U.S. patent application number 14/369601 was filed with the patent office on 2015-02-12 for method of operating tandem ion traps.
The applicant listed for this patent is DH Technologies Development Pte. Ltd.. Invention is credited to Mircea Guna.
Application Number | 20150041639 14/369601 |
Document ID | / |
Family ID | 48696418 |
Filed Date | 2015-02-12 |
United States Patent
Application |
20150041639 |
Kind Code |
A1 |
Guna; Mircea |
February 12, 2015 |
METHOD OF OPERATING TANDEM ION TRAPS
Abstract
A method is provided comprising accumulating ions in a first ion
trap at a first time; transmitting a first plurality of ions out of
the first ion trap through a timed-ion selector; applying a pulsed
DC voltage to the timed-ion selector; transmitting the first
portion of selected ions into a second ion trap at a second time;
retaining a second plurality of ions in the first ion trap at the
second time; transmitting the first portion of selected ions out of
the second ion trap at a third time; transmitting the second
plurality of ions out of the first ion trap through a timed-ion
selector; applying a pulsed DC voltage to the timed-ion selector
transmitting a second portion of selected ions into the second ion
trap at a fourth time; and transmitting the second portion of
selected ions out of the second trap.
Inventors: |
Guna; Mircea; (North York,
CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
DH Technologies Development Pte. Ltd. |
Singapore |
|
SG |
|
|
Family ID: |
48696418 |
Appl. No.: |
14/369601 |
Filed: |
December 4, 2012 |
PCT Filed: |
December 4, 2012 |
PCT NO: |
PCT/IB2012/002583 |
371 Date: |
June 27, 2014 |
Current U.S.
Class: |
250/283 |
Current CPC
Class: |
H01J 49/0031 20130101;
H01J 49/061 20130101; H01J 49/4265 20130101; H01J 49/26 20130101;
H01J 49/004 20130101 |
Class at
Publication: |
250/283 |
International
Class: |
H01J 49/42 20060101
H01J049/42; H01J 49/26 20060101 H01J049/26; H01J 49/06 20060101
H01J049/06; H01J 49/00 20060101 H01J049/00 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 29, 2011 |
US |
13/340176 |
Claims
1. A method of operating a mass spectrometer system having a first
ion trap and a second ion trap, the method comprising: a)
accumulating ions in the first ion trap at a first time; b)
transmitting a first plurality of ions out of the first ion trap
through a timed-ion selector; c) applying a pulsed DC voltage to
the timed-ion selector for deflecting a first group of unwanted
ions from the first plurality of ions, leaving a first portion of
selected ions having masses within a first mass range; d)
transmitting the first portion of selected ions out of the
timed-ion selector and into the second ion trap at a second time;
e) retaining a second plurality of ions in the first ion trap at
the second time, the second plurality of ions having masses within
a second mass range different from the first mass range; f)
transmitting the first portion of selected ions out of the second
ion trap at a third time; and, g) transmitting the second plurality
of ions out of the first ion trap through a timed-ion selector; h)
applying a pulsed DC voltage to the timed-ion selector for
deflecting a second group of unwanted ions from the second
plurality of ions, leaving a second portion of selected ions having
masses within a second mass range; i) transmitting a second portion
of selected ions out of the timed-ion selector and into the second
ion trap at a fourth time; and j) transmitting the second portion
of selected ions out of the second trap.
2. The method of claim 1, wherein the steps of (b) and (g) comprise
transmitting ions out of the first ion trap during a first sliding
transmission window, the ions transmitted during the first sliding
transmission window having masses within a first variable mass
range, the first variable mass range corresponding to different
mass ranges at different operating times, such that the first
variable mass range corresponds to the first mass range at the
second time and the second mass range at the third time; wherein
step (c) comprises transmitting the first variable mass range
through the timed-ion-selector to select a portion of the first
variable mass range; wherein step (d) comprises transmitting the
portion of the first variable mass range into the second ion trap;
and wherein step (f) comprises transmitting the portion of the
first variable mass range out of the second trap during a second
sliding transmission window, the ions transmitted during the second
sliding transmission window having masses within a second variable
mass range, the second variable mass range corresponding to
different mass ranges at different operating times, such that the
second variable mass range corresponds to the first mass range at
the third time.
3. The method of claim 2 further comprising, over an operating time
interval, scanning the first variable mass range and the second
variable mass range over an operating mass range.
4. The method of claim 3, wherein over the operating time interval,
the second sliding transmission window is time delayed relative to
the first sliding transmission window by a delay time interval such
that the first variable mass range at any operating time
substantially corresponds to the second variable mass range at the
operating time plus the delay time interval.
5. The method of claim 3, wherein over the operating time interval,
the second sliding transmission window is time delayed relative to
the first sliding transmission window by a delay time interval such
that the first variable mass range at any operating time equals the
second variable mass range at the operating time plus the delay
time interval.
6. The method of claim 4, wherein the first variable mass range is
changed at a first scan rate, and the second variable mass range is
changed at a second scan rate, the first scan rate and the second
scan rate being substantially equal.
7. The method of claim 6 further comprising controlling the first
scan rate using a first RF voltage provided to the first ion trap
and controlling the second scan rate using a second RF voltage
provided to the second ion trap, such that during the operating
time, the first RF voltage at any operating time substantially
corresponds to the second RF voltage at the operating time plus the
delay time interval.
8. The method of claim 7, wherein the first and second RF voltages
are independently provided to the first and second ion traps.
9. The method as defined in claim 6, comprising controlling the
first scan rate using a first RF voltage and a first auxiliary AC
voltage provided to the first ion trap, and controlling the second
scan rate using a second RF voltage and a second auxiliary AC
voltage provided to the second ion trap, such that during the
operating time a ratio of the first RF voltage to the second RF
voltage is substantially constant.
10. The method as defined in claim 9, wherein the first and second
ion traps are capacitively coupled using one or more coupling
capacitors, and ratio of the first RF voltage to the second RF
voltage is controlled by selecting the capacitances of the one or
more coupling capacitors.
11. The method as defined in claim 10, wherein the first auxiliary
AC voltage and the second auxiliary AC voltage are determined,
based on the ratio of the first RF voltage to the second RF
voltage, such that the first scan rate substantially equals the
second scan rate.
12. The method as defined in claim 9, wherein the first and second
RF voltages are independently provided to the first and second ion
traps.
13. The method as defined in claim 4, further comprising selecting
a second space charge level for the second ion trap; and then
determining a cooling time interval for retaining ions in the
second ion trap to provide the space charge level, wherein the
delay time interval substantially equals the cooling time
interval.
14. The method as defined in claim 6, wherein, the ions in the
first ion trap have a starting mass range; the ions in the second
ion trap have a variable operating mass range, the variable
operating mass range at any operating time after the delay time
interval being substantially equal to the first scan rate
multiplied by the delay time interval; and the variable operating
mass range is less than half of the starting mass range.
15. The method as defined in claim 16 wherein the variable
operating mass range is less than a fifth of the starting mass
range.
16. The method as defined in claim 16 wherein the variable
operating mass range is less than a tenth of the starting mass
range.
Description
RELATED APPLICATIONS
[0001] The present application claims benefit of and priority to
U.S. patent application Ser. No. 13/340,176, filed on Dec. 29,
2011, entitled "Method of Operating Tandem Ion Traps," which claims
priority to co-pending U.S. patent application Ser. No. 12/480,160,
filed on Jun. 8, 2009, entitled "Method of Operating Tandem Ion
Traps," which claims benefit of and priority to Provisional
Application 61/059,962, filed Jun. 9, 2008, and Provisional
Application 61/120,674, filed Dec. 8, 2008, the entire disclosures
of which are herein incorporated by reference.
FIELD
[0002] The applicant's teachings relate generally to ion traps, and
more particularly to tandem ion trap mass spectrometer
configurations, and methods of operating the same.
INTRODUCTION
[0003] Conventional ion trap mass spectrometers, of the kind
described in U.S. Pat. No. 2,939,952, can include three electrodes,
namely a ring electrode, and a pair of end cap electrodes.
Appropriate RF/DC voltages can be applied to the electrodes to
establish a three dimensional field that traps ions within a
specified mass-to-charge range. Linear quadrupoles may also be
configurable as ion trap mass spectrometers, with radial ion
confinement being provided by an applied RF voltage and axial ion
confinement by DC potential barriers at each end of the rod set.
Mass selective detection of ions trapped within a linear ion trap
can utilize radial ejection of ions, as taught by U.S. Pat. No.
5,420,425, or axial ejection of ions (MSAE), as taught by U.S. Pat.
No. 6,177,668. Fourier Transform techniques can also be utilized
for in situ detection of ions, as taught by U.S. Pat. No.
4,755,670.
SUMMARY
[0004] In accordance with various aspects of the applicant's
teachings, there is provided a method of operating a mass
spectrometer system having a first ion trap and a second ion trap,
the method comprising a). accumulating ions in the first ion trap
at a first time; b). transmitting a first plurality of ions out of
the first ion trap through a timed-ion selector; c). applying a
pulsed DC voltage to the timed-ion selector for deflecting a first
group of unwanted ions from the first plurality of ions, leaving a
first portion of selected ions having masses within a first mass
range; d). transmitting the first portion of selected ions out of
the timed-ion selector and into the second ion trap at a second
time; e). retaining a second plurality of ions in the first ion
trap at the second time, the second plurality of ions having masses
within a second mass range different from the first mass range; f).
transmitting the first portion of selected ions out of the second
ion trap at a third time; and, g). transmitting the second
plurality of ions out of the first ion trap through a timed-ion
selector; h). applying a pulsed DC voltage to the timed-ion
selector for deflecting a second group of unwanted ions from the
second plurality of ions, leaving a second portion of selected ions
having masses within a second mass range; i). transmitting a second
portion of selected ions out of the timed-ion selector and into the
second ion trap at a fourth time; and g). transmitting the second
portion of selected ions out of the second trap.
[0005] In various aspects, the steps of (b) and (g) comprise
transmitting ions out of the first ion trap during a first sliding
transmission window, the ions transmitted during the first sliding
transmission window having masses within a first variable mass
range, the first variable mass range corresponding to different
mass ranges at different operating times, such that the first
variable mass range corresponds to the first mass range at the
second time and the second mass range at the third time; wherein
step (c) comprises transmitting the first variable mass range
through the timed-ion-selector to select a portion of the first
variable mass range; wherein step (d) comprises transmitting the
portion of the first variable mass range into the second ion trap;
and wherein step (f) comprises transmitting the portion of the
first variable mass range out of the second trap during a second
sliding transmission window, the ions transmitted during the second
sliding transmission window having masses within a second variable
mass range, the second variable mass range corresponding to
different mass ranges at different operating times, such that the
second variable mass range corresponds to the first mass range at
the third time.
[0006] In various embodiments, the method further comprises over an
operating time interval, scanning the first variable mass range and
the second variable mass range over an operating mass range.
[0007] In various aspects, over the operating time interval, the
second sliding transmission window is time delayed relative to the
first sliding transmission window by a delay time interval such
that the first variable mass range at any operating time
substantially corresponds to the second variable mass range at the
operating time plus the delay time interval.
[0008] In various aspects, over the operating time interval, the
second sliding transmission window is time delayed relative to the
first sliding transmission window by a delay time interval such
that the first variable mass range at any operating time equals the
second variable mass range at the operating time plus the delay
time interval.
[0009] In various aspects, the first variable mass range is changed
at a first scan rate, and the second variable mass range is changed
at a second scan rate, the first scan rate and the second scan rate
being substantially equal.
[0010] In various embodiments, the method further comprises
controlling the first scan rate using a first RF voltage provided
to the first ion trap and controlling the second scan rate using a
second RF voltage provided to the second ion trap, such that during
the operating time, the first RF voltage at any operating time
substantially corresponds to the second RF voltage at the operating
time plus the delay time interval.
[0011] In various aspects, the first and second RF voltages are
independently provided to the first and second ion traps.
[0012] In comprising embodiments, the method comprises controlling
the first scan rate using a first RF voltage and a first auxiliary
AC voltage provided to the first ion trap, and controlling the
second scan rate using a second RF voltage and a second auxiliary
AC voltage provided to the second ion trap, such that during the
operating time a ratio of the first RF voltage to the second RF
voltage is substantially constant.
[0013] In various aspects, the first and second ion traps are
capacitively coupled using one or more coupling capacitors, and
ratio of the first RF voltage to the second RF voltage is
controlled by selecting the capacitances of the one or more
coupling capacitors.
[0014] In various aspects, the first auxiliary AC voltage and the
second auxiliary AC voltage are determined, based on the ratio of
the first RF voltage to the second RF voltage, such that the first
scan rate substantially equals the second scan rate.
[0015] In various aspects, the first and second RF voltages are
independently provided to the first and second ion traps.
[0016] In various embodiments, the method further comprises
selecting a second space charge level for the second ion trap; and
then determining a cooling time interval for retaining ions in the
second ion trap to provide the space charge level, wherein the
delay time interval substantially equals the cooling time
interval.
[0017] In various aspects, the ions in the first ion trap have a
starting mass range; the ions in the second ion trap have a
variable operating mass range, the variable operating mass range at
any operating time after the delay time interval being
substantially equal to the first scan rate multiplied by the delay
time interval; and the variable operating mass range is less than
half of the starting mass range.
[0018] In various aspects, the variable operating mass range is
less than a fifth of the starting mass range.
[0019] In various aspects, the variable operating mass range is
less than a tenth of the starting mass range.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] A detailed description of various embodiments is provided
herein below with reference to the following drawings, in
which:
[0021] FIG. 1 is a block diagram illustrating a tandem linear ion
trap mass spectrometer system that can be configured to implement a
method according to an aspect of an embodiment of the present
invention;
[0022] FIG. 2A is a timing diagram of exemplary RF voltage and
auxiliary AC excitation frequency waveforms suitable for
mass-selective axial ejection of ions when the applied auxiliary AC
excitation frequency is held constant according to an aspect of an
embodiment of the present invention;
[0023] FIG. 2B is a timing diagram of exemplary RF voltage and
auxiliary AC excitation frequency waveforms suitable for
mass-selective axial ejection of ions according to an aspect of an
embodiment of the present invention;
[0024] FIG. 2C is a timing diagram of exemplary RF voltage and
auxiliary AC excitation frequency waveforms suitable for
mass-selective axial ejection of ions according to an aspect of an
embodiment of the present invention;
[0025] FIG. 3 is a timing diagram of starting and operating mass
ranges for two linear ions traps operated in tandem according to an
aspect of an embodiment of the present invention;
[0026] FIG. 4 is a block diagram illustrating a tandem linear ion
trap mass spectrometer system that can be configured to implement a
method according to an aspect of an alternative embodiment of the
present invention;
[0027] FIG. 5 is a block diagram illustrating a tandem linear ion
trap mass spectrometer system that can be configured to implement a
method according to an aspect of an alternative embodiment of the
present invention;
[0028] FIG. 6 is a block diagram illustrating a tandem linear ion
trap mass spectrometer system that can be configured to implement a
method according to an aspect of an alternative embodiment of the
present invention;
[0029] FIG. 7 is a block diagram illustrating a tandem linear ion
trap mass spectrometer system that can be configured to implement a
method according to an aspect of an alternative embodiment of the
present invention;
[0030] FIG. 8A is a block diagram illustrating an alternative
configuration of a tandem linear ion trap mass spectrometer system
according to an aspect of the applicant's teachings;
[0031] FIG. 8B is a sectional diagram showing the details of the
timed ion selector in FIG. 8A;
[0032] FIG. 9A is a block diagram illustrating an alternative
configuration of a tandem linear ion trap mass spectrometer system
according to an aspect of the applicant's teachings;
[0033] FIG. 9B is a sectional diagram of the electrodes in FIG. 9B
operating as a timed ion selector;
[0034] FIG. 10 is a block diagram illustrating an alternative
configuration of a tandem linear ion trap mass spectrometer system
according to an aspect of the applicant's teachings;
[0035] FIG. 11 is a block diagram illustrating an alternative
configuration of a tandem linear ion trap mass spectrometer system
according to an aspect of the applicant's teachings; and
[0036] FIG. 12 is a block diagram illustrating an alternative
configuration of a tandem linear ion trap mass spectrometer system
according to an aspect of the applicant's teachings.
DESCRIPTION OF VARIOUS EMBODIMENTS
[0037] It will be understood by those skilled in the art that the
drawings and associated descriptions to follow are intended to be
exemplary in nature only and not to limit the scope of the present
invention in any way. For convenience like reference numerals will
be repeated where available to describe like features of the
drawings.
[0038] The spectral resolution of ion trap mass spectrometers may
depend on the density, or space charge, of trapped ions. Using
conventional techniques, the spectral resolution of ion trap mass
spectrometers may decline sharply once the space charge of the
trapped ions reaches or exceeds a certain threshold level. In
extreme cases, mass spectral peaks can be lost entirely due to
space charge effects. Other undesirable space charge effects can
include spontaneous emptying of the ion trap, shifts in mass
calibration in the spectrometer and other forms of spectral
distortion.
[0039] Reference is first made to FIG. 1, which is a block diagram
illustrating a triple quadrupole mass spectrometer system 10
configured to implement a method according to an aspect of an
embodiment of the present invention. The mass spectrometer system
10 comprises ion source 20, which generates and directs a focused
ion stream toward curtain plate 22. In some embodiments the ion
source 20 may be an ion spray or electrospray device, for example.
Ions passing through an aperture in the curtain plate 22 can enter
into curtain chamber 23, formed between curtain plate 22 and
orifice plate 24. A flow of curtain gas into curtain chamber 23 can
reduce the influx of unwanted neutral particles into the analyzing
sections of mass spectrometer system 10. Ions can leave curtain
chamber 23 through an aperture in orifice plate 24, passing through
rod set 26 and entering into quadrupole rod set 30 by way of an
aperture in interquad barrier 28. One function of quadrupole rod
set 30 can be to collect and focus ions for transmission to
downstream detection stages of mass spectrometer system 10. A
secondary function of quadrupole rod set 30 can be further
extraction of neutral particles from the ion stream that
inadvertently passed through curtain chamber 23.
[0040] Ions collected and focused in quadrupole rod set 30 can exit
through an aperture in interquad barrier 32 and pass through RF
stubby rod set 34 (otherwise known as a Brubaker lens) into
quadrupole rod set 36, which can be configured as a mass filter. As
is known to those skilled in the art, a mass filter can be
configured by applying a combination of quadrupolar RF and direct
current (DC) potentials to a quadruple rod set that selectively
stabilizes or destabilizes ions passing through the rod set. By
controlling the amplitude and the ratio of the DC and RF
potentials, it is possible to isolate ions having masses that fall
inside of a range of interest for transmission to downstream
detection stages, in that ions having masses that fall outside of
the range of interest are destabilized and ejected. In this manner,
quadrupole rod set 36 can substantially isolate a mass range of
interest.
[0041] RF stubby rod set 38 guides ions ejected out of quadrupole
rod set 36 into quadrupole rod set 40. Collision cell 42 encloses
quadrupole rod set 40 and is maintained at a desired high pressure
by pumping in a suitable collision gas, such as nitrogen or argon.
Collision cell 42 also comprises entrance aperture 39 and exit
aperture 43 for letting ions into and out of the collision cell 42,
respectively. RF stubby rod set 44 guides ions exiting collision
cell 42 through exit aperture 43 into quadrupole rod set 46, which
can be maintained at a lower pressure than quadrupole rod set 40.
Finally, ions ejected out of quadrupole rod set 46 pass through
exit lens 48 for mass detection by a suitable detector.
[0042] It will be understood by those skilled in the art that the
representation of FIG. 1 is schematic only. Additional elements may
need to be assembled to complete the mass spectrometer system 10.
For example, a plurality of power supplies might be used for
delivering DC and RF voltages to different elements of the system,
including quadrupole rod sets 36, 40, 46, exit aperture 43 and exit
lens 48. In addition, a gas pump or other arrangement might be used
to maintain different chambers of the system at desired pressure
levels, including collision cell 42 as described. One or more ion
detectors may also be provided. One or more coupling capacitors may
also be provided.
[0043] In the mass spectrometer system 10 shown in FIG. 1,
quadrupole rod set 40 can be configured as a first linear ion trap
40 by applying appropriate RF/DC containment voltages and AC
excitation voltages, such that it can provide mass-selective axial
ejection (MSAE) of ions as disclosed in U.S. Pat. No. 6,177,668. In
like fashion, quadrupole rod set 46 can be configured as a second
linear ion trap 46 also operable for MSAE. As mentioned previously,
quadrupole rod set 36 can be configured as mass filter 36 for
isolating a desired mass range of interest. Moreover, first and
second linear ion traps 40, 46 can be coupled together using
capacitor Ca, while second linear ion trap 46 can be coupled to RF
stubby rod set 44 using Capacitor Cb.
[0044] Ions having masses falling within a mass range of interest
can be selectively filtered by mass filter 36 and accumulated in
first ion trap 40. For example, the masses of the accumulated ions
fall within a mass range defined by a lower and an upper bound ion
mass. Alternatively, the ions that are selected by the mass filter
36 can be transferred at high collision energy into collision cell
42. These ions may as a result be fragmented through collision with
the collision gas molecules pumped into the collision cell 42. A
delay period can be used to cool the fragmented ions formed through
collision assisted dissociation (CAD) and trapped in linear ion
trap 40. At the end of the delay period, first ion trap 40 can
begin to transmit ions by way of RF stubby rod set 44 into second
ion trap 46 using one of the techniques for MSAE taught by U.S.
Pat. No. 6,177,668. Ions that are mass-selectively ejected out of
first ion trap 40 can be accumulated and cooled in second ion trap
46. After another delay period ions can be ejected from linear ion
trap 46 again using one of the MSAE techniques taught by U.S. Pat.
No. 6,177,668. In this fashion, first and second ion traps 40, 46
can be operated in tandem.
[0045] Multiple different techniques for MSAE are known. One such
method involves providing a constant DC trapping field and then
providing an additional auxiliary AC field to the downstream end of
the ion trap. That is, a DC trapping field can be created at the
downstream end of the ion trap by applying a DC offset voltage that
is higher than the DC offset voltage applied to the quadrupole rods
of the ion trap. With these DC voltages so applied, ions that are
stable within the radial RF containment field can encounter the DC
potential barrier created at the downstream end of the ion trap and
be axially trapped as well. In the configuration of FIG. 1, for
example, the requisite DC potential barrier can be created in first
linear ion trap 40 by providing the appropriate DC offset voltage
in the vicinity of exit aperture 43, and likewise in second linear
ion trap 46 by providing the appropriate DC offset voltage to exit
lens 48.
[0046] Ions clustered around the centre of the ion trap can
experience RF containment fields that are near perfectly
quadrupolar. However, ions in the vicinity of the downstream end
can experience imperfectly quadrupolar fields on account of the
RF/DC fields terminating at the end of the quadrupole rod set.
These imperfect fields (commonly referred to as "fringing fields")
tend to couple the radial and axial components of motion of the
trapped ions. In other words, the trapped ions' radial and axial
components of motion may cease to be essentially mutually
orthogonal, unlike the ions clustered around the centre of the ion
trap that have essentially uncoupled, or only very loosely coupled,
components of motion. Because of the fringing fields formed near
the downstream end of the ion trap, ions in the vicinity can be
mass-dependently scanned out of the ion trap by application of a
low voltage auxiliary AC field of the appropriate frequency. The
applied auxiliary AC field couples to both the radial and axial
secular ion motions. By absorbing energy from the auxiliary AC
field, ions can become sufficiently excited such that that they are
able to overcome the DC potential barrier formed at the downstream
end of the ion trap. Ions not sufficiently excited by the auxiliary
AC field can remain contained in the ion trap until the frequency
of the auxiliary AC field is changed to match their secular
frequency, at which point they too can be mass-selectively ejected
out of the ion trap.
[0047] Other techniques for mass-selective axial ejection of ions
can also be implemented on a linear quadrupole rod set. For
example, rather than scanning the frequency of the auxiliary AC
field provided to the exit aperture, the amplitude of the main RF
containment field provided to the quadrupole rods can instead be
scanned. A q value of only about 0.2 to 0.3 can be used for axial
ejection, which is well below the q value of about 0.907 typically
used for radial ejection. Thus, few if any ions may be lost due to
radial ejection when the amplitude of the main RF voltage is
scanned. As described with reference to the drawings, mass
spectrometer system 10 can mass-selectively eject ions by scanning
the main RF containment field over a range of amplitudes. Of
course, it will be appreciated by those skilled in the art that
mass spectrometer system 10 can be adapted or reconfigured for
other MSAE techniques without limiting the scope of the present
invention. It will also be appreciated by those skilled in the art
that different MSAE techniques can be used in combination. For
example, the amplitude of the RF containment voltage can be scanned
in combination with scanning of the applied auxiliary AC excitation
field frequency. Alternatively, other ion traps involving axial
transmission can be used such as, for example, those described in
U.S. Pat. No. 5,783,824 and U.S. Patent Publication No.
2005/0269504 A1.
[0048] Reference is now made to FIG. 2A, which illustrates
exemplary RF voltage and auxiliary AC excitation frequency
waveforms suitable for mass-selective axial ejection of ions for
first and second ion traps 40, 46 in mass spectrometer system 10.
Waveform 110 represents the RF containment voltage applied to first
ion trap 40, while waveform 115 represents the RF containment
voltage applied to second ion trap 46. Accordingly, waveforms 110,
115 may be suitable for MSAE in which the amplitude of the RF
containment voltage is scanned and the frequency of the applied
auxiliary AC excitation field is held constant (represented by
constant line 105). Waveforms 110, 115 may also be provided
independently to first and second ion traps 40, 46 by one or more
voltage sources (not shown).
[0049] As illustrated, both waveforms 110, 115 can comprise an
accumulation/cooling phase, wherein the applied RF voltage is
constant, followed by a mass-selective ejection phase, wherein the
applied RF voltage is linearly scanned. Waveforms 110, 115 can also
comprise a reset phase, wherein the applied RF containment voltages
can be reset to their pre-scan levels and stray ions still trapped
in the mass spectrometer system 10 can be evacuated by lowering the
DC trapping barriers in the first and second ion traps 40,46.
Waveform 115 can be time-delayed relative to waveform 110 by a
delay time interval .DELTA.t, as shown in FIG. 2A and discussed
further below.
[0050] Ions filtered by mass filter 36 can be transmitted into
first ion trap 40 starting at time T0 wherein they can be
accumulated and cooled until time T1. The mass range of ions that
accumulate in first ion trap 40 between times T0 and T1 can be
referred to as the starting mass range 220 of first ion trap 40, as
shown in FIG. 3. At time T1, ions can begin to be mass-selectively
scanned out of the first ion trap 40 into the second ion trap 46 at
a first scan rate, defined in units of Daltons per second (Dais).
The slope of waveform 110 during the mass-selective ejection phase
represents this first scan rate. For example, ions can be scanned
out at a rate of 1000 Da/s, such that after 25 ms of scanning, a 25
Da mass range will have accumulated in second ion trap 46. After a
delay time interval, .DELTA.t in FIG. 2A, ions accumulated in the
second ion trap 46 can begin to be mass-selectively scanned at a
second scan rate. As shown in FIG. 2A, scanning of the first ion
trap 40 commences at T1 and concludes at T3, while scanning of the
second ion trap 46 commences at T2 and concludes at T4. The reset
phase then begins at the end of the mass-selective ejection
phase.
[0051] By setting the second scan rate to substantially equal the
first scan rate, the rate of ions entering the second ion trap 40
can be kept substantially equal to the rate of ions ejected from
it. Thus, over an operating time interval of mass spectrometer
system 10, the mass range of ions trapped in the second ion trap 46
can substantially equal the ion mass range that initially
accumulated in the second ion trap 46 during the delay time
interval .DELTA.t between times T1 and T2. This mass range can be
referred to as the variable operating mass range 222 of the second
ion trap 46. In other words, over the operating time interval of
the mass spectrometer system 10, the mass range of the second ion
trap may approximately equal the scan rate of the first ion trap 40
(1000 Da/s in the example) multiplied by the delay time interval
.DELTA.t between times T1 and T2 (25 ms in the example).
[0052] If ions are scanned out of second ion trap 46 at
substantially the same scan rate as the scan rate of the first ion
trap 40, only time-delayed by the delay time interval .DELTA.t,
then the variable operating mass range 222 of the second ion trap
46 can be set narrower than the starting mass range 220 of the
first ion trap 40 by selecting the appropriate delay time interval
.DELTA.t. Again in terms of the above example, at any point after
the 25 ms delay time interval, the ions in the second ion trap 46
may have a mass range of approximately 25 Da. Thus, if the starting
mass range 220 of the first ion trap 40 is 1000 Da, then the
variable operating mass range 222 of the second ion trap 46 may be
only approximately 2.5% of the starting mass range of first ion
trap 46. If the starting mass range 220 of the first ion trap 40
were 500 Da instead, then the variable operating mass range 222 of
the second ion trap 46 may be only approximately 5% of the starting
mass range 222 of the first ion trap 40. By having a narrower ion
mass range during the operating time interval of the mass
spectrometer system 10, the second ion trap 46 may be less
susceptible to space charge effects relative to the first ion trap
40. As a result ions can be scanned out of second ion trap 46 with
higher resolution than they otherwise could have been scanned out
of first ion trap 40. Being less susceptible to space charge
effects, the second ion trap 46 may also have a shorter length,
relative to first ion trap 40, in alternative embodiments of the
present invention.
[0053] As described above, waveforms 110, 115 may be suitable for
MSAE in which the amplitude of the RF containment voltage is
scanned and the frequency of the applied auxiliary AC field is held
constant. As it will be appreciated by those skilled in the art,
the Mathieu q-value for a linear quadrupole ion trap may be given
by:
q = 4 e V m r 0 2 .OMEGA. 2 , ( 1 ) ##EQU00001##
where m and e are the ion mass, and charge, respectively, r.sub.0
is the field radius of the quadrupole trap, .OMEGA. is the angular
drive frequency of the quadrupole, and V is the amplitude of the RF
radial containment field measured pole to ground. Also, ion
fundamental resonant frequency can be represented by:
.omega. = ( 2 n + .beta. ) .OMEGA. 2 , ( 2 ) ##EQU00002##
which, by setting n=0 and using the relationship defined in
equation 1, can be re-written as:
.omega. .apprxeq. q .OMEGA. 8 , for q < 0.4 . ( 3 )
##EQU00003##
Alternatively, equation 3 can be expressed explicitly in terms of
the frequency of the applied auxiliary AC field, .omega., and the
RF amplitude of the radial containment field, V as:
.omega. V .apprxeq. 2 e m r 0 2 .OMEGA. , for q < 0.4 . ( 4 )
##EQU00004##
[0054] Resonant excitation of an ion occurs when the frequency of
the auxiliary AC field applied to the quadruple coincides with the
ion fundamental resonant frequency, .omega.. Thus, it will be
appreciated how equation 4 may define an overall relationship, for
each ion trap 40, 46, between the frequency of the applied
auxiliary AC field, equal to .omega., and the RF amplitude of the
radial containment field, V, that results in resonant excitation of
ions having mass, m, and charge, e, trapped in a quadrupole field
of radius, r.sub.0, and drive frequency, .OMEGA.. This overall
relationship, moreover, may be used as part of a control system for
first and second ion traps 40, 46. In particular, if the same
auxiliary AC field is applied to each ion trap 40, 46, then
resonant excitation of ions may occur for the same applied RF
amplitude, V. As illustrated by waveform 105 in FIG. 2A, the
auxiliary AC excitation frequency applied to each of first and
second ion traps 40, 46 may be constant and equal. In that case,
controlling the rate at which the RF amplitudes for first and
second ion traps 40, 46 are scanned, therefore, may provide a way
of controlling the times at which ions of particular masses and
charges are ejected. For example, the RF amplitude of the second
ion trap 46 may be scanned at the same rate as the RF amplitude of
the first ion trap 40, only time-delayed by the delay time
interval, as seen in waveforms 110, 115. These waveforms may also
be provided independently to first and second ion traps 40, 46 by
one or more voltage sources. The selected delay-time interval may
also substantially correspond to a cooling time of the ions.
[0055] Reference is now made to FIG. 2B, which illustrates
exemplary RF voltage and auxiliary AC excitation frequency
waveforms suitable for mass-selective axial ejection of ions for
first and second ion traps 40, 46 in mass spectrometer system 10
according to an aspect of an alternative embodiment of the present
invention. In this alternate embodiment, MSAE of ions may be
provided using constant RF containment fields, and by scanning the
frequency of the auxiliary AC excitation fields applied to first
and second ion traps 40, 46. Waveform 120 in FIG. 2B represents the
amplitude of the RF containment field applied to second ion trap
46, while waveform 125 represents the amplitude of the RF
containment field applied to first ion trap 40. As illustrated,
waveforms 120 and 125 have different amplitudes, but they may also
have the same amplitude. The RF containment voltages may be
provided independently by one or more voltage sources or using
capacitive coupling, as described below. In general, waveforms for
the first ion trap are represented using a dashed line, while
waveforms for the second ion trap are represented using a solid
line.
[0056] Waveforms 130 and 135 represent the auxiliary AC frequency
waveforms that may be suitable for MSAE of ions. Waveform 130
represents the frequency of the auxiliary AC excitation field
applied to second ion trap 46, while waveform 135 represents the
frequency of the auxiliary AC excitation field applied to first ion
trap 40. As illustrated, waveform 130 is a scaled and time-delayed
version of waveform 135 during the mass-selective ejection phase.
That is, waveform 130 is time-delayed by the delay time interval
and scaled, according to equation 4, in the same proportion as
waveforms 120 and 125 are scaled. By setting this particular
relationship between waveforms 130 and 135, ions of a certain mass
ejected out of first ion trap 40, into second ion trap 46, may then
also be ejected from second ion trap 46 after having been cooled in
second ion trap 46 for a period of time equal to the delay time
interval .DELTA.t.
[0057] Reference is now made to FIG. 2C, which illustrates
exemplary RF voltage and auxiliary AC excitation frequency
waveforms suitable for mass-selective axial ejection of ions for
first and second ion traps 40, 46 in mass spectrometer system 10
according to an aspect of an alternate embodiment of the present
invention. Waveform 140 represents the RF containment voltage
applied to second ion trap 46, while waveform 145 represents the RF
containment voltage applied to first ion trap 40. Similar to
waveforms 110, 115 shown in FIG. 2A, waveforms 140, 145 each
comprise an accumulation/cooling phase, a mass-selective ejection
phase and a reset phase. The ratio 150 of the amplitude of waveform
140 to the amplitude of waveform 145 can be substantially constant
over an operating time interval, for example between times T0 and
T4.
[0058] Waveforms 140, 145 may represent RF containment voltages
suitable for MSAE of ions in which, as is known from U.S. Pat. No.
6,177,668, the frequency of the applied auxiliary AC field is
scanned in addition to the amplitude of the ion trap RF containment
voltage. As illustrated, the amplitudes of waveforms 140, 145 may
be scanned, not at the same rate, but in approximately the same
proportion. That is, the ratio 150 of the amplitudes may be
substantially fixed.
[0059] Waveforms 140, 145 may be applied independently to second
and first ion traps 46, 40 by one or more voltage sources, but
waveforms 140, 145 may also be applied using capacitive coupling
between first and second ion traps 40, 46. For example, as
illustrated in FIG. 1, capacitor Ca may couple first ion trap 40
with second ion trap 46, and capacitor Cb may couple second ion
trap 46 with RF stubby 44. Together with additional circuit
elements as may be needed, capacitors Ca and Cb set up an AC
voltage divider between first and second ion traps 40, 46.
Accordingly, as is known, the ratio 150 can be selected by
selecting appropriate values for Ca and Cb. For example, the ratio
150 of waveform 140 to waveform 145, representing the amplitudes of
the RF containment voltages applied to second and first ion traps
46, 40, respectively, may be approximately equal to 2 over an
operating interval of the mass spectrometer 10.
[0060] According to equation 1, assuming that first and second ion
traps have the same quadrupole field radius, r.sub.0, the q value
of the first ion trap 40 will be approximately half of the q value
of second ion trap 40 for a ratio 150 approximately equal to 2.
Similarly, according to equation 3, the ion fundamental resonant
frequency, .omega., of the first ion trap 40 will be approximately
half that of the second ion trap 46. So, for example, if second ion
trap 46 is operated at q=0.846 over the operating interval, then
the auxiliary AC excitation frequency applied to first ion trap 40
may correspond to some value q<0.423. The relationship is
expressed as an inequality to reflect the fact that ions of a
certain mass may be excited out of second ion trap 46 some delay
time interval after they are ejected out of first ion trap 40 (and
into second ion trap 46). Controlling the delay time interval may
be accomplished by controlling the auxiliary excitation frequency,
.omega., applied to the first ion trap 40. The lower the q value at
which ions may be ejected from first ion trap 40, the lower the
excitation frequency, w, and correspondingly the bigger the delay
time interval. That delay time interval, again, may correspond to a
cooling time of the ions.
[0061] Stated in slightly different terms, for each of first and
second ion traps 40, 46, equation 4 may provide an overall
relationship, between the RF amplitudes, V.sub.1, V.sub.2 and the
auxiliary AC excitation frequencies, .omega..sub.1, .omega..sub.2.
Given RF amplitudes V1, V2, for example as represented by waveforms
145, 140, respectively, equation 4 therefore provides auxiliary
excitation frequencies .omega..sub.1, .omega..sub.2 suitable for
MSAE of ions. Waveforms 155 and 160, for example, illustrate
exemplary auxiliary AC excitation frequencies, as a function of
time, suitable for MSAE of ions. In particular, .omega..sub.1,
.omega..sub.2 may be scanned such that, over a mass range of ions
and an operating interval of mass spectrometer 10, ions are ejected
out of second ion trap 46 a delay time interval after being ejected
out of first ion trap 10 (and into second ion trap 46). As
illustrated by waveform 160, the auxiliary AC excitation frequency
for first ion trap 40 may be selected to scan linearly during the
mass-selective ejection phase of first ion trap 40, as defined by
line times T1 and T3. Equation 4 may then provide a means of
determining how to scan the auxiliary AC excitation frequency for
second ion trap 46, illustrated by waveform 155. In such a case,
the scan rate of second ion trap may be non-linear. During times T1
and T2, when second ion trap 46 is accumulating ions ejected from
first ion trap 40, the auxiliary AC excitation frequency may,
according to equation 4, be any value such that, given the
amplitude of the RF containment field applied to second ion trap
46, the fringing fields in second ion trap 46 do not cause any
appreciable resonant excitation of ions until at least time T2. At
time T2, however, when second ion trap 46 may commence MSAE of
ions, then the value of the auxiliary AC excitation frequency may
be controlled for MSAE, again according to equation 4, for example.
When first and second ion traps 40, 46 are operated such that both
RF amplitude and auxiliary AC excitation frequency are scanned,
then scanning of (Di, (02 can be thought of as serving a
compensatory function to correct for the different, though
proportionate, scan rates of V.sub.1, V.sub.2, and which, without
this compensatory function, would result in different ion ejection
rates for first and second ion traps 40, 46. Again, as described
previously, the delay time interval may correspond to a cooling
time of ions.
[0062] Reference is now made to FIG. 3, which shows examples of ion
mass ranges for first and second ion traps 40, 46 when excited
using RF voltage waveforms such as those shown in FIGS. 2A-2C.
Region 205 represents the mass range of ions trapped in first ion
trap 40 as a function of time. Similarly, region 210 represents the
mass range of ions trapped in second ion trap 46 as a function of
time. FIG. 3 is not necessarily drawn to scale and is figurative
only. As illustrated, region 205 has a starting mass range 220
defined by a lower and upper bound mass (M.sub.LOW and M.sub.UPP
respectively). As shown, region 205 is bounded vertically by
horizontal lines 206 and 207 at .omega..sub.LOW and M.sub.UPP
respectively, on the left by the Y axis at time T0, and on the
right by a sloping 208 line extending from (T1, M.sub.LOW) to (T3,
M.sub.UPP). During the accumulation/cooling phase, i.e. between
times T0 and T1, the mass range of first ion trap 40 remains
substantially constant at the starting mass range 220. However, as
ions begin to be mass-selectively scanned out of first ion trap 40
starting at time T1, the mass range of trapped ions begins to
narrow over time. As the amplitude of waveform 110 is scanned, ions
of increasingly greater mass are ejected out of first ion trap 40
until time T3 by which point no or only a negligible number of ions
may remain in first ion trap 40.
[0063] In the second ion trap, initially (before time T1) there may
be no or only a negligible number of ions because scanning of ions
out of first ion trap 40 has not yet commenced. But during the
delay time interval .DELTA.t between times T1 and T2, ions of
increasingly greater mass, i.e. those ejected out of first ion trap
40, can be accumulated until second ion trap 46 reaches its
operating mass range 222 at time T2. At that point, since the
injection and ejection rates of second ion trap 46 can be
approximately equal, the range of ion masses trapped in second ion
trap 46 can remain substantially constant, though the ion masses
themselves can increase over time. By time T3 first ion trap 40 has
ejected all or substantially all the ions trapped within it, at
which point the mass range of ions trapped in second ion trap 46
can begin to narrow, as shown in FIG. 3, until eventually all or
substantially all the ions can be ejected from second ion trap 46,
which occurs at time T4. As shown in FIG. 3, and as can be inferred
from what is described above, the region 210 has a lower bound
defined by horizontal line 206 extending from (T1, M.sub.LOW) to
(T2, M.sub.LOW), and is bounded at its upper end by horizontal line
207 extending from (T3, M.sub.UPP) to (T4, M.sub.UPP). Region 210
is also bounded on the left by the sloped line 208 extending from
(T1, M.sub.LOW) to (T3, M.sub.UPP), and is bounded on the right by
a sloped line 209 extending from (T2, M.sub.LOW) to (T4,
M.sub.UPP).
[0064] The main RF containment voltage and/or auxiliary AC
excitation frequency, depending as the case may be on how
mass-selective axial ejection is being implemented, may be either
continuously or discontinuously scanned. Where the voltage is
continuously scanned it may be either linearly or non-linearly
scanned. Different RF/AC voltage waveforms are suitable for this
purpose. FIGS. 2A-2C illustrate RF pairs of voltage waveforms 110
and 115, 120 and 125, and 140 and 145, respectively, that may be
suitable for continuous and linear scanning of ions. FIG. 3 may
then represent the resulting mass ranges for first and second ion
traps 40, 46, according to any of these applied RF/AC voltages. It
will be appreciated that, as described above, the auxiliary AC
excitation frequencies for first and second ion traps 40, 46 may be
scanned in addition to the RF containment voltages according to
aspects of some embodiments of the present invention. Waveforms
140, 145 in FIG. 2C may represent those RF containment voltages. It
may also be the case that only the auxiliary AC excitation
frequencies are scanned, as illustrated by waveforms 130, 135 in
FIG. 2B. Finally, it will also be appreciated that other RF/AC
voltage waveforms can be suitable according to alternative
embodiments of the present invention, which can produce different
resulting mass ranges.
[0065] Referring again to FIG. 2A, as discussed previously, ions
can be scanned out of first and second ion traps 40, 46 using mass
selection axial ejection techniques as taught, for example, in U.S.
Pat. No. 6,177,668. To operate first and second ion traps 40, 46
for tandem MSAE, the main RF containment voltages applied to the
first and second ion traps 40, 46 can be scanned in tandem. In
particular, the RE voltage 115 applied to the second ion trap 46
can substantially correspond to the RF voltage 110 applied to the
first ion trap 40 only time-delayed by a delay time interval
.DELTA.t, such that mass-selection ion ejection in the second ion
trap 46 lags behind mass-selective ion ejection in the first ion
trap 40 by that delay time interval .DELTA.t. For this purpose,
independent RF voltages can be applied to first and second ion
traps 40, 46 using separate power supplies.
[0066] Alternatively, RF containment voltages can be applied to
first and second ion traps 40, 46 using one or more coupling
capacitors, such as those illustrated in FIG. 1. In these
configurations of mass spectrometer 10, capacitance values can be
chosen to establish different proportions between the RF
containment voltages applied to first and second ion traps 40, 46.
FIGS. 2B and 2C illustrate suitable pairs of waveforms 120, 125 and
140, 145. By selecting values for coupling capacitors Ca, Cb, and
controlling the applied RF containment and auxiliary AC excitation
frequencies applied to first and second ion traps 40, 46, over a
mass range of ions and an operating time interval of the mass
spectrometer 10, ions of a certain mass can be ejected from second
ion trap 46 a delay-time interval after being ejected out of first
ion trap 40. The delay time interval moreover can be chosen to
substantially correspond to the cooling time of ions accumulated in
second ion trap 46, which in turn depends on characteristics of the
ions (mass, initial energy, etc.) as well as characteristics of the
ion trap (volume, pressure, etc.) The delay-time interval could be
greater than the cooling time of the ions, but doing so reduces the
duty cycle of the mass spectrometer system and thus may generally
be undesirable.
[0067] Various aspects of embodiments of the present invention are
described below with reference to FIGS. 2A-2C and 3. A method of
operating a tandem mass spectrometer system can be described by
reference to the state of the mass spectrometers or the ion traps
included in the system at different times. For example, at a first
time, between T0 and T1, ions can be accumulated in the first ion
trap 40. Then, at a second time, at any time between T1 and T3 as
shown in FIGS. 2A and 3, a first plurality of ions can be
transmitted from the first ion trap 40 and into the second ion trap
46. The first plurality of ions would have masses within a first
mass range. Also at this second time, a second plurality of ions
could be retained in the first ion trap 40. The second plurality of
ions would have masses within a second mass range different from
the first mass range. Now consider a third time, after the second
time somewhere between T2 and T3 shown in FIGS. 2A and 3. During
this third time, the first plurality of ions could be transmitted
out of the second ion trap 46, while the second plurality of ions
could be transmitted from the first ion trap 40 into the second ion
trap 46.
[0068] The foregoing description can be seen as a series of three
snapshots taken at three different times throughout a method in
accordance with an aspect of an embodiment of the present
invention. For clarity, this description is repeated with specific
reference to FIG. 3, in which the first time, second time and third
time are designated using reference numerals 212, 214 and 216
respectively. Specifically, as shown, at the first time 212, ions
are accumulating in the first ion trap 40. Alternatively, ions may
have been accumulating in the first ion trap before time T0. Then,
at a second time 214, a first plurality of ions having a mass range
defined by upper bound M.sub.1 can be transmitted from the first
ion trap 40 to the second ion trap 46, while a second plurality of
ions, having a second mass range from just above M.sub.1 to M.sub.2
can be retained in the first ion trap. Note that, as illustrated in
FIG. 3, second time 214 falls between T1 and T2 though it may also
fall between T2 and T3. At a third time 216, the first plurality of
ions, having a maximum mass M.sub.1, can now be ejected from the
second ion trap 46, while the second plurality of ions, having a
mass range between just above M.sub.1 and M.sub.2, can be
transmitted from the first ion trap 40 to the second ion trap
46.
[0069] The foregoing description can be seen as a series of
snapshots of a method in accordance with an aspect of the present
invention at different times. As described above, it can be
advantageous to maintain a much higher first space charge density
in the first ion trap 40 at the second time 214 relative to the
second space charge density in the second ion trap 46 at the second
time 214. Where, as described above, the second time 214 is close
to T1, the first space charge density may be 5, 10, or 20 times the
second space charge density. Of course, as the second time 214
moves from T1 toward T3, the relative difference in the space
charge densities of the first and second ion traps 40, 46 may well
diminish.
[0070] While some aspects of embodiments of the present invention
can perhaps be better described through a series of snapshots,
other aspects of embodiments of the present invention are perhaps
better described by using a more dynamic vocabulary to describe how
the method operates over time analogous to, say, a video, rather
than a series of snapshots. As shown in FIG. 3, the variable
operating mass range 222 between lines 208 and 209, for operating
times falling between T1 and T3, can be seen as an instance of a
first sliding transmission window having an upper bound defined by
the height of line 208. The upper bound of the first sliding
transmission window is related to the RF voltage and auxiliary AC
excitation frequency applied to the first ion trap 40 for MSAE. In
particular, according to equations 1 and 3, for a given RF voltage
level and auxiliary AC excitation frequency, the upper bound of the
first sliding transmission window may define the heaviest ion mass
that will, for that RF voltage level and auxiliary AC excitation
frequency, be sufficiently excited for MSAE out of the first ion
trap 40. As the RF voltage level is scanned, according to aspects
of some embodiments of the present invention, the upper bound of
the first sliding transmission window increases. Thus, between T1
and T3, over which the RF voltage waveform 110 is scanned, the
upper bound of the first sliding transmission window will change.
In particular, as shown in FIG. 3, at the second time 214, the
first sliding transmission window will have an upper bound at
M.sub.1, while at the third time 216, the first sliding
transmission window will have an upper bound at M.sub.2. In other
embodiments, the auxiliary AC excitation frequency applied to first
ion trap 40 is also scanned between T1 and T3 as the upper bound of
the first sliding transmission window changes.
[0071] Similarly, consider a second sliding transmission window
representing those ions that are transmitted out of the second ion
trap 46. As with the first sliding transmission window, the upper
bound of the second sliding transmission window, represented by
sloped line 209, will change over time as the amplitude of RF
voltage waveform 115 is scanned between T2 and T4. Thus, until the
third time 216, the second ion trap 46 would be operable to retain
the first plurality of ions having a mass of at least M.sub.1;
however, at the third time 216, the upper bound of the second
sliding transmission window will reach ions of mass M.sub.1, such
that these ions can now be ejected from the second ion trap 46. As
with the first sliding transmission window, according to aspects of
some embodiments of the present invention, the RF voltage waveform
115 is scanned between T2 and T4, while in other embodiments the
auxiliary AC excitation frequency applied to second ion trap 46 is
also scanned.
[0072] As shown in FIG. 3, the first variable mass range covered by
the first sliding transmission window and the second variable mass
range covered by the second sliding transmission window can be
linearly scanned at substantially the same rate. Over an operating
time interval from T2 to T3, for example, the second sliding
transmission window can be time-delayed relative to the first
sliding transmission window by a delay time interval, shown as
.DELTA.t in FIG. 3, such that the first variable mass range at any
operating time during the operating time interval can substantially
correspond to the second variable mass range at the operating time
plus the delay time interval .DELTA.t. For example, as shown in
FIG. 3, the points at which a horizontal line representing M.sub.1
intersects slope lines 208 and 209 are separated by approximately
.DELTA.t. In some embodiments, as shown, the first scan rate
represented by the slope of line 208, can substantially equal the
second scan rate, represented by the slope of line 209.
[0073] Optionally, a second space charge level can be selected for
the second ion trap 46, and a cooling time interval selected for
retaining ions in the second ion trap 46 to provide the second
space charge level. In that case, the delay time interval .DELTA.t
may substantially equal the cooling time interval.
[0074] As described above, the first scan rate can be represented
in FIG. 3 by a slope of line 208. Multiplying this slope by the
delay time interval .DELTA.t, can yield the vertical distance
between lines 208 and 209 at any point between T2 and T3, assuming,
of course, that the slopes 208 and 209 are equal (in other words,
that the scan rates of the first ion trap 40 and the second ion
trap 46 are equal). This vertical difference is, of course, the
variable operating mass range 222 of second ion trap 46.
Optionally, to improve resolution and reduce the space charge
problems, this variable operating mass range 222 can be kept
relatively small as compared to the starting mass range 220. For
example, it can be less than half of the starting mass range 220,
or even less than the fifth or a tenth of the starting mass range
220.
[0075] According to some embodiments of the present invention, the
first ion trap and the second ion trap can be capacitively coupled.
In some such embodiments, the first scan rate from the first ion
trap can be controlled by adjusting the first RF voltage and the
first auxiliary AC voltage provided to the first ion trap. Then, as
a result of the capacitive coupling, a second RF voltage can be
automatically applied to the second ion trap. Again, as a result of
the capacitive coupling, the ratio of the first RF voltage applied
to the first ion trap and the second RF voltage applied to the
second ion trap can be kept substantially constant over the
operating time of tandem ion traps. Specifically, the ratio of the
first RF voltage and the second RF voltage can be controlled by
selecting the capacitances of the one or more coupling
capacitors.
[0076] As described above, it can be desirable for the first scan
rate from the first ion trap to equal the second scan rate from the
second ion trap. To provide this in embodiments in which the ion
traps are capacitatively coupled, the first auxiliary AC voltage
applied to the first ion trap and the second auxiliary AC voltage
applied to the second trap can be determined based on the ratio of
the first RF voltage to the second RF voltage such that the first
scan rate substantially equals the second scan rate. Of course,
according to other embodiments, as described above, the first RF
voltage and the second RF voltage can be independently provided to
the first and second ion traps respectively.
[0077] Reference is now made to FIGS. 4-7, which are block diagrams
illustrating different possible configurations of a triple
quadrupole mass spectrometer system according to alternative
embodiments of the present invention. These alternative embodiments
function in the same or a similar manner to mass spectrometer
system 10 illustrated in FIG. 1. Accordingly, only differences in
the alternative embodiments will be explained in detail. For
clarity, elements of the alternative embodiments illustrated in
FIGS. 4-7 are designated using the reference numerals used to
designate similar or analogous elements in the mass spectrometer
system 10 of FIG. 1.
[0078] FIG. 4 illustrates a block diagram of mass spectrometer
system 100 configured according to an alternative embodiment of the
present invention. Mass spectrometer system 100 comprises skimmer
plate 52 instead of quadrupole rod set 26 and interquad barrier 28,
both of which are included in mass spectrometer system 10. Ions
exiting curtain chamber 23 through the aperture in orifice plate 24
pass through skimmer plate 52 into quadrupole rod set 30. Mass
spectrometer. system 100 also comprises additional interquad
barrier 50.
[0079] Triple quadrupole mass spectrometer system 100 is operated
as a tandem linear ion trap mass spectrometer by configuring RF
stubby 44 to act as a first ion trap and quadrupole rod set 46 to
act as a second ion trap. Indeed additional interquad barrier 50 is
included in mass spectrometer system 100 as one possible
configuration for setting up a DC trapping field in RF stubby 44.
An auxiliary AC field can also be provided to interquad barrier 50.
Optionally, the frequency of the applied auxiliary AC field can be
scanned if that mode of MSAE is being implemented. Otherwise
interquad barrier 50 can receive a DC potential and substantially
constant auxiliary AC excitation frequency, while the main RF
containment voltage applied to the quadrupole rods of RF stubby 44
can be scanned to provide MSAE of ions. In mass spectrometer system
100, collision cell 40 can be maintained at a relatively high
pressure to assist with ion cooling, though first and second ion
traps 44, 46 can both maintained at low pressure. For example, the
operating pressure in collision cell 40 can be maintained between
5.times.10.sup.-5 Torr and 20 mTorr, while the operating pressure
in ion traps 44, 46 can be maintained between 6.times.10.sup.-6
Torr and 5.times.10.sup.-4 Torr. Also, coupling capacitors Ca, Cb
can be utilized as part of a voltage divider for setting the ratio
of RF containment voltages applied to first and second ion traps
44, 46, which, together with appropriate scanning of applied
auxiliary AC excitation frequencies, can provide tandem MSAE of
ions out of first and second ion traps 44, 46 according to aspects
of some embodiments of the present invention.
[0080] FIG. 5 illustrates a block diagram of mass spectrometer
system 200 configured according to an alternative embodiment of the
present invention. Mass spectrometer system 200 comprises skimmer
plate 52 instead of quadrupole rod set 26 and interquad barrier 28
in like fashion to mass spectrometer system 100, and further has
quadrupole rod set 36 configured as a first ion trap and quadrupole
rod set 46 configured as a second ion trap. Thus, in mass
spectrometer system 200, ions can pass through high-pressure
collision cell after ejection from first ion trap 36 and before
accumulation in second ion trap 46. First and second ion traps 36,
46 can both be maintained at low pressure. Note also that in the
configuration of mass spectrometer system 200, RF containment
voltages can be supplied independently to first and second ion
traps 36, 46 because, as illustrated, no capacitive coupling is
provided between them. Of course, mass spectrometer 200 system in
other embodiments can be reconfigured to provide capacitive
coupling between first and second ion traps 36, 46.
[0081] FIG. 6 illustrates a block diagram of mass spectrometer
system 300 configured according to an alternative embodiment of the
present invention. Mass spectrometer system 300 comprises skimmer
plate 52 instead of quadrupole rod set 26 and interquad barrier 28
in like fashion to mass spectrometer system 100 and 200, and
further has quadrupole rod set 30 configured as a first ion trap
and quadrupole rod set 36 configured as a second ion trap.
Capacitor Ca now couples first and second ion traps 30, 36, while
capacitor Cb similarly couples RF stubby 34 and second ion trap 36.
Thus, mass spectrometer system 300 is configured to have the RF
containment voltages provided to first and second ion traps 36, 46
using capacitive coupling and one or more voltage sources (not
shown).
[0082] FIG. 7 illustrates a block diagram of mass spectrometer
system 400 configured according to an alternative embodiment of the
present invention. Mass spectrometer system 400 differs from mass
spectrometer system 300 in terms of the detection method used to
detect ions mass-selectively ejected from second ion trap 36. In
particular, mass spectrometer system 400 comprises on orthogonal
time-of-flight mass spectrometer 54 that can be used to detect and
distinguish ions as is known to those skilled in the art.
[0083] FIG. 8A illustrates an alternative configuration of a tandem
linear ion trap mass spectrometer system 500 according to an aspect
of the applicant's teachings. In system 500, RF stubby rod set 34
can be used as a timed-ion-selector (TIS). The pulsed DC voltage 60
applied to the RF stubby rod set 34 can be applied to eject
unwanted ions. The pulsed DC voltage 60 can be, but is not limited,
to a quadrupolar voltage or a dipolar voltage. The DC applied to
the RF stubby rod set 34 can be but is not limited to one pair of
rods or two pairs of rods that ejects or deflects ions when the
pulsed DC voltage 60 is applied. When applied in a quadrupole
fashion, the pulsed DC voltage 60 can be mass-dependent and can
correspond to the Mathieu parameter a having a value that causes
the ion trajectory of the unwanted ion to become unstable in the
TIS (i.e. the RF stubby rod set 34). Because unwanted ions are
ejected, only the ions of interest are transferred to the
quadrupole rod set 36 where they will accumulate when it acts as an
ion trap. FIG. 8B is a sectional drawing showing the details of RF
stubby rod set 34 configured to operate as a TIS.
[0084] FIG. 9A illustrates an alternative configuration of a tandem
linear ion trap mass spectrometer system 600 according to an aspect
of the applicant's teachings. In system 600, the RF stubby rod set
34 is configured to operate as a low pressure trap. Additional
lenses or interquad barriers 62, 63 are positioned between the
stubby rod set 34 and quadrupole rod set 36. In this example,
auxiliary electrodes 66 can be a set of four plates arranged in a
quadrupole configuration to form a transmission window for the ion
beam. Auxiliary electrodes 66 are oriented generally orthogonal to
trajectory of the ion beam and are positioned between the interquad
barriers 62, 63, such that the electrodes are substantially
parallel to the axis of the ion beam. In another example, the
auxiliary electrodes 66 can be a top pair of parallel plates and a
substantially bottom pair of parallel plates forming a rectangular
transmission window. It can be appreciated that other orientations
and configurations of the auxiliary electrodes 66 are also possible
as long as a transmission of suitable size and shape is defined.
For example, plates of different cross-sectional shapes can also be
used.
[0085] FIG. 9B is a cross sectional diagram detailing the auxiliary
electrodes of FIG. 9A acting as a TIS. The auxiliary electrodes can
be coupled to a controllable voltage source (not shown), which can
be configured to provide a pulsed DC voltage 64, i.e. a square wave
pulse train. Application of the pulsed DC voltage 64 to the
auxiliary electrodes 66 can establish an ion ejection or deflection
field between the electrodes during time intervals when the pulsed
DC voltage is high. Unwanted ions are deflected or ejected from the
ion beam. Ions of interest are left and transferred to the second
trap or quadrupole rod set 36. After a short cooling period, ions
of interest are mass selective axially ejected from the quadrupole
rod set 36.
[0086] As it is known in the art, the configurations of FIGS. 8A
and 9A may be used in combination with other kinds of devices such
as a time-of-flight spectrometer. FIG. 10 shows a mass spectrometer
system 700 having the configuration of FIG. 8A in combination with
an orthogonal time-of-flight mass spectrometer 54 that can be used
to detect and distinguish ions as is known to those skilled in the
art. In this example, the tandem trap system 600 is used as a
multiplexing device in conjunction with a TOF mass
spectrometer.
[0087] FIG. 11 shows an alternative configuration of a tandem
linear ion trap mass spectrometer system 800 according to an aspect
of the applicant's teachings. In mass spectrometer system 800,
quadrupole set 40 acts as an ion trap and the RF stubby rod set 44
can be used as a TIS to select product ions from the collision cell
42 while ejecting unwanted fragmented ions. The pulsed DC voltage
70 can be, but is not limited to, a quadrupolar voltage or a
dipolar voltage. The DC applied to the RF stubby rod set 44 can be
but is not limited applied to one pair of rods that ejects or
deflects ions when the pulsed DC voltage 70 is applied. Because
unwanted ions are ejected, only product ions of interest are
accumulated in the quadrupole rod set 46 when it acts as an ion
trap.
[0088] FIG. 12 shows an alternative configuration of a tandem
linear ion trap mass spectrometer system 900 according to an aspect
of the applicant's teachings. In mass spectrometer system 900, the
RF stubby rod set 44 is configured to operate as a low pressure
trap. Additional lenses or interquad barriers 82, 84 are positioned
between the stubby rod set 44 and quadrupole rod set 44. Auxiliary
electrodes 86 can be but are not limited to a set of four plates
arranged in a quadrupole configuration to form a transmission
window positioned between the interquad barriers 82, 84. This
transmission window is oriented such that the electrodes are
substantially parallel to the axis of the ion beam. In another
example, the auxiliary electrodes 86 can be a top pair of parallel
plates and a substantially bottom pair of parallel plates forming a
rectangular transmission window. It can be appreciated that other
orientations and configurations of the auxiliary electrodes 66 are
also possible as long as a transmission of suitable size and shape
is defined. For example, plates of different cross-sectional shapes
can also be used.
[0089] The auxiliary electrodes can be coupled to a controllable
voltage source (not shown), which can be configured to provide a
pulsed DC voltage 80, i.e. a square wave pulse train. Application
of the pulsed DC voltage 80 to the auxiliary electrodes 86 can
establish an ion ejection or deflection field between the
electrodes during time intervals when the pulsed DC voltage is
high. Unwanted ions are deflected or ejected from the ion beam.
Ions of interest are left and transferred to the second trap or
quadrupole rod set 36. After a short cooling period, which is
dependent on the scan rate of the traps, ions of interest are mass
selective axially ejected from the quadrupole rod set 46 toward the
detector.
[0090] Both the configurations of FIGS. 11 and 12 can be used in
enhanced MS (EMS) scan mode when quadrupole rod set 36 transmits
the whole mass range or large windows of the ions with the selected
mass-to-charge range. In this case, ions are not fragmented in
quadrupole set 40.
[0091] Other variations and modifications of the invention are
possible. For example, multipoles other than quadrupoles can be
used to implement different aspects of the invention. Further, mass
spectrometer or ion trap configurations in addition to those
described above can also be used to implement different aspects of
the invention. For example, instead of mass selective axial
ejection ions can be radially ejected from one linear ion trap to
another ion trap. Radial ejection can be performed through one of
the rods out of the main RF poles, as described by the U.S. Pat.
No. 5,420,425B1, or through a slot in an auxiliary rod interposed
between the main RF poles as described by U.S. Pat. No.
6,770,871B1. In addition, techniques of mass selective axial
ejection other than those described above can also be employed,
i.e. U.S. Pat. No. 5,783,824, WO7072038A2, US2007045533 and U.S.
Pat. No. 7,084,398B2. In the case of the last mentioned technique
where the ions get ejected out of the first trap from high to low
mass, the second trap can be scanned from high to low mass. All
such modifications and variations are believed to be within the
sphere and scope of the invention as defined by the claims.
* * * * *