U.S. patent number 3,629,573 [Application Number 05/065,574] was granted by the patent office on 1971-12-21 for monopole/quadrupole mass spectrometer.
This patent grant is currently assigned to The Bendix Corporation. Invention is credited to John P. Carrico, Patrick F. McGinnis.
United States Patent |
3,629,573 |
Carrico , et al. |
December 21, 1971 |
MONOPOLE/QUADRUPOLE MASS SPECTROMETER
Abstract
This invention combines an auxiliary apertured electrode with a
quadrupole type mass filter. The assembly comprises a quadrupole
arrangement with at least one of the poles of the filter extending
beyond the others. Beneath the extended electrode is a V-shaped
electrode having an aperture therein for the passage of ions. The
extended rod and the V-shaped electrode forms a monopole
configuration so that ions to be analyzed are introduced through
the aperture in the V-shaped electrode and into the analyzing
region at an angle to the central axis (Z-axis) of the quadrupole
structure thereby reducing the effect of the fringing field.
Inventors: |
Carrico; John P. (Royal Oak,
MI), McGinnis; Patrick F. (Pittsford, NY) |
Assignee: |
The Bendix Corporation
(N/A)
|
Family
ID: |
22063655 |
Appl.
No.: |
05/065,574 |
Filed: |
August 20, 1970 |
Current U.S.
Class: |
250/292 |
Current CPC
Class: |
H01J
49/4215 (20130101) |
Current International
Class: |
H01J
49/42 (20060101); H01J 49/34 (20060101); H01j
039/36 () |
Field of
Search: |
;250/41.9DS |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Lever; IMB Technical Discl. Bulletin; Vol. 8, No. 1, June 1965; pp.
179, 180..
|
Primary Examiner: Birch; Anthony L.
Claims
Having described the invention, what is claimed is:
1. In combination with a mass filter of the type having four
axially elongated electrodes having coextensive portions and
arranged about a central axis for creating a time periodical
electric field therebetween to separate ions having different
mass-to-charge ratios by causing certain ions to perform
oscillations of limited amplitudes and other ions to perform
oscillations of increasing amplitudes depending on the
mass-to-charge ratios of the ions, thereby separating certain ions
from the others, the improvement comprising:
an auxiliary electrode having an aperture therein for the passage
of ions, said auxiliary electrode aligned with and adjacent to at
least one of said elongated electrodes which extends beyond the
other three so that ions passing through said auxiliary electrode
aperture enter said time periodical electric field.
2. The combination as recited in claim 1 wherein said auxiliary
electrode comprises:
two axially elongated conductive and electrically interconnected
surface members forming an angle with each other and extending
along and in spaced relationship to said central axis between said
elongated electrodes, said aperture therein extending along a
portion of at least one surface member.
3. The combination as recited in claim 2 wherein said plurality of
elongated electrodes arranged about the central axis for creating a
time periodical electric field therebetween are cylindrically
shaped electrodes.
4. The combination as recited in claim 2 wherein said auxiliary
electrode includes a screen covering said aperture.
5. The combination as recited in claim 1 wherein said auxiliary
electrode comprise a planar electrode having an aperture therein
for the passage of ions.
6. In combination with a mass filter of the type having four
axially elongated electrodes arranged about a central axis for
creating a time periodical electric field therebetween to separate
ions having different mass-to-charge ratios, the improvement
wherein one of said electrodes extends beyond the ends of the other
electrodes, and a fifth electrode having an aperture is disposed
opposite the extended portion of said extended electrode on the
side of said central axis so that ions may pass through the
aperture in said fifth electrode and into said time periodical
electric field between said electrodes.
7. The combination as recited in claim 6 wherein said fifth
electrode comprises:
two axially elongated conductive and electrically interconnected
surface members forming an angle with each other and extending
along and in spaced relationship to said central axis between said
elongated electrodes, said aperture therein extending along a
portion of at least one surface member.
8. The combination as recited in claim 7 wherein said elongated
electrodes arranged about the central axis are cylindrically shaped
electrodes.
9. The combination as recited in claim 6 wherein said auxiliary
electrode includes a screen covering said aperture.
10. An apparatus for separating ions having different
mass-to-charge ratios, comprising:
an evacuable enclosure;
four axially elongated electrodes located within said evacuable
enclosure and arranged about a central axis, one electrode
extending beyond the other three;
an auxiliary electrode having an aperture therein for the passage
of ions, said auxiliary electrode disposed in parallel axial
alignment with and adjacent to said one electrode extending beyond
the other three on the side of the central axis;
means for holding said electrodes in spaced relation;
means for applying a voltage having a periodical function of time
f(t) to said elongated electrodes to create a time periodical
electric field therebetween;
means for creating and introducing ions into said electric field
through said aperture in said auxiliary electrode whereby said
electric field causes certain ions to perform oscillations of
limited amplitude and other ions to perform oscillations of
increasing amplitude, depending upon the respective specific
charges on the ions, thereby separating certain ions from others;
and
means for detecting said ions having oscillations of limited
amplitude said detecting means disposed at one of the ends of said
elongated electrodes.
11. The apparatus as recited in claim 10 wherein said auxiliary
electrode having an aperture therein comprises:
two axially elongated conductive and electrically interconnected
surface members forming an angle with each other and extending
along and in spaced relationship to said central axis between said
elongated electrodes, said aperture therein extending along a
portion of at least one surface member.
12. The apparatus as recited in claim 10 wherein said detecting
means includes:
a second auxiliary electrode having an aperture therein for the
passage of said ions having oscillations of limited amplitude, said
second auxiliary electrode disposed in axial alignment with at
least one of said elongated electrodes; and
means for collecting ions passing through said second auxiliary
electrode aperture, said collecting means disposed adjacent the
central axis of said elongated electrodes.
13. The apparatus as recited in claim 11 wherein said detecting
means includes:
a second auxiliary electrode having an aperture therein for the
passage of said ions having oscillations of limited amplitude, said
second auxiliary electrode disposed in axial alignment with at
least one of said elongated electrodes; and
means for collecting ions passing through said second auxiliary
electrode aperture, said collecting means disposed adjacent the
central axis of said elongated electrodes.
14. The combination as recited in claim 10 wherein said auxiliary
electrode comprises a planar electrode having an aperture therein
for the passage of ions.
Description
BACKGROUND OF THE INVENTION
This invention relates to a nonmagnetic mass analyzer, and more
specifically to a monopole-quadrupole mass filter apparatus for
separating charged particles of different specific charges.
In the operation of a nonmagnetic mass analyzer, such as a
quadrupole mass filter of the type described in U.S. Pat. No.
2,939,952 to W. Paul et al., it has been found that exposure of
charged particles to be analyzed to the fringing electric fields
existing at the extrance end of the analyzer for more than two or
three cycles of the AC voltage applied to the field-forming
electrodes results in an undesirable radial impulse being imparted
to the particles. If the impulse is sufficiently high, the charged
particles contact the electrodes where they are discharged, thereby
reducing the quantity of charged particles which would otherwise be
transmitted by the analyzer and creating several other undesirable
side effects.
A first approach to reducing the undesirable impulse producing
effects of the fringing field has been to impart a relatively high
injection energy to particles as they are directed towards the
entrance end of the filter so as to reduce the transient time of
the particles through the fringing fields to a minimum. A high
injection energy, however, limits the maximum power which can be
obtained and imposes additional electric power requirements on the
instrument. To restore some of the lost resolving power, an
analyzer of increased length is often provided in order to permit
the analyzing or resolving action of the electric fields within the
analyzer to be exerted over a longer distance and particle
transient time. Increased analyzer length is frequently undesirable
especially when the analyzer is made part of the instrumentation in
a space vehicle.
A second approach has been to provide auxiliary electrodes adjacent
to the entrance end of the instrument. The ratio of the voltages
connected to these electrodes is then arranged such that particles
entering the analyzer encounter an intermediate ratio in the
transition from the region of zero field outside the analyzer to
the region of very strong electric fields in the center of the
analyzer. This is done by reducing the amplitude of the DC voltage
connected to the auxiliary electrodes while leaving the amplitude
of the AC voltage unchanged. By this means, a substantial increase
in the transmission efficiency of the analyzer is achieved. Such an
approach is described in U.S. Pat. No. 3,129,327 to W. M.
Brubaker.
In the preceding approaches, and in general, it has been common
practice to inject ions into the analyzing region of a mass filter
from a point parallel to the central axis (Z-axis) of the analyzing
region. See U. VonZahn, review of Scientific Instruments 34,
(1963). This practice subjected ions to the undesirable effects of
the fringing electromagnetic field at the ends of the electrodes
forming the analyzing region and reduced the sensitivity of the
mass filter.
SUMMARY OF THE INVENTION
To increase the sensitivity of a nonmagnetic mass filter,
especially a quadrupole type filter, ions are introduced into the
filter through an aperture in an auxiliary electrode.
The invention is characterized by a mass filter which includes an
auxiliary electrode having an aperture therein for the passage of
ions. The auxiliary electrode is disposed in axial alignment with
at least one of the electrodes of the mass filter so that ions
passing through the aperture of the auxiliary electrode will become
subject to a time periodical electric field between at least one of
the mass filter electrodes and the auxiliary electrode. In one
embodiment of the invention the auxiliary electrode is
characterized by two axial elongated conductive and electrically
interconnected surface members forming an angle with each other
(V-shaped) and extending along and in spaced relationship to the
central axis between the quadrupole electrodes. Located along a
portion of at least one surface member of the auxiliary electrode
is an aperture for the passage of ions. This type of arrangement
permits ions to be introduced into the quadrupole analyzer field
without subjecting the ions to the adverse effects of the
fringe-field associated with the ends of the mass filter
electrodes.
Accordingly, it is an object of this invention to introduce ions
into the quadrupole mass filter without subjecting them to the
electric field associated with the end portions of the quadrupole
electrodes.
It is another object of this invention to increase the sensitivity
of a quadrupole mass filter.
It is a further object of this invention to combine the advantages
of both a monopole and quadrupole structure in a single mass
filter.
It is still a further object of this invention to combine the
advantages of both a duopole and quadrupole structure in one mass
filter.
It is still another object of this invention to reduce the fringing
field scattering of ions at both the entrance and exit ends of a
quadrupole filter by the use of an apertured auxiliary electrode
for the passage of ions.
It is still a further object of this invention to improve the
overall operation and performance of a quadrupole mass
analyzer.
The above and other objects and features of the invention will
become apparent from the following detailed description taken in
conjunction with the accompanying drawings and claims which form a
part of this specification.
BRIEF DESCRIPTION OF THE DRAWINGS
Prior Art
FIG. 1 illustrates an ideal shape for electrodes used in a
quadrupole mass spectrometer.
FIG. 2 shows an arrangement of four cylindrical rod electrodes of
circular cross section, which sufficiently approximate the field
generated by the hyperbolic electrodes shown in FIG. 1.
FIG. 3 shows the arrangement and shape of the electrodes used in a
duopole mass spectrometer.
FIG. 4 shows the arrangement and shape of electrodes used in a
monopole mass spectrometer.
FIG. 5 is a block diagram of a mass analyzer of the type shown in
FIG. 4.
Invention
FIG. 6 is a diagrammatic view of a preferred embodiment of the
invention showing the monopole-quadrupole combination.
FIG. 7 is a side view of the electrodes shown in FIG. 6.
FIG. 8 is a diagrammatic view of a preferred embodiment of the
invention showing a duopole-quadrupole combination.
FIG. 9 is a side view of the electrodes shown in FIG. 8.
FIG. 10 is a diagrammatic view of another preferred embodiment of
the invention which shows an apertured auxiliary electrode in
combination with quadrupole electrodes.
FIG. 11 is a side view of the electrodes shown in FIG. 10.
FIG. 12 is a diagrammatic view of another preferred embodiment of
the invention which shows an additional apertured electrode at both
the input (source) end and the detector end of a quadrupole mass
filter.
DETAILED DESCRIPTION OF THE DRAWINGS
Prior Art
FIG. 1 illustrates four electrodes A,A,B,B of hyperboloidal shape
arranged at a distance r.sub.o from the Z-axis of an X,Y,Z
coordinate system. The electrodes receive a potential which creates
a time varying symmetrical electric field between them. The
potential of the field is periodic in time, symmetrical with
respect to the Z-axis, and depends quadratically on x and y:
.phi.=(U+V.sup.. f(t) ) (ax.sup.2 -by.sup.2).
f(t) is an arbitrary periodic function of time (t) and the field is
assumed to be quasistatic, so that Laplace's equation
.DELTA..phi.=0 requires that the constants a and b should satisfy
the condition a=b. The two electrodes A,A are electrically
interconnected, and the electrodes B, B are electrically
interconnected. A time-periodical voltage U=U.sub.0 +V cos.omega.t
is applied to electrodes A and U=-U.sub.o -V cos.omega.t to
electrode B whereby an electric field is created between the
electrodes. This field is independent of z, its center of symmetry
being the Z-axis.
FIG. 2 shows a schematic diagram of how voltage sources are coupled
to the electrodes 1 of a quadrupole filter. Four elongated or
cylindrical rod-shaped electrodes 1, which sufficiently approximate
the field generated by hyperboloidal electrodes, are arranged to
make a quadrupole mass filter. An electric generator 9 creates a
high-frequency voltage V cos.omega.t which is applied to the
electrodes 1 through two capacitors 14. In this arrangement, the
high-frequency voltage is rectified and smoothed by a rectifier 10.
The direct voltage so created is divided by a potentiometer 15 and
coupled to the electrodes 1 through inductors 13 whereby the ratio
(u) of the direct voltage U to the alternating voltage V is
substantially independent of alterations of the alternating
voltage. As an alternative, a direct current voltage may be
supplied to the electrodes 1 by an independent voltage source 11
and an electric two-way switch 11A. This electrode arrangement
establishes an electric field between the electrodes whereby ions
brought into such a field have equations of motion that are
differential equations with periodical coefficients, the equations
being characterized by having ranges of stable and unstable
solutions. Thus, there exists two different kinds of ion paths
(stable and unstable); either the ions perform oscillations around
the center of symmetry of the field (Z-axis) because the amplitudes
of the oscillations remain smaller than a certain maximum value
(stable paths) or the amplitudes of the oscillations increase until
they exceed a certain maximum value (unstable paths) whereby the
ions impinge upon the field generating electrodes and are
removed.
FIG. 3 is a schematic and perspective view of a duopole mass
filter. Sources of charged particles 42 and 43 are arranged at one
end of a pair of rod-shaped electrodes 3 and 5 which are arranged
in parallel relationship to a planar electrode 4. The electrodes 3,
4 and 5 are oriented so as to direct charged particles along axis
44 and 45 (in the Z-plane) toward collector 46 and 47. The
collectors 46 and 47 are shown as planar electrodes which are of
prime advantage with this type of apparatus because of the
elimination of induced voltages in the collector. Ions emerging
from the exit of the filter impinge on the collectors 46 and 47
whereby signals are conducted to amplifiers 48 and 49 and hence to
recorders 40 and 41 such as strip chart recorders. As is typical of
conventional quadrupole mass filters, this apparatus is also
energized with symmetrical AC and DC potentials. A more detailed
description of a duopole filter may be found in U.S. Pat. No.
3,418,464 to W. M. Brubaker et al.
FIG. 4 is a schematic and perspective representation of a monopole
mass filter. The cylindrical rod-shaped electrode 3 is arranged in
parallel and spaced relation from two axially elongated conductive
and electrically interconnected surface members 4 and 6 forming an
angle with each other and extending in parallel in spaced relation
to an axis (Z) between an ion source (not shown) and an ion
collector (not shown).
FIG. 5 is a schematic block diagram of the monopole mass filter
shown in FIG. 4. An ion source 42 is connected to a current-supply
unit 21 which is preferably energized from a utility line. The ion
accelerating voltage, for example about 30 volts, is then connected
between the ion source and the angle structure comprised of two
surface members 4 and 6. The ions are produced by electron
collision in the ion source 42. The field electrode 3 is connected
to a high-frequency generator 23 and a direct voltage source 25
through a circuit containing a lead 15, a capacitor 22 and an
inductance coil 24. The high-frequency generator 23 and the direct
voltage source 25 are preferably energized by the same power source
coupled to current supply 21. A collector 47 is grounded through an
input stage of an amplifier 26 whose output circuit is connected to
a recording instrument 27. In this filter, as in the previously
described filters, the ions are introduced into the filter from a
position which is generally along the Z-axis and, therefore ions
must pass the field at the ends of the electrodes (and the fringing
field) when entering and leaving the mass filter. A more detailed
description of a monopole mass filter may be found in U.S. Pat. No.
3,197,633 to U. VonZahn.
Preferred Embodiments
FIG. 6 illustrates a mass filter which utilizes the principles of
the invention. Four elongated electrodes 3, 5, 7 and 9 are
symmetrically arranged about a central axis. Preferably the
elongated electrodes have hyperbolic surfaces but cylindrical
rod-shaped electrodes are also acceptable. Disposed in axial
alignment with at least one of the elongated electrodes 3 is an
auxiliary electrode 2 having an aperture 8 therein. In this
embodiment, the cylindrical rod-shaped electrodes 5, 7 and 9 are
approximately the same length, whereas the fourth electrode 3
extends beyond the other electrodes so that the extended portion of
electrode 3 and the auxiliary electrode 2 form a monopole
configuration. Located on one side of the aperture of electrode 2
is an ion source 42 which injects ions 50 through the aperture 8
and into the space between electrodes 2 and 3. The ions thus
injected have components of momentum both perpendicular and
parallel to the Z axis. Once in this space, the ions 50 are
subjected to an electric field, first between the monopole
electrodes 2 and 3 and then between the quadrupole electrodes 3, 5,
7, 9. The ends of electrodes 5, 7, 9 overlap auxiliary electrode 2
so that ions 50, traveling into the quadrupole field, travel in the
space between the auxiliary electrode 2 and quadrupole electrode 3
and, therefore, are not subjected to the fringing fields associated
with the ends of electrodes 5, 7, 9 on the other side of the
auxiliary electrode 2. The entrance end of the filter can be
further modified by locating a plate perpendicular to the ends of
the electrodes 5, 7, 9 to further shield ions entering the filter.
Disposed at the other end of the quadrupole mass filter is a
detector 47 which detects the ions which exit from the quadrupole
filter. As an additional feature, the aperture 8 in the auxiliary
electrode 2 may have a wire mesh or screen (not shown) placed over
the opening so that ions passing through the aperture 8 will be
more uniformly arranged. Further, the wire mesh may be grounded or
biased to obtain other advantages.
FIG. 7 is a side view of the preferred embodiment illustrated in
FIG. 6. From this view it can be seen that auxiliary electrode 2 is
disposed in axial alignment with quadrupole electrode 3. The
auxiliary electrode 2 comprises two axially elongated conductive
and electrically interconnected surface members forming an angle
with each other and having an aperture therein which extends along
a portion of at least one surface member. Alternatively, the
aperture 8 in the auxiliary electrode could extend along a portion
of both surface members as is shown in FIG. 6.
FIG. 8 is a diagrammatic representation of a duopole mass filter
and a quadrupole mass filter. Electrodes 3, 5, 7 and 9 are arranged
about a central axis with electrodes 3 and 5 aligned in a plane
that is in parallel relationship to electrodes 7 and 9. In this
embodiment, the auxiliary electrode 2 has a wire screen 6 covering
the aperture 8 therein. Ions from introduced from an ion source 42
so that the ions 50 enter the electric field between the auxiliary
electrode 2 and the duopole electrodes 3 and 5 through aperture 8.
The ions so entering then pass into the quadrupole mass filter
defined by electrodes 3, 5 7 and 9.
FIG. 9 is a side view of a preferred embodiment shown in FIG. 8.
This figure illustrates the arrangement of the auxiliary electrode
2 with respect to the quadrupole electrodes 3, 5, 7 and 9. It is
preferred that the planar electrode 2 be located in a plane that is
parallel to a first plane located between the center axis of
electrode 3 and 5 and parallel to a plane located between the
center axis of electrodes 7 and 9.
FIG. 10 is an alternate embodiment of the invention. Four elongated
electrodes 3, 5, 7 and 9 are symmetrically arranged about a central
axis. In this embodiment, the cylindrical rod-shaped electrodes 5,
7 and 9 extend beyond electrode 3 and are in axial alignment and
spaced relationship to auxiliary electrode 2. Ions introduced
through the aperture 8 in the auxiliary electrode 2 will be
subjected to the electric field established between auxiliary
electrode 2 and quadrupole electrodes 5, 7 and 9.
FIG. 11 is a side view of the embodiment illustrated in FIG.
10.
FIG. 12 is an illustration of how auxiliary apertured electrodes 2
may be used at the entrance end and exit end of a quadrupole
filter. In this embodiment, a potential is applied to electrodes 3,
5, 7 and 9 to create a time varying electric field. A first
auxiliary electrode 2, having an aperture 8 therein, is arranged so
that it forms generally a monopole configuration with one of the
quadrupole electrodes 3. An ion source 42 accelerates ions 50 into
the field between the first auxiliary electrode 2 and one of the
quadrupole electrodes 3. This arrangement introduces ions into the
electric field at an angle to the central (Z) axis of the
quadrupole structure. Similiarly, a second auxiliary electrode 12
having an aperture 18 therein is located at the exit end of the
quadrupole filter. A detector 47 is located on one side of the
second auxiliary electrode to detect ions that leave the quadrupole
field by passing into the field between the second auxiliary
electrode 12 and the quadrupole electrode 3 and then through the
aperture 18 where they are detected by the detector 47. Although
this embodiment uses V-shaped electrodes at both the input and
output of the mass filter, planar electrodes or planar electrodes
in combination with a V-shaped electrode may be used. Other
modifications such as, biasing the detector 47 to draw ions 50 out
of the filter and locating a plate over the ends of each electrode
5, 7 and 9 at the source and/or detector end may also be made to
this embodiment.
Operation
Ideally, a quadrupole has a mass analyzing region that consists of
four electrically conducting, parallel hyperbolic surfaces in which
opposite pairs of surfaces are connected together. Applied to the
two pairs of surfaces are equal but opposite polarity potentials
(each potential having DC and RF voltage components) so that
equipotential surfaces in the analyzing region between the four
poles appear as oscillating hyperbolic potentials.
If an ion is now injected down the Z-axis of the four surfaces, it
will undergo transverse motion in the X-Y plane in addition to its
motion down the Z-axis. The exact trajectories of the ions are
solutions of Mathieu's differential equation and contain either an
oscillatory factor or an exponential factor, depending upon, among
other factors, the charge-to-mass ratio of the ion in question.
For singly charged ions and a specific frequency and set of
voltages, only one mass of ion will undergo oscillatory motion that
allows transmission through the analyzing region. All other ions
will be swept radially outward from the center Z-axis and be
neutralized when they strike one of the surfaces. Thus, the
apparatus functions as a mass filter by transmitting only one
particular mass of ion from a collection of injected ions.
Referring now to FIG. 12 the advantages of the invention are as
follows: To avoid the undesirable effect, e.g., rejection, in the
fringe-fields at the entrance of the mass filter, ions 50 from ion
source 42 are injected into the region between the V-shaped
electrode 2 and the quadrupole electrode 3 through aperture 8. An
ion in this region is subjected to forces similar to those in a
monopole filter; the ion 50 having an oscillatory motion and
traveling along Z-axis. As the ions 50 leave the monopole
configuration, they enter the quadrupole filter where ions having
unstable oscillation are neutralized when they strike one of the
quadrupoles. The remaining ions, having stable oscillations, travel
in a direction along the Z-axis. To minimize the fringe-fields at
the exit end of the mass filter, an arrangement similar to that at
the entrance end is located at the exit end of the analyzer region.
Here another apertured electrode 2 allows the ions to pass out of
the quadrupole field without being affected by the fringing fields
associated with the ends of the electrodes. A detector 47 disposed
on one side of the aperture electrode receives the ions as they
pass through the aperture 18. From this embodiment, it can be seen
that the fringe-field effects on ions as they enter and leave the
mass filter are eliminated.
While a preferred embodiment of the invention has been disclosed,
it will be apparent to those skilled in the art that changes may be
made to the invention as set forth in the appended claims, and, in
some cases, certain features of the invention may be used to
advantage without corresponding use of other features. For example,
the auxiliary electrode having the aperture therein may be covered
by a wire mesh which is either grounded or electrically biased to
reflect or accelerate ions. Further, the rod electrodes and/or one
electrode such as, electrode 3, may be comprised of a plurality of
electrically separate electrodes for electrical biasing to achieve
certain effects. Accordingly, it is intended that the illustrative
and descriptive materials herein be used to illustrate the
principles of the invention and not to limit the scope thereof.
* * * * *