Note: Descriptions are shown in the official language in which they were submitted.
CA 02430531 2003-05-30
MASS SPECTROMETER
The present invention relates to a mass
spectrometer and a method of mass spectrometry.
Mass spectrometers are known having an RE' ion guide
which comprises a multipole rod set wherein ions are
radially confined within the ion guide by the
application of an RF voltage to the rods. The RF
voltage applied between neighbouring electrodes produces
a pseudo-potential well or valley which radially
confines ions within the ion guide.
RF ion guides are used, for example, to transport
ions from an atmospheric pressure ion source through a
vacuum chamber maintained at an intermediate pressure
e.g. 0.001-10 mbar to a mass analyser maintained in a
vacuum chamber at a relatively low pressure. Mass
analysers which must be operated in a low pressure
vacuum chamber include quadrupole ion traps, quadrupole
mass filters, Time of Flight mass analysers, magnetic
sector mass analysers and Fourier Transform Ion
Cyclotron Resonance ("FTICR") mass analysers. The RF
ion guides can efficiently transport ions despite the
ions undergoing many collisions with gas molecules which
cause the ions to be scattered and to lose energy since
the RF radial confinement ensures that ions are not lost
from the ion guide.
It is desired to provide an improved ion guide.
According to an aspect of the present invention
there is provided a mass spectrometer comprising:
a device which repeatedly generates or releases
packets of ions in a substantially pulsed manner; and
an ion guide comprising a plurality of electrodes,
the ion guide being arranged to receive packets of ions
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generated or released from the device and wherein in use
one or more packets of ions generated or released from
the device are trapped in one or more axial trapping
regions within the ion guide and wherein the one or more
axial trapping regions are translated along at least a
portion of the axial length of the ion guide and ions
are then released from the one or more axial trapping
regions so that ions exit the ion guide in a
substantially pulsed manner.
A characteristic of the preferred ion guide that
distinguishes it from other ion guides is that ions exit
the ion guide in a pulsed manner. This will be true
irrespective of whether the ion beam entering the ion
guide is continuous or pulsed. Hence the preferred ion
guide may be used to convert a continuous beam of ions
into a pulsed beam of ions. Furthermore, the preferred
ion guide may be used to transport a series of ion
packets without allowing the ions to become dispersed
and merged one with the next.
The pulsed nature of ions emitted from the ion
guide advantageously allows the detection system to be
phase locked with the ion pulses. For example, the
detection system response may be modulated or pulsed in
the same way the ion beam is modulated or pulsed. This
provides a means of improving the signal to noise of the
ion detection system since any continuous noise, white
noise or DC offset in the detection system can be
essentially eliminated from the detected signal.
The preferred ion guide may be advantageously
interfaced with a discontinuous mass analyser. For
example, the pulsing of an orthogonal acceleration Time
of Flight mass spectrometer may be arranged to be
synchronised with the frequency of a DC potential
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waveform passing along the ion guide to maximise the
duty cycle for ions of a particular range of mass to
charge ratios. The range of masses for which the duty
cycle is maximised will be determined by the distance
from the exit of the ion guide to the orthogonal
acceleration region, the energy of the ions and the
phase shift between that of the travelling DC waveform
applied to the ion guide and that of the pulsing of the
orthogonal acceleration Time of Flight mass
spectrometer.
According to a first main embodiment a mass
spectrometer is provided having an ion guide downstream
of a device which repeatedly generates or releases
packets of ions in a substantially pulsed manner. For
is example, the device may comprise a pulsed ion source,
such as a Laser Desorption or ablation source or a
Matrix Assisted Laser Desorption Ionisation. ("MALDI")
ion source. Alternatively, the device may comprise an
ion trap wherein ions are released from the ion trap in
a pulsed manner.
According to another aspect of the present
invention there is provided a mass spectrometer
comprising:
a device which generates or provides ions in a
substantially continuous manner; and
an ion guide comprising a plurality of electrodes,
the ion guide being arranged to receive the ions from
the device and wherein in use the ions received from the
device are trapped in one or more axial trapping regions
within the ion guide and wherein the one or more axial
trapping regions are translated along at least a portion
of the axial length of the ion guide and ions are then
released from the one or more axial trapping regions so
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that ions exit the ion guide in a substantially pulsed
manner; and an ion detector, said ion detector being arranged
to be substantially phase locked in use with the pulses of
ions emerging from the exit of the ion guide.
According to the second main embodiment of the
present invention the device may comprise a continuous
ion source e.g. an Electrospray ("ESI") ion source, an
Atmospheric Pressure Chemical Ionisation ("APCI") ion
source, an Atmospheric Pressure Photo Ionisation
("APPI") ion source, an Inductively Coupled Plasma
("ICP") ion source, an Electron Impact ("EI") ion
source, an Chemical Ionisation ("CI") ion source, a Fast
Atom Bombardment ("FAB") ion source or a Liquid
Secondary Ions Mass Spectrometry ("LSIMS") ion source.
The device may according to a less preferred
embodiment comprise a pulsed ion source in combination
with a dispersing means for dispersing ions emitted by
the pulsed ion source. The dispersed ions may therefore
arrive at the ion guide in a substantially continuous or
pseudo-continuous manner.
According to both main embodiments ions being
transmitted through the ion guide are preferably
substantially not fragmented within the ion guide.
Accordingly, at least 50%, 60%, 70%, 80%, 90% or 95% of the
ions entering the ion guide are arranged to have, in use, an
energy less than 10 eV for a singly charged ion or less than
20 eV for a doubly charged ion such that the ions are
substantially not fragmented within the ion guide.
A potential barrier between two or more trapping
regions may be removed so that the two or more trapping
regions become a single trapping region.
A potential barrier between two or more trapping
regions may be lowered so that at least some ions are
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able to be move between the two or more trapping
regions.
According to the preferred embodiment one or more
transient DC voltages or one or more transient DC
voltage waveforms are progressively applied to the
electrodes so that ions trapped within one or more axial
trapping regions are urged along the ion guide.
An axial voltage gradient may be maintained along
at least a portion of the length of the ion guide
wherein the axial voltage gradient varies with time
whilst ions are being transmitted through the ion guide.
The ion guide may comprise a first electrode held
at a first reference potential, a second electrode held
at a second reference potential, and a third electrode
held at a third reference potential, wherein:
at a first time tl a first DC voltage is supplied
to the first electrode so that the first electrode is
held at a first potential above or below the first
reference potential;
at a second later time t2 a second DC voltage is
supplied to the second electrode so that the second
electrode is held at a second potential above or below
the second reference potential; and
at a third later time t3 a third DC voltage is
supplied to the third electrode so that the third
electrode is held at a third potential above or below
the third reference potential.
Preferably, at the first time tl the second
electrode is at the second reference potential and the
third electrode is at the third reference potential;
at the second time t2 the first electrode is at the
first potential and the third electrode is at the third
reference potential;
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at the third time t3 the first electrode is at the
first potential and the second electrode is at the
second potential.
Alternatively, at the first time tl the second
electrode is at the second reference potential and the
third electrode is at the third reference potential;
at the second time t2 the first electrode is no
longer supplied with the first DC voltage so that the
first electrode is returned to the first reference
potential and the third electrode is at the third
reference potential; and
at the third time t3 the second electrode is no
longer supplied with the second DC voltage so that the
second electrode is returned to the second reference
potential and the first electrode is at the first
reference potential.
The first, second and third reference potentials
are preferably substantially the same. Similarly, the
first, second and third DC voltages may be substantially
the same. The first, second and third potentials may
also be substantially the same.
The ion guide may comprise -3, 4, 5, 6, 7, 8, 9, 10,
11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24,
25, 26, 27, 28, 29, 30 or >30 segments, wherein each
segment comprises 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12,
13, 14,.15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26,
27, 28, 29, 30 or >30 electrodes and wherein the
electrodes in a segment are maintained at substantially
the same DC potential. A plurality of segments may be
maintained at substantially the same DC potential.
Each segment may be maintained at substantially the
same DC potential as the subsequent nth segment wherein
n is 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16,
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17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30
or >30.
Ions are preferably confined radially within the
ion guide by an AC or RF electric field. Ions are
preferably radially confined within the ion guide in a
pseudo-potential well and are constrained axially by a
real potential barrier or well.
According to the preferred embodiment the transit
time of ions through the ion guide is selected from the
group consisting of: (i) less than or equal to 20 ms;
(ii) less than or equal to 10 ms; (iii) less than or
equal to 5 ms; (iv) less than or equal to 1 ms; and (v)
less than or equal to 0.5 ms.
The ion guide is preferably maintained at a
pressure selected from the group consisting of: (i)
greater than or equal to 0.0001 mbar; (ii) greater than
or equal to 0.0005 mbar; (iii) greater than or equal to
0.001 mbar; (iv) greater than or equal to 0.005 mbar;
(v) greater than or equal to 0.01 mbar; (vi) greater
than or equal to 0.05 mbar; (vii) greater than or equal
to 0.1 mbar; (viii) greater than or equal to 0.5 mbar;
(ix) greater than or equal to 1 mbar; (x) greater than
or equal to 5 mbar; and (xi) greater than or equal to 10
mbar.
The ion guide is preferably maintained at a
pressure selected from the group consisting of: (i) less
than or equal to 10 mbar; (ii) less than or equal to 5
mbar; (iii) less than or equal to 1 mbar; (iv) less than
or equal to 0.5 mbar; (v) less than or equal to 0.1
mbar; (vi) less than or equal to 0.05 mbar; (vii) less
than or equal to 0.01 mbar; (viii) less than or equal to
0.005 mbar; (ix) less than or equal to 0.001 mbar; (x)
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less than or equal to 0.0005 mbar; and (xi) less than or
equal to 0.0001 mbar.
The ion guide is preferably maintained, in use, at
a pressure selected from the group consisting of: (i)
between 0.0001 and 10 mbar; (ii) between 0.0001 and 1
mbar; (iii) between 0.0001 and 0.1 mbar; (iv) between
0.0001 and 0.01 mbar; (v) between 0.0001 and 0.0'01 mbar;
(vi) between 0.001 and 10 mbar; (vii) between 0.001 and
1 mbar; (viii) between 0.001 and 0.1 mbar; (ix) between
0.001 and 0.01 mbar; (x) between 0.01 and 10 mbar; (xi)
between 0.01 and 1 mbar; (xii) between 0.01 and 0.1
mbar; (xiii) between 0.1 and 10 mbar; (xiv) between 0.1
and 1 mbar; and (xv) between 1 and 10 mbar.
According to the preferred embodiment the ion guide
is maintained, in use, at a pressure such that a viscous
drag is imposed upon ions passing through the ion guide.
Preferably, one or more transient DC voltages or
one or more transient DC voltage waveforms are initially
provided at a first axial position and are then
subsequently provided at second, then third different
axial positions along the ion guide.
Preferably, one or more transient DC voltages or
one or more transient DC voltage waveforms move in use
from one end of the ion guide to another end of the ion
guide so that. ions are urged along the ion guide.
The one or more transient DC voltages may create:
(i) a potential hill or barrier; (ii) a potential well;
(iii) multiple potential hills or barriers; (iv)
multiple potential wells; (v) a combination of a
potential hill or barrier and a potential well; or (vi)
a combination of multiple potential hills or barriers
and multiple potential wells. The one or more transient
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DC voltage waveforms may comprise a repeating waveform
such as a square wave.
The amplitude of the one or more transient DC
voltages or the one or more transient DC voltage
waveforms may remain substantially constant with time.
Alternatively, the amplitude of the one or more
transient DC voltages or the one or more transient DC
voltage waveforms may vary with time. For example, the
amplitude of the one or more transient DC voltages or
the one or more transient DC voltage waveforms may
either: (i) increase with time; (ii) increase then
decrease with time; (iii) decrease with time; or (iv)
decrease then increase with time.
The ion guide may comprise an upstream entrance
region, a downstream exit region and an intermediate
region, wherein:
in the entrance region the amplitude of the one or
more transient DC voltages or the one or more transient
DC voltage waveforms has a first amplitude;
in the intermediate region the amplitude of the one
or more transient DC voltages or the one or more
transient DC voltage waveforms has a second amplitude;
and
in the exit region the amplitude of the one or more
transient DC voltages or the one or more transient DC
voltage waveforms has a third amplitude.
The entrance and/or exit region may comprise a
proportion of the total axial length of the ion guide
selected from the group consisting of: (i) < 5%; (ii) 5-
10%; (iii) 10-15%; (iv) 15-20%; (v) 20-25%; (vi) 25-30%;
(vii) 30-35%; (viii) 35-40%; and (ix) 40-45%.
Preferably, the first and/or third amplitudes are
substantially zero and the second amplitude is
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substantially non-zero. The second amplitude is
preferably larger than the first amplitude and/or the
second amplitude is preferably larger than the third
amplitude.
5 Preferably, the one or more transient DC voltages
or one or more transient DC voltage waveforms pass in
use along the ion guide with a first velocity and
wherein the first velocity: (i) remains substantially
constant; (ii) varies; (iii) increases; (iv) increases
10 then decreases; (v) decreases; (vi) decreases then
increases; (vii) reduces to substantially zero; (viii)
reverses direction; or (ix) reduces to substantially
zero and then reverses direction.
The one or more transient DC voltages or the one or
more transient DC voltage waveforms may cause ions
within the ion guide to pass along the ion guide with a
second velocity.
The difference between the first velocity and the
second velocity is preferably less than or equal to 100
m/s, 90 m/s, 80 m/s, 70 m/s, 60 m/s, 50 m/s, 40 m/s, 30
m/s, 20 m/s, 10 m/s, 5 m/s or 1 m/s.
Preferably, the first velocity is selected from the
group consisting of: (i) 10-250 m/s; (ii) 250-500 m/s;
(iii) 500-750 m/s; (iv) 750-1000 m/s; (v) 1000-1250 m/s;
(vi) 1250-1500 m/s; (vii) 1500-1750 m/s; (viii) 1750-
2000 m/s; (ix) 2000-2250 m/s; (x) 2250-2500 m/s; (xi)
2500-2750 m/s; and (xii) 2750-3000 m/s.
Preferably, the second velocity is selected from
the group consisting of: (i) 10-250 m/s; (ii) 250-500
m/s; (iii) 500-750 m/s; (iv) 750-1000 m/s; (v) 1000-1250
m/s; (vi) 1250-1500 m/s; (vii) 1500-1750 m/s; (viii)
1750-2000 m/s; (ix) 2000-2250 m/s; (x) 2250-2500 m/s;
(xi) 2500-2750 m/s; and (xii) 2750-3000 m/s.
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The second velocity is preferably substantially the
same as the first velocity.
The one or more transient DC voltages or the one or
more transient DC voltage waveforms preferably have a
frequency, and wherein the frequency: (i) remains
substantially constant; (ii) varies; (iii) increases;
(iv) increases then decreases; (v) decreases; or (vi)
decreases then increases.
The one or more transient DC voltages and the one
or more transient DC voltage waveforms preferably have a
wavelength, and wherein the wavelength: (i) remains
substantially constant; (ii) varies; (iii) increases;
(iv) increases then decreases; (v) decreases; or (vi)
decreases then increases.
According to an embodiment two or more transient DC
voltages or two or more transient DC voltage waveforms
pass simultaneously along the ion guide.
The two or more transient DC voltages or the two or
more transient DC voltage waveforms may be arranged to
move: (i) in the same direction; (ii) in opposite
directions; (iii) towards each other; (iv) away from
each other.
Preferably, one or more transient DC voltages or
one or more transient-DC voltage waveforms are
repeatedly generated and passed in use along the ion
guide, and wherein the frequency of generating the one
or more transient DC voltages or the one or more
transient DC voltage waveforms: (1) remains
substantially constant; (ii) varies; (iii) increases;
(iv) increases then decreases; (v) decreases; or (vi)
decreases then increases.
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The mass spectrometer preferably further comprises
a Time of Flight mass analyser comprising an electrode
for injecting ions into a drift region, the electrode
being arranged to be energised in use in a substantially
synchronised manner with the pulses of ions emerging
from the exit of the ion guide.
The mass spectrometer may further comprise an ion
trap arranged downstream of the ion guide, the ion trap
being arranged to store and/or release ions from the ion
trap in a substantially synchronised manner with the
pulses of ions emerging from the exit of the ion guide.
The mass spectrometer may further comprise a mass
filter arranged downstream of the ion guide, wherein a
mass to charge ratio transmission window of the mass
filter is varied in a substantially synchronised manner
with the pulses of ions emerging from the exit of the
ion guide.
The ion guide may comprise an ion funnel comprising
a plurality of electrodes having apertures therein
through which ions are transmitted, wherein the diameter
of the apertures becomes progressively smaller or
larger. Alternatively, the ion guide may comprise an
ion tunnel comprising a plurality of electrodes having
apertures therein through which ions are transmitted,
wherein the diameter of the apertures remains
substantially constant. The ion guide may comprise a
stack of plate, ring or wire loop electrodes.
Each electrode preferably has an aperture through
which ions are transmitted in use. Each electrode
preferably has a substantially circular aperture. Each
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electrode preferably has a single aperture through which
ions are transmitted in use.
The diameter of the apertures of at least 50%, 60%,
70%, 80%, 90% or 95% of the electrodes forming the ion
guide is preferably selected from the group consisting
of: (i) less than or equal to 10 mm; (ii) less than or
equal to 9 mm; (iii) less than or equal to 8 mm; (iv)
less than or equal to 7 mm; (v) less than or equal to 6
mm; (vi) less than or equal to 5 mm; (vii) less than or
equal to 4 mm; (viii) less than or equal to 3 mm; (ix)
less than or equal to 2 mm; and (x) less than or equal
to 1 mm.
Preferably, at least 50%, 60%, 70%, 80%, 90% or 95%
of the electrodes forming the ion guide have apertures
which are substantially the same size or area.
According to a less preferred embodiment the ion
guide may comprise a segmented rod set.
The ion guide may consist of: (i) 10-20 electrodes;
(ii) 20-30 electrodes; (iii) 30-40 electrodes; (iv) 40-
50 electrodes; (v) 50-60 electrodes; (vi) 60-70
electrodes; (vii) 70-80 electrodes; (viii) 80-90
electrodes; (ix) 90-100 electrodes; (x) 100-110
electrodes; (xi) 110-120 electrodes; (xii) 120-130
electrodes; (xiii) 130-140 electrodes; (xiv) 140-150
electrodes; or (xv) more than 150 electrodes.
Preferably, the thickness of at least 50%, 60%,
70%, 80%, 90% or 95% of the electrodes is selected from
the group consisting of: (i) less than or equal to 3 mm;
(ii) less than or equal to 2.5 mm; (iii) less than or
equal to 2.0 mm; (iv) less than or equal to 1.5 mm; (v)
less than or equal to 1.0 mm; and (vi) less than or
equal to 0.5 mm.
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The ion guide preferably has a length selected from
the group consisting of: (i) less than 5 cm; (ii) 5-10
cm; (iii) 10-1.5 cm; (iv) 15-20 cm; (v) 20-25 cm; (vi)
25-30 cm; and (vii) greater than 30 cm.
Preferably, at least 10%, 20%, 30%, 40%, 50%, 60%,
70%, 80%, 90%, or 95% of the electrodes are connected to
both a DC and an AC or RF voltage supply.
Preferably, axially adjacent electrodes are
supplied with AC or RF voltages having a phase
difference of 180 .
According to another aspect of the present
invention there is provided a method of mass
spectrometry comprising:
repeatedly generating or releasing packets of ions
in a substantially pulsed manner;
receiving one or more packets of ions in an ion
guide comprising a plurality of electrodes;
trapping the one or more packets of ions in one or
more axial trapping regions within the ion guide;
translating the one or more axial trapping regions
along at least a portion of the axial length of the ion
guide;
releasing ions from the one or more axial trapping
regions so that ions exit the ion guide in a substantially
pulsed manner; and phase locking an ion detector to the
pulses of ions emerging from the exit of the ion guide.
According to another aspect of the present
invention there is provided a method of mass
spectrometry comprising:
generating or providing ions in a substantially
continuous manner;
receiving the ions in an ion guide comprising a
plurality of electrodes;
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trapping the ions in one or more axial trapping
regions within the ion guide;
translating the one or more axial trapping regions
along at least a portion of the axial length of the ion
guide;
releasing ions from the one or more axial trapping
regions so that ions exit the ion guide in a
substantially pulsed manner; and phase locking an ion
detector to the pulses of ions emerging from the exit of
the ion guides.
Preferably, the method further comprises
synchronising the energisation of an electrode for
injecting ions into a drift region of a Time of Flight
mass analyser to pulses of ions emerging from the exit
of the ion guide.
Preferably, the method further comprises
synchronising the storing and/or releasing of ions in an
ion trap arranged downstream of the ion guide with the
pulses of ions emerging from the exit of the ion guide.
Preferably, the method further comprises
synchronising varying the mass to charge ratio
transmission window of a mass. filter arranged downstream
of the ion guide with the pulses of ions emerging from
the exit of the ion guide.
A repeating pattern of electrical DC potentials may
be superimposed along the length of the ion guide so
that a DC periodic waveform is formed. The DC potential
waveform is arranged to travel along the ion guide in
the direction and at a velocity at which it is desired
to move ions along the ion guide.
The preferred ("travelling wave") ion guide may
comprise an AC or RF ion guide such as a multipole rod
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16 _
set or stacked ring set which is segmented in the axial
direction so that independent transient DC potentials
may be applied to each segment. The transient DC
potentials are superimposed on top of the RF confining
voltage and any constant DC offset voltage- The DC
potentials are changed temporally to generate a
travelling DC potential wave in the axial direction.
At any instant in time a voltage gradient is
generated between segments which acts to push or pull
ions in a certain direction. As the voltage gradient
moves in the required direction so do the ions. The
individual DC voltages on each of the segments may be
programmed to create a required waveform. Furthermore,
the individual DC voltages on each of the segments may
be programmed to change in synchronism so that the DC
potential waveform is maintained but shifted in the
direction in which it is required to move the ions.
The DC potential waveform may be superimposed on
any nominally imposed constant axial. DC voltage offset.
No constant axial DC voltage gradient is required
although the travelling DC wave may less preferably be
provided in conjunction with an axial DC voltage
gradient.
The transient DC voltage applied to each segment
may be above or below that of a constant DC voltage
offset applied to the electrodes forming the ion guide.
The transient DC voltage causes the ions to move in the
axial direction.
The transient DC voltages applied to each segment
may be programmed to change continuously or in a series
of steps. The sequence of voltages applied to each
segment may repeat at regular intervals or at intervals
that may progressively increase or decrease. The time
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over which the complete sequence of voltages is applied
to a particular segment of the ion guide is the cycle
time T. The inverse of the cycle time is the wave
frequency f. The distance along the AC or RF ion guide
over which the travelling DC waveform repeats itself is
the wavelength A. The wavelength divided by the cycle
time is the velocity vwave of the travelling DC potential
wave. Hence, the travelling wave velocity vd,:,,,C:
vw~ =T=A,f
Under correct operation the velocity of the ions
will be equal to that of the travelling DC potential
wave. For a given wavelength the travelling DC wave
velocity may be controlled by selection of the cycle
time. If the cycle time T progressively increases then
the velocity of the travelling DC wave will
progressively decrease. The optimum velocity of the
travelling DC. potential wave may depend upon the mass of
the ions and the pressure and composition of the
background gas.
The travelling wave ion guide may be used at
intermediate pressures between 0.0001 and 100 mbar,
preferably between 0.001 and 10 mbar, for which the gas
density will be sufficient to impose a viscous drag on
the ions. The gas at these pressures will appear as a
viscous medium to the ions and will act to slow the
ions. The viscous drag resulting from frequent
collisions with gas molecules will prevent the ions from
building up excessive velocity. Consequently the ions
will tend to ride on the travelling DC wave rather than
run ahead of the wave and execute excessive oscillations
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within the travelling or translating potential wells
which could lead to ion fragmentation.
The presence of the gas will impose a maximum
velocity at which the ions will travel through the gas
for a given field strength. The higher the gas pressure
the more frequent the ion-molecule collisions and the
slower the ions will travel for a given field strength.
Furthermore, the energy of the ions will be dependent
upon their mass and the square of their velocity. If
fragmentation is to be avoided then the energy of the
ions is preferably kept below a particular value usually
below 5-10 eV. This consideration may impose a limit on
the travelling wave velocity.
Since the preferred ion guide produces a pulsed
beam of ions the repetition rate of the ion guide can be
tailored to that of a mass analyser in terms of scan
rates and acquisition times. For example, in a scanning
quadrupole system the repetition rate may be high enough
to prevent pulsing across the mass range. In a triple
quadrupole tandem mass spectrometer operating in a MRM
mode the repetition frequency may be compatible with the
reaction monitoring dwell times- In a quadrupole Time
of Flight tandem mass spectrometer the repetition
frequency may be synchronised with the pusher pulses of
the Time of Flight mass analyser to maximise ion
sampling duty cycle and hence sensitivity.
Under conditions of intermediate gas pressures
where ion-molecule collisions are likely to occur the
travelling wave ion guide provides a means of ensuring
ions exit the RF ion guide and of reducing their transit
times.
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Various embodiments of the present invention will
now be described, by way of example only, and with
reference to the accompanying drawings in which:
Fig. 1 shows a preferred ion guide; and
Fig. 2A shows a waveform with a single potential
hill or barrier, Fig. 2B shows a waveform with a single
potential well, Fig. 2C shows a waveform with a single
potential well followed by a potential hill or barrier,
Fig. 2D shows a DC potential waveform with a repeating
potential hill or barrier and Fig. 2E shows another DC
potential waveform;
Fig. 3 illustrates how a repeating transient DC
voltage waveform may be generated;
Fig. 4 shows an embodiment of the present
invention; and
Fig. 5 shows a graph illustrating the arrival time
T1 of ions arriving at a preferred ion guide, the time T2
that the ions exit the preferred ion guide and the
arrival time T3 of the ions at a pusher electrode of an
orthogonal acceleration Time of Flight mass analyser for
ions of varying mass to charge ratio.
As shown in Fig. 1 the preferred embodiment relates
to an AC or RF ion guide I comprising a plurality of
electrodes 2. Ions arrive at an entrance 3 to the ion
guide 1 and leave the ion guide 1 via an exit 4. The
ion guide 1 may comprise a plurality of segments, each
segment comprising one or more electrodes 2. The DC
voltage applied to each segment may be programmed to
change continuously or in a series of steps. The
sequence of DC voltages applied to each segment may
repeat at regular intervals or at intervals which may
progressively increase or decrease. The time over which
the complete sequence of DC voltages is applied to a
CA 02430531 2003-05-30
particular segment is the cycle time T. The inverse of
the cycle time is the wave frequency f. The distance
along the AC or RF ion guide 1 over which the DC
potential waveform repeats itself is the wavelength X.
5 The wavelength divided by the cycle time is the velocity
vW,,ve of the wave. Hence, the travelling wave velocity:
"wavef
10 According to the preferred embodiment the velocity
of the DC potential waveform which is progressively
applied along the length of the ion guide 1 is arranged
to substantially equal that of the ions arriving at the
ion guide. For a given wavelength, the travelling wave
15 velocity may be controlled by selection of the cycle
time. If the cycle time T progressively increases then
the velocity of the DC potential waveform will
progressively decrease. The optimum velocity of the
travelling DC potential waveform may depend on the mass
20 of the ions and the pressure and composition of the gas
in the ion guide 1.
The travelling wave ion guide 1 may be operated at
intermediate pressures between 0.0001 and 100 mbar,
preferably between 0.001 and 10 mbar, wherein the gas
density will be sufficient to impose a viscous drag on
the ions. The gas at these pressures will appear as a
viscous medium to the ions and will act to slow the
ions. The viscous drag resulting from frequent
collisions with gas molecules prevents the ions from
building up excessive velocity. Consequently, the ions
will tend to ride on or with the travelling DC potential
waveform rather than run ahead of the DC potential
CA 02430531 2003-05-30
- 21 -
waveform and execute excessive oscillations within the
potential wells which are being translated along the
length of the ion guide 1.
The presence of a gas in the ion guide 1 imposes a
maximum velocity at which the ions will travel through
the gas for a given field strength. The higher the gas
pressure the more frequent the ion-molecule collisions
and the slower the ions will travel for a given field
strength. Furthermore, the energy of the ions will be
dependent upon their mass and the square of their
velocity. If fragmentation is not desired, then the
energy of the ions is preferably kept below about 5-10
eV. This may impose a limit on the velocity of the DC
potential waveform. Consequently, the optimum DC
potential wave velocity will vary with the mass of the
ion, the gas pressure and whether it is desired to
transport ions with minimal fragmentation or to fragment
ions.
A feature of the preferred ion guide 1 is that it
emits a pulsed beam of ions. The repetition rate of the
pulses of ions can be tailored to a mass analyser
downstream of the ion guide 1 in terms of scan rates and
acquisition times. For example, in a scanning
quadrupole system the repetition rate can be made high
enough to prevent pulsing across the mass range. In a
triple quadrupole tandem mass spectrometer operating in
a MRM mode the repetition frequency may be made
compatible with the reaction monitoring dwell times.
With a quadrupole Time of Flight tandem mass
spectrometer, the repetition frequency may be
synchronised with the pusher pulses on the Time of
Flight mass analyser to maximise ion sampling duty cycle
and hence sensitivity.
CA 02430531 2003-05-30
- 22 -
The pulses of ions emitted from the ion guide 1 may
also be synchronised with the operation of an ion trap
or mass filter.
According to one embodiment the transient DC
potential waveform applied to the ion guide 1 may
comprise a square wave. The amplitude of the DC
waveform may become progressively attenuated towards the
entrance of the ion guide 1 i.e. the amplitude of the
travelling potential DC waveform may grow to its full
amplitude over the first few segments of the travelling
wave ion guide 1. This allows ions to be introduced
into the ion guide 1 with minimal disruption to their
sequence. A continuous ion beam arriving at the
entrance 3 to the ion guide 1 will advantageously exit
the ion guide 1 as a series of pulses.
One example of an advantage to be gained from
converting a continuous beam of ions into a pulsed beam
of ions is that it allows the detection system to be
phase locked with the ion pulses. The detection system
response may be modulated or pulsed in the same way the
ion beam is modulated or pulsed. This provides a means
of improving the signal to noise of the ion detection
system since any continuous noise, white noise, or DC
offset in the detection system may be substantially
eliminated from the detected signal.
Another example of an advantage to be gained from
converting a continuous beam of ions into a pulsed beam
of ions is that gained when the travelling wave ion
guide 1 is interfaced to a discontinuous mass analyser.
For example, the pulsing of an orthogonal acceleration
Time of Flight mass spectrometer may be synchronised
with the travelling wave frequency to maximise the duty
cycle for ions having a particular range of mass to
CA 02430531 2003-05-30
23
charge ratios. The range of masses for which the duty
cycle is maximised will be determined by the distance
from the exit of the travelling wave ion guide 1 to the
orthogonal acceleration region, the energy of the ions
and the phase shift between that of the travelling
waveform and that of the pulsing of the orthogonal
acceleration Time of Flight mass spectrometer.
A further advantage of the preferred ion guide 1 is
that a pulse of ions arriving at the entrance to the
travelling wave ion guide 1 can be arranged to also exit
the ion guide 1 as a pulse of ions. The pulse of ions
arriving at the travelling wave ion guide 1 is
preferably synchronised with the travelling waveform so
that the ions arrive at the optimum phase of-that
waveform. In other words, the arrival of the ion pulse
should preferably coincide with a particular phase of
the waveform. This characteristic of the travelling
wave ion guide 1 is an advantage when used with a pulsed
ion source, such as a laser ablation source or MALDI
source or when ions have been released from an ion trap
and it is desired to substantially prevent the pulse of
ions from becoming dispersed and broadened. The
preferred embodiment is therefore particularly
advantageous for transporting ions to an ion trap or to
a discontinuous mass analyser such as a quadrupole ion
trap, FTICR mass analyser or Time of Flight mass
analyser.
An ion guide 1 according to a preferred embodiment
comprises a stacked ring AC or RF ion guide. The
complete stacked ring set is preferably 180 mm long and
is made from 120 stainless steel rings each preferably
0.5 mm thick and spaced apart by 1 mm. The internal
aperture in each ring is preferably 5 mm in diameter.
CA 02430531 2003-05-30
24 -
The frequency of the RF supply is preferably 1.75 MHz
and the peak RF voltage may be varied up to 500. The
stacked ring ion guide 1 may be mounted in an enclosed
collision cell chamber positioned between two quadrupole
mass filters in a triple quadrupole mass spectrometer.
The pressure in the enclosed collision cell chamber may
be varied up to 0.01 mbar. The stacked ring RF ion
guide is preferably electrically divided into 15
segments each 12 mm long and consisting of 8 rings.
Three different DC voltages may be connected to every
third segment so that a sequence of voltages applied to
the first three segments is repeated a further four
times along the whole length of the stacked ring set.
The three DC voltages applied to every third segment may
be independently programmed up to 40 volts. The
sequence of voltages applied to each segment preferably
creates a waveform with a potential hill, repeated five
times throughout the length of the stacked ring set.
Hence the wavelength of the travelling waveform is
preferably 36 mm (3 x 12 mm). The cycle time for the
sequence of voltages on any one segment is preferably 23
usec and hence the wave velocity is preferably 1560 m/s
(36 mm/23 us).
The operation of a travelling wave ion guide 1 will
now be described with reference to Fig. 3. The
preferred embodiment preferably comprises 120 electrodes
but 48 electrodes are shown in Fig. 3 for ease of
illustration.
Alternate electrodes are preferably fed with
opposite phases of a RF supply (preferably 1 MHz and 500
V p-p). The ion guide 1 may be divided into separate
groups of electrodes (6 groups of electrodes are shown
in Fig. 3). The electrodes in each group may be fed
CA 02430531 2003-05-30
- 25 -
from separate secondary windings on a coupling
transformer as shown in Fig. 3. These are connected so
that all the even-numbered electrodes are 1800 out of
phase with all the odd-numbered electrodes. Therefore,
at the point in the RF cycle when all the odd numbered
electrodes are at the peak positive voltage, all the
even-numbered electrodes are at the peak negative
voltage.
Groups of electrodes at each end of the ion guide 1
(e.g. electrodes #1-6 and #43-48) may be supplied with
RF only potentials whereas the central groups (e.g.
electrodes #7-12, #13-18, #19-24, #25-30, #31-36 and
#37-42) may be supplied with both RF and DC potentials.
Electrodes #1, #3, #5, #43, #45 and #47 may be connected
to one pole of the secondary winding CTS and electrodes
#2, #4, #6, #44, #46 and #48 may be connected to the
opposite end of winding CT7 to ensure the correct RF
phasing of the electrodes. The other ends of these
windings are connected to the 0 V DC reference so that
only RF potentials are applied to the end groups of
electrodes. Electrodes #7, #13, #19, #24, #31 and #37
which are the first electrodes of each of the central
groups are connected together and fed from secondary
winding CT6. Windings CTS, CT4, CT3, CT2 and CT1
respectively supply the second through sixth electrodes
of each of central groups. Each of windings CT1-6 is
referred to a different DC reference point shown
schematically by the 2-gang switch in Fig. 3 so that the
first through sixth sets of electrodes of the central
groups of electrodes can be supplied with a DC potential
selected by the switch, as well as the RF potentials.
In the preferred mode of operation only one set of
interconnected electrodes comprised in the central
CA 02430531 2003-05-30
- 26 -
groups is supplied with a DC voltage at any given
instant. All the other windings are referenced to OV DC
at that particular instant. For example, with the
switch in the position illustrated in Fig. 3, winding
CT6 of the transformer may be connected to the DC supply
biasing all the first electrodes (e.g. electrodes #7,
#13, #19 etc.) of the central groups relative to all
other electrodes.
If the switch is then moved to the next position,
winding CT5 is connected to the DC supply, biasing all
the second electrodes (e.g. electrodes #8, #14, #20
etc.) while the first electrodes (e.g. electrodes #7,
#13, #19 etc.) are returned to 0 V DC.
When used as a travelling wave ion guide 1 the
switch can be effectively rotated continuously biasing
in turn the first through sixth electrodes and then
repeating the sequence without interruption. A
mechanical switch is shown in Fig. 3 for sake of
illustration. Electronic switching may more preferably
be used to carry out the switching. Each transformer
winding CT1--8 may be fed by a Digital to Analogue
Converter which can apply the desired DC potential to
the winding under computer control.
Typical operating conditions may have an RF peak-
to-peak voltage of 500 V, an RF frequency of 1 MHz, a DC
bias of +5 V (for positive ions) and a switching
frequency of 10-100 kHz.
If a positive ion enters the ion guide 1 when the
switch is in the position shown in Fig. 3 and. a positive
DC potential is applied to electrode #7 then the ion
will encounter a potential barrier at electrode #7 which
prevents its further passage along the ion guide 1
(assuming that its translational kinetic energy is not
CA 02430531 2003-05-30
- 27 -
too high). As soon as the switch moves to the next
position, however, this potential barrier will shift to
electrode #8 and then electrode #9, #10, #11 and #12
upon further rotation of the switch.. This allows the
ion to move further along the ion guide 1. on the next
cycle of operation of the switch, the barrier in front
of the ion moves to electrode #13 and a new potential
barrier now appears on electrode #7 behind the ion. The
ion therefore becomes contained or otherwise trapped in
a potential well between the potential barriers on
electrodes #7 and #13. Further rotation of the switch
moves this potential well from electrodes #7-13 to
electrodes #8-14, then #9-15, through to #12-18. A
further cycle of the switch moves this potential well in
increments of one electrode from electrodes #12-18
through to electrodes 418-24. The process repeats
thereby pushing the ion along the ion guide ]. in its
potential well until it emerges into the RF only exit
group of electrodes #43-48 and then subsequently leaves
the ion guide 1.
As a potential well moves along the ion guide 1,
new potential wells capable of containing more ions may
be created and moved along behind it. The travelling
wave ion guide 1 therefore carries individual packets of
ions along its length in the travelling potential wells
while simultaneously the strong focusing action of the
RF field tends to confine the ions to the axial region.
According to a particularly preferred embodiment a
mass spectrometer is provided having two quadrupole mass
filters/analysers and a collision cell. A travelling
wave ion guide 1 may be provided upstream of the first
mass filter/analyser. A transient DC potential waveform
may be applied to the travelling wave ion guide 1 having
CA 02430531 2003-05-30
28
a wavelength of 14 electrodes. The DC voltage is
preferably applied to neighbouring pairs of electrodes 2
and is preferably stepped in pairs. Hence, according to
the preferred embodiment there are seven steps in one
cycle. Therefore, at any one time there are two
electrodes with a transient applied DC voltage followed
by 12 electrodes with no applied DC voltage followed by
two electrodes with a transient applied DC voltage
followed by 12 electrodes with no applied DC voltage
etc.
A buffer gas (typically nitrogen or helium) may be
introduced into the travelling wave ion guide 1. If the
ion guide 1 is used to interface a relatively high
pressure source to a high-vacuum mass analyser or is
used as a collision cell then gas will already be
present in the ion guide 1. The buffer gas is a viscous
medium and is preferably provided to dampen the motion
of the ions. The presence of gas tends to thermalise
the ion translational energies. Therefore, ions
entering the ion guide 1 may become thermalised by
collisional cooling irrespective of the kinetic energy
possessed by the ions and they will be confined in their
potential wells as they travel through the ion guide 1.
Assuming that the potential barriers are sufficiently
high to ensure the ions remain in the potential well,
their transit time through the ion guide 1 will be
independent of both their initial kinetic energy and the
gas pressure. The ion transit time will therefore be
determined solely by the rate at which the potential
wells are moved or translated along the ion guide 1 and
will be a function of the switching rate of the
electrode potentials. This property can be exploited
advantageously in a number of applications and leads to
CA 02430531 2003-05-30
29 -
improvements in performance when compared to instruments
using conventional rod-set guides in which this control
is unavailable.
A particularly preferred embodiment is shown in
Fig. 4. The travelling wave ion guide 1 advantageously
allows the ion transit time to be controlled unlike
other ion guides and in particular allows a MALDI-TOF
instrument to be operated in a very efficient way with
virtually a 100% ion transmission and analysis
efficiency.
A sample to be analysed is coated on a target 10
and is bombarded with photons from a laser 11. Ions so
produced pass through an aperture in an extraction
electrode 12 and then through a travelling wave ion
guide 1 according to the preferred embodiment. On
exiting the travelling wave ion guide.1 they pass
through an exit electrode 13 and enter the pulser 14 of
a Time of Flight mass analyser 15. A linear or a
reflecting Time of Flight mass analyser 15 may be
provided. An orthogonal reflecting type is preferred
and is shown in Fig. 4. Operation of the pulser 14 and
Time of Flight mass analyser 15 is conventional. Gas
(e.g. nitrogen) may be introduced into the travelling
wave ion guide 1 at e.g. a pressure of between 10-3 and 1
mbar in order to provide collisional cooling of the ions
as they are carried through the travelling wave ion
guide 1.
An accelerating region is preferably provided
between the target 10 and the extraction electrode 12
and a 10 V potential gradient may be provided to
accelerate positive ions as shown. This region is
preferably followed by a field-free region 16 between
the extraction electrode 12 and the entrance of the
CA 02430531 2003-05-30
- 30 -
travelling wave ion guide 1. According to an embodiment
the length of the field free region 16 is 250 mm.
Another accelerating field may be provided between
the travelling wave ion guide exit electrode 13 and the
Time of Flight pulser 14, as shown. A 40 V potential
gradient may, for example, be provided in this region.
The accelerating fields and the field-free region
16 interact with the operation of the travelling wave
ion guide 1 to enable a mode of operation which is
highly efficient. The ion source, acceleration regions
and field-free region 16 are preferably maintained at
relatively high vacuum.
It is known that the majority of ions ejected from
the MALDI target 10 will have a range of velocities
typically between about 0.5 and 2.0 times the speed of
sound, on average about 300-400 m/s. This spread in
velocities accounts for the relatively large spread in
ion energies. In the embodiment shown in Fig. 4 an
accelerating field exists between the target 10 and the
extraction electrode 12 so that the ions gain an equal
amount of kinetic energy on passing through the field
which adds a mass dependent component of velocity to
their approximately constant ejection velocity. Since
kinetic energy KE:
KE= MV 2
2
then if the energy is constant, the added velocity is
proportional to 1/-Im.
The ions then enter a field-free drift region 16
between the extraction electrode 12 and the entrance of
the travelling wave ion guide 1 in which they begin to
CA 02430531 2003-05-30
31 -
separate according to their mass to charge ratios
because of the different mass-dependent velocities
imparted to them during the prior acceleration stage.
Consequently, the lightest ions arrive first at the
entrance to the travelling wave ion guide 1. These ions
will enter the travelling wave ion guide 1 and become
trapped in a DC potential well. As that DC potential
well moves or is translated along the length of
travelling wave ion guide 1, a second DC potential well
opens behind it into which some slightly heavier ions
will become trapped. These ions will have taken
slightly longer to reach the travelling wave ion guide
entrance because they will have moved slightly more
slowly through the field free region 16 than the
lightest ions. Thus it will be seen that the combined
effect of the accelerating region, field-free region 16
and the travelling DC potential wells of the travelling
wave ion guide 1 results in a series of DC potential
wells reaching the end of the travelling wave ion guide
1 with each potential well or trapping region containing
ions of similar mass to charge ratios. The first
potential well or trapping region arriving at the exit
of the travelling wave ion guide 1 will contain the
lightest ions, the following potential wells or trapping
regions will contain ions of steadily increasing mass to
charge ratios and the last potential well or trapping
region will contain the heaviest ions from any
particular laser pulse.
Since the ions remain trapped in their potential
wells during their passage or translation through the
traveling wave ion guide 1, the ions preferably do not
mix with ions in different potential wells. Since gas
is present in the travelling wave ion guide 1 this
CA 02430531 2003-05-30
32
results in collisional cooling of the ions in each
potential well whilst the travelling potential well
continues to push the ions forward at a velocity equal
to that of the potential well. Consequently, by the
time the ions reach the end of the travelling wave ion
guide 1 the ions in each potential well will have lost
most of their initial velocity spread even though they
have a bulk velocity equal to that of the potential
well. In other words, their initial relatively large
spread in energy is reduced to that of the thermal
energy of the buffer gas.
When the first potential well (containing the
lightest ions with substantially only thermal energies)
reaches the end of the travelling wave ion guide 1 the
front potential barrier disappears and the rear
potential barrier pushes the ions out of the travelling
wave ion guide 1 into another accelerating field between
the exit of the travelling wave ion guide 1 and the
pusher electrodes of the Time of Flight mass analyser
15. Typically, a gradient of about 40 V may be applied.
This field rapidly accelerates the ions into the pusher
region 14, but because they all start with similar (very
low) kinetic energy and because the potential. well
contains only ions having a limited range of masses, the
ions do not significantly separate in space during this
acceleration. The slowest ions released from the
potential well will therefore still enter the pusher
region 14 before the fastest ions can exit the pusher
region 14. Consequently, if the pusher voltage is
applied at this precise time then all the ions contained
in a particular potential well or trapping region can be
analysed by the Time of Flight mass analyser 15 without
loss. Advantageously, a single TOF push, synchronised
CA 02430531 2003-05-30
33 -
with but delayed from the arrival of a potential well at
the exit of the travelling wave ion guide l may be used
to analyse all the ions in a potential well. The
preferred embodiment is therefore capable of mass
analysing all the ions from a given laser pulse with
virtually a 100% efficiency.
The preferred embodiment can be yet further refined
by varying the travelling wave ion guide switching speed
during the arrival of ions at the travelling wave ion
guide 1 following a laser pulse. The collection of ions
into individual potential wells will proceed with least
disruption to their grouping by mass to charge ratio if
the velocity of the potential wells is arranged to
substantially match the velocities of the ions arriving
at the entrance to the travelling wave ion guide 1. The
ions arriving at the travelling wave ion guide 1 from
each laser pulse will have progressively slower
velocities as the elapsed time from the laser pulse
increases as their velocity is simply the length of the
field free region 16 from the target plate 10 to the
travelling wave ion guide 1 divided by the elapsed time.
Accordingly, the velocity of the potential wells in the
travelling wave ion guide 1 may be continuously reduced
so as to continuously match the velocity of the ions
arriving at the entrance of the travelling wave ion
guide 1. This can be achieved by arranging the
travelling wave ion guide switching time intervals to
increase linearly with elapsed time from the laser
pulse.
As a consequence, the velocities of the ions within
potential wells within the travelling wave ion guide 1
will also preferably continuously reduce. Since the
ions have a natural tendency to slow due to the viscous
CA 02430531 2003-05-30
- 34 -
drag of the collision gas, by appropriate selection of
gas type and pressure the natural slowing of ions due to
viscous drag can be made to substantially match the
slowing velocity of the potential wells in the
travelling wave ion guide 1 thereby reducing the chances
of any ions fragmenting unintentionally in the ion guide
1.
Another advantage of this arrangement is that the
energy of the ions leaving the travelling wave ion guide
1 is approximately constant (otherwise, the energy of
the ions would increase with the increasing mass of the
ions in the later arriving potential wells). The ions
therefore leave the travelling wave ion guide 1 with
substantially the velocity of the potential barriers
moving along the travelling wave ion guide 1. If the
traveling DC wave velocity is kept constant then ions
with higher masses will have greater kinetic energies
than ions with lower masses. However, ions entering an
orthogonal Time of Flight mass analyser 15 should
preferably all have approximately the same energy in
order to avoid spatial separation of ions when they
arrive at the ion detector 17. It is therefore
necessary for all ions to have substantially the same
energy in order to ensure that all the ions ultimately
hit the ion detector 17. This can be achieved by
reducing the velocity of the potential barriers as the
heavier masses arrive at and leave the travelling wave
ion guide 1. If the velocity of the potential wells is
reduced by arranging the travelling wave ion guide
switching time intervals to increase linearly with
elapsed time from the laser pulse, then the ions all
advantageously exit the travelling wave ion guide 1 with
approximately the same energy independent of their mass.
CA 02430531 2003-05-30
in order to allow for the lower velocity of the
higher mass ions, the delay between the arrival of a
potential well at the exit of the travelling wave ion
guide 1 and the operation of the Time of Flight pulser
5 14 is preferably increased in synchronism with the
increased switching time intervals of the travelling
wave ion guide operation.
A theoretical treatment of the effect of gas
collisions in the travelling wave ion guide 1 or the
10 transport of ions in the potential well shows that the
potential well translation velocity (i.e. the switching
speed of the travelling wave ion guide) should be
reduced exponentially during the time the laser desorbed
ions are arriving at the travelling wave ion guide.
15 Fig. 5 illustrates how ions of differing mass to
charge ratios will arrive at the travelling wave ion
guide 1 shown in Fig. 4 as a function of time T1. Fig.
5 also illustrates the exit time Tz of the ions from the
travelling wave ion guide 1 and the arrival time T3 of
20 the ions at the orthogonal acceleration Time of Flight
mass analyser 15.
The curves shown in Fig. 5 assume that ions are
released or generated at time T=0 and are accelerated by
a voltage V1 of 10 V. The ions will therefore have an
25 energy of E1 (eV) where E1 = 10. The distance L1 (m)
from the pulsed ion source 10,11 to the entrance of the
travelling wave ion guide 1 is 0.25 in. The arrival time
T1 for ions at the entrance to the travelling wave ion
guide 1 is therefore given by:
Ti = 72L,
CA 02430531 2003-05-30
36 -
The velocity v (m/s) of the transient DC voltage
waveform and/or of the ions arriving at the travelling
wave ion guide 1 is given by:
v= L`.106
The length L2 (in) of the travelling wave ion guide
is 0.25 m. The time T2 at which ions exit the
travelling wave ion guide 1 is given by:
Tz = T, e
The velocity vX of the transient DC voltage
waveform and/or the ions at the exit of the travelling
wave ion guide 1:
Vx =Ve (L.)
The energy E2 (eV) of ions at the exit of the
travelling wave ion guide 1 is:
E2 = E,e'2(4
and hence:
E2 =1.353
The ions are further accelerated by a voltage V3
(V) at the exit of the travelling wave ion guide 1:
CA 02430531 2003-05-30
- 37 -
V3 = 38.647
The energy E3 (eV) of the ions therefore after
acceleration:
-E3 = E2 `4' " V3
where E3 = 40_ The path length L3 (m) from the
travelling wave ion guide 1 to the orthogonal
acceleration pusher region is 0.15 in. The flight time
T,s from the exit of the travelling wave ion guide 1 to
the orthogonal acceleration pusher region 14:
T,, _72L FT3
The arrival time T3 at the orthogonal acceleration
pusher region:
T3 =T2+T
Although the present invention has been described
with reference to preferred embodiments, it will be
understood by those skilled in the art that various
changes in form and detail may be made without departing
from the scope of the invention as set forth in the
accompanying claims.
