U.S. patent application number 12/762783 was filed with the patent office on 2010-10-07 for method and apparatus for detecting positively charged and negatively charged ionized particles.
This patent application is currently assigned to Ionics Mass Spectrometry Group Inc.. Invention is credited to Lisa Cousins, Gholamreza Javahery, Charles JOLLIFFE.
Application Number | 20100252729 12/762783 |
Document ID | / |
Family ID | 39135460 |
Filed Date | 2010-10-07 |
United States Patent
Application |
20100252729 |
Kind Code |
A1 |
JOLLIFFE; Charles ; et
al. |
October 7, 2010 |
METHOD AND APPARATUS FOR DETECTING POSITIVELY CHARGED AND
NEGATIVELY CHARGED IONIZED PARTICLES
Abstract
An ion detector includes collision surfaces for converting both
positively and negatively charged ions into emitted secondary
electrons. Secondary electrons may be detected using an electron
detector, than may, for example include an electron multiplier.
Conveniently, secondary electrons (or electrons emitted by the
multiplier) may be detected using an electron pulse counter.
Inventors: |
JOLLIFFE; Charles;
(Schomberg, CA) ; Cousins; Lisa; (Woodbridge,
CA) ; Javahery; Gholamreza; (Kettleby, CA) |
Correspondence
Address: |
SMART & BIGGAR
438 UNIVERSITY AVENUE, SUITE 1500, BOX 111
TORONTO
ON
M5G 2K8
CA
|
Assignee: |
Ionics Mass Spectrometry Group
Inc.
|
Family ID: |
39135460 |
Appl. No.: |
12/762783 |
Filed: |
April 19, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11467720 |
Aug 28, 2006 |
7728292 |
|
|
12762783 |
|
|
|
|
Current U.S.
Class: |
250/282 ;
250/281 |
Current CPC
Class: |
H01J 49/025 20130101;
H01J 49/0095 20130101 |
Class at
Publication: |
250/282 ;
250/281 |
International
Class: |
H01J 49/02 20060101
H01J049/02 |
Claims
1. A method of detecting charged particles, comprising guiding said
charged particles toward first and second electrodes; biasing said
first and second electrodes, at potentials with said first
electrode biased to attract positive ones of said charged
particles, and said second electrode biased to attract negatively
charged ones of said charged particles; wherein said first and
second electrodes each emit secondary electrons in response to
collisions by ones of said charged particles; attracting said
secondary electrons to an electron multiplier, and causing said
electron multiplier to emit electrons in response thereto; and
detecting said electrons emitted by said electron multiplier, at a
detection surface biased at a potential above said first and second
electrodes, to detect said electrons emitted by said electron
multiplier, and thereby said charged particles.
2. The method of claim 1 wherein said biasing said first electrode
comprises applying a bias voltage of between about +1 kV to +10
kV.
3. The method of claim 1 wherein said biasing said second electrode
comprises applying a bias voltage of between about -1 kV to
-10kV.
4. The method of claim 1, wherein a voltage of about 0.1 kV and 1
kV are applied to said detection surface.
5. The method of claim 1, further comprising heating at least one
of said first and second electrodes to a temperature between about
200.degree. C. and 800.degree. C.
6. An ion detector, comprising a first electrode that emits
secondary electrons when collided by a charged ion; a second
electrode that emits secondary electrons when collided by a charged
ion; an electron detector for detecting emitted secondary
electrons, said electron detector having a detection surface; and
at least one voltage source to bias said first electrode at a
potential above ground, said second electrode at a potential below
ground, and said detection surface of said detector at a potential
above said first electrode.
7. The ion detector of claim 6, wherein said first electrode is
biased at a potential to cause said first electrode to emit
secondary electrons in response to collisions by negatively charged
ions.
8. The ion detector of claim 6, wherein said electron detector
comprises an electron multiplier that emits tertiary electrons in
response to said secondary electrons, and wherein said detection
surface detects said tertiary electrons.
9. The ion detector of claim 6, wherein said first electrode is
formed of one of metal and semi-conductor material.
10. The ion detector of claim 9, wherein said second electrode is
formed of one of metal and semi-conductor material.
11. The detector of claim 6, wherein said first electrode is formed
of stainless steel.
12. The ion detector of claim 6, wherein said electron detector
comprises a channel electron multiplier.
13. The ion detector of claim 12, wherein said channel electron
multiplier comprises a ceramic channel.
14. The ion detector of claim 12, wherein said electron multiplier
comprises a glass channel.
15. The ion detector of claim 12, wherein said channel electron
multiplier has an inlet and an exit proximate said detection
surface and wherein said channel electron multiplier proximate said
inlet is biased at a lower potential than said channel electron
multiplier proximate said exit.
16. The ion detector of claim 6, wherein said electron detector
comprises a discrete dynode electron multiplier
17. The ion detector of claim 6, wherein said detection surface
comprises a photo-emissive surface.
18. The ion detector of claim 7, wherein said first electrode is
biased at a voltage between about +1 kV to +10 kV.
19. The ion detector of claim 7, wherein said second electrode is
biased at a voltage between about -1 kV to -10 kV.
20. (canceled)
21. (canceled)
22. (canceled)
23. (canceled)
24. (canceled)
25. (canceled)
26. (canceled)
27. (canceled)
28. (canceled)
29. A method of detecting charged particles, comprising guiding
said charged particles toward first and second collision surfaces;
biasing said first and second collision surfaces, at potentials
with said first collision surface biased to attract positive ones
of said charged particles, and said second collision surface biased
to attract negatively charged ones of said charged particles;
wherein said first and second collision surfaces each emit
secondary electrons in response to collisions by ones of said
charged particles; and detecting emission of said electrons by said
collision surfaces to detect said charged particles.
30. (canceled)
31. (canceled)
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a Continuation of U.S. patent
application Ser. No. 11/467,720 filed Aug. 28, 2006, the contents
of which are hereby incorporated herein by reference.
FIELD OF THE INVENTION
[0002] The present invention relates generally to ion detection,
and more particularly to a method and device for detecting
positively charged ionized particles, as well as negatively charged
ionized particles.
BACKGROUND OF THE INVENTION
[0003] Mass spectrometry has proven to be an effective analytical
technique for identifying unknown compounds and for determining the
precise mass of known compounds. Advantageously, compounds can be
detected or analysed in minute quantities allowing compounds to be
identified at very low concentrations in chemically complex
mixtures. Not surprisingly, mass spectrometry has found practical
application in medicine, pharmacology, food sciences,
semi-conductor manufacturing, environmental sciences, security, and
many other fields.
[0004] A typical mass spectrometer includes an ion source that
ionizes particles of interest. The ions are passed to an analyser
region, where they are separated according to their mass
(m)-to-charge (z) ratios (m/z). The separated ions are detected at
a detector. A signal from the detector may be sent to a computing
or similar device where the m/z ratios may be stored together with
their relative abundance for presentation in the format of a m/z
spectrum.
[0005] Typical ion sources are detailed in "Ionization Methods in
Organic Mass Spectrometry", Alison E. Ashcroft, The Royal Society
of Chemistry, UK, 1997; and the references cited therein.
Conventional ion sources may create ions by atmospheric pressure
chemical ionisation (APCI); chemical ionisation (CI); electron
impact (EI); electrospray ionisation (ESI); fast atom bombardment
(FAB); field desorption/field ionisation (FD/FI); matrix assisted
laser desorption ionisation (MALDI); or thermospray ionization
(TSP).
[0006] Ionized particles may be separated by quadrupoles,
time-of-flight (TOF) analysers, magnetic sectors, and Fourier
transform and quadrupole ion traps. Most ion sources are capable of
producing ionized particles of positive or negative in polarity.
For example, ESI transfers ions that are created in an acidic or
basic solution directly into the gas phase. These ions are
typically products of acid base reactions, such as protonated
molecular adducts that tend to have basic sites, or negatively
charged ions that are slightly acidic. APCI creates negative or
positive ions in the gas phase, through chemical reactions.
[0007] The ion detector in a mass spectrometer typically amplifies
the ion signal striking a detection surface in order to provide
sufficient signal-to-noise to measure intensity as a function of
mass. Typical ion detectors include discrete electrodes with a
resistive chain or a continuous channel with a resistive surface.
Ions strike the first electrode, causing secondary electrons to be
emitted from the surface and undergo a cascade of amplification as
they are accelerated down the tube. The electron acceleration
potential is the difference between the voltage on the first
electrode and the last electrode.
[0008] The emission of secondary electrons is velocity dependent,
with higher velocity ions producing more emission. Ions of
different mass-to-charge ratios are accelerated to the same energy
(for the same charge state), and since E=1/2 mv.sup.2 the
velocities and therefore the detection efficiency is mass
dependent.
[0009] Two common approaches to detection are used: pulse counting
and analog current detection. In pulse counting detection,
individual ion pulses are amplified, typically with a gain between
1.times.10.sup.6 and 100.times.10.sup.6, and detected as a current
pulse. In analog current detection, the individual ion pulses are
amplified with a gain between 1,000 and 10,000 and measured as a DC
current.
[0010] In some applications such as pharmaceutical drug discovery
and drug development, it is desirable to investigate both positive
and negative ions generated by one or more ion sources at
approximately the same time. Therefore the mass analyser and ion
detector must be able to rapidly switch from a mode that samples
one polarity (e.g. negative ions) to another (e.g. positive
ions).
[0011] Such switching typically requires reversal of polarity of
large applied voltages. To do so, a power supply having a high
voltage range that is capable of quick switching is required.
Moreover, extreme care must be taken to limit the noise resulting
from power supply switching, and to ensure the output signal is not
distorted, and that the detector is not damaged. Typically,
providing a suitable supply and integrating it in an ion detector
is costly, and complex.
[0012] At least one ion detector that may be used to simultaneously
detect both positively and negatively charged ions uses two
conversion electrodes (also referred to as dynodes). Incoming
positive ions strike one conversion electrode, held at high
negative voltage, causing ejection of electrons. Incoming negative
ions are attracted to, and strike the second conversion electrode,
held at high positive voltage, causing ejection of a positive ion.
Positive ions, and electrons emitted by the conversion electrodes
are attracted to, and strike the inlet of a glass or similar
electron multiplier, that is kept at a voltage above that of the
conversion electrodes. Incident ions and electrons cause the
emission of electrons, within the multiplier. Measurement of
emitted electrons and associated energies allows for detection of
ions incident on the conversion electrodes.
[0013] By design, emitted electrons are detected at ground
potential, and may thus be detected by an analog detector. Not
surprisingly, conversion of ions to electrons at electrodes is
dependent on the mass of the ions. Unfortunately, conversion of
negative to positive ions at a conversion electrode is not well
understood and may exhibit poor sensitivity for certain compounds.
Thus, negative ion detection in such a detector is mass and
compound dependent.
[0014] Further, as positive ions are heavier than electrons, the
electrons are accelerated more quickly to the multiplier, than
positive ions. The relatively slow speed of the positive ion can
impede high speed operation of the detector.
[0015] Accordingly, there is a need for an improved ion detector,
and method capable of quickly and efficiently detecting both
positively and negatively charged ionized particles.
SUMMARY OF THE INVENTION
[0016] In accordance with an aspect of the present invention, an
ion detector includes collision surfaces for converting both
positively and negatively charged ions into electrons. The
collision surfaces may be formed as conversion electrodes. Emitted
secondary electrons may be detected using an electron detector that
may, for example, include an electron multiplier. Conveniently,
secondary electrons (or electrons emitted by the multiplier) may be
detected using an electron pulse counter.
[0017] In accordance with an embodiment of the present invention, a
method of detecting charged particles, comprises guiding the
charged particles toward first and second electrodes; biasing the
first and second electrodes, at potentials with the first electrode
biased to attract positive ones of the charged particles, and the
second electrode biased to attract negatively charged ones of the
charged particles. Secondary electrons are emitted by the first and
second electrodes. The secondary electrons are attracted to an
electron multiplier, and cause the electron multiplier to emit
electrons. Electrons emitted by the electron multiplier, are
detected at a detection surface biased at a potential above the
first and second electrodes, to detect the electrons emitted by the
electron multiplier, and thereby the charged particles.
[0018] In accordance with a further embodiment, an ion detector
comprises first and second electrodes that emit secondary electrons
when collided by a charged ion. An electron detector having a
detection surface detects emitted secondary electrons. At least one
voltage source biases the first electrode at a potential above
ground, the second electrode at a potential below ground, and the
detection surface of the detector at a potential above the first
electrode.
[0019] In a further embodiment, a charged particle detector
comprises first and second conversion electrodes that emit
electrons when collided by charged particles. An electron
multiplying detector multiplies the emitted electrons. The
multiplying detector has a detection surface. At least one voltage
source biases the first electrode at a potential above ground, the
second electrode at a potential below ground, and the detection
surface of the electron multiplier at a potential above the first
and second electrodes.
[0020] In accordance with yet a further embodiment, a method of
detecting charged particles, comprises guiding the charged
particles toward first and second collision surfaces; biasing the
first and second collision surfaces, at potentials with the first
collision surface biased to attract positive ones of the charged
particles, and the second collision surface biased to attract
negatively charged ones of the charged particles; wherein the first
and second collision surfaces each emit secondary electrons in
response to collisions by ones of the charged particles; and
detecting emission of the electrons by the collision surfaces to
detect the charged particles.
[0021] In accordance with yet another embodiment, a method of
detecting charged particles, comprises biasing first and second
collision surfaces, at potentials with the first collision surface
biased to attract positive ones of the charged particles, and the
second collision surface biased to attract negatively charged ones
of the charged particles; wherein the first and second collision
surfaces each emit secondary electrons in response to collisions by
ones of the charged particles; guiding charged particles of a
single first polarity toward first and second collision surfaces;
detecting emission of the electrons by the collision surfaces to
detect the charged particles of the first polarity; after the
detecting, guiding charged particles of a second, opposite,
polarity toward first and second collision surfaces; detecting
emission of the electrons by the collision surfaces to detect the
charged particles of the second polarity.
[0022] Other aspects and features of the present invention will
become apparent to those of ordinary skill in the art upon review
of the following description of specific embodiments of the
invention in conjunction with the accompanying figures.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] In the figures which illustrate by way of example only,
embodiments of the present invention,
[0024] FIG. 1 is a schematic block diagram of an ion detector,
exemplary of an embodiment of the present invention;
[0025] FIG. 2 is a schematic block diagram of an ion detector,
exemplary of another embodiment of the present invention;
[0026] FIG. 3 is a schematic block diagram of an ion detector,
exemplary of yet another embodiment of the present invention;
and
[0027] FIG. 4 is a schematic block diagram of an ion detector,
exemplary of a further embodiment of the present invention.
DETAILED DESCRIPTION
[0028] FIG. 1 schematically illustrates an ion detector 100,
exemplary of an embodiment of the present invention. Ion detector
100 typically forms part of a mass spectrometer. Ions enter
detector 100, from an upstream stage (typically referred to as a
mass analyser) of the mass spectrometer. The mass analyser (not
shown) may take the form of a sector, time of flight, quadrupole,
quadrupole ion trap, fourier transform, orbitrap, or other mass
analyser, known to those of ordinary skill.
[0029] As illustrated, ion detector 100 includes two conversion
electrodes 102, 104. Conversion electrodes 102 and 104 provide
collision surfaces that emit electrons in response to collisions by
particles, such as molecules, ions, electrons and the like. The
number of emitted electrons will be dependent on the energies of
incident particles. Example conversion electrodes 102, 104 may, for
example, be dynodes formed of metal or semi-conductor material. For
example, conversion electrodes 102, 104 may be formed of stainless
steel bars. Alternatively, conversion electrodes may be formed of
alloys, or coated materials. Optional heating device may be in
thermal communication with electrodes 102 and 104, to heat these to
as suitable temperature to further facilitate the emission of
electrons. A suitable temperature may, for example, be between
200.degree. C. and 800.degree. C.
[0030] An electron detector 110 is positioned downstream of
conversion electrodes 102 and 104 that detects the emission of
secondary electrons by electrodes 102 and 104. In the depicted
embodiment, electron detector 110 includes an electron multiplier
112, having an inlet 108 and an outlet 120 connecting a channel 122
that provides electrons to a detection surface 114. Typically, a
capacitor 116, transmits electron pulses emitted by electron
multiplier 112 to a pulse counter, such as pulse
amplifier/discriminator/counter 124. Capacitor 116 isolates the
high voltage of detection surface 114 from the (usually) ground
potential of the amplifier/discriminator/counter 124.
[0031] Of course, electron detector 110 could be embodied as any
suitable electron detector. Electron detector 110 could, for
example, accelerate the electrons (perhaps after several stages of
amplification) into a photo-emissive detection surface which
provides resulting photons into a photomultiplier or avalanche
photodiode. Other suitable electron detectors will be apparent to
those of ordinary skill.
[0032] In any event, detection surface 114 is typically a
conductive or semi-conductive surface on which receives electrons
to be detected. Surface 114 may, for example, be stainless
steel.
[0033] Pulse amplifier/discriminator/counter 124 is an example of
any suitable high sensitivity electron pulse counting apparatus. An
example pulse amplifier/discriminator/counter 124 is available from
ORTEC of Oak Ridge, Tenn., under model number Model Number 9302.
Other suitable electron pulse counting devices will be apparent to
those of ordinary skill.
[0034] Electron multiplier 112 may be a channel electron
multiplier, and as such, channel 122 may be a ceramic channel, a
semi-conductor channel, a glass channel, or the like. Again, the
channel may be coated, with a material that facilitates emission of
electrons. Alternatively, electron multiplier 112 may be a discrete
dynode electron multiplier, a multi-channel plate multiplier, or
any other suitable electron multiplier, known to those of ordinary
skill.
[0035] Electric power supplies 118a, 118d apply DC voltages to the
conversion electrodes 102 and 104, respectively. Similarly,
supplies 118b and 118c apply front and rear potentials to regions
proximate inlet 108 and outlet 120 of electron multiplier 112.
Supply 118e provides a DC voltage to plate 114. Supplies 118a,
118b, 118c, 118d and 118e may be conventional DC supplies. Multiple
ones of supplies 118a, 118b, 118c, 118d and 118e may be combined.
For example, one or two physical DC power supplies and suitable
resistor network may be used to provide voltages of supplies 118a,
118b, 118c, 118d and 118e.
[0036] In operation, positive and negative ions are sequentially
produced by a suitable ion source upstream of detector 100. Ions
(positive or negative) enter a region proximate conversion dynodes
102, 104. Positively charged ions are attracted to conversion
electrode 102, at a negative voltage, and collide therewith.
Conversion electrode 102 emits secondary electrons, at energies
close to the voltage of power supply 118d. As the inlet 108 of
electron multiplier is at a more positive potential than electrode
102, secondary electrons are accelerated to inlet 108 of electron
multiplier 112.
[0037] Negative ions are similarly attracted by conversion
electrode 104. Upon impact, these negative ions cause the emission
of secondary electrons by conversion electrode 104. The secondary
electrons, emitted by conversion electrode 104 are similarly
attracted to inlet 108 of multiplier 112, which is also at a higher
potential than conversion electrode 104.
[0038] Supplies 118a and 118d provide DC biases to attract incident
ions. In the depicted embodiment, supplies 118a and 118d apply DC
apply biases of +4 kV and -6 kV to conversion electrodes 104 and
102, respectively. Supply 118b applies a fixed voltage of +6 kV to
inlet 108. As such, secondary electrons emitted by conversion
electrodes 104 and 102 are respectively accelerated through
potentials of 2 kV and 12 kV to inlet 108 of electron multiplier
112. Of course, other voltages could be applied to conversion
electrodes 104, 102 and electron multiplier 112. For example,
suitable voltages in the range of about +3 kV and +10 kV above the
energies of ions to be detected, could be applied to conversion
electrode 104. Similarly, voltages in the range of about -2 kV and
-10 kV below the energies of ions to be detected could be applied
to conversion electrode 102, depending upon the maximum mass
detected. Corresponding voltages above that applied to conversion
electrode 104 could be applied proximate the inlet 108 of electron
multiplier 112. In the depicted embodiment, supplies 118a-118e
provide the indicated voltages relative to ground. Of course,
voltages would typically be provided relative to the potentials at
which the ions are introduced into detector 100. For example, ions
typically leave the upstream mass analyser at an elevated potential
of, for example, between about 150V and -150V. Supplies 118a-118e
may be biased accordingly, above the potential of the output of the
mass analyser.
[0039] Power supply 118c applies a voltage higher than that
proximate inlet 108. As such, secondary electrons, from both
conversion electrode 102 and 104, at inlet 108, are accelerated to
outlet 120 at a higher potential than inlet 108. The emission
electrons, incident at inlet 108 further cause the emission of a
cascade of tertiary electrons by electron multiplier 112 resulting
in the electrons at output 120.
[0040] Electrons at outlet 120 are incident on detection surface
114. In order to attract electrons, detection surface 114 is
maintained at a voltage higher than outlet 120. Surface 114 is
maintained more positive than electrode 104 (e.g. at least +100V
more positive than electrode 104, and in the depicted embodiment
about +200V more positive than outlet 120), by supply 118e. Pulse
detector 124, in turn, detects the output electrons. In the
depicted embodiment, electron detector 110 takes the form of a
pulse counting detector. As such, it may provide its output to a
computing device (not shown), that in turn may tabulate counted
pulses, and their masses and display measured results.
[0041] Conveniently, although the output of multiplier 112 and
detection surface 114 are maintained at positive voltages, above
ground, pulses may be easily detected by a pulse counting detector.
Alternatively, current could be measured directly. However, high
speed, sub-picoamp current detection at about the potential of
outlet 120, is difficult and costly.
[0042] Conveniently, ion detector 100 allows for the detection of
both positively charged and negatively charged ions. No switching
of power supplies 118 is required and the sensitivity is not
compromised.
[0043] Moreover, ion to electron conversion efficiencies of both
conversion electrodes 102, 104 (and electron multiplier 112) are
not dependent on the particular structure of incident
molecules.
[0044] After ions of one polarity have been detected, ions of the
opposite polarity may be introduced to detector 100, and
detected.
[0045] As will be appreciated, applied voltages on electrodes 102,
104 and electron multiplier 112 (and surface 114) may be adjusted
by a small amount in dependence on the polarity of ions to be
detected, to aid in the formation, extraction and focusing of
electrons, and remain within the scope of the invention. For
example, for negative ions the voltage of electrode 104 may be made
more positive by between 0 to 25% from the voltage applied for
positive ions, and the voltage applied to electrode 102 may be made
more negative by between 0 to 25%. For positive ions, the voltages
applied to electrodes 102, 104 may again be respectively raised for
electrode 104 and lowered for electrode 102.
[0046] In an alternate mode of operation, positive and negative
ions may be detected concurrently by detector 100. For example,
both positive and negative ions may be introduced to detector 100,
as described above. Both types (i.e. positive and negative) may be
detected as described above: they are attracted to one of
conversion electrodes 102, 104 causing emission of secondary
electrons that are attracted to and detected by electron detector
110. Discriminating detection of positive ions from negative ions
may, however, not be possible as both positive and negative ions
result in the detection of electrons at detection surface 114.
[0047] As will now be appreciated, conversion electrode 104 of
detector 100 could actually be integrated with electron multiplier
112. In this way, detector 100 may be modified to form an alternate
detector 100' depicted in FIG. 2. Unmodified elements of detector
100 forming detector 100' are identified using numerals identical
used in FIG. 1. As illustrated in FIG. 2, inlet 108' of electron
multiplier 112' acts as conversion electrode 104'. In operation,
incident negatively charged ions would impact inlet 108' directly,
causing emission of secondary (and tertiary electrons) within
channel 122, as described above. Power supply 118a may be
eliminated. Positively charged ions may be detected as in detector
100 (FIG. 1)
[0048] In further embodiments, an ion detector 100'' illustrated in
FIG. 3, may be formed with electrodes 102'', 104'' identical to
electrodes 102, 104 but tilted, so that collision surfaces of
electrodes 102'' and 104'' are at an angle .alpha. relative to an
axis 140 parallel to the central axis approximately normal to a
plane of inlet 108 of electron multiplier 112. In the depicted
embodiment, the planes of the collision surfaces 102'' and 104''
are at an angle of between about 30.degree. and 90.degree. relative
to axis 140.
[0049] In yet a further embodiment, an ion detector 100'''
illustrated in FIG. 4, includes electrodes 102''', 104''' having
non-planar collision surfaces 142 and 144, respectively. As
illustrated, electrodes 102''', 104''' may have non-planar
collision surfaces 142, 144 to aid in the formation, extraction and
focusing of electrons including concave surfaces, as illustrated,
or convex surfaces, ridged, or corrugated surfaces are possible.
Again, detectors 102''' and 104''' may be formed of metal or
semiconductor, or other suitable material.
[0050] Detectors 100', 100'', and 100''' of FIGS. 2-4 may be
operated to sequentially or concurrently to detect positive and
negative ions, in much the same way as these may be detected using
detector 100.
[0051] A person of ordinary skill will now appreciate that
detectors 100, 100', 100'', and 100''' may be used to detect
particles other than ions. For example, positrons, or other charged
particles could be detected.
[0052] Of course, the above described embodiments are intended to
be illustrative only and in no way limiting. The described
embodiments of carrying out the invention are susceptible to many
modifications of form, arrangement of parts, details and order of
operation. The invention, rather, is intended to encompass all such
modification within its scope, as defined by the claims.
* * * * *