Note: Descriptions are shown in the official language in which they were submitted.
CA 02487790 2004-11-16
227-12 CA
MASS SPECTROMETER
The present invention relates to a mass filter and
a mass spectrometer incorporating a mass filter.
It is known to use a mass filter in a mass
spectrometer to select parent ions having a certain mass
to charge ratio. The selected parent ions may then, for
example, be fragmented in a collision or fragmentation
cell and the resulting fragment ions can then be mass
analysed by a mass analyser. The mass filter most
commonly used to select parent ions having a certain
mass to charge ratio is a quadrupole rod set mass
analyser. However, other types of mass filters are
known including Wien filters and Bradbury-Nielsen ion
gates.
A Wien filter operates by passing a beam of ions
through crossed electric and magnetic fields. Ions
having a mass m, charge q and velocity v will pass
undeviated through the filter if:
Eq =Bqv
where E and B are the electric and magnetic field
strengths respectively. Accordingly, if all the ions in
an ion beam have essentially the same energy, then only
ions having a particular mass to charge ratio will have
the required velocity to pass through the filter
undeflected. However, disadvantageously, the resolution
of a Wien filter is dependent upon the absolute
magnitude of the crossed electric and magnetic fields
experienced by the ion beam. Since large magnetic
fields require very large electromagnets then the
ultimate resolution of a mass spectrometer incorporating
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a Wien filter is, in practice, fairly restricted,
particularly at higher mass to charge ratios. A maximum
mass to charge ratio resolution of approximately 400 is
common for known mass spectrometers which incorporate a
Wien filter. The mass to charge ratio resolution R may
be defined as:
R= ____________
Am
where Lm is a mass to charge ratio window transmitted at
a mass to charge ratio m. The large physical size of
the various components necessary to form a Wien filter
in addition to its limited resolution has relegated its
use to certain specialised areas such as atomic physics
and ion implantation.
Quadrupole rod set mass filters, by contrast, are
relatively compact and are commonly used in commercial
mass spectrometers. A quadruple rod set mass filter
comprises two electrically connected pairs of
cylindrical rod electrodes to which both RF and DC
voltages are applied. For a given RF frequency and at
appropriate setting of the RF and DC voltages, only ions
having a very limited range of mass to charge ratios
will have stable trajectories through the quadrupole rod
set mass filter. Accordingly, only ions having a
certain mass to charge ratio will be transmitted by the
quadrupole rod set mass filter. Ions having other mass
to charge ratios will have unstable trajectories within
the rod set mass filter and will collide with the
cylindrical rod electrodes and hence become lost to the
system.
Quadrupole rod set mass filters are particularly
advantageous in that they can have resolutions of
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several thousand. However, disadvantageously, in order
to operate effectively quadrupole rod set mass filters
require that the ion beam which is to be mass filtered
should have a relatively low energy. Quadrupole rod set
mass filters also have a relatively limited mass to
charge ratio range and must be manufactured and
constructed to very high tolerances. Furthermore,
quadrupole rod set mass filters suffer from the problem
that the particular RF power supplies which are used
with such mass filters are physically relatively large.
This is particularly problematic when seeking to provide
a compact bench-top mass spectrometer.
A Bradbury-Nielsen ion gate can be used as a mass
filter. The ion gate may, for example, be provided in a
flight region of a mass spectrometer wherein ions take
different times to traverse the flight region depending
upon their mass to charge ratio. The ion gate may be
arranged so as only to allow ions having a relatively
small range of mass to charge ratios to be transmitted.
This is achieved by rapidly opening and then closing the
electrostatic ion gate at a time equal to the arrival
time of ions having mass to charge ratios of interest.
Bradbury-Nielsen ion gates comprise parallel
electrodes between which an ion beam is directed. An
electric field is created in use between the electrodes
of the ion gate. The electric field, when created, is
sufficient to deflect the beam of ions away from their
original path and hence the ion gate can be considered
to be closed or otherwise to have a transmission of 0%
when an electric field is created. In order to open the
gate or otherwise to provide a transmission of 100%, the
electric field maintained between the electrodes is
switched OFF or is otherwise reduced to zero for a very
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short period of time. This enables ions having a
desired mass to charge ratio to pass through the ion
gate without being deflected by an electric field. As
soon as ions having the desired mass to charge ratio
have been transmitted, the electric field is restored
and ions subsequently arriving at the ion gate are
deflected away from their original path.
In theory, the mass to charge ratio range of a
Bradbury-Nielsen ion gate is unlimited. However, in
practice, the resolution achievable with a Bradbury-
Nielsen ion gate tends to be disappointingly low e.g.
approximately 20-50 for dual-electrode arrangements and
of the order of 100-200 for multi-electrode
arrangements. The placement of electrodes very close to
the path of an ion beam also tends to lead to a loss in
ion transmission even when the ion gate is not being
used as a mass filter since some ions will still tend to
strike the electrodes. As a result, Bradbury-Nielsen
ion gates are not commonly used as mass filters in
commercial mass spectrometers.
Time of flight mass filters are also known which,
like Wien filters, transmit all ions having a certain
specific velocity. However, disadvantageously, ions
having different mass to charge ratios but which happen
to have substantially the same velocity will be
simultaneously transmitted by such mass filters. This
can be problematic in a number of different scenarios.
For example, if a precursor or parent ion fragments
(either spontaneously due to Post Source Decay or due to
Collision Induced Dissociation in a collision or
fragmentation cell), the resulting fragment ions will
retain essentially the same velocity as the
corresponding precursor or parent ion had. Accordingly,
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if a precursor or parent ion fragments upstream of a
time of flight mass filter, then fragment ions together
with corresponding unfragmented parent ions will be
simultaneously transmitted by the time of flight mass
filter. Accordingly, the time of flight mass filter
will transmit ions having substantially different mass
to charge ratios at substantially the same time.
It is therefore apparent that there are a number of
problems associated with conventional mass filters.
It is therefore desired to provide an improved mass
filter.
According to an aspect of the present invention
there is provided a mass filter comprising:
one or more electrodes associated with an entrance
region of said mass filter, wherein, in use, one or more
first voltage pulses are applied to the one or more
electrodes in order to orthogonally accelerate at least
some ions away from the one or more electrodes of said
entrance region; and
one or more ion mirrors for reflecting at least some
Ions which have been orthogonally accelerated away from said
entrance region such that the reflected ions move generally
towards an exit region of the mass filter disposed at a
distance from said entrance region;
wherein, in use, first ions having a desired mass or
mass to charge ratio or having masses or mass to charge
ratios within a first desired range are orthogonally
decelerated or otherwise orthogonally retarded by one or more
electric fields as said first ions approach said exit region
of the mass filter.
The ions are preferably arranged to enter the mass
filter substantially in an axial direction, the axial
direction being substantially orthogonal to an orthogonal
direction.
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The one or more electrodes preferably comprise one
or more pusher and/or puller electrodes for orthogonally
accelerating the at least some ions in an orthogonal
direction.
The one or more first voltage pulses preferably
have an amplitude selected from the group consisting of:
(i) < 50 V; (ii) 50-100 V; (iii) 100-150 V; (iv) 150-200
V; (v) 200-250 V; (vi) 250-300 V; (vii) 300-350 V;
(viii) 350-400 V; (ix) 400-450 V; (x) 450-500 V; (xi)
500-550 V; (xii) 550-600 V; (xiii) 600-650 V; (xiv) 650-
700 V; (xv) 700-750 V; (xvi) 750-800 V; (xvii) 800-850
V; (xviii) 850-900 V; (xix) 900-950 V; (xx) 950-1000 V;
(xxi) 1000-1050 V; (xxii) 1050-1100 V; (xxiii) 1100-1150
V; (xxiv) 1150-1200 V; (xxv) 1200-1250 V; (xxvi) 1250-
1300 V; (xxvii) 1300-1350 V; (xxviii) 1350-1400 V;
(xxix) 1400-1450 V; (xxx) 1450-1500 V; (xxxi) 1500-1550
V; (xxxii) 1550-1600 V; (xxxiii) 1600-1650 V; (xxxiv)
1650-1700 V; (xxxv) 1700-1750 V; (xxxvi) 1750-1800 V;
(xxxvii) 1800-1850 V; (xxxviii) 1850-1900 V; (xxxix)
1900-1950 V; (xxxx) 1950-2000 V; and (xxxxi) > 2000 V.
The one or more first voltage pulses preferably
have a duration tpulse, wherein tpuise is preferably
selected from the group consisting of: (i) < 1 is; (ii)
1-2 s; (iii) 2-3 s; (iv) 3-4 s; (v) 4-5 is; (vi) 5-
6 s; (vii) 6-7 is; (viii) 7-8 s; (ix) 8-9 s; (x) 9-
10 s; (xi) 10-11 s; (xxii) 11-12 s; (xxiii) 12-13 s;
(xiv) 13-14 s; (xv) 14-15 is; (xvi) 15-16 s; (xvii)
16-17 s; (xviii) 17-18 s; (xix) 18-19 s; (xx) 19-
20 s; (xxi) 20-21 s; (xxii) 21-22 s; (xxiii) 22-
23 s; (xxiv) 23-24 s; (xxv) 24-25 s; (xvi) 25-26 s;
(xvii) 26-27 s; (xviii) 27-28 s; (xxix) 28-29 s;
(xxx) 29-30 s; and (xxxi) > 30 s.
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The one or more first voltage pulses are preferably
applied after a delay period having a duration t
-start,
wherein t
-start is preferably selected from the group
consisting of: (i) < 1 s; (ii) 1-2 s; (iii) 2-3 s;
(iv) 3-4 s; (v) 4-5 s; (vi) 5-6 s; (vii) 6-7 s;
(viii) 7-8 s; (ix) 8-9 s; (x) 9-10 is; (xi) 10-11 s;
(xxii) 11-12 s; (xxiii) 12-13 s; (xiv) 13-14 s; (xv)
14-15 is; (xvi) 15-16 s; (xvii) 16-17 s; (xviii) 17-
18 s; (xix) 18-19 s; (xx) 19-20 s; (xxi) 20-21 is;
(xxii) 21-22 s; (xxiii) 22-23 is; (xxiv) 23-24 s;
(xxv) 24-25 s; (xvi) 25-26 s; (xvii) 26-27 s; (xviii)
27-28 s; (xxix) 28-29 s; (xxx) 29-30 is; and (xxxi) >
30 is.
The delay period t
-start is preferably measured from
when ions are first generated in an ion source or in an
ion generating region.
The one or more first voltage pulses preferably
comprise a square wave(s). However, according to other
embodiments the one or more first voltage pulses may
comprise voltage pulses having a linear, ramped,
stepped, non-linear, sinusoidal or curved waveform or
voltage profile.
According to the preferred embodiment ions entering
the mass filter preferably have a non-zero component of
velocity in an axial direction and preferably have a
substantially zero component of velocity in an
orthogonal direction. The orthogonal direction is
preferably at 90 to the axial direction. At least some
of the first ions are preferably orthogonally
decelerated or otherwise orthogonally retarded by the
one or more electric fields so as to have a
substantially zero component of velocity in an
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orthogonal direction. Preferably, at least some of the
first ions are orthogonally decelerated or otherwise
orthogonally retarded by the electric field but maintain
a substantially non-zero component of velocity in an
axial direction.
At least some ions other than the first ions are
preferably only partially orthogonally decelerated or
otherwise only partially orthogonally retarded by one or
more electric fields so that these ions preferably
continue with a substantially non-zero component of
velocity in an orthogonal direction. Preferably, at
least some ions other than the first ions are only
partially orthogonally decelerated or otherwise only
partially orthogonally retarded by one or more electric
fields but maintain a substantially non-zero component
of velocity in an axial direction.
According to an embodiment at least some ions other
than the first ions are not substantially orthogonally
decelerated or otherwise orthogonally retarded so that
the ions continue with a substantially non-zero
component of velocity in an orthogonal direction.
Preferably, at least some ions other than the first ions
are not substantially orthogonally decelerated or
otherwise orthogonally retarded so that the ions
continue whilst maintaining a substantially non-zero
component of velocity in an axial direction.
The first ions preferably have a mass to charge
ratio or have mass to charge ratios falling within one
or more ranges x, wherein x is selected from the group
consisting of: (i) < 50; (ii) 50-100; (iii) 100-150;
(iv) 150-200; (v) 200-250; (vi) 250-300; (vii) 300-350;
(viii) 350-400; (ix) 400-450; (x) 450-500; (xi) 500-550;
(xii) 550-600; (xiii) 600-650; (xiv) 650-700; (xv) 700-
!
1
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750; (xvi) 750-800; (xvii) 800-850; (xviii) 850-900;
(xix) 900-950; (xx) 950-1000; (xxi) 1000-1050; (xxii)
1050-1100; (xxiii) 1100-1150; (xxiv) 1150-1200; (xxv)
1200-1250; (xxvi) 1250-1300; (xxvii) 1300-1350; (xxviii)
1350-1400; (xxix) 1400-1450; (xxx) 1450-1500; (xxxi)
1500-1550; (xxxii) 1550-1600; (xxxiii) 1600-1650;
(xxxiv) 1650-1700; (xxxv) 1700-1750; (xxxvi) 1750-1800;
(xxxvii) 1800-1850; (xxxviii) 1850-1900; (xxxix) 1900-
1950; (xxxx) 1950-2000; and (xxxxi) > 2000.
The first ions preferably exit the mass filter
wherein, in use, ions other than the first ions are
preferably substantially attenuated or lost within the
mass filter. Preferably, at least some of the first
ions exit the mass filter with a non-zero component of
velocity in an axial direction. Preferably, at least
some of the first ions exit the mass filter with a
substantially zero component of velocity in an
orthogonal direction.
The mass filter preferably comprises one or more
flight regions arranged between the one or more
electrodes and the one or more ion mirrors. One or more
potential gradients are preferably maintained across at
least a portion of the flight region as ions move from
the one or more electrodes towards the one or more ion
mirrors. The one or more potential gradients preferably
act so as to further accelerate at least some ions
towards the one or more ion mirrors. One or more
potential gradients are preferably maintained across at
least a portion of the flight region as ions move from
the one or more ion mirrors towards the one or more
electrodes. The one or more potential gradients
preferably act so as to decelerate at least some ions as
they approach the one or more electrodes.
,
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According to a less preferred embodiment, at least
a portion of the flight region may comprise one or more
field free regions. Ions in the one or more field free
regions are preferably neither accelerated nor
decelerated as they move in the one or more field free
regions towards the one or more ion mirrors. Ions in
the one or more field free regions are also preferably
neither accelerated nor decelerated as they move in the
one or more field free regions from the one or more ion
mirrors towards the one or more electrodes.
According to a preferred embodiment the one or more
ion mirrors comprise one or more reflectrons. A linear
or non-linear electric field gradient may be maintained
within one or more of the reflectrons or ion mirrors.
Preferably, at least some second ions having
undesired masses or mass to charge ratios having been
reflected by the one or more ion mirrors approach the
exit region of the mass filter and are reflected by one
or more electric fields. At least some of the second
ions are preferably reflected by the one or more
electric fields into a flight region. Preferably, at
least some of the second ions are reflected by the one
or more electric fields away from the exit region of the
mass filter.
The second ions preferably include ions having a
mass to charge ratio selected from the group consisting
of: (i) < 50; (ii) 50-100; (iii) 100-150; (iv) 150-200;
(v) 200-250; (vi) 250-300; (vii) 300-350; (viii) 350-
400; (ix) 400-450; (x) 450-500; (xi) 500-550; (xii) 550-
600; (xiii) 600-650; (xiv) 650-700; (xv) 700-750; (xvi)
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750-800; (xvii) 800-850; (xviii) 850-900; (xix) 900-950;
(xx) 950-1000; (xxi) 1000-1050; (xxii) 1050-1100;
(xxiii) 1100-1150; (xxiv) 1150-1200; (xxv) 1200-1250;
(xxvi) 1250-1300; (xxvii) 1300-1350; (xxviii) 1350-1400;
(xxix) 1400-1450; (xxx) 1450-1500; (xxxi) 1500-1550;
(xxxii) 1550-1600; (xxxiii) 1600-1650; (xxxiv) 1650-
1700; (xxxv) 1700-1750; (xxxvi) 1750-1800; (xxxvii)
1800-1850; (xxxviii) 1850-1900; (xxxix) 1900-1950;
(xxxx) 1950-2000; and (xxxxi) > 2000.
According to the preferred embodiment at least some
third ions having undesired masses or mass to charge
ratios having been reflected by the one or more ion
mirrors approach the exit region of the mass filter and
are only partially orthogonally decelerated or otherwise
only partially orthogonally retarded. At least some of
the third ions preferably continue through the exit
region of the mass filter. Preferably, at least some of the
third ions do not exit from the mass filter. According to
the preferred embodiment at least some of the third ions
impinge upon the one or more electrodes.
Preferably, at least some of the third ions are substantially
attenuated or lost within the mass filter.
The third ions preferably include ions having a mass to
charge ratio selected from the group consisting of: (i) < 50;
(ii) 50-100; (iii) 100-150; (iv) 150-200; (v) 200-250; (vi)
250-300; (vii) 300-350; (viii) 350-400; (ix) 400-450; (x)
450-500; (xi) 500-550; (xii) 550-600; (xiii) 600-650; (xiv)
650-700; (xv) 700-750; (xvi) 750-800; (xvii) 800-850; (xviii)
850-900; (xix) 900-950; (xx) 950-1000; (xxi) 1000-1050;
(xxii) 1050-1100; (xxiii) 1100-1150; (xxiv) 1150-1200; (xxv)
1200-1250; (xxvi) 1250-1300; (xxvii) 1300-1350; (xxviii)
1350-1400; (xxix) 1400-1450; (xxx) 1450-1500; (xxxi) 1500-
1550; (xxxii) 1550-1600; (xxxiii) 1600-1650; (xxxiv) 1650-
1700; (xxxv) 1700-1750; (xxXvi) 1750-1800; (xxxvii)
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1800-1850; (xxxviii) 1850-1900; (xxxix) 1900-1950;
(xxxx) 1950-2000; and (xxxxi) > 2000.
According to an embodiment at least some fourth
ions having masses or mass to charge ratios within a
fourth range pass through the mass filter without being
orthogonally accelerated whilst at least some other ions
having different masses or mass to charge ratios are
orthogonally accelerated. The fourth ions preferably
include ions having a mass to charge ratio selected from
the group consisting of: (i) < 50; (ii) 50-100; (iii)
100-150; (iv) 150-200; (v) 200-250; (vi) 250-300; (vii)
300-350; (viii) 350-400; (ix) 400-450; (x) 450-500; (xi)
500-550; (xii) 550-600; (xiii) 600-650; (xiv) 650-700;
(xv) 700-750; (xvi) 750-800; (xvii) 800-850; (xviii)
850-900; (xix) 900-950; (xx) 950-1000; (xxi) 1000-1050;
(xxii) 1050-1100; (xxiii) 1100-1150; (xxiv) 1150-1200;
(xxv) 1200-1250; (xxvi) 1250-1300; (xxvii) 1300-1350;
(xxviii) 1350-1400; (xxix) 1400-1450; (xxx) 1450-1500;
(xxxi) 1500-1550; (xxxii) 1550-1600; (xxxiii) 1600-1650;
(xxxiv) 1650-1700; (xxxv) 1700-1750; (xxxvi) 1750-1800;
(xxxvii) 1800-1850; (xxxviii) 1850-1900; (xxxix) 1900-
1950; (xxxx) 1950-2000; and (xxxxi) > 2000.
At least some of the fourth ions are preferably
onwardly transmitted to the exit of the mass filter and
preferably emerge or are emitted from the mass filter.
According to an embodiment, at least some fifth
ions having masses or mass to charge ratios within a
fifth range pass through the mass filter without being
orthogonally accelerated whilst at least some other ions
having different masses or mass to charge ratios are
orthogonally accelerated. Preferably, the fifth ions
have a mass to charge ratio selected from the group
consisting of: (i) < 50; (ii) 50-100; (iii) 100-150;
=
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(iv) 150-200; (v) 200-250; (vi) 250-300; (vii) 300-350;
(viii) 350-400; (ix) 400-450; (x) 450-500; (xi) 500-550;
(xii) 550-600; (xiii) 600-650; (xiv) 650-700; (xv) 700-
750; (xvi) 750-800; (xvii) 800-850; (xviii) 850-900;
(xix) 900-950; (xx) 950-1000; (xxi) 1000-1050; (xxii)
1050-1100; (xxiii) 1100-1150; (xxiv) 1150-1200; (xxv)
1200-1250; (xxvi) 1250-1300; (xxvii) 1300-1350; (xxviii)
1350-1400; (xxix) 1400-1450; (xxx) 1450-1500; (xxxi)
1500-1550; (xxxii) 1550-1600; (xxxiii) 1600-1650;
(xxxiv) 1650-1700; (xxxv) 1700-1750; (xxxvi) 1750-1800;
(xxxvii) 1800-1850; (xxxviii) 1850-1900; (xxxix) 1900-
1950; (xxxx) 1950-2000; and (xxxxi) > 2000.
At least some of the fifth ions are preferably
onwardly transmitted to the exit of the mass filter and
preferably emerge or are emitted from the mass filter.
According to an embodiment at least some sixth ions
having masses or mass to charge ratios within a sixth
range are orthogonally accelerated substantially
immediately upon entering the mass filter. At least
some of the sixth ions are preferably arranged to
collide with a plate or electrode forming part of the
entrance region of the mass filter. At least some of
the sixth ions are preferably substantially attenuated
or lost within the mass filter. The sixth ions
preferably include ions having a mass to charge ratio
selected from the group consisting of: (i) < 50; (ii)
50-100; (iii) 100-150; (iv) 150-200; (v) 200-250; (vi)
250-300; (vii) 300-350; (viii) 350-400; (ix) 400-450;
(x) 450-500; (xi) 500-550; (xii) 550-600; (xiii) 600-
650; (xiv) 650-700; (xv) 700-750; (xvi) 750-800; (xvii)
800-850; (xviii) 850-900; (xix) 900-950; (xx) 950-1000;
(xxi) 1000-1050; (xxii) 1050-1100; (xxiii) 1100-1150;
(xxiv) 1150-1200; (xxv) 1200-1250; (xxvi) 1250-1300;
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(xxvii) 1300-1350; (xxviii) 1350-1400; (xxix) 1400-1450;
(xxx) 1450-1500; (xxxi) 1500-1550; (xxxii) 1550-1600;
(xxxiii) 1600-1650; (xxxiv) 1650-1700; (xxxv) 1700-1750;
(xxxvi) 1750-1800; (xxxvii) 1800-1850; (xxxviii) 1850-
1900; (xxxix) 1900-1950; (xxxx) 1950-2000; and (xxxxi) >
2000.
According to an embodiment one or more second
voltage pulses are applied, in use, to the one or more
electrodes prior to the one or more first voltage
pulses. The one or more second voltage pulses
preferably have a duration t(1)0N, wherein t(1)0N is
preferably selected from the group consisting of: (i) <
1 is; (ii) 1-2 vls; (iii) 2-3 Ils; (iv) 3-4 is; (v) 4-
5 s; (vi) 5-6 vis; (vii) 6-7 is; (viii) 7-8 is; (ix) 8-
9 s; (x) 9-10 .is; (xi) 10-11 vis; (xxii) 11-12 is;
(xxiii) 12-13 vs; (xiv) 13-14 vis; (xv) 14-15 vs; (xvi)
15-16 s; (xvii) 16-17 vs; (xviii) 17-18 s; (xix) 18-
19 vs; (xx) 19-20 vs; (xxi) 20-21 vs; (xxii) 21-22 vs;
(xxiii) 22-23 vs; (xxiv) 23-24 vs; (xxv) 24-25 is; (xvi)
25-26 ils; (xvii) 26-27 vis; (xviii) 27-28 is; (xxix) 28-
29 vis; (xxx) 29-30 s; and (xxxi) > 30 vis.
The voltage applied to the one or more electrodes
is preferably reduced for a period of time t(l)OFF after
the one or more second voltage pulses are applied to the
one or more electrodes and prior to the one or more
first voltage pulses. Preferably, t(l)OFF is selected
from the group consisting of: (i) < 1 s; (ii) 1-2 ms;
(iii) 2-3 lis; (iv) 3-4 s; (v) 4-5 is; (vi) 5-6 vs;
(vii) 6-7 is; (viii) 7-8 is; (ix) 8-9 vs; (x) 9-10 vs;
(xi) 10-11 vis; (xxii) 11-12 vs; (xxiii) 12-13 vis; (xiv)
13-14 is; (xv) 14-15 vs; (xvi) 15-16 pis; (xvii) 16-
!
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17 gs; (xviii) 17-18 gs; (xix) 18-19 gs; (xx) 19-20 gs;
(xxi) 20-21 gs; (xxii) 21-22 gs; (xxiii) 22-23 gs;
(xxiv) 23-24 gs; (xxv) 24-25 is; (xvi) 25-26 is; (xvii)
26-27 gs; (xviii) 27-28 gs; (xxix) 28-29 gs; (xxx) 29-
30 gs; and (xxxi) > 30 gs.
According to an embodiment at least some seventh
ions having masses or mass to charge ratios within a
seventh range are orthogonally accelerated substantially
immediately upon entering the mass filter. At least
some of the seventh ions are preferably arranged to
collide with a plate or electrode forming part of the
entrance region of the mass filter. Preferably, at
least some of the seventh ions are substantially
attenuated or lost within the mass filter. The seventh
ions preferably include ions having a mass to charge
ratio selected from the group consisting of: (i) < 50;
(ii) 50-100; (iii) 100-150; (iv) 150-200; (v) 200-250;
(vi) 250-300; (vii) 300-350; (viii) 350-400; (ix) 400-
450; (x) 450-500; (xi) 500-550; (xii) 550-600; (xiii)
600-650; (xiv) 650-700; (xv) 700-750; (xvi) 750-800;
(xvii) 800-850; (xviii) 850-900; (xix) 900-950; (xx)
950-1000; (xxi) 1000-1050; (xxii) 1050-1100; (xxiii)
1100-1150; (xxiv) 1150-1200; (xxv) 1200-1250; (xxvi)
1250-1300; (xxvii) 1300-1350; (xxviii) 1350-1400; (xxix)
1400-1450; (xxx) 1450-1500; (xxxi) 1500-1550; (xxxii)
1550-1600; (xxxiii) 1600-1650; (xxxiv) 1650-1700; (xxxv)
1700-1750; (xxxvi) 1750-1800; (xxxvii) 1800-1850;
(xxxviii) 1850-1900; (xxxix) 1900-1950; (xxxx) 1950-
2000; and (xxxxi) > 2000.
One or more third voltage pulses are preferably
applied, in use, to the one or more electrodes
subsequent to the one or more first voltage pulses. The
CA 02487790 2004-11-16
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one or more third voltage pulses preferably have a
duration t(2)ON, wherein t(2)0N is preferably selected
from the group consisting of: (i) < 1 s; (ii) 1-2 s;
(iii) 2-3 s; (iv) 3-4 s; (v) 4-5 s; (vi) 5-6 is;
(vii) 6-7 is; (viii) 7-8 s; (ix) 8-9 s; (x) 9-10 s;
(xi) 10-11 s; (xxii) 11-12 s; (xxiii) 12-13 s; (xiv)
13-14 s; (xv) 14-15 is; (xvi) 15-16 s; (xvii) 16-
17 is; (xviii) 17-18 is; (xix) 18-19 s; (xx) 19-20 s;
(xxi) 20-21 s; (xxii) 21-22 s; (xxiii) 22-23 s;
(xxiv) 23-24 s; (xxv) 24-25 is; (xvi) 25-26 s; (xvii)
26-27 s; (xviii) 27-28 is; (xxix) 28-29 s; (xxx) 29-
30 s; and (xxxi) > 30 s.
The voltage applied to the one or more electrodes
is preferably reduced for a period of time t(2)OFF after
the one or more first voltage pulses are applied to the
one or more electrodes and prior to the one or more
third voltage pulses being applied to the one or more
electrodes. Preferably, t(2)0FF is selected from the
group consisting of: (i) < 1 s; (ii) 1-2 s; (iii) 2-
3 s; (iv) 3-4 s; (v) 4-5 is; (vi) 5-6 .is; (vii) 6-
7 is; (viii) 7-8 s; (ix) 8-9 is; (x) 9-10 .is; (xi) 10-
11 is; (xxii) 11-12 s; (xxiii) 12-13 s; (xiv) 13-
14 s; (xv) 14-15 s; (xvi) 15-16 s; (xvii) 16-17 is;
(xviii) 17-18 is; (xix) 18-19 s; (xx) 19-20 s; (xxi)
20-21 s; (xxii) 21-22 s; (xxiii) 22-23 s; (xxiv) 23-
24 s; (xxv) 24-25 s; (xvi) 25-26 s; (xvii) 26-27 s;
(xviii) 27-28 s; (xxix) 28-29 s; (xxx) 29-30 s; and
(xxxi) > 30 s.
A preferred feature of the present invention is
that the first ions preferably have a first range of
CA 02487790 2004-11-16
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angular divergence 491 immediately prior to or upon
entering the mass filter. Preferably, the first ions
have a second range of angular divergence AO2
immediately prior to or upon exiting the mass filter.
The ratio of the first range of angular divergence to
the second range of angular divergence 481/462 is
preferably selected from the group consisting of (i) >
1; (ii) 1-1.1; (iii) 1.1-1.2; (iv) 1.2-1.3; (v) 1.3-1.4;
(vi) 1.4-1.5; (vii) 1.5-1.6; (viii) 1.6-1.7; (ix) 1.7-
1.8; (x) 1.8-1.9; (xi) 1.9-2.0; and (xii) > 2.
According to an aspect of the present invention
there is provided a mass spectrometer comprising a mass
filter as described above.
The mass spectrometer preferably comprising an ion
source arranged upstream of the mass filter. The ion
source is preferably selected from the group consisting
of: (i) an Electrospray ("ESI") ion source; (ii) an
Atmospheric Pressure Chemical Ionisation ("APCI") ion
source; (iii) an Atmospheric Pressure Photo Ionisation
("APPI") ion source; (iv) a Laser Desorption Ionisation
("LDI") ion source; (v) an Inductively Coupled Plasma
("ICP") ion source; (vi) an Electron Impact ("El") ion
source; (vii) a Chemical Ionisation ("CI") ion source;
(viii) a Field Ionisation ("Fl") ion source; (ix) a Fast
Atom Bombardment ("FAB") ion source; (x) a Liquid
Secondary Ion Mass Spectrometry ("LSIMS") ion source;
(xi) an Atmospheric Pressure Ionisation ("API") ion
source; (xii) a Field Desorption ("FD") ion source;
(xiii) a Matrix Assisted Laser Desorption Ionisation
("MALDI") ion source; (xiv) a Desorption/Ionisation on
Silicon ("DIOS") ion source; and (xv) a Desorption
Electrospray Ionisation ("DESI") ion source.
CA 02487790 2012-04-23
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The ion source may comprises a continuous ion
source or a pulsed ion source. The mass spectrometer
preferably further comprises a mass analyser which is
preferably arranged downstream of the mass filter. The
mass analyser is preferably selected from the group
consisting of: (i) an orthogonal acceleration Time of
Flight mass analyser; (ii) an axial acceleration Time of
Flight mass analyser; (iii) a quadrupole mass analyser;
(iv) a Penning mass analyser; (v) a Fourier Transform
Ion Cyclotron Resonance ("FTICR") mass analyser; (vi) a
2D or linear quadrupole ion trap; (vii) a Paul or 3D
quadrupole ion trap; and (viii) a magnetic sector mass
analyser.
According to another aspect of the present
invention there is provided a method of mass filtering
ions comprising:
providing one or more electrodes associated with an
entrance region of a mass filter;
applying one or more first voltage pulses to the
one or more electrodes in order to orthogonally
accelerate at least some ions away from the one or more
electrodes of said entrance region;
reflecting at least some ions which have been
orthogonally accelerated away from said entrance region
such that the ions move generally towards an exit region
of the mass filter disposed at a distance from said
entrance region; and
orthogonally decelerating or otherwise orthogonally
retarding by means of one or more electric fields first
CA 02487790 2012-04-23
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ions having a desired mass or mass to charge ratio or
having masses or mass to charge ratios within a first
desired range as the first ions approach the exit region
of the mass filter.
The preferred embodiment relates to a new type of
mass filter. The preferred mass filter differs from
known time of flight mass filters in that the preferred
mass filter does not utilise the axial velocity of ions
in order to isolate or otherwise select ions having a
particular mass to charge ratio. Instead, the mass
filter according to the preferred embodiment preferably
orthogonally accelerates (i.e. accelerates ions in an
orthogonal direction which is substantially 902 to the
initial axial direction of the ions) ions out of a
primary acceleration region and into a flight region.
The ions preferably travel towards and then enter an ion
mirror. The ion mirror preferably reflects the ions
back into the flight region and back towards the primary
acceleration region. The ions are preferably partially
decelerated after having been reflected by the ion
mirror as they pass through the flight region towards
the primary acceleration region. Ions which return to
the primary acceleration region at a certain precise
time are preferably arranged to be further orthogonally
decelerated or retarded by a time varying electric field
maintained across a portion of the primary acceleration
region. Ions having a desired mass to charge ratio are
preferably retarded or otherwise orthogonally
decelerated such that their component of velocity in an
orthogonal direction is preferably reduced to
substantially zero whilst their component of velocity in
an axial direction preferably remains substantially non-
zero. The selected ions are then preferably emitted and
CA 02487790 2012-04-23
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onwardly transmitted from the mass filter. Since the
mass filtering mode of operation of the preferred mass
filter preferably does not depend upon the axial
velocity of the ions, then the preferred mass filter is
preferably substantially unaffected by the initial
axial, spatial, energy and time distributions of the
ions which are to be mass filtered. The preferred mass
filter is therefore particularly advantageous compared
to known mass filters.
The preferred mass filter may, in one embodiment,
orthogonally accelerate ions out of the primary
acceleration region by the application of a preferably
relatively long, preferably relatively high voltage
pulse to one or more orthogonal acceleration electrodes
arranged in the primary acceleration region.
Accordingly, all ions in an ion beam will gain
essentially the same energy. The ions are then
preferably accelerated towards an ion mirror and are
then reflected back towards the primary acceleration
region by the ion mirror. As ions having the desired
mass to charge ratio approach the primary acceleration
region, these particular ions are then preferably fully
orthogonally decelerated by arriving at the primary
acceleration region at a precise time when the high
voltage pulse which initially orthogonally accelerated
the ions is now falling from a maximum voltage to zero
in a finite period of time. By switching the voltage
pulse applied to the one or more orthogonal acceleration
electrodes OFF at a certain precise time, ions having a
certain mass to charge ratio arriving at the primary
acceleration region will experience a deceleration in
the orthogonal direction of substantially the same
magnitude as the magnitude of the orthogonal
CA 02487790 2012-04-23
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acceleration which the ions initially experienced.
Accordingly, ions having a certain desired mass to
charge ratio will have their component of velocity in
the orthogonal direction reduced back to zero and hence
will return to their original axial path through the
mass filter.
Ions having a particular mass to charge ratio are
therefore preferably selected by the accurate timing of
the length or duration of one or more preferably
relatively high voltage pulses applied to one or more
orthogonal acceleration electrodes preferably arranged
in a primary acceleration region of the mass filter.
Whilst ions having a desired mass to charge ratio will
preferably be onwardly transmitted by the mass filter,
ions having a relatively smaller mass to charge ratio
are preferably arranged such that they are reflected by
the ion mirror and arrive at the primary acceleration
region at a time when the one or more orthogonal
acceleration electrodes are still being energised by the
application of a voltage pulse to the one or more
primary acceleration electrodes. The ions therefore
arrive at a time when an electric field is present in
the primary acceleration region. The electric field
will cause the ions having a relatively small mass to
charge ratio to be orthogonally decelerated, reflected
and then orthogonally re-accelerated back into the
flight region. Such ions will then preferably become
lost to the system.
Ions having a relatively high mass to charge ratio
are preferably arranged to arrive at the primary
acceleration region (having been reflected by the ion
mirror) at a time when the one or more orthogonal
acceleration electrodes are preferably no longer being
CA 02487790 2012-04-23
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energised i.e. when no voltage pulse is preferably being
applied to the one or more orthogonal acceleration
electrodes. The ions will therefore preferably arrive
at the primary acceleration region at a time when no
electric field is present in the primary acceleration
region. Accordingly, ions having a relatively high mass
to charge ratio, although partially decelerated in an
orthogonal direction as the ions pass back through the
flight region towards the primary acceleration region
are not further or completely orthogonally decelerated
in the primary acceleration region. As a result, these
ions will continue to travel with a non-zero component
of velocity in an orthogonal direction and hence are not
returned to having a purely axial component of velocity.
According to an embodiment such ions may be arranged to
collide with one of the orthogonal acceleration
electrodes or another part of the mass filter and hence
become lost to the system.
The preferred mass filter has a number of
advantages compared with known mass filters. Since the
preferred mass filter does not select ions having a
particular mass to charge ratio based upon the axial
velocity of ions, then axial energy distributions and
time distributions preferably do not adversely effect
the operation of the preferred mass filter. As a
result, undesired fragment ions resulting from a
dissociation event after corresponding parent ions have
been accelerated to their final energy or velocity
preferably are advantageously not onwardly transmitted
by the preferred mass filter unlike conventional time of
flight mass filters. Another advantage of the preferred
mass filter is that the preferably high voltage pulse(s)
applied to the one or more orthogonal acceleration
CA 02487790 2012-04-23
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electrodes preferably do not require very fast rise
and/or fall times and hence complex and expensive fast
electronic voltage supplies are not required.
When the mass filter is not in use or is otherwise
arranged to act as an ion guide with a high (e.g. 100%)
ion transmission in a non-mass filtering mode of
operation, no electrodes are present sufficiently close
to the path of an ion beam passing through the mass
filter as to interfere with the ion beam. Since ions
will not therefore collide with any electrodes in the
mass filter, the mass filter preferably will have a
substantially 100% ion transmission efficiency when used
as an ion guide in a non-mass filtering mode of
operation. This is not the case with other known mass
filters such as Bradbury-Nielson ion gates wherein ions
can collide with the electrodes which form the ion gate,
and hence such ion gates typically have an ion
transmission efficiency < 100% when used in a non-mass
filtering mode of operation.
Another advantage of the preferred mass filter is
that by correctly timing the length and/or duration of
one or more high voltage pulse(s) applied to the one or
more orthogonal acceleration electrodes, it is possible
to reduce the divergence of an ion beam being mass
filtered by the mass filter and hence the preferred mass
filter advantageously focuses an ion beam. The mass
filter can therefore be used to increase the
transmission of ions through subsequent stages of a mass
spectrometer which are preferably arranged downstream of
the preferred mass filter.
Various embodiments of the present invention will
now be described, by way of example only, and with
reference to the accompanying drawings in which:
CA 02487790 2012-04-23
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Fig. 1A shows a SIMION (RTM) simulation of three
ions having different mass to charge ratios being
orthogonally accelerated by a mass filter according to a
first embodiment, Fig. 1B shows a corresponding voltage
timing diagram illustrating the delay time and pulse
duration of a high voltage pulse applied to an
orthogonal acceleration electrode of a preferred mass
filter and Fig. 1C shows a corresponding potential
energy diagram illustrating the potential gradient
maintained across the primary acceleration region,
flight region and within the ion mirror during and after
an orthogonal acceleration pulse is applied to one or
more orthogonal acceleration electrodes in the primary
acceleration region;
Fig. 2A shows a SIMION (RTM) simulation of a second
embodiment wherein ions having relatively low and
relatively high mass to charge ratios are not
orthogonally accelerated by the mass filter but instead
pass straight through the mass filter and Fig. 2B shows
a corresponding voltage timing diagram illustrating the
delay time and pulse duration of a high voltage pulse
applied to an orthogonal acceleration electrode of a
mass filter according to the second embodiment;
Fig. 3A shows a SIMION (RTM) simulation of a third
embodiment wherein ions having relatively low and
relatively high mass to charge ratios are arranged to
collide with an inlet aperture of the mass filter and
Fig. 3B shows a corresponding voltage timing diagram
illustrating the delay times and pulse duration of the
high voltage pulses applied to an orthogonal
acceleration electrode of a mass filter according to the
third embodiment;
Fig. 4 illustrates the different trajectories
CA 02487790 2012-04-23
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through a preferred mass filter of ions having the same
mass to charge ratio but a range of initial axial
energies;
Fig. 5 shows a SIMION (RTM) simulation of the
different trajectories of six groups of ions through a
preferred mass filter when the ions arriving at the mass
filter had a distribution of initial kinetic energies
and positions;
Fig. 6A shows in tabular form the initial kinetic
energies and positions for six groups of ions simulated
in Fig. 5, and Fig. 6B illustrates the distribution of
initial trajectories which ions in a particular group
were modelled as having; and
Fig. 7 shows the angular divergence of all the ions
modelled in the simulation shown in Fig. 5 both before
and after being orthogonally accelerated by the
preferred mass filter.
A preferred embodiment of the present invention
will now be described with reference to Fig. 1A. Fig.
lA shows a SIMION (RTM) simulation of a mass filter
according to a preferred embodiment. An ion source 1 is
shown arranged upstream of a mass filter according to a
preferred embodiment. The mass filter comprises an
entrance aperture 5a, a primary acceleration region 2
including one or more orthogonal acceleration electrodes
9, a flight region 3 arranged adjacent to the primary
acceleration region 2, an ion mirror or reflectron 4
(arranged to receive ions exiting from the flight region
3 and to reflect them back into the flight region 3) and
an exit aperture 5b. The mass filter was modelled by
theoretically surrounding the mass filter in a grounded
chamber 12 in order to mimic the effects of a vacuum
chamber. It will be appreciated, however, that the
CA 02487790 2012-04-23
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grounded chamber 12 is merely provided and shown for
purposes of modelling the passage of ions through the
mass filter in the simulation and is not actually
required in a real mass filter according to the
preferred embodiment.
The trajectories of three ions 6,7,8 having
different mass to charge ratios were simulated as they
entered and passed through the mass filter. The three
ions 6,7,8 had mass to charge ratios of 1000, 1500 and
2000 respectively. The respective trajectories of the
ions 6,7,8 through the mass filter are shown in Fig. 1A.
An axial or x-direction is shown which is preferably at
902 to an orthogonal or y-direction.
The three ions 6,7,8 in the simulation were
modelled as being accelerated from +500 V to 0 V in the
region of the ion source 1. At a time 2.5 ps after the
ions 6,7,8 had been emitted from or otherwise generated
in the ion source 1, a +750 V voltage pulse having a
duration of 8.374 ps was applied to the one or more
orthogonal acceleration electrodes 9 arranged in the
primary acceleration region 2. The voltage pulse
applied to the one or more orthogonal acceleration
electrodes 9 had the effect of raising the potential of
the one or more orthogonal acceleration electrodes 9
from 0 V to +750 V for a time period of 8.374 ps. The
voltage pulse applied to the one or more orthogonal
acceleration electrodes 9 thus had the effect of
generating an electric field which orthogonally
accelerated the ions 6,7,8 out of the primary
acceleration region 2 and into the adjacent flight
region 3. The applied voltage pulse in the embodiment
shown and described in relation to Figs. 1A-1C was
modelled as having a rise time of 50 ns i.e. it took 50
CA 02487790 2012-04-23
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ns for the potential of the one or more orthogonal
acceleration electrodes 9 to increase or rise from 0 V
to +750 V. Similarly, the applied voltage pulse was
modelled as having a fall time of 50 ns i.e. it took 50
ns for the potential of the one or more orthogonal
acceleration electrodes 9 to fall or reduce from +750 V
to 0 V.
Fig. 1B shows a voltage timing diagram showing the
timing of a high voltage pulse applied to the one or
more orthogonal acceleration electrodes 9 according to a
preferred embodiment. The high voltage pulse was
applied to the one or more orthogonal acceleration
electrodes 9 after a certain delay time tstar-_ after the
formation, generation or release of ions from the ion
source 1 or an ion generating region otherwise arranged
upstream of the mass filter. For the particular
simulation shown in Fig. 1A the delay time tstart was 2.5
is. The rise time trise and the fall time tfal- were 50
ns. The duration tptise of the relatively high voltage
pulse is preferably taken to be the time (t,,,) for the
voltage pulse to rise or increase from zero to a maximum
voltage and then to remain at this maximum voltage
without reducing in amplitude. In the particular
embodiment shown and described with reference to Figs.
1A-1C, the voltage pulse had a duration tpuise of 8.374
s.
It will be appreciated that the delay time t
-start,
rise time trj,e, voltage pulse duration t
fall time
ttall and the amplitude of the voltage pulse applied to
the one or more orthogonal acceleration electrodes 9 may
vary and differ from the embodiment described with
reference to Figs. 1A-1C depending upon the mass to
charge ratio of ions to be selected and the overall
CA 02487790 2012-04-23
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geometry of the mass filter. It will also be
appreciated that the voltage pulse may have a negative
polarity and that the one or more orthogonal
acceleration electrodes 9 may be maintained at a
potential above or below 0 V when a voltage pulse is not
applied to the one or more orthogonal acceleration
electrodes 9. A person skilled in the art will also
appreciate that the absolute voltages at which the one
or more orthogonal acceleration electrodes 9 are
maintained is less important than the fact that there is
a relative change in the potential at which the one or
more orthogonal acceleration electrodes 9 are maintained
in use.
The flight region 3 according to the preferred
embodiment is preferably not a field free region but
rather as can be seen from Fig. 1C preferably comprises
a region wherein ions which have been orthogonally
accelerated out of the primary acceleration region 2 are
preferably further orthogonally accelerated due to a
non-zero potential gradient being maintained across the
flight region 3 as the ions pass through the flight
region 3 towards the ion mirror or reflectron 4. The
three ions 6,7,8 modelled in Fig. 1A are therefore
preferably further orthogonally accelerated (i.e.
accelerated in the y-direction shown in Fig. 1A) upon
entering the flight region 3 towards the entrance of the
ion mirror or reflectron 4. The ion mirror or
reflectron 4 is preferably arranged adjacent to the
flight region 3 and preferably receives ions exiting the
flight region 3. The ion mirror or reflectron 4
preferably reflects the ions 6,7,8 back into the flight
region 3 and hence preferably directs the ions 6,7,8
back towards the primary acceleration region 2 and in
CA 02487790 2012-04-23
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the general direction of the exit or exit region of the
mass filter. However, other embodiments are
contemplated wherein ions may be arranged to exit the
mass filter in a different manner to that shown in Fig.
1A by, for example, being further deflected within the
mass filter.
In the particular embodiment shown and described
above with relation to Figs. 1A-1C, the entrance region
of the ion mirror or reflectron 4 (or the electrodes
forming the entrance region or portion of the ion mirror
or reflectron 4) are preferably held or maintained at a
potential of -2750 V. The rearmost region or portion of
the ion mirror or reflectron 4 (or the electrodes of the
ion mirror or reflectron 4 located at the rearmost
region of the ion mirror or reflectron 4) are preferably
held at a potential of +4000 V. Electrodes located
within the ion mirror or reflectron 4 between the
entrance region and the rearmost region of the ion
mirror or reflectron 4 are preferably held or maintained
at intermediate potentials between - 2750 V and + 4000
V. The profile of the potential gradient maintained
within the ion mirror or reflectron 4 is shown for ease
of illustration as being linear in Fig. 1C. However, in
practice and/or according to other embodiments, the
potential gradient within the ion mirror or reflectron 4
may comprise a stepped, curved, exponential or otherwise
non-linear potential gradient profile.
Once the ions 6,7,8 enter the ion mirror or
reflectron 4, the ions 6,7,8 are preferably subjected to
a retarding potential field within the ion mirror or
reflectron 4 such that the ions 6,7,8 are reflected
within the ion mirror or reflectron 4. The ions 6,7,8
will then preferably exit the ion mirror or reflectron 4
CA 02487790 2012-04-23
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such that they then re-enter the flight region 3. The
ions 6,7,8 upon re-entering the flight region 3 then
preferably pass back through the flight region 3 as they
head towards the primary acceleration region 2 and the
general direction of the exit of the mass filter. As
the ions 6,7,8 pass back through the flight region 3
having been reflected by the ion mirror and reflectron
4, the ions 6,7,8 are preferably partially orthogonally
decelerated in the y-direction only by the retarding
potential gradient which is preferably maintained across
the flight region 3. The potential gradient maintained
across the flight region which served to initially
further orthogonally accelerate the ions 6,7,8 when they
were travelling from the primary acceleration region 2
towards the ion mirror or reflectron 4, now therefore
preferably serves to partially orthogonally decelerate
the ions 6,7,8 as they head back towards the primary
acceleration region 2. The axial component of velocity
of the ions 6,7,8 preferably remains substantially the
same throughout the primary acceleration region 2,
flight region 3 and ion mirror 4. The partially
orthogonally decelerated ions 6,7,8 then preferably re-
enter the primary acceleration region 2 as can be seen
more clearly with reference to Fig. 1A.
The voltage pulse applied to the one or more
orthogonal acceleration electrodes 9 preferably has an
amplitude of +750 V and a duration of 8.374 ps. The
potential of the one or more orthogonal acceleration
electrodes 9 then preferably returns to 0 V (or less
preferably to another different potential or voltage) at
the end of the voltage pulse duration.
The application of the relatively high voltage
pulse to the one or more orthogonal acceleration
CA 02487790 2012-04-23
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electrodes 9 preferably affects the ions 6,7,8 having
different mass to charge ratios in different ways. Ions
6 having the lowest mass to charge ratio of 1000 will
preferably have travelled further into the entrance
region of the mass filter than the ions 7,8 having
higher mass to charge ratios of 1500 and 2000 when the
voltage pulse is applied to the one or more orthogonal
acceleration electrodes 9. Ions 6 having the lowest
mass to charge ratio of 1000 will also have the fastest
flight time through the flight region 3 once they have
been orthogonally accelerated. Accordingly, ions 6
having a mass to charge ratio of 1000 will exit the
flight region 3 having been reflected by the ion mirror
or ref lectron 4 and will arrive at the primary
acceleration region 2 before other ions 7,8 which have
comparatively higher mass to charge ratios.
The duration of the high voltage pulse applied to
the one or more orthogonal acceleration electrodes 9 is
preferably such that ions 6 having a mass to charge
ratio of 1000 will preferably exit the flight region 3
and arrive at the primary acceleration region 2 at a
time when the one or more orthogonal acceleration
electrodes 9 are still preferably being energised by the
+750 V voltage pulse. Accordingly, ions 6 having a
mass to charge ratio of 1000 which approach the primary
acceleration region 2 having been reflected by the ion
mirror on reflectron 4 will preferably be orthogonally
decelerated or retarded but will then also be reflected
back out into the flight region 3 by the electric field
maintained across the primary acceleration region 2.
Upon re-entering the flight region 3 the ions 6 having a
mass to charge ratio of 1000 are preferably allowed or
arranged to become lost to the system by, for example,
CA 02487790 2012-04-23
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colliding with a part of the mass filter.
Ions 8 having the highest mass to charge ratio of
2000 will have the slowest flight time through the
flight region 3. The duration of the high voltage pulse
applied to the one or more orthogonal acceleration
electrodes 9 is preferably such that ions 8 having a
mass to charge ratio of 2000 will preferably exit the
flight region 3 and arrive at the primary acceleration
region 2 at a time when the one or more orthogonal
acceleration electrodes 9 are preferably no longer being
energised by the high voltage pulse i.e. when the one or
more orthogonal acceleration electrodes 9 are preferably
maintained at 0 V (or some other potential or voltage).
Accordingly, although ions 8 having a mass to charge
ratio of 2000 will have been partially orthogonally
decelerated or retarded as they pass from the ion mirror
or reflectron 4 back through the flight region 3, the
ions 8 will not experience any further orthogonal
deceleration or orthogonal retardation in the orthogonal
or y-direction in the primary acceleration region 2.
This is because at the time when the ions 8 arrive at
the primary acceleration region 2 the potential gradient
across the primary acceleration region 2 will preferably
be substantially zero. Accordingly, the ions 8 will
therefore possess a non-zero component of velocity in
the orthogonal or y-direction as they enter and pass
through the primary acceleration region 2. These ions 8
will therefore preferably continue through the primary
acceleration region 2 before preferably colliding with
either one of the orthogonal acceleration electrodes 9
or with another part of the mass filter. The ions 8 are
therefore preferably arranged or allowed to become lost
to the system.
CA 02487790 2012-04-23
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The duration of the relatively high voltage pulse
applied to the one or more orthogonal acceleration
electrodes 9 is preferably such that ions 7 having a
mass to charge ratio of 1500 are arranged to have a
flight time through the flight region 3 such that when
the ions 7 exit the flight region 3 having been
reflected by the ion mirror 4 and approach the primary
acceleration region 2, the potential gradient maintained
across the primary acceleration region 2 will preferably
begin to vary (i.e. decrease) with time as the ions 7
further approach the primary acceleration region 2.
Since the voltage pulse applied to the one or more
orthogonal acceleration electrodes 9 preferably has a
finite fall time (e.g. 50 ns according to the preferred
embodiment), then a retarding potential gradient will
preferably be maintained across the primary acceleration
region 2 which will reduce in intensity or amplitude to
preferably zero (or less preferably to a low value) over
the finite fall time of the voltage pulse applied to the
one or more orthogonal acceleration electrodes 9.
Accordingly, ions 7 having a mass to charge ratio of
1500 are preferably arranged to experience a retarding
impulse or orthogonal deceleration in the orthogonal or
y-direction only in the primary acceleration region 2
which will have precisely the opposite effect to the
accelerating impulse or orthogonal acceleration which
originally orthogonally accelerated the ions 6,7,8 into
the flight region 3. As a result of receiving an equal
and opposite impulse to the impulse which originally
orthogonally accelerated the ions 6,7,8 into the flight
region 3, the ions 7 having a mass to charge ratio of
1500 will preferably have their component of velocity in
an orthogonal or y-direction preferably reduced to zero
CA 02487790 2012-04-23
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(or less preferably to near zero) and hence will be
returned to their original, preferably axial, path or
heading 10 through the mass filter as indicated by the
x-direction in Fig. 1A. The result of the decelerating
impulse is therefore preferably that the orthogonal
component of velocity of the desired ions 7 having a
mass to charge ratio of 1500 is reduced to zero (or less
preferably to near zero) whilst the component of
velocity of the desired ions 7 in an axial or x-
direction is preferably unaffected. The desired ions 7
therefore preferably return to having a purely axial
component of velocity. The ions 7 having a desired mass
to charge ratio will then preferably exit the mass
filter, preferably but not necessarily in an axial or x-
direction, via an exit aperture 5b which preferably
forms part of a downstream portion of the mass filter.
A beam of ions 7 corresponding to ions 7 is shown in
Fig. lA exiting the mass filter.
Fig. 1C illustrates the potential gradient
maintained across the primary acceleration region 2, the
flight region 3 and the ion mirror 4 according to a
preferred embodiment of the present invention.
According to this embodiment the primary acceleration
region 2 is preferably initially maintained at 0 V. The
one or more orthogonal acceleration electrodes 9 are
then preferably pulsed from 0 V to +750 v so that a 750
V potential gradient is preferably maintained across the
primary acceleration region 2. This potential gradient
preferably causes ions 6,7,8 to be substantially
orthogonally accelerated in the orthogonal or y-
direction out from the primary acceleration region 2 and
into the flight region 3. The ions 6,7,8 having passed
into the flight region 3 are then preferably further
CA 02487790 2012-04-23
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orthogonally accelerated in the orthogonal or y-
direction as they pass through the flight region 3 due
to an accelerating potential gradient which is
preferably maintained across the flight region 3.
The ions 6,7,8 then preferably reach the ion mirror
4, whereupon the ions 6,7,8 are then preferably
decelerated within the ion mirror 4. The ions 6,7,8 are
then preferably reflected and accelerated out of the ion
mirror 4 such that the ions 6,7,8 preferably re-enter
the flight region 3. As the ions 6,7,8 re-enter the
flight region 3, the ions 6,7,8 preferably experience
the same potential gradient which had previously further
orthogonally accelerated them towards the ion mirror 4.
However, the potential gradient maintained across the
flight region 3 now acts to partially retard or
partially orthogonally decelerate the ions 6,7,8 in the
orthogonal or y-direction. The ions 6,7,8 having been
partially orthogonally decelerated in the orthogonal or
y-direction then preferably exit the flight region 3 and
re-enter the primary acceleration region 2. The
duration of the high voltage pulse applied to the one or
more orthogonal acceleration electrodes 9 is preferably
such that ions having a desired mass to charge ratio
experience in the primary acceleration region 2 a
retarding potential gradient which rapidly decreases
with time or an impulse such that the ions having a
desired mass to charge ratio are further orthogonally
decelerated until or such that their component of
velocity in the orthogonal or y-direction is preferably
reduced to zero. Ions having a desired mass to charge
ratio will therefore preferably be arranged to end up
having a non-zero axial (or x-direction) component of
velocity and preferably a substantially zero orthogonal
CA 02487790 2012-04-23
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(or y-direction) component of velocity in the primary
acceleration region 2. Less preferred embodiments are
contemplated wherein the desired ions which are emitted
or which emerge from the mass filter may have a non-zero
component of velocity in the orthogonal direction if,
for example, the desired ions are then further deflected
and/or accelerated and/or decelerated within the mass
filter.
According to the particular embodiment shown and
described with reference to Figs. 1A-1C, ions
irrespective of their mass to charge ratio will
preferably be orthogonally accelerated into the flight
region 3 but only ions having a desired mass to charge
ratio will preferably have their orthogonal component of
velocity reduced to zero and hence will preferably
emerge from the mass filter and be onwardly transmitted
therefrom.
A variation of the embodiment shown and described
with reference to Figs. 1A-1C will now be described with
reference to Figs. 2A and 2B. According to this second
embodiment, the ion source 1 is preferably located
further away from the mass filter than in the first
embodiment shown and described with reference to Figs.
1A-1C. The extended region between the ion source 1 and
the mass filter preferably acts as an additional flight
region such that ions emitted from the ion source 1 will
preferably arrive at the entrance to the mass filter at
different times depending upon their mass to charge
ratio i.e. ions will preferably become temporally
separated or dispersed according to their mass to charge
ratio as they pass from the ion source 1 to the entrance
of the mass filter.
The particular embodiment shown and described in
CA 02487790 2012-04-23
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relation to Figs. 2A and 23 differs from the first
embodiment shown and described in relation to Figs. 1A-
1C in that ions having relatively low mass to charge
ratios are preferably transmitted straight through the
mass filter without ever being orthogonally accelerated
into the flight region 3. This is achieved by arranging
that ions having a relatively low mass to charge ratio
pass through and exit the mass filter before a high
voltage pulse is preferably applied to the one or more
orthogonal acceleration electrodes 9.
In a similar manner, ions having relatively high
mass to charge ratios are also preferably transmitted
straight through the mass filter without ever being
orthogonally accelerated into the flight region 3. This
is achieved by preferably arranging that ions having a
relatively high mass to charge ratio arrive at the mass
filter only after a high voltage pulse has been applied
to the one or more orthogonal acceleration electrodes 9
and the one or more orthogonal acceleration electrodes 9
are no longer being energised.
It will be apparent therefore that according to the
second embodiment disclosed and described in relation to
Figs. 2A and 2B, ions having relatively low mass to
charge ratios and ions having relatively high mass to
charge ratios are preferably transmitted straight
through the mass filter without ever being orthogonally
accelerated into the flight region 3. Ions having
intermediate mass to charge ratios are, however,
preferably orthogonally accelerated within the mass
filter and are therefore preferably subjected to the
preferred method of mass filtering.
In the particular embodiment shown in Fig. 2A the
ion source 1 was modelled as being arranged 90 mm
CA 02487790 2012-04-23
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further away from the entrance 5a of the mass filter
than in the first embodiment shown and described in
relation to Fig. 1A. In the particular simulation shown
and described in relation to Figs. 2A and 2B, three ions
having mass to charge ratios of 400, 1500 and 7000 were
modelled as being accelerated to an energy of 500 eV by
or within the ion source 1. The mass filter was then
operated in a similar mode of operation to the mode of
operation described above in relation to the first
embodiment shown with reference to Figs. 1A-1C except
that the start or delay time tstart was increased. In
particular, the start or delay time t
-sLart relates to the
time from when ions are generated in the ion source 1 to
the time when a high voltage pulse is first applied to
the one or more orthogonal acceleration electrodes 9.
In the second embodiment shown and described in relation
to Fig. 2B, the start or delay time t
-start was preferably
increased from 2.5 s to 14.5 ps. The increase in the
start or delay time tsLatt allowed ions having a
relatively low mass to charge ratio of 400 to pass
straight through the mass filter and reach the exit of
the mass filter before a voltage pulse was applied to
the one or more orthogonal acceleration electrodes 9.
The start or delay time t
-start was also set such that ions
having a desired mass to charge ratio of 1500 were
arranged to enter the mass filter and be orthogonally
accelerated into the flight region 2 due to the presence
of an electric field resulting from the application of a
high voltage pulse to the one or more orthogonal
acceleration electrodes 9. The start or delay time tstart
and the length or duration of the voltage pulse tpuiõ
were preferably arranged such that ions having a
relatively high mass to charge ratio of 7000 reach the
CA 02487790 2012-04-23
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entrance of the mass filter only after the high voltage
pulse is no longer being applied to the one or more
orthogonal acceleration electrodes 9. Accordingly, ions
having a mass to charge ratio of 7000 are transmitted
straight through the mass filter without ever being
orthogonally accelerated into the flight region 3. The
simulation shows that all three ions having mass to
charge ratios of 400, 1500 and 7000 were onwardly
transmitted by the mass filter.
A voltage timing diagram showing the timing of the
high voltage pulse applied to the one or more orthogonal
acceleration electrodes 9 in the second embodiment
described in relation to Fig. 2A is shown in Fig. 2B.
For ease of illustration only, the finite rise and fall
time of the high voltage pulse is not shown. However,
the rise time and the fall time are both preferably 50
ns.
A variation of the second embodiment described
above in relation to Figs. 2A and 23 will now be
described with reference to Figs. 3A and 33. According
to this third embodiment, the one or more orthogonal
acceleration electrodes 9 are preferably initially
maintained at a voltage of +750 V (as opposed to 0 V).
The one or more orthogonal acceleration electrodes 9
preferably remain at this relatively high potential for
a certain period of time t(1)0N which is preferably 11.5
ps. As a result, ions which arrive at the entrance of
the mass filter whilst the high voltage pulse is being
applied to the one or more orthogonal acceleration
electrodes 9 during the time period t(1)0N will
preferably be deflected or otherwise orthogonally
accelerated immediately upon entering the mass filter.
The entrance aperture 5a of the mass filter is
CA 02487790 2012-04-23
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preferably arranged such that ions which are immediately
deflected or otherwise orthogonally accelerated upon
entering the mass filter are preferably prevented from
passing into the flight region 3 but are instead
preferably arranged to collide with a portion of the
entrance aperture 5a of the mass filter and hence become
lost to the system. Other less preferred embodiments
are, however, contemplated wherein the ions may
initially enter the flight region 3 but wherein the ions
are arranged such that they collide with a plate or
electrode positioned in the flight region 3 (or another
region of the mass filter) and hence become lost to the
system.
After the initial time period t(1)0N during which a
high voltage pulse is preferably applied to the one or
more orthogonal acceleration electrodes 9, the voltage
applied to the one or more orthogonal acceleration
electrodes 9 is then preferably reduced to 0 V (or a
relatively low potential) for a period of time t(l)0F
which is preferably 3 ps. The potential of the one or
more orthogonal acceleration electrodes 9 is therefore
preferably reduced to zero (or a relatively low
potential) immediately prior to the arrival of ions
having intermediate mass to charge ratios (which
preferably include ions having mass to charge ratios of
interest) at the entrance aperture 5a of the mass
filter.
By appropriate setting of the time periods t(1)0N
and t(l)oF, ions having mass to charge ratios less than
a certain mass to charge ratio are preferably
immediately deflected at the entrance aperture 5a of the
mass filter and hence are lost to the system whereas
ions having mass to charge ratios within an intermediate
CA 02487790 2012-04-23
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range are preferably allowed to enter further into the
mass filter such that they are then preferably
orthogonally accelerated and subjected to the preferred
method of mass filtering. After the time period t(1) OFF
the one or more orthogonal acceleration electrodes 9 are
preferably then subsequently pulsed or maintained at a
relatively high potential in a similar manner to the
first and second embodiments described above in relation
to Figs. 1A-1C and Figs. 2A-2B. The one or more
orthogonal acceleration electrodes 9 are therefore
preferably maintained at a relatively high voltage of
e.g. 750 V for a time period tpulse which is preferably
8.374 ps. Accordingly, ions having mass to charge
ratios within an intermediate range are preferably
orthogonally accelerated in the orthogonal or y-
direction into the flight region 3 with the result that
certain desired ions will be selected by the preferred
mass filtering process of orthogonally accelerating and
then fully orthogonally decelerating desired ions. The
desired ions will therefore preferably emerge from the
exit of the mass filter whilst ions having other mass to
charge ratios are preferably arranged to be lost to the
system. After ions having desired mass to charge ratios
have preferably been returned to the axial or x-
direction, the voltage applied to the one or more
orthogonal acceleration electrodes 9 is then preferably
maintained at 0 V (or a relatively low potential or
voltage) for a period of time t(2)01,-T which is preferably
3 ps to enable the desired ions to exit the mass filter.
After the time period t(2)oFF, the potential of the one
or more orthogonal acceleration electrodes 9 is then
preferably raised to a relatively high voltage of e.g.
+750 V once again. The relatively high voltage applied
CA 02487790 2012-04-23
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to the one or more orthogonally acceleration electrodes
9 then preferably remains ON for a further time period
t(2)01,1 which may, for example, be 10 Ps or longer. The
result of reapplying a high voltage to the one or more
orthogonal acceleration electrodes 9 is that ions having
relatively high mass to charge ratios which are only
just approaching or arriving at the entrance of the mass
filter (after being generated approximately 26 ps
previously) will then preferably be deflected or
orthogonally accelerated immediately upon entering the
entrance 5a of the mass filter. According to the third
embodiment, therefore, ions having relatively low mass
to charge ratios and also ions having relatively high
mass to charge ratios are preferably arranged such that
they do not pass into the flight region 3 but rather are
preferably arranged such that they collide with a
portion of the entrance aperture 5a of the mass filter
or another part of the mass filter and hence become lost
to the system. Other less preferred embodiments are
contemplated wherein ions having very low and/or very
high mass to charge ratios may be allowed to enter the
flight region 3 but then collide with a plate or
electrode positioned in the flight region 3 or in
another region of the mass filter. Embodiments are also
contemplated wherein ions having very low and/or very
high mass to charge ratios are deflected to a different
portion or region of the mass filter.
Fig. 3B shows a timing diagram for the voltages
applied to the one or more orthogonal acceleration
electrodes 9 for the third embodiment modelled and
described above in relation to Fig. 3A. For simplicity
the finite rise and fall times of the high voltage
pulses are not shown but according to a preferred
CA 02487790 2012-04-23
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embodiment the voltage pulses have rise and/or fall
times of 50 ns.
It can be seen from Fig. 3B that the voltage
applied to the one or more orthogonal acceleration
electrodes 9 preferably remain initially ON or high for
a time period t(1)0N of 11.5 ps. The voltage applied to
the one or more orthogonal acceleration electrodes is
then preferably switched OFF or remains low for a delay
time period t(l)0F of preferably 3 is. The one or more
orthogonal acceleration electrodes 9 are then preferably
energised for a time period t
-pulse Of 8.374 ps in a
similar manner to the second embodiment described above
in relation to Fig. 2B. The voltage applied to the one
or more orthogonal acceleration electrodes 9 is then
preferably switched OFF or remains low for a further
delay time period t(2)= which is preferably 3 ps. The
voltage applied to the one or more orthogonal
acceleration electrodes 9 is then preferably switched ON
or remains high for a further period of time t(2)0N which
is preferably at least 10 ps.
The width of the two short delay time periods
t(l)OFF and t(2)01..T when the potential of the one or more
orthogonal acceleration electrodes 9 is preferably zero
(or otherwise relatively low) preferably effectively
determines a time window during which ions are able to
enter and leave the mass filter. Although Fig. 3B shows
that the amplitude of the voltage pulse applied to the
one or more orthogonal acceleration electrodes 9 is
preferably the same during time periods t(1)0N, tpuise and
t(2)0N, according to other embodiments the amplitude of
the voltage pulse may vary or differ such that the
amplitude during the time period t(1)0N and/or during the
time period t1se and/or during the time period t(2)0N
CA 02487790 2012-04-23
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are all different. Similarly, it will be appreciated
that the one or more orthogonal acceleration electrodes
9 may be maintained at potentials other than 750 V and 0
V during the time periods u(1)0N, t)l) OFF , tpilse (2 ) OFF
and t(2)0N.
Known time of flight mass filters and known mass
filters incorporating an ion gate suffer from the
problem that their overall resolution is reduced due to
the ions having an initial finite spread of axial
energies or velocities. An important advantage of a
mass filter according to the preferred embodiment is
that the preferred mass filter is relatively if not
substantially wholly immune to any effects due to the
ions having an initial spread of axial velocities. Fig.
4 shows a SIMION (RTM) simulation of the trajectories of
ten ions having the same mass to charge ratio but having
a relatively wide range of initial axial velocities.
The ions were orthogonally accelerated in the orthogonal
or y-direction within the mass filter according to the
preferred embodiment. In the example shown in Fig. 4,
the ten ions had a spread of axial energies ranging from
0 eV to 45 eV. The ten ions were then orthogonally
accelerated by a voltage pulse applied to the one or
more orthogonal acceleration electrodes 9. Such a large
spread in axial ion energies is much larger than would
be experienced in practice, but the results shown in
Fig. 4 serve to illustrate that the mass filter
according to the preferred embodiment is nonetheless
able to effectively select ions having a desired mass to
charge ratio even when the ions to be selected have a
wide range of initial axial energies or velocities. As
can be seen from Fig. 4, despite the fact that the ions
have a wide range of axial energies, all of the ions
CA 02487790 2012-04-23
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were orthogonally accelerated and then subsequently
orthogonally decelerated such that they returned to
their original (axial) path and subsequently emerged
from the mass filter. Simulating ions having the same
mass to charge ratio and the same initial axial energy
but with different creation times led to similar
results.
Fig. 5 shows the result of a simulation of the
performance of a mass filter according to a preferred
embodiment when the ions filtered by the mass filter had
an initial distribution of energies and positions such
as might be encountered experimentally. A total of 540
ions all having a mass to charge ratio of 1500 but
having different initial energies and positions were
simulated. The ions which were simulated were arranged
in six different groups of ions, each group comprising
90 ions. The six groups of ions represent two different
starting energies and three different starting
positions. The initial starting conditions of the
different groups of ions are summarised in Fig. 6A i.e.
the ions either had initial relative positions of -0.1
mm, 0 mm or + 0.1 mm and either had initial kinetic
energies of 0.2 eV or 0.5 eV. All 90 ions within a
group were modelled as being initially distributed so as
to have an approximate cos2e distribution of initial ion
trajectories. The initial ion trajectories were
oriented about the normal to the ion source 1. Such a
distribution of initial ion trajectories is shown in
Fig. 68. It is apparent from Fig. 5 that all of the 540
ions were onwardly transmitted through the exit aperture
5b of the mass filter.
For the particular conditions modelled in Fig. 5
the size of the virtual object from which the ions
CA 02487790 2012-04-23
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appear to originate after exiting the mass filter is
increased. By tracing back the final trajectories of
the ions, the size of the virtual object was determined
to be approximately 1.3 mm for the particular conditions
simulated. This represents approximately a x6 increase
in the size of the object prior to mass selection and
results in the brightness of the ion beam being reduced.
The brightness of an ion beam is defined as the
current density per unit solid angle in the axial
direction. As a result, brightness is inversely
proportional to the product of the cross sectional area
of the beam and the square of the beam divergence.
Accordingly, an increase in the width of the ion beam
will lead to a decrease in its brightness.
Fig. 7 shows a plot of the angular divergence of
all 540 ions in the simulation described above in
relation to Fig. 5 and Figs. 6A-6B. The angular
divergence of the ions is shown both prior to being mass
filtered by the preferred mass filter and also
subsequent to being mass filtered by the preferred mass
filter. Prior to mass selection, the ions had a spread
of angular divergences which range from approximately +
1.7 to - 1.7 for ions having a kinetic energy of 0.5
eV and a spread of angular divergences which range from
approximately + 1.1 to - 1.1 for ions having a kinetic
energy of 0.2 eV.
After mass selection it can be seen that the
angular divergence of the ion beam has now been
significantly reduced. The angular divergence now
ranges from + 1.1 to - 1.0 for ions having a kinetic
energy of 0.5 eV and from + 1.1 to - 0.1 for ions having
a kinetic energy of 0.2 eV. Accordingly, the mass
filter according to the preferred embodiment has the
CA 02487790 2012-04-23
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effect of reducing the angular divergence of ions having
a kinetic energy of 0.5 eV by 38% and of reducing the
angular divergence of ions having a kinetic energy of
0.2 eV ions by 45%.
For ions generated from a point ion source 1 as
shown in the simulation shown in Fig. 5, it is possible
to achieve optimal focussing and reduce the angular
divergence of the ions by a factor of x2 or more. For
ions created at different spatial positions, further
embodiments are contemplated wherein a dynamic voltage
pulse may be applied to the one or more orthogonal
acceleration electrodes 9 in order to improve the
overall focussing of the ions. For example, a linear
ramp, a sinusoidal or an exponential voltage waveform
may be superimposed on the DC level of a square wave or
other voltage pulse applied to the one or more
orthogonal acceleration electrodes 9.
An additional advantage of the preferred mass
filter therefore is that the mass filter may be used to
select ions having a certain mass to charge ratio from
an ion beam whilst at the same time reducing the angular
divergence (and hence velocity spread) of the selected
ions. This enables the effect of turn around time to be
reduced if the ions are then subsequently passed to an
orthogonal acceleration Time of Flight mass analyser for
mass analysis. As a result, the preferred mass filter
can lead to a significant improvement in the mass
resolution of a Time of Flight mass analyser when such a
mass analyser is used in conjunction with a mass filter
according to the preferred embodiment.
Embodiments are contemplated wherein a high voltage
pulse may be applied to the one or more orthogonal
acceleration electrodes 9 as a series of two or more
CA 02487790 2012-04-23
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short pulses rather than a single long pulse.
Further embodiments are contemplated wherein
instead of using a single voltage pulse which remains ON
to orthogonally accelerate or orthogonally decelerate
ions, two separate voltage pulses may be used, one which
starts low and pulses high to accelerate the ions, and
one which starts high and pulses low to decelerate the
ions.
According to an embodiment the primary acceleration
region 2 may be split into two or more regions in order
to reduce the capacitance of the electrodes.
In an embodiment a relatively short voltage pulse
may be applied to the one or more orthogonally
acceleration electrodes 9 in order to initially
accelerate the ions giving them all constant momentum.
A relatively long voltage pulse may then be applied to
orthogonally decelerate the ions once they return to the
primary acceleration region 2. According to another
embodiment, the ions may be initially accelerated using
a relatively long voltage pulse but then orthogonally
decelerated using a relatively short voltage pulse which
only starts once substantially all of the desired ions
having a desired mass to charge ratio have re-entered
the primary acceleration region 2.
According to a less preferred embodiment one or
more grids or grid electrodes may be provided in the
flight region 3 so that the ions travel through a field
free region before and/or after reaching the ion mirror
or reflectron 4.
According to another less preferred embodiment,
instead of reflecting the ions, the ions may
alternatively be decelerated in a second accelerating
region offset in the y direction which would result in
CA 02487790 2012-04-23
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an offset between the filtered and unfiltered beam.
Embodiments are also contemplated wherein a mass
filter according to the preferred embodiment may be
coupled to another device such as an ion trap. The mass
filter may be used primarily to reduce the divergence of
an ion beam and indeed the mass filter may be operated
in a non-mass filtering mode of operation wherein the
device acts solely as an ion guide and transmits
substantially all ions received at the entrance to the
mass filter.
