles fixed to a substrate for emitting the electron beams, a
first control grid configured to control emission of the electron
beams, and a second control grid configured to control energies of
the electron beams; an ionization chamber having an electron-beam
inlet to allow the electron beams to enter the ionization chamber,
a sample inlet for sample introduction, and an ion-beam outlet to
provide an exit for ionized sample molecules; an electron lens
disposed between the electron emitter assembly and the ionization
chamber to focus the electron beams; and at least one electrode
disposed proximate the ion-beam outlet to focus the ionized sample
molecules exiting the ionization chamber.
2. The ion source of claim 1, wherein the carbon nanotube bundles
comprise one selected from single-walled carbon nanotubes,
multi-walled carbon nanotubes, and a combination thereof.
3. The ion source of claim 1, further comprising an ion repeller
disposed inside the ionization chamber to help the ionized sample
molecules exit the ionization chamber.
4. The ion source of claim 1, further comprising a trap electrode
to capture a portion of the electron beams exiting the ionization
chamber to provide a feedback control of electron beam
emission.
5. The ion source of claim 1, wherein the at least one electrode
comprises at least one selected from a focusing half plate, a
source slit plate, an alpha plate, an extracting lens, and a
collimating lens.
6. The ion source of claim 1, wherein the second control grid are
adapted to connect to an electrical source such that the energies
of the electron beams are about 70 electron volts.
7. An ion source for use in a mass spectrometer, comprising: an
ionization chamber comprising carbon nanotube bundles for emitting
electron beams, wherein the carbon nanotube bundles are fixed on a
conductive surface on a first wall of the ionization chamber; a
sample inlet disposed on the ionization chamber for sample
introduction; an ion-beam outlet disposed on the ionization chamber
to provide an exit for ionized sample molecules; and at least one
electrode disposed proximate the ion-beam outlet to focus the
ionized sample molecules exiting the ionization chamber, wherein
the conductive surface on the first wall and an electron-energy
plate on a second wall of the ionization chamber are adapted to
connect to an electrical source such that an electrical field is
established to induce electron beam emission from the carbon
nanotube bundles.
8. The ion source of claim 7, wherein the carbon nanotube bundles
comprise one selected from single-walled carbon nanotubes,
multi-walled carbon nanotubes, and a combination thereof.
9. The ion source of claim 7, further comprising an ion repeller
disposed inside the ionization chamber to help the ionized sample
molecules exit the ionization chamber.
10. The ion source of claim 7, wherein the at least one electrode
comprises at least one selected from a focusing half plate, a
source slit plate, an alpha plate, an extracting lens, and a
collimating lens.
11. A mass spectrometer, comprising: a carbon nanotube-based ion
source; a mass filter operatively coupled to the carbon
nanotube-based ion source for separating ionized sample molecules
based on their mass-to-charge ratios; and an ion detector
operatively coupled to the mass filter for detecting the ionized
sample molecules.
12. The mass spectrometer of claim 11, wherein the carbon
nanotube-based ion source comprises: an electron emitter assembly
configured to emit electron beams, wherein the electron emitter
assembly comprises carbon nanotube bundles fixed to a substrate for
emitting the electron beams, a first control grid configured to
control emission of the electron beams, and a second control grid
configured to control energies of the electron beams; an ionization
chamber having an electron-beam inlet to allow the electron beams
to enter the ionization chamber, a sample inlet for sample
introduction, and an ion-beam outlet to provide an exit for ionized
sample molecules; an electron lens disposed between the electron
emitter assembly and the ionization chamber to focus the electron
beams; and at least one electrode disposed proximate the ion-beam
outlet to focus the ionized sample molecules exiting the ionization
chamber.
13. The mass spectrometer of claim 12, wherein the carbon nanotube
bundles comprise one selected from single-walled carbon nanotubes,
multi-walled carbon nanotubes, and a combination thereof.
14. The mass spectrometer of claim 12, wherein the carbon
nanotube-based ion source further comprising an ion repeller
disposed inside the ionization chamber to help the ionized sample
molecules exit the ionization chamber.
15. The mass spectrometer of claim 12, wherein the carbon
nanotube-based ion source further comprising a trap electrode to
capture a portion of the electron beams exiting the ionization
chamber and to provide a feedback control of electron beam
emission.
16. The mass spectrometer of claim 12, wherein the at least one
electrode comprises at least one selected from a focusing half
plate, a source slit plate, an alpha plate, an extracting lens, and
a collimating lens.
17. The mass spectrometer of claim 12, wherein the second control
grid are adapted to connect to an electrical source such that the
energies of the electron beams are about 70 electron volts.
18. The mass spectrometer of claim 11, wherein the carbon
nanotube-based ion source comprises: an ionization chamber
comprising carbon nanotube bundles for emitting electron beams,
wherein the carbon nanotube bundles are fixed on a conductive
surface on a first wall of the ionization chamber; a sample inlet
disposed on the ionization chamber for sample introduction; an
ion-beam outlet disposed on the ionization chamber to provide an
exit for ionized sample molecules; and at least one electrode
disposed proximate the ion-beam outlet to focus the ionized sample
molecules exiting the ionization chamber, wherein the conductive
surface on the first wall and an electron-energy plate on a second
wall of the ionization chamber are adapted to connect to an
electrical source such that an electrical field is established to
induce electron beam emission from the carbon nanotube bundles.
19. The mass spectrometer of claim 18, wherein the carbon nanotube
bundles comprise one selected from single-walled carbon nanotubes,
multi-walled carbon nanotubes, and a combination thereof.
20. The mass spectrometer of claim 18, further comprising an ion
repeller disposed inside the ionization chamber to help the ionized
sample molecules exit the ionization chamber.
21. The mass spectrometer of claim 18, wherein the at least one
electrode comprises at least one selected from a focusing half
plate, a source slit plate, an alpha plate, an extracting lens, and
a collimating lens.
22. The mass spectrometer of claim 11, wherein the mass filter is
based on a mechanism selected from magnetic sector, electrostatic
sector, quadrupole, ion trap, and time-of-flight.
23. The mass spectrometer of claim 11, further comprising an
electronic module operatively coupled to the carbon nanotube-based
ion source for controlling electron beam emission.
24. The mass spectrometer of claim 11, further comprising a
computer operatively coupled to the mass spectrometer.
25. The mass spectrometer of claim 24, wherein the computer
comprises a program for monitoring a performance of the ion source.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] Not Applicable.
BACKGROUND OF INVENTION
[0002] 1. Field of the Invention
[0003] The invention relates generally to ion sources for mass
spectrometers, and, more particularly, to carbon nanotube-based ion
sources for mass spectrometers.
[0004] 2. Background Art
[0005] Mass spectrometers are powerful instruments for the analysis
of a wide variety of samples. In order to perform mass analysis,
the samples need to be vaporized. The gas molecules are then
ionized by an ion source. An efficient ion source will convert as
many sample molecules into ions as possible and produce an optimal
beam for the specific type of analyzer. The most common ion source
is the electron ionization (EI) source. In an EI source, electrons
are produced by thermal emission from a hot filament, which is
heated by a current flowing through it, located outside the
ionization chamber. The electrons are accelerated by an electric
field to a desired level of energy. This energy level is typically
round 70 eV, but can vary from about 10 eV to upwards of 150 eV, as
defined by the potential difference between the filament and the
ionization chamber. When the electrons collide with sample gas
molecules in the ionization chamber, the gas molecules each lose an
electron and become positively charged. Once the sample molecules
acquire positive charges, they can be accelerated out of the
ionization chamber and guided into the entrance of the mass
spectrometer by an applied electrostatic field.
[0006] While various configurations have been developed for EI
sources used in mass spectrometers, the configuration originally
design by Nier and the variants thereof are the most common. FIG. 1
shows two views of a basic Nier design ion source that uses a hot
wire filament 10 to produce an electron beam 19; one view (1 A) is
perpendicular to the xz plane, while the other view (1 B) is in the
xy plane, where the x-axis is the direction of motion of ions
leaving the ion source and the y-axis is in the direction of mass
separation and the z-axis is perpendicular to both the x- and
y-axes. The electron beam 19 is typically accelerated to about 70
eV of energy. The electron beam 19 is designed to interact with
molecules introduced into the ionization chamber 11, under high
vacuum. The interactions produce molecular ions and fragment ions
that can be accelerated out of the ionization chamber 11.
[0007] Because the electron beam is somewhat divergent, a pair of
permanent magnets 14 is added to force the electron beam 19 to
travel in a spiral path, which constrains the motion of the
electrons to a narrow beam. Any component of electron motion which
is perpendicular to the magnetic flux acts to deflect the electrons
into a spiral trajectory. This has the effect of increasing the
probability of the interactions between the electron beam 19 and
the molecules in the ionization chamber 11 in the region where they
are extracted as positive ions. In this way good sensitivity and
resolution (low ion energy spread) are achieved.
[0008] Once ionized, the newly charged particles are repelled by
the ion repeller 12 to move towards an exit of the ionization
chamber 11. In addition, the charged particles are accelerated by
the accelerating potential 15, focused by the focusing half plate
16, and filtered by the alpha slit 17 to form a focused ion beam
18. The focused ion beam 18 is then introduced into a mass filter
(not shown), where they are separated according to their
mass-to-charge ratios.
[0009] Interactions between the sample gas and the hot filament may
result in changes in the electron work function of the filament. In
order to provide a constant intensity of the electron beam 19, an
electron trap 13 is typically provided in an EI source. The
electron trap 13 is to capture the proportion of the electron beam
19 that exits the ionization chamber 11. In addition, the electron
trap 13 may also be used to monitor the intensity of the electron
beam 19 in order to provide a feedback control to the current
flowing through the filament 10. The feedback control enables the
filament 10 to produce a constant intensity electron beam 19 as
measured at the electron trap 13.
[0010] In a typical EI source, the filament 10 is a wire and made
of a refractory metal. The current heats the filament 10 to a
temperature (about 2000.degree. C.) at which thermionic emission of
electrons occurs. The filament 10 is typically held at a negative
electric field relative to the ionization chamber 11 (e.g., by
applying an potential difference across the filament 10 and the
ionization 11) so that the emitted electrons are accelerated from
the hot filament 10 in the direction of the gradient of the
electric field. The translational energy of the electron beams
affects the nature of the interactions between the gaseous sample
molecules and the electrons.
[0011] Although a typical ion source design is based upon well
established principles, the performance of an ion source depends
upon the interactions of many subtle design characteristics. There
are several problems associated with the filament assemblies used
in electron impact or chemical ionization source. The primary
problem is that the origin and trajectory of the electrons are ill
defined. Additionally, the electron emission relies on the
vaporization of material, which results in a limited filament
lifetime. Interactions between the sample gas and the hot filament
may result in changes in the electron work function of the
filament. As noted above, a trap electrode (shown as 13 in FIG. 1)
may be used in a feedback circuit to regulate the electron beam 19
intensity. However, regulation of the trap current will alter
filament temperature. This can lead to fluctuation in the
temperature distribution in the ion source and cause the assembly
to become misaligned. These effects lead to changes in absolute
sensitivity, relative sensitivity, and the degree of molecular
fragmentation. As a result, it is often difficult, if not
impossible, to de-convolute a mass spectrum of a complex mixture
sample, due to inevitable uncertainties in the contributions from
the components in the mixture.
[0012] Thus, to avoid mass analysis complications, it is desirable
to have an ion source that can produce a stable stream of electrons
with predictable trajectories and uniform density.
SUMMARY OF INVENTION
[0013] One aspect of the invention relates to ion sources for use
in a mass spectrometer. An ion source in accordance with aspects of
the invention includes an electron emitter assembly configured to
emit electron beams, wherein the electron emitter assembly
comprises carbon nanotube bundles fixed to a substrate for emitting
the electron beams, a first control grid configured to control
emission of the electron beams, and a second control grid
configured to control energies of the electron beams; an ionization
chamber having an electron-beam inlet to allow the electron beams
to enter the ionization chamber, a sample inlet for sample
introduction, and an ion-beam outlet to provide an exit for ionized
sample molecules; an electron lens disposed between the electron
emitter assembly and the ionization chamber to focus the electron
beams; and at least one electrode disposed proximate the ion-beam
outlet to focus the ionized sample molecules exiting the ionization
chamber.
[0014] Another aspect of the invention relates to ion sources for
use in a mass spectrometer, wherein the CNT emitter is incorporated
within a micromachined ionization chamber. An ion source in
accordance with embodiments of the invention includes an ionization
chamber comprising carbon nanotube bundles for emitting electron
beams, wherein the carbon nanotube bundles are fixed on a
conductive surface on a first wall of the ionization chamber; a
sample inlet disposed on the ionization chamber for sample
introduction; an ion-beam outlet disposed on the ionization chamber
to provide an exit for ionized sample molecules; and at least one
electrode disposed proximate the ion-beam outlet to focus the
ionized sample molecules exiting the ionization chamber, wherein
the conductive surface on the first wall and an electron-energy
plate on a second wall of the ionization chamber are adapted to
connect to an electrical source such that an electrical field is
established to induce electron beam emission from the carbon
nanotube bundles.
[0015] One aspect of the invention relates to mass spectrometers. A
mass spectrometer in accordance with embodiments of the invention
includes a carbon nanotube-based ion source; a mass filter
operatively coupled to the carbon nanotube-based ion source for
separating ionized sample molecules based on their mass-to-charge
ratios; and an ion detector operatively coupled to the mass filter
for detecting the ionized sample molecules.
[0016] Other aspects and advantages of the invention will be
apparent from the following description and the appended
claims.
BRIEF DESCRIPTION OF DRAWINGS
[0017] FIGS. 1A and 1B illustrate a conventional ion source based
on Nier design.
[0018] FIG. 2A illustrates a closed-end multi-walled carbon
nanotube (MWCNT) and
[0019] FIG. 2B illustrates the key components of the external CNT
emitter assembly in accordance with one embodiment of the
invention.
[0020] FIGS. 3A and 3B illustrate a carbon nanotube-based ion
source in accordance with one embodiment of the invention.
[0021] FIGS. 4A-4C illustrate a carbon nanotube-based ion source in
accordance with another embodiment of the invention.
[0022] FIG. 5 shows a schematic of a mass spectrometer in
accordance with one embodiment of the invention.
DETAILED DESCRIPTION
[0023] Embodiments of the present invention relate to ion sources
for mass spectrometry. An ion source in accordance with embodiments
of the invention is based on carbon nanotubes and can provide
reliable electron beams for a long life time.
[0024] Carbon nanotubes (CNT) are seamless tubes of graphite sheets
with full fullerene caps which were first discovered as multi-layer
concentric tubes (i.e., multi-walled carbon nanotubes, MWCNT), as
shown in FIG. 2A. Subsequently, single-walled carbon nanotubes
(SWCNT) were prepared in the presence of transition metal
catalysts. CNT have shown promising potentials in applications
including nanonscale electronic devices, high strength materials,
electron field emission, tips for scanning probe microscopy, gas
storage, etc.
[0025] As electron field emitters, carbon nanotubes have the
characteristics of low work function, durability, and thermal
stability. Accordingly, an electron field emitter based on CNT can
be driven at low voltage. In addition, the resistance of such
devices to reactions with gases, which are generated during the
operation of the device, is improved, thereby increasing the life
span of the emitters. Examples of the use of CNT as electron field
emitters and the methods of preparing CNT-based field emission
arrays, for example, may be found in U.S. Pat. No. 6,440,761 issued
to Choi.
[0026] FIG. 2B illustrates a CNT field emitter assembly 200 which
consists of a substrate 20 with a conducting layer to which the
parallel CNT bundles 21 are fixed. Immediately above, and insulated
from, the ends of the CNT array is a first grid assembly (an
emission control grid) 22 aligned in such a way as to provide the
necessary field emission and to permit passage of the electrons
through the second grid (an energy control grid) 23 that is
connected to the energy controlling potential V.sub.e. Electrons
are ejected through the second grid 23 towards the ionization
region (e.g., the ionization chamber 34 in FIG. 3A) having a
current intensity controlled by the potential V.sub.i. The
potential V.sub.i controls the emission current density via a
feedback circuit that incorporates the trap electrode (e.g.,
electron trap 35 in FIG. 3A). Potential V.sub.e is used to modify
the electron energy, which is typically controlled at the 70 eV
level. The CNT emitter assembly 200 thereby provides a
monoenergetic electron beam of uniform density with a predetermined
spatial origin and a fixed trajectory.
[0027] FIG. 3A shows a view in the XZ plane of an ion source 300
including a CNT emitter assembly 30 in accordance with one
embodiment of the invention. This view illustrates the location of
a CNT electron emitter assembly 30 and the electron lens 31 with
respect to the ionization chamber 34 and the electron trap
electrode 35. As shown, the CNT electron emitter assembly 30 is
configured to produce an electron beam 32 via the field emission
effect. The electron beam 32 is focused by the electron lens 32
into a narrow beam, which then passes the electron-beam inlet 38
into the ionization chamber 34. The electron beams 32 interacts
with gas sample molecules in the ionization chambers 34 to produce
ionized sample molecules. The ionized sample molecules may include
molecule ions and fragment ions. The ionized sample molecules may
be repelled by a repeller electrode 36 to exit the ionization
chamber 34 via the ion-beam exit 29. The ionized sample molecules
exiting the ionization chamber 34 are focused by at least one
electrode/plate 37, which may include, for example, a focusing half
plate 37a, a source slit plate 37b, and an alpha plate 37c, into a
narrow ion beam 33. The ion bean 33 may then be introduced into a
mass filter/analyzer (not shown) for analysis.
[0028] Also shown in FIG. 3A is an electron trap 35, which
functions to capture the portion of the electron beam 32 that exit
the ionization chamber 34. In some embodiments, the electron trap
35 may be coupled to a feedback circuit 35a and an electrical
source 35b to regulate the emission of the electron beam 32 from
the CNT electron emitter assembly 30. The electron trap 35 together
with the feedback circuit 35a makes it possible to control the
emission of the electron beam 32 at a constant level, as determined
by the electron beams 32 captured by the electron trap 35.
[0029] FIG. 3B illustrates another view of the ion source 300 shown
in FIG. 3A. As shown, the ionization chamber 34 includes a sample
inlet 28 for the introduction of sample gas and an ion-beam outlet
29 to allowed the ionized sample molecules (i.e., ion beam 33) to
exit the ionization chamber 34. At least one electrode/plate 37 is
provided around the ion-beam outlet 29 to focus the exiting ion
beam 33. The at least one electrode 37 may function to extract and
focus the ionized sample molecules as an ion beam 33. The
extraction of the ionized sample molecules may also be facilitated
by the ion repeller 36 and an accelerating potential applied across
the ionization chamber 34 and the source slit plate 37b. The at
least one electrode 37, for example, may include a focusing half
plate 37a, a source slit plate 37b, and an alpha slit 37c. Once out
of the ionization chamber 34, the ion beam 33 may be focused by the
focusing half plate 37a, the source slit plate 37b, and/or the
alpha slit 37c, before the ion beam 33 is allowed into a mass
filter/analyzer (not shown).
[0030] FIGS. 4A-4C show a CNT-based ion source in accordance with
one embodiment of the invention. The CNT-based ion source 400 is
particularly suitable for use in a miniature mass spectrometer.
FIG. 4A shows a side view in the xz plane of the CNT-based ion
source 400. FIG. 4B shows an enlarged section of a CNT electron
emitter/ion source assembly 40 included in the ion source 400 shown
in FIG. 4A, while FIG. 4C shows a view of the ion source 400 in the
xy plane.
[0031] Referring to FIGS. 4A and 4B, the CNT electron emitter 40
comprises a substrate layer 41, on which a layer of carbon
nanotubes 42 is formed. A preferred embodiment of the CNT electron
emitter 40 for use in a miniaturized mass spectrometer, for
example, may include a CNT array of approximately 0.1 cm.sup.2 and
will contain of the order of 108 MWCNTs. However, one of ordinary
skill in the art would appreciate that other dimensions and
densities of the CNT array may also be used without departing from
the scope of the invention. There are three main approaches to the
synthesis of CNT: laser ablation of carbon, electric arc discharge
of graphite rod, and chemical vapor deposition (CVD) of
hydrocarbons, as disclosed in U.S. Pat. No. 6,333,016 B1 issued to
Resasco et al. and references cited therein. Among these
approaches, CVD coupled with photolithography has been found to be
the most versatile in the preparation of various CNT devices. Many
commercial sources now supply high quality CNT devices in various
configurations. The CNT suitable for use with embodiments of the
invention, for example, may be obtained from Molecular Nanosystems
(Palo Alto, Calif.).
[0032] The layer of carbon nanotubes 42 may comprise a highly
ordered array of defect-free, parallel carbon nanotubes (CNT).
These can be single-walled (SWCNT), multi-walled (MWCNT) CNT, or a
combination thereof. Both MWCNT and SWCNT can be manufactured to
have narrow size distributions, large-scale periodicities, and high
array densities. These attributes result in a very stable,
predictable, and uniformly dense electron beam. In preferred
embodiments of the invention, the carbon nanotube layer 42 comprise
MWCNT.
[0033] When an electrical source 44 (for generating an electric
field) is applied across the layer of substrate 41 and the electron
energy plate 43 of the ionization chamber 64, electrons emitted
from the CNT layer 42 are accelerated towards the electron energy
layer 43. As in a conventional EI source, this electric field 44
may be controlled to provide the electron beams with a desired
energy level to cause fragmentation of the molecules. In a typical
application this electric field may be maintained at between 10 and
150 eV. In preferred embodiments of the invention, this field is
maintained to provide the electron beams with an energy equivalent
to about 70 eV.
[0034] In operation, the sample in gaseous state is introduced into
the ionization chamber 64 (see FIG. 4B or 4C) via a sample inlet
46. The sample inlet 46 may be a laser drilled leak assembly or any
suitable machined opening. Once inside the ionization chamber 64,
the sample molecules 45 interact with the electron beams 47 emitted
by the CNT layer 42. As a result of the interactions, the sample
molecule 45 become ionized (charged). The ionized sample molecules
45 may then be repelled by the ion repeller 48, which comprise an
electrode disposed inside the ionization chamber 64, to move
towards an ion-beam outlet 66. An extraction lens L1 is provided
near the outlet. The extraction lens L1, which comprise one or more
electrodes, is provided with an electrical potential to help the
charged molecules 45 move out of the ion-beam outlet 66. Once the
charged molecules 45 exit the ionization chamber 64, they are
focused by a series of collimating lens L2, which comprises at
least one electrode, to form a highly focused molecular ion beam
49, which is then introduced into the mass filter (mass analyzer)
portion of a mass spectrometer (not shown) that separates and
detects these charged ions 45 based on their mass-to-charge ratios
(m/z).
[0035] FIG. 5 shows a schematic of a mass spectrometer using a CNT
based ion source (see FIG. 3 and FIG. 4) in accordance with one
embodiment of the invention. As shown, a mass spectrometer 50
comprises an ion source 51, a mass filter 52, and a detector 53.
The ion source 51 may comprise a CNT-based ion source shown in
either FIG. 3 or FIG. 4. The ion source 51 ionize sample molecules
and focuses them into a narrow ion beam 57 (see also FIGS. 3 and
4).
[0036] The narrow ion beam 57 is introduced into the mass filter
52, where the charged particles are separated according to their
m/z. Embodiments of the invention may use any mass filter known in
the art. These mass filters may be based on, for example, magnetic
sector, electrostatic sector, quadrupole, ion trap, time-of-flight,
etc. The separated charge particles are then detected by ion
detector 53. Again, any suitable ion detector may be used with
embodiments of the invention.
[0037] In addition, the mass spectrometer 50 includes an electronic
module 54 that can control the entry of the ion beam 57 into the
mass filter 52. The electronic module 54 may accomplish this
function by two mechanisms: by controlling the production of
electron beams (shown as 47 in FIG. 4) from the CNT layer (shown as
42 in FIG. 4) or by controlling the introduction of the ion beam 57
into the mass filter 52. In preferred embodiments of the invention,
the electronic module 54 controls the generation of the electron
beams from the CNT layer (see FIG. 3 and FIG. 4). The advantage of
blocking the electron beam formation from the CNT layer, as opposed
to blocking the entry of the molecular ion beam 57 into the mass
filter 52, is that much smaller transition times are needed for
blocking the electron beam, as compared to blocking the ion beam,
all other factors being equal. This is due to the fact that the
molecular ions are at least 10,000 times more massive than the
electrons, and, therefore, electrons travel at least 100 times
faster than ions at comparable energies. Consequently, transition
times in the stopping and starting of electron beams are at least
100 times faster than for ion beams.
[0038] In addition, a CNT-based electron emitter in accordance with
embodiments of the invention permits fast switching (i.e., turning
on and off at high frequency), which makes pulsed ionization
possible. For example, referring to FIG. 3A and FIG. 4B,
alternately switching on the electron beams 47 and the ion repeller
48 allows the source designer to select a relatively high repeller
48 voltage that can be used to ensure rapid ion extraction, without
distorting the electron beam 47 energy or trajectory.
[0039] Referring again to FIG. 5, the mass spectrometer 50 may be
controlled by a computer 55. The computer 55 may be a general
purpose computer or specifically designed computer that may include
interfaces 56 and programs to control sample ionization by the ion
source 51, the operation of the mass filter 52 (e.g., ramping the
electrostatic field or the magnetic field), and the detection of
the charged molecules by the ion detector 53. In addition, the
computer 55 also controls the electronic module 54 that, for
example, may permit pulsed ionization or alternate switching on the
ion source and the repeller potential, as noted above. In
alternative embodiments of the invention, the electronic modules
may be part of the computer 55, rather than part of the mass
spectrometer 50.
[0040] The above examples are for illustration only. One of
ordinary skill in the art would appreciate that various
modifications are possible without departing from the scope of the
invention. For example, while the ion source shown in FIG. 4 has a
generally square (box) shaped ionization chamber 64 and a
disk-shaped CNT layer, other geometries may also be used.
[0041] The advantages of the invention may include one or more of
the following. In designing an ion source in accordance with
embodiments of the invention, a computer program may be used to
simulate the ion trajectory and reduce the number of variables
associated with the performance characteristics of the ion source.
The same program can also be used to model and monitor changes in
the performance of the CNT-based ion source, once it is in service.
The ability to monitor and account for changes in the performance
of the ion source in turn allows for easy calibration of the
instruments, for example, using application gas library with
software correction provided by measuring a single certified
calibration gas blend. This would provide a significant reduction
in the complexity of operation, as compared to a traditional EI
equipped mass spectrometer. Because the CNT-based ion source design
removes the need for regular filament replacement, it is possible
to design an extremely capable and reliable general-purpose
industrial gas analyzer with no moving parts. This is made possible
because the traditional turbo molecular vacuum pump can be replaced
with an ion pump because there is no longer a requirement for a
fast pump-down time--the vacuum does not need to be interrupted for
routine maintenance. The CNT-based ion sources in accordance with
the invention can be operated with minimal thermal perturbation and
can respond to fast voltage regulation. This makes it possible to
run the mass analysis in a pulsed mode.
[0042] While the invention has been described with respect to a
limited number of embodiments, those skilled in the art, having
benefit of this disclosure, will appreciate that other embodiments
can be devised which do not depart from the scope of the invention
as disclosed herein. Accordingly, the scope of the invention should
be limited only by the attached claims.
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