Note: Descriptions are shown in the official language in which they were submitted.
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Title: AXIAL EJECTION IN A MULTIPOLE MASS SPECTROMETER
FIELD OF THE INVENTION
This invention relates to a multipole elongated rod ion trap
mass spectrometer with axial ejection.
BACKGROUND OF THE INVENTION
Conventional ion traps, of the kind described in U.S. patent
2,939,952, are generally composed of three electrodes, namely a ring
electrode, and a pair of end caps, with appropriate RF and DC voltages
applied to these electrodes to establish a three-dimensional field which
traps ions within a mass range of interest in the relatively small volume
- between the ring electrode and the end caps. The electrodes may be
hyperbolic, producing a theoretically perfect three-dimensional
quadrupole field, or they may deviate from hyperbolic geometry, giving
rise to additional multipole fields superimposed on the quadrupole field
and which can produce improved results.
Usually ion trap mass spectrometers are filled in an
essentially mass-independent manner and are emptied mass-dependently
by manipulating the RF and DC voltages applied to one or more of the
electrodes. The ion storage and fast scanning capabilities of the ion trap are
advantageous in analytical mass spectrometry. High analysis efficiency,
compared to typical beam-type mass spectrometers, can be achieved if the
time to eject and detect ions from the trap is smaller than the time
required to fill a trap. If this condition is met, then very few ions are
wasted.
However an inherent disadvantage of ion traps is that ion
transport into the trap is usually of very low efficiency, e.g. one to ten
. percent, primarily due to the relatively small volume of the trap and the
very demanding ion energetic constraints for trap acceptance of externally
generated ions. The relatively small volume of the ion trap means that
the number of ions that can be accepted before space charge effects become
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serious is also relatively small. Increasing the radial dimension of the
volume of the trapping chamber of a conventional ion trap partially
overcomes this limitation, but with the additional disadvantages of
reduced analytical utility and/or increased costs (e.g. reduced mass range,
larger power supplies). The small volume of the ion trapping chamber
will also tend to limit the linear response range (i.e. dynamic range), again
because of the effects of space charge at high ion densities.
An additional problem is that when a conventional ion trap
is performing an analysis, no additional ions can be accepted. For many
modern ion sources such as electrospray, ion spray (disclosed in U.S.
patent 4,861,988), or corona discharge, this can be a considerable
disadvantage because the trap fill time is usually short compared with the
analysis time. Consequently, and as described in U.S. patent 5,179,278
assigned to the assignee of the present invention, many ions can be wasted
during the analysis time, resulting in relatively low duty cycles.
It is known that ions can be trapped and stored very efficiently
in a two-dimensional RF quadrupole. In some cases ions have been
admitted into and then trapped in a two-dimensional quadrupole for
purposes of releasing them into a conventional ion trap, as shown in U.S.
patent 5,179,278. More generally ions have been admitted into a
pressurized linear cell or a two-dimensional RF quadrupole for the
purpose of studying ion molecule reactions. Generally the ions enter the
device from a mass selective source such as a resolving quadrupole, are
trapped for a specified period of time, and then are ejected mass-
independently for subsequent mass analysis.
U.S. patent 5,420,425 teaches that ions can be trapped and
stored in a two-dimensional RF quadrupole and scanned out mass-
dependently, using the technique of mass selective instability. According
to that patent the device disclosed therein was conceived in order to
improve ion sensitivities, detection limits, and dynamic range, by
increasing the volume of the trapping chamber in the axial dimension.
The mass selective instability mode of ion ejection (and all other mass
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analysis scanning modes described in U.S. patent 5,420,425) involve
ejecting ions out of the trapping chamber in a direction orthogonal to the
center axis of the device, i.e. radially.
There are several disadvantages of radial ejection of ions
' 5 from a two-dimensional RF quadrupole. One disadvantage is that radial
ejection expels ions through or between the quadrupole (or higher order
multipole) rods. This forces the ions through regions of space for which
there are significant RF field imperfections. The effect of these
imperfections is to eject ions at points not predicted by the normal stability
diagram.
Radial ejection from a two-dimensional RF quadrupole has
the further disadvantage of providing a poor match between the
dimensions of the plug of ejected ions and conventional ion detectors. In
a linear or curved rod structure, radially ejected ions will exit throughout
the length of the device, i.e. with a rectangular cross-section of length
corresponding to the rods themselves. Most conventional ion detectors
have relatively small circular acceptance apertures (e.g. less than 2 cm2)
that are not well-suited for elongated ion sources.
Mass selective instability for radial ion ejection of ions from a
two-dimensional RF quadrupole has additional problems. Ions ejected
radially from such a device will exit with a diverging spatial profile with a
characteristic solid angle. Some of the ejected ions will hit the rods and be
lost. In addition, radially ejected ions will leave the trapping structure in
opposite directions. Multiple ion detectors would be required to collect all
of the ions made unstable by this and similar techniques. Ions ejected
away from the detectors) or which encounter one of the electrodes are lost
and therefore do not contribute to the measured ion signal. Therefore
only a small fraction of trapped ions would normally be collected, despite
the very high storage ability of this device.
BRIEF SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention in one of
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its aspects to provide an elongated multipole mass spectrometer which has
a high injection efficiency and an enlarged trapping volume, and in which
ions are ejected along the major axis of the device, thus allowing a good
geometric match with commonly used ion detectors.
In one of its aspects the invention provides a method of
operating a mass spectrometer having an elongated rod set, said rod set
having an entrance end and an exit end and a longitudinal axis, said
method comprising:
(a) admitting ions into said entrance end of said rod set,
(b) trapping at least some of said ions in said rod set by
producing a barrier field at an exit lens adjacent to the
exit end of said rod set and by producing an RF field
between the rods of said rod set adjacent at least the
exit end of said rod set,
(c) said RF and barrier fields interacting in an extraction
region adjacent to said exit end of said rod set to
produce a fringing field,
(d) energizing ions in said extraction region to mass
selectively eject at least some ions of a selected mass to
charge ratio axially from said rod set past said barrier
field,
(e) and detecting at least some of the ejected ions.
Further objects and advantages of the invention will be
apparent from the following description, taken together with the
accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
In the drawings:
Fig. 1 is a diagrammatic view of a simple mass spectrometer
apparatus with which the present invention may be used;
Fig. 1a is an end view of a rod set of Fig. 1 and showing
electrical connections to such rod set;
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Fig. 2 is a diagrammatic view of a modification of a part of the
apparatus of Fig. 1;
Fig. 3 is a diagrammatic view of a further modification of the
apparatus of Fig. 1;
Fig. 4 is a diagrammatic view of another modification of the
Fig. 1 apparatus;
Fig. 5 is a graph showing results obtained with the apparatus
of Fig. 4;
Fig. 6 is a graph showing further results obtained with the Fig.
4 apparatus;
Fig. 7 is an end view of rods which can be used as an exit lens;
and
Fig. 8 is a plan view of a modified exit lens.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENT
Reference is first made to Fig. 1, which shows a mass analyzer
system 10 with which the invention may be used. The system 10 includes
a sample source 12 (normally a liquid sample source such as a liquid
chromatography from which sample is supplied to a conventional ion
source 14. Ion source 14 may be an electrospray, an ion spray, or a corona
discharge device, or any other known ion source. An ion spray device of
the kind shown in U.S. patent 4,861,988 issued August 29, 1989 to Cornell
Research Foundation Inc. is suitable.
Ions from ion source 14 are directed through an aperture 16 in
an aperture plate 18. Plate 18 forms one wall of a gas curtain chamber 19
which is supplied with curtain gas from a curtain gas source 20. The
curtain gas can be argon, nitrogen or other inert gas and is described in the
above-mentioned U.S. patent 4,861,988. The ions then pass through an
orifice 22 in an orifice plate 24 into a first stage vacuum chamber 26
evacuated by a pump 28 to a pressure of about 1 Torr.
The ions then pass through a skimmer orifice 30 in a
skimmer plate 32 and into a main vacuum chamber 34 evacuated to a
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pressure of about 2 milli-Torr by a pump 36.
The main vacuum chamber 34 contains a set of four linear
conventional quadrupole rods 38. The rods 38 may typically have a rod
radius r=0.470cm, an inter-rod dimension ro=0.415cm, and an axial length
1=20cm.
Located about 2mm past the exit ends 40 of the rods 38 is an
exit lens 42. The lens 42 is simply a plate with an aperture 44 therein,
allowing passage of ions through aperture 44 to a conventional detector 46
(which may for example be a channel electron multiplier of the kind
conventionally used in mass spectrometers).
The rods 38 are connected to the main power supply 50 which
applies a DC rod offset to all the rods 38 and also applies RF in
conventional manner between the rods. The power supply 50 is also
connected (by connections not shown) to the ion source 14, the aperture
and orifice plates 18 and 24, the skimmer plate 32, and to the exit lens 42.
By way of example, for positive ions the ion source 14 may
typically be at +5,000 volts, the aperture plate 18 may be at +1,000 volts,
the
orifice plate 24 may be at +250 volts, and the skimmer plate 32 may be at
ground (zero volts). The DC offset applied to rods 38 may be -5 volts. The
axis of the device, which is the path of ion travel, is indicated at 52.
Thus, ions of interest which are admitted into the device
from ion source 14 move down a potential well and are allowed to enter
the rods 38. Ions that are stable in the applied main RF field applied to the
rods 38 travel the length of the device undergoing numerous momentum
dissipating collisions with the background gas. However a trapping DC
voltage, typically -2 volts DC, is applied to the exit lens 42. Normally the
ion transmission efficiency between the skimmer 32 and the exit lens 42 is
very high and may approach 100%. Ions that enter the main vacuum
chamber 34 and travel to the exit lens 42 are thermalized due to the
numerous collisions with the background gas and have little net velocity
in the direction of axis 52. The ions also experience forces from the main
RF field which confines them radially. Typically the RF voltage applied is
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in the order of about 450 volts (unless it is scanned with mass) and is of a
frequency of the order of about 816 kHz. No resolving DC field is applied
to rods 38.
When a DC trapping field is created at the exit lens 42 by
applying a DC offset voltage which is higher than that applied to the rods
38, the ions stable in the RF field applied to the rods 38 are effectively
trapped.
However ions in region 54 in the vicinity of the exit lens 42
will experience fields that are not entirely quadrupolar, due to the nature
of the termination of the main RF and DC fields near the exit lens. Such
fields, commonly referred to as fringing fields, will tend to couple the
radial and axial degrees of freedom of the trapped ions. This means that
there will be axial and radial components of ion motion that are not
mutually orthogonal. This is in contrast to the situation at the center of
rod structure 38 further removed from the exit lens and fringing fields,
where the axial and radial components of ion motion are not coupled or
are minimally coupled.
Because of the fringing fields couple the radial and axial
degrees of freedom of the trapped ions, ions may be scanned out of the ion
trap constituted by rods 38, by the application to the exit lens 42 of a low
voltage auxiliary AC field of appropriate frequency. (An example of the
frequencies that may be used is given later in this description.) The
auxiliary AC field may be provided by an auxiliary AC supply 56, which for
illustrative purposes is shown as forming part of the main power supply
50.
The auxiliary AC field is an addition to the trapping DC
voltage supplied to exit lens 42 and couples to both the radial and axial
secular ion motions. The auxiliary AC field is found to excite the ions
sufficiently that they surmount the axial DC potential barrier at the exit
lens 42, so that they can leave axially in the direction of arrow 58. The
deviations in the field in the vicinity of the exit lens 42 lead to the above
described coupling of axial and radial ion motions enabling the axial
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ejection at radial secular frequencies. This is in contrast to the situation
existing in a conventional ion trap, where excitation of radial secular
motion will generally lead to radial ejection and excitation of axial secular
motion will generally lead to axial ejection, unlike the situation described
above.
Therefore, ion ejection in a sequential mass dependent
manner can be accomplished by scanning the frequency of the low voltage
auxiliary AC field. When the frequency of the auxiliary AC field matches a
radial secular frequency of an ion in the vicinity of the exit lens 42, the
ion
will absorb energy and will now be capable of traversing the potential
barrier present on the exit lens due to the radial/axial motion coupling.
When the ion exits axially, it will be detected by detector 46. After the ion
is ejected, other ions upstream of the region 54 in the vicinity of the exit
lens are energetically permitted to enter the region 54 and be excited by
subsequent AC frequency scans.
Ion ejection by scanning the frequency of the auxiliary AC
voltage applied to the exit lens is desirable because it does not empty the
trapping volume of the entire elongated rod structure 38. In a
conventional mass selective instability scan mode for rods 38, the RF
voltage on the rods would be ramped and ions would be ejected from low
to high masses along the entire length of the rods when the q value for
each ion reaches a value of 0.907. After each mass selective instability scan,
time is required to refill the trapping volume before another analysis can
be performed. In contrast, when an auxiliary AC voltage is applied to the
exit lens as described above, ion ejection will normally only happen in the
vicinity of the exit lens because this is where the coupling of the axial and
radial ion motions occurs and where the auxiliary AC voltage is applied.
The upstream portion 60 of the rods serves to store other ions for
subsequent analysis. The time required to refill the volume 54 in the
vicinity of the exit lens with ions will always be shorter than the time
required to refill the entire trapping volume. Therefore fewer ions will be
wasted.
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As an alternative, instead of scanning the auxiliary AC
voltage applied to end lens 42, the auxiliary AC voltage on end lens 42 can
be fixed and the main RF voltage applied to rods 38 can be scanned in
amplitude, as will be described. While this does change the trapping
conditions, a q of only about 0.2 to 0.3 is needed for axial ejection, while a
q
of about 0.8 to 0.907 is needed for radial ejection. Therefore, as will be
explained, few if any ions are lost to radial ejection if the RF voltage is
scanned through an appropriate amplitude range, except possibly for very
low mass ions.
As a further alternative, and instead of scanning either the RF
voltage applied to rods 38 or the auxiliary AC voltage applied to end lens
42, a further supplementary or auxiliary AC dipole voltage may be applied
to rods 38 (as indicated by dotted connection 57 in Fig. 1) and scanned, to
produce varying fringing fields which will eject ions axially in the manner
described. As is well known, the dipole voltage is usually applied between
an opposed pair of the rods 38, as indicated in Fig. 1a.
Alternatively, a combination of some or all of the above three
approaches (namely scanning an auxiliary AC field applied to the end lens
42, scanning the RF voltage applied to the rod set 38 while applying a fixed
auxiliary AC voltage to end lens 42, and applying an auxiliary AC voltage
to the rod set 38 in addition to that on lens 42 and the RF on rods 38) can be
used to eject ions axially and mass dependently past the DC potential
barrier present at the end lens 42.
The device illustrated may be operated in a continuous
fashion, in which ions entering the main RF containment field applied to
rods 38 are transported by their own residual momentum toward the exit
lens 42 and ultimate axial ejection. Thus, the ions which have reached the
extraction volume in the vicinity of the exit lens have been
preconditioned by their numerous collisions with background gas,
eliminating the need for an explicit cooling time (and the attendant delay)
as is required in most conventional ion traps. At the same time as ions are
entering the region 60, ions are being ejected axially from region 54 in the
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mass dependent manner described.
It is noted that the extraction volume 54 in the vicinity of the
exit lens is quite small. The exit lens 42 is normally placed very close to
the ends of the rods 38, e.g. 2mm from the rod ends (as mentioned). The
penetration of the fringing fields from the exit lens 42 into the space
between the rods 38 is believed to be very small, typically of the order of
between 0.5mm and l.Omm, so the extent of volume or region 54 is
exaggerated in Fig. 1, for clarity of illustration.
As a further alternative, the DC offset applied to all four rods
38 (which in the example given is -5 volts) can be modulated at the same
frequency as the AC which would have been applied to exit lens 42. In that
case no AC is needed on exit lens 42 since modulating the DC offset is
equivalent to applying an AC voltage to the exit lens, in that it creates an
AC field in the fringing region. Of course the DC potential barrier is still
applied to the exit lens 42. The amplitude of the modulation of the DC
offset will be the same as the amplitude of the AC voltage which otherwise
would have been applied to the exit lens 42, i.e. it is set to optimize the
axially ejected ion signal. Then, either the RF amplitude is scanned to
bring ions sequentially into resonance with the AC field created by the DC
modulation, or else the frequency of the modulation is scanned so that
again, when such frequency matches a radial secular frequency of an ion in
the fringing fields in the vicinity of the exit lens, the ion will absorb
energy
and be ejected axially for detection. The rod offset would not be modulated
until after ions have been injected and trapped within the rods, since the
modulation would otherwise interfere with ion injection, so this process
would be a batch process. This is in contrast to the continuous process
possible when AC is placed on the exit lens, in which case ions can be
ejected from the extraction region 54 at the same time as ions are entering
region 60 (because the AC field on exit lens 42 does not affect ion
injection).
The highest efficiency in a continuous mode operation is
achieved when the ion ejection rate is faster than the rate at which ions of
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the desired mass/charge ratio are injected into the rods and travel along them
to
the exit lens 42. Ion ejection processes can require some tens of
milliseconds.
The time required for ions to travel from one end to the other of the rods 38
depends on the lengths of the rods themselves, the initial energy of the ions,
and
the pressure in the vacuum chamber 34. In some cases the end-to end transit
time
will dominate, but more often the time required to extract the ions from the
region
in the vicinity of the exit lens 42 will be more important. Thus it may be
desirable
to manipulate the axial energy of the ions to afford an optimum match between
the time required for the ions to reach the extraction region 54, and the time
for
the ejection process itself to occur. Further, it may be of utility to be able
to
control the concentration of ions in the vicinity of the exit lens to reduce
or
enhance the local charge density in that region, depending on the application.
One method of matching the time required for ions to travel from one end
to the other of the rods 38 to the time required to eject the ions is to apply
an axial
field along the rods 38. Methods for imposing axial fields of this kind are
described in U. S. Patent No. 5,847,386 and entitled "Spectrometer with Axial
Field" assigned to the assignee of this application. An applied axial field in
the
direction of the exit lens 42 will tend to concentrate ions in the region of
rods 3 8
in the vicinity of lens 42, i.e. in the volume 54 of the device from which the
ions
are extracted. An applied axial field in the opposite direction will tend to
deplete
ions from the extraction volume 54, and may also be desirable in some cases.
Several techniques are available as described in the above-identified U. S.
Patent No. 5,847,386 for providing an axial field by modification of the
electrode
geometries. Such arrangements include tapering the rods, or locating one pair
of
rods nearer the center line at one end of the device and the other pair of
rods
nearer the center line at the other end of the device, or segmenting the rods
axially
and applying different DC offsets to successive segments. The disclosure and
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drawings of said application are hereby incorporated by reference into this
application.
One typical electrode geometry which may be used is that
depicted in Fig. 2, where primed reference numerals indicate parts
corresponding to those of Fig. 1. Here, the rods 38' are divided into
segments 38-1' to 38-5', with DC offsets V1 to V6 which increase in negative
value applied to successive segments from 38-1' to 38-6'. This arrangement
will provide an axial field as described in said application. Such an
arrangement will strongly concentrate ions in the volume near the exit
lens 42' and will increase the coupling of axial and radial ion motion.
If desired, the axial field can be oscillated as described in the
above mentioned copending application. Such oscillation may enhance
axial ejection of ions trapped in the volume near the exit lens 42. It can
also be used for ion dissociation as described in said application, by
oscillating the ion population trapped in the rod structure about their
equilibrium positions.
The system described can be considered as being an open-
ended three-dimensional ion trap, where the open end is an integrated
high efficiency ion injection device (supplied by ion source 14). The ions
in the vicinity of the exit lens 42 experience a three-dimensional trapping
field comprised of radial and axial components. Radially the ions are
contained by the main RF field applied to the linear rods 38. Axially the
ions are contained at the exit end of the device by the DC potential on the
exit lens 42, and are contained at the entrance end of the device by the
potential gradient from the applied axial field (or from the skimmer 32}.
The ions are also to some extent contained in the trapping region or
volume 54 by the field created by the build-up of charge density upstream
of that region. It will therefore be appreciated that the actual trapping
volume 54 is variable along the axial or Z direction.
If desired, the RF on the rods 38 can be scanned to eject ions
mass dependently, while keeping a DC potential barrier on end lens 42 but
with no AC field on end lens 42, no modulation of the DC offset on rods
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38, and no dipole AC field on rods 38. In that case, ions in the fringing
fields at the downstream ends of rods 38 will leave axially and be detected,
but most of the ions between rods 38 (those in region 60) will leave radially
and will be wasted. The wastage can be reduced by segmenting rods 38 as
shown in Fig. 2, making the last set of segments 38-5' very short (e.g. less
than 1cm), and scanning the RF for ejection only on segments 38-5'. In this
way, a higher proportion of the ions between rods 38-5' will be ejected
axially mass dependently, for detection.
Often many of the ions from conventional ion sources are of
little or no analytical utility. Examples of such ions are low mass solvent
and cluster ions. These ions simply serve to increase the overall charge
density within the ion trap at the expense of optimum performance.
Various techniques may be used to eliminate such unwanted ions from
the linear ion trap described. One such method is to operate the main RF
voltage from power supply 50 at a level where the analytes of interest are
stable within the rod structure 38, but the unwanted ions are unstable. For
example if the unwanted ions are in the mass to charge range 10 to 100,
and the ions of interest are in the mass to charge range 200 to 1,000, then
the main RF voltage can be operated at 214 volts peak to peak.
Another method of eliminating unwanted ions from the ion
trap is to apply an additional auxiliary AC voltage between opposite pairs
of the rods 38, to resonantly eject the unwanted ions radially out of the rod
set. This technique is well known, as mentioned. In the technique an
auxiliary AC voltage, of magnitude equal to about 10% of the level of the
main RF voltage and of much lower frequency, is typically applied
between opposite pairs of rods. The auxiliary AC voltage, of appropriate
amplitude and frequency, may be scanned to resonantly eject unwanted
ions radially.
The use of resonant ejection to eject ions is disclosed in
Langmuir U.S. patent 3,334,225, in Syka et al. U.S. patent Re34,000, and in
Kelley U.S. patent 5,381,007 and is also disclosed in Douglas U.S. patent
5,179,278 assigned to the assignee of this application.
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Another technique for removing unwanted ions is to apply
low voltage DC to opposite pairs of rods to make the rods 38 act as a low
resolution mass spectrometer. The magnitude of the DC applied will be
such that the combination of AC and DC ejects ions only in the Iow mass
range which is not of interest.
Finally, unwanted ions can be eliminated from rods 38 by the
application of a filtered noise field to the rods, such that only ions of
interest are stable within the rod structure and can be stored. The use of
such a filtered noise field is disclosed in Langmuir U.S. patent 3,334,225
issued August 1, 1967 and in Kelley U.S. patent 5,381,007 issued January 10,
1995.
One of the disadvantages of most ion traps is that as
mentioned, they cannot accept additional ions while performing an
analysis. When using continuous ion sources this leads to reduced duty
cycles and decrease overall sensitivity, since many or in fact most of the
ions generated by the source are not analyzed and are wasted. Douglas U.S.
patent 5,179,278 teaches that a multipole inlet system can reduce these
problems by accepting and storing ions from the ion source while the ion
trap is performing an analysis. This can dramatically enhance the overall
system sensitivity by increasing duty cycle. Figure 3 illustrates a device
which uses these principles.
In the Fig. 3 device, in which double primed reference
numerals indicate parts corresponding to those of Fig. 1, the rods 38" have
been divided into three sets of rods 38a, 38b and 38c. Rods 38a are used to
pre-trap ions from the continuous ion source 14". The rods 38b are used as
an RF rod mirror that can reflect or transmit ions, by changing the DC
offset of rods 38b. The rods 38c and lens 42" serve as the open-ended ion
trap previously described, for analysis of ions which are injected into rods
38c through the rods 38b.
In operation, the RF and DC voltages on rod set 38a are set to
accept ions within a mass range of interest, while the AC and DC voltages
on rod set 38b are set to reflect ions, so that a population of ions
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accumulates in rod set 38a. (The operation is exactly as described for rod
set 44 in U.S. patent 5,179,278.) After a predetermined time, the voltages
on rod set 38b are changed to allow passage of the accumulated ions in rod
set 38a through rod set 38b to rod set 38c. The RF voltage and DC offset
voltage applied to rod set 38c, and the AC and DC voltages applied to lens
42", are set such that rod set 38c operates as an ion trap with axial ejection
as described in connection with Fig. 1. Thus, ions are axially ejected in a
mass dependent manner from rod set 38c, as previously described, for
detection in detector 46".
While mass analysis is being performed in rod set 38c, the
voltages on rod set 38b return to the ion reflection mode and further ions
from the source 14" are stored in rod set 38a. An advantage of this
configuration is that while ions are being analyzed in rod set 38c, ions
from the continuous source are accumulated for subsequent analysis in
the pre-trapping region, i.e. rod set 38a, and are not lost. As U.S. patent
5,179,278 teaches, proper optimization of the time to collect sufficient ions
to fill the analysis region of the device, the time to empty the pre-trap
region, and the time to perform analysis can result in very high duty
cycles, and thus high overall sensitivity. Of course even in the Fig. 1
arrangement, ions are collected in region 60 of rods 38 while ions from the
extraction region 54 are being ejected, so that some ions can be collected
while the ion trap constituted by rods 38 and lens 42 is scanning out ions.
However the Fig. 3 version allows storage of more ions since a larger
volume can be used. In addition, some DC can conveniently be applied
between the pairs of rods of rod set 38a to eliminate unwanted ions, thus
reducing space charge effects in rod set 38c.
By way of example, 0.1~,M (micro moles) of reserpine (having
mass to charge ratio 609) was introduced using the well known ion spray
source (not shown) into a conventional mass spectrometer model API 300
produced by Sciex Division of MDS Health Group Limited of Concord,
Ontario, Canada. A simplified diagrammatic view of the model API 300
ion optical path is shown in Fig. 4, where the gas curtain entrance plate is
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indicated at 70, the gas curtain exit plate is indicated at 72, the skimmer
plate is shown at 74, and four sets of rods are indicated as Q0, Q1, Q2 and
Q3, with orifice plates IQ1 between rod sets QO and Q1, IQ2 between Q2 and
Q3, and IQ3 between Q2 and Q3. The exit lens is indicated at 76 and the
detector (a channel electron multiplier) is indicated at 78.
In the Fig. 4 example, the pressures were 2.2 Torr in chamber
80, 8 milli-Torr in chamber 82 and 2 x 10-5 Torr in the remainder of the
vacuum chamber 84. The applied DC voltages were: ground at skimmer
plate 80; -5 volts DC at Q0, -7 volts DC at IQ1, -10 volts at Q1, -20 volts at
IQ2, -7 volts DC at Q2, -3 volts DC at IQ3 (which served as the equivalent of
the exit lens 42); -15 volts DC on Q3, and 0 volts on the final exit plate 76.
All resolving DC voltages were removed from the quadrupoles.
Q2, which was normally a collision cell, was configured to
trap ions and had a cell pressure of 1 x 10-3 Torr. In addition, an auxiliary
AC voltage was applied to the exit lens IQ3. The auxiliary AC power
supply could produce 100 volts peak to peak, at frequency one-ninth that of
the main RF frequency, and was phase locked to the main RF frequency.
(The main RF frequency was 816 kHz so that of the auxiliary AC voltage
was 91.67 kHz.)
The auxiliary AC voltage was held at 47 volts peak to peak, its
frequency was held constant at 91.67 kHz for the experiment, and the RF
voltage applied to the Q2 rods was scanned to obtain a mass spectrum.
Although scanning the amplitude of RF voltage changes the trapping
conditions and could eject very low mass ions, the q of the device is so low
under the conditions described that ions of interest trapped in the rod set
Q2 are not normally ejected (unless the experimenter is interested in
extremely low mass ions).
The sequence of events in the experiment was:
(a) A short pulse of ions was allowed to pass from QO into
Q2 (Ql performed no function except as an ion pipe
during this experiment) by changing the DC lens
voltage on IQl from +20 volts (which stopped ions) to -
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7 volts {for ion transmission).
(b) Ions that were trapped in Q2 were then scanned out
axially by ramping the RF applied to the Q2 rods,
typically from 924 volts peak to peak to 960 volts peak
to peak.
(c) Q2 was then emptied of any residual ions by reducing
the RF applied to its rods to a very low voltage,
typically 20 volts peak to peak.
(d) The sequence was then repeated.
A typical spectrum produced using this technique is shown in
Fig. 5, which shows a peak 100 at mass 529.929. Since the spectrum was not
mass calibrated, the reported peak of 529.929 was incorrect; the true mass
was 609.
It will be seen that the peak width at half height, corrected
manually for the mass calibration offset, is 0.42 AMU. This yields an
M/OM resolution value at M/Z609 of about 1450, which is a very high
resolution. In the example shown, good resolution was best obtained by
scanning slowly, at a scan speed in this example of 78 AMU per second.
However with optimization, higher scan rates are expected to be achieved.
In another experiment performed with the Fig. 4 apparatus,
fragmentation was performed under the following conditions:
(a) Two volts trapping DC were applied to lens IQ3.
(b) 62 volts (peak to peak) AC at 91.67 kHz were applied to
lens IQ3.
(c) Q1 was set to a resolving mode (RF and DC were
applied) to allow only transmission of a selected parent
ion, namely renin substrate tetra decapeptide (M +
3H)3+ at m/z 587.
(d) Q3 was set to RF only (the resolving DC was removed).
(e) Q2 was pressurized with 1 x 10-3 Torr helium.
The experimental steps were:
1. A pulse of m/z 587 ions was allowed to pass from QO to
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Q2 by changing the voltage on lens IQ1 from 20 volts to
-7 volts.
2. Ions within Q2 were excited by setting the RF rod
voltage to 897.8 volts peak to peak for 50 ms. This was
an excitation step. Ions in Q2 which were in resonance
with the applied AC field (on IQ3) were excited
(absorbed power) and, because of their increased kinetic
energy, were either ejected from the trap or fragmented
due to collisions with the background gas.
3. Immediately after the 50 ms excitation step, ions in Q2
were scanned out axially by scanning the Q2 RF rod
voltage from 800 volts peak to peak to 1,422 volts peak
to peak. This step simply took a "snapshot" of the ions
that had remained in the trap after the excitation step.
The observed ions are shown at peaks 104, 106, 108 in Fig. 6.
Again the mass spectrum was not calibrated and is shown corrected. Peak
108 is the parent ion at m/z 587 (triply charged), while the fragment ions
m/z 697 and m/z 720 are shown at 106, 108 (they have higher m/z ratios
because they are only doubly charged).
Thus, it will be seen that collisional fragmentation can readily
be performed in the linear trap described, and the fragment ions can be
scanned axially in a mass dependent manner for detection and analysis.
While the invention has been described in connection with a
quadrupole rod structure, other multipole rod geometries may be used, for
example octopoles and hexapoles. In addition, although the exit lens 42
has been described as a plate with an aperture, other configurations of exit
lenses may be used, for example a short RF-only rod array such as that
indicated at 102 in Fig. 7 and having A and B poles 102a, 102b. The rod
offsets of the A and B poles 102a, 102b may then be resonated at the
resonance frequency of the ion to be ejected, producing axial ejection as
was achieved by the auxiliary AC field applied to exit Iens 42.
While linear rod sets have been described and illustrated, if
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desired curved rod sets may be used. In addition, while end lenses in the
form of a perforated plate, or a short set of rods, have been shown, other
forms of end lenses may be used. For example, as shown in Fig. 8, the end
lens 110 may be a segmented plate having wedge-shaped segments 110-1,
110-2, 110-3, 110-4 and an aperture 112. This allows different fields to be
applied to each segment, to optimize the results, while still limiting the
quantity of gas which can leave the part of the vacuum chamber upstream
of lens 110.
While preferred embodiments of the invention have been
described, it will be appreciated that various changes may be made within
the scope of the invention, and all such changes are intended to be
included in the accompanying claims.
