Note: Descriptions are shown in the official language in which they were submitted.
CA 02281405 1999-09-02
BP #571-580
BERESKIN & PARK CANADA
Title: MASS SPECTROMETER WITH TAPERED ION GUIDE
Inventor(s): Charles Jolliffe, Bruce Thomson
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Title: MASS SPECTROMETER WITH TAPERED ION GUIDE
FIELD OF THE INVENTION
This invention relates to a mass spectrometer system having
a tapered ion guide for producing a flat thin sheet of charge suitable for
introduction into a mass analyzer such as a time of flight (TOF)
instrument.
BACKGROUND OF THE INVENTION
TOF mass analyzers have existed for many years and are
noted for their high resolution. However for optimum results, they
require an input ion beam in which the initial conditions, namely the start
time, location and energy of the ions, have minimum dispersion. If the
ions introduced into the TOF analyzer are, for example, dispersed spatially,
then the resolution will suffer.
It has become common practice to introduce ions into a TOF
analyzer using the method of orthogonal extraction from a quadrupole or
tandem mass spectrometer, such as the collision cell of a tandem mass
spectrometer. Unfortunately, the ion beam from the collision cell of a
conventional mass spectrometer has a relatively large diameter, typically
about 2 mm. The spatial dispersion inherent in this beam adversely affects
the resolution of a following TOF analyzer. It is difficult to reduce the
spatial dispersion, and doing so tends to increase space charge effects,
which also adversely impact the performance of the instrument.
BRIEF SUMMARY OF THE INVENTION
It is therefore an object of the invention to provide an ion
guide which will accept an ion beam from a preceding stage and will
reduce the diameter of the ion beam while spreading it out, to produce a
relatively flat, thin, sheet of charge which is more suited for introduction
into an orthogonal extraction TOF instrument.
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In one of its aspects the invention provides a method of
operating a mass spectrometer system having a time of flight (TOF)
analyzer having an ion extraction region and a flight tube, comprising:
providing ions to be analyzed, and passing said ions through a tapered ion
guide to configure the shape of the volume occupied by said ions into a
substantially flat thin sheet of ions, directing said flat thin sheet of ions
into said extraction region of said TOF analyzer, and extracting said flat
thin sheet of ions in a direction substantially orthogonal to the plane of
said flat thin sheet of ions into said flight tube of said TOF analyzer.
In another aspect the invention provides a mass spectrometer
system comprising a time of flight (TOF) analyzer, and an ion guide for
introducing ions into said TOF analyzer along a path of travel, said ion
guide tapering in height in the direction of said path of travel so that ions
entering or located in said ion guide are configured into a volume in the
form of a substantially flat thin sheet, and an ion transmission path for
conducting said flat thin sheet of ions into said TOF analyzer.
Further aspects and advantages of the invention will appear
from the following description, taken together with the accompanying
drawings.
BRIEF SUMMARY OF THE DRAWINGS
In the drawings:
Fig. 1 is a schematic view of a mass spectrometer system
incorporating an ion guide according to the invention;
Fig. 2 is a diagrammatic perspective view of an ion guide
according to the invention;
Fig. 3 is a diagrammatic view showing the path of an ion in
an ion guide according to the invention; and
Fig. 4 is a side sectional view showing a portion of an ion
guide according to the invention, constructed from printed circuit boards;
Fig. 5 is a diagrammatic end view of the ion guide of Fig. 4;
and
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Fig. 6 is a diagrammatic view of a modified mass analyzer
system according to the invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
Reference is first made to Fig. 1, which shows a conventional
ion source 10 which supplies ions 12 along a path of travel through an
aperture 14 in an aperture plate 16, through a skimmer opening 18 in a
skimmer 20, into an RF-only ion transmission quadrupole QO in a
chamber evacuated by pump 22.
The ions 12 pass through Q0, and through a lens IQO into a
resolving quadrupole Q1 (also in an evacuated chamber) where ions of a
desired mass to charge ratio are selected, and undesired ions are rejected
radially. From quadrupole Q1 the selected ions pass through lens IQ1 into
a collision cell Q2 supplied with collision gas from source 30. In
quadrupole Q2 the ions (referred to as parent ions) are fragmented to
produce daughter ions. (It is assumed in this disclosure that the parent
ions are injected into Q2 with sufficient kinetic energy to fragment.) The
daughter ions leave Q2 axially and passes through lens IQ2 in an ion beam
32 which is typically about 2 mm in diameter, together with gas from Q2
(since Q2 is at the relatively high pressure of about 5 millitorr). The
arrangement so far described is entirely conventional and is well known.
It would normally be desired next to introduce the ion beam
32 into inlet 33 of a TOF analyzer 34. In TOF 34, the ion beam is extracted
orthogonally to its path of travel 36, by extraction electrodes 38. Thus, ions
from ion beam 32 are pulsed sideways down the flight tube 40, in the
direction of arrow 42, to a detector 44 where they are detected for analysis.
Flight tube 40 is evacuated by pump 46.
According to the invention, a tapered ion guide 50 is inserted
between Q2 (or lens IQ2) and the inlet 33 of TOF 34. The tapered ion guide
50 may comprise (as shown in Fig. 2) a set of rectangular lenses or rings
52-1 to 52-8 inclusive, i.e. each lens or ring has a rectangular opening.
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While the lenses could be formed from wire, more commonly they will be
formed conventionally from metal plates each having a rectangular hole
therein. Ring 52-1 serves as an inlet ring while 52-8 serves as an exit ring.
The number of rings may of course vary and will commonly be greater
than that shown.
It will be seen that the height of the ion guide rings decreases
or tapers from the entrance ring 52-1 to the exit ring 52-8. The decrease i n
height may be linear as shown, or may be of other geometric form to
produce the best results.
In addition, as the rings 52-1 to 52-8 taper in height, they also
increase outwardly in width from the entrance ring 52-1 to the exit ring
52-8, as also shown in Fig. 2.
RF from an RF supply 60 (which forms part of an RF and DC
source 62) is applied to the rings 52-1 to 52-8, with alternating poles 60A,
60B of the RF supply 60 connected to each alternate ring as shown in Fig. 2.
Therefore, one pole 60A is connected (through capacitors C) to rings 52-1,
52-3, 52-5 and 52-7, while the other pole 60B is connected through further
capacitors C to rings 52-2, 52-4, 52-6 and 52-8. This produces what is seen by
the ions to be an alternating RF field as the ions travel through the rings
from one end to the other. Ion guides consisting of a stack of spaced rings
with RF and/or DC connected thereto are known and are described for
example in an article entitled "Stacked-Ring Electrostatic Ion Guide" by
Guan and Marshall, 1996 J Am Soc Mass Spectrum 1996, pages 101-106, and
in the article entitled "A Novel Ion Funnel for Focusing Ions at Elevated
Pressure Using Electrospray Ionization Mass Spectrometry", by Richard
Smith et al., Rapid Comm. Mass Spectrom. 11, 1813-1817 (1997).
To ensure that the ions will continue moving through the
ion guide 50, a small DC drag field is provided, produced by DC potentials
V1 to V8 applied respectively to the rings. Each DC potential V2 to V8 is
slightly higher than the preceding potential (typically at a gradient of about
10 volts per meter). Potentials V1 to V8 are obtained from DC supply 66,
which forms part of source 62.
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As ions enter the ion guide 50, they tend to follow a path such
as that shown at 70 in Fig. 3. The path 70 is not sinusoidal, but rather is
more like that of the path of a bouncing ball, being reflected back and forth
along the path of travel until, at the exit ring 52-8 the amplitude of the
path becomes very small and the frequency of oscillations become higher.
To deal with this path, the spacing along the length of the path of travel
between the rings is made smaller in the direction of the ion movement,
so that the rings become closer together at the exit end of the ion guide 50.
As the ions are squeezed in one dimension, and spread out in
the other dimensions, from a beam into a shape approximating a thin
sheet, the ion temperature tends to increase, by a factor of as much as
three. However the presence of gas within the ion guide tends to cool the
ions by collisional damping, reducing the ion temperature. This is so even
though much of the gas is removed from the ion guide 50 by pump 74, so
that minimal gas will enter the TOF analyzer 34.
In addition, although the ion beam is squeezed in height, it is
as mentioned allowed to expand widthwise in direction W, as the width of
the rings 52-1 to 52-8 increases. Thus, space charge effects which would
otherwise occur are minimized, and the cylinder ion beam is transformed
to have a shape which is generally that of a wide, flat, thin sheet of charge.
Such a sheet of charge is more suited to being extracted into the flight tube
40 of the TOF analyzer 34.
It is found, when a tapered ion guide of the kind shown is
used, that the ion beam can be reduced in height from 2 mm to
approximately .2 mm, or a ten times improvement, resulting in an
increase in resolution in the TOF. The width W of the ion sheet can be as
desired, but may for example be 40 mm in the case where the diameter of
flight tube 40 is 228 mm. The dimensions of inlet 33 of TOF 34 will of
course be made generally rectangular and of a size to admit the sheet of
charge.
Although the ion guide rings 52-1 to 52-8 have been shown as
increasing in width from the entrance to the exit of the ion guide, if
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desired they can all be of equal widths, namely the width of the exit ring
52-8. The ion beam 32 will then simply expand widthwise to fill the space
available. However the height will still taper from the entrance to the exit
end of the ion guide 50, to transform the ion beam into a flat, thin sheet of
charge.
An advantage of spreading the ion beam over this essentially
larger area perpendicular to the axis of the TOF instrument is that the ions
then more uniformly cover the area of the detector. For example, it is
common to use a detector area of up to about 40 mm in diameter. In such
cases it is useful to increase the beam width (i.e. to produce a sheet of
charge) which is about 40 mm in width, in order to spread the beam across
the surface of the detector and avoid local saturation effects on the
detector. This helps to increase the dynamic range of the detector.
It can also be advantageous to tailor the gas density along the
tapered ion guide in the direction of ion motion, which is possible
provided that the ion guide is partially enclosed. With a lower gas density,
a lower axial field is required to ensure continued movement of the ions,
and a lower axial field is generally helpful because it reduces the energy
involved in ion collisions with neutrals, thus reducing ion heating,
particularly in the extraction direction. In practice, the gas pressure can be
reduced near the exit end of the ion guide, after the ion beam has been
reshaped into a thin sheet and the ions have been cooled by collisional
damping. Once that has occurred, the gas within the ion guide is less
important and can be removed (e.g. by increased ventilation between the
ion guide rings). In that case, the axial field between the ion guide rings or
lenses can also be made non-uniform, e.g. it can be reduced toward the exit
end of the guide where there is less gas and where therefore a smaller axial
field is needed to assist movement of the ions through the guide.
While separate rings have been shown, if desired, and as
indicated diagrammatically in Fig. 4, printed circuit boards 82, 84 can be
used, with circuit tracks 86, 88 laid on them to form the equivalent,
electrically, of the rings 52-1 to 52-8. Four such printed circuit board
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elements will be used, one for each of the top, bottom and each side of the
ion guide, as shown diagrammatically in Fig. 5 where the printed circuit
boards at the sides of the guide are indicated at 90, 92 respectively, with
circuit tracks 94 shown on the board 90 in Fig. 4. (More than four printed
circuit boards can be used if desired, to produce a guide having at least at
its
entrance a non-rectangular cross-section.) The circuit tracks on each
printed circuit board are connected together by any desired means, e.g. by
separate connector pieces (not shown) joining each pair of printed circuit
boards together. Any other conventional connecting means may also be
used. An appropriate opening or set of openings 96 is provided at least in
the upper printed circuit board 82 to allow gas in the ion guide to be
withdrawn by pump 74.
If desired, in the arrangement of Fig. 1 Q2 can be eliminated
and ions from Q1 can be injected directly into the ion guide 50, with
sufficient energy to fragment them as indicated in Fig. 6. Collision gas
from source 30 is injected into ion guide 50 near its inlet end, as shown.
The gas is removed either by a pump (not shown) between the outlet of
ion guide 50 and the inlet 33 of the TOF 34, or by providing a separate
pump 102 to exhaust the extraction region of TOF 34. The advantage of
this arrangement is that as the daughter ions are formed in the ion guide
50 (which acts as a collision cell), they are also reconfigured into a flat
thin
sheet of charge by the ion guide 50, thus eliminating the need for one
component (namely Q2) of the mass spectrometer system.
When printed circuit boards are used to form the ion guide as
shown in Figs. 4 and 5, the surface of the printed circuit boards between
circuit tracks may normally be made weakly conducting to prevent
charging of the surface of the boards.
While the ion beam processed by the tapered ion guide 50 has
been shown as originating from a collision cell, or from a mass analyzer
with fragmentation to be performed in ion guide 50, it will be realized that
the tapered ion guide 50 can be used to reconfigure an ion beam from any
ion source or the like into a flat thin sheet of charge suitable for entry
into
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a TOF analyzer.
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.
