Note: Descriptions are shown in the official language in which they were submitted.
A-4195~ JT
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LOW NOISE TANDEM QUADRUPOLE
MASS SPECTROMETERS AND METHOD
Background of Invention
This invention relates generally to tandem quadrupole mass
spectrometers and more particul~rly to a low noise tandem
quadrupole mass spectrometer. -~
Tandem quadrupole mass spectrometers are known in the prior
art. Such tandem quadrupole mass spectrometers have been
used in the study of ion molecule reactions. A center RF
only quadrupole has been added to tandem quadrupole mass
spectrometers for study of photo dissociation and fsr metas-
table ion studies.
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~Patent 4,234,791 and 4,329,582 describe tandem quadrupole
mass spectrometers including a highly efficient intermediate
fragmentation stage which employs collision induced ~issoci-
ation (CI~) in the form of an RF only quadrupole.
All of the prior art tandem quadrupole mass spectrometer
systems are noisy. It is believed that the noise is due to
excited and fast neutral particles and fast ions traveling
directly to the region of the detector where they strike
surfaces in the vicinity of the detector and generate sec-
ondary ions. These secondary ions produce an interfering
ion current which is detected by the detector.
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Obiects and Summarv of the Invention
It is a general object of the present inven~ion to
provide an improved tandem quadrupole mass spectrometer.
It is another object of the present invention to provide
a tandem quaclrupole mass spectrometer having low neutral particle
and fast ion noise.
It is a further object of the present invention to
provide a tandem quadrupole mass spectrometer including a bent RF
only intermediate quadrupole stage.
The foregoing and other objects of the invention are
achieved by a mass spectrometer which includes an ion source, a
lens for directing ions from said source along a predetermined
path, at least one quadrupole filter or mass analyzer for
filtering or analyzing said ions, a detection means for detecting
said ions and quadrupole means for directing ions away from said
predetermined path so they impinge upon said detector.
In accordance with a broad aspect of the invention there
is provided a mass spectrometer including:
an ion source for providing ions of a sample,
means for directing ions from said ion source along a
predetermined path;
a multipole collision cell comprising a plurality of spaced
rods for receiving said ions and generating daughter ions and ion
fragments, said collision cell having an input end aligned to
receive the ions from said source and an output end for delivering
ions, said rods being positioned or bent whereby neutral
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61051-2047
particles or ions travelling in a straight line from said ion
source are not able to transit past the collision cell without
striking the rods or the surrounding structure;
a quadrupole filter or analyzer having an entrance aperture
for receiving the daughter ions and ion fragments;
detector means for receiving the output of said quadrupole
filter or analyzer and provide an output signal.
In accordance with another broad aspect of the invention
there is provided a mass spectrometer including:
an ion source for providing sample ions;
means for directing ions from said ion source along a
predetermined path;
a quadrupole filter or analyzer for receiving said ions and
providing outpu~ ions within a selected mass-to-charge range;
a multiple collision cell for receiving said ions of selected
mass-to-charge range and generating fragment and daughter ions,
said collision cell having an input aperture aligned with said
path and an output end for delivering ions out of line with
respect to said path;
a quadrupole filter or analyzer having an entrance aperture
for receiving said fragments and daughter ions, said entrance
aperture being positioned out of line with respect to said path,
said filter or analyzer providing output daughter ions or fragment
ions within a selected mass-to-charge range; and detector means
for receiving said fragment ions and daughter ions within said
selected range and provide an output signal.
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61051-2047
B~L~ L~ the Drawings
Figure l is a schematic view of a tandem quadrupole mass
spectrometer in accordance with the prior ar~.
Figure 2 is a schematic view of a low noise tandem
quadrupole mass spectrometer in accordance with the present
invention.
Figure 3 is a schematic representation of another
embodiments of the present invention.
Figure 4 is a schematic view of a tandem sector
quadrupole mass spectrometer in accordance with the prior art.
Figure 5 is a schematic view of a tandem sector
quadrupole mass spectrometer in accordance with the present
invention.
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Figure 6 is a schematic view showing another embodiment of
the tandem quadrupole spectrometer of Figure 2.
Figure 7 is a schematic view showing still another embodi-
ment of the invention.
5 Figure 8 is a schematic view showing a further embodiment.
Detailed Description
A tandem quadrupole mass spectrometer of the prior art is
shown in Figure 1. It includes an ion source 11 shown as
including a chamber 12 with an electron source 13 and col-
10 lector 14. The ion source 11 may be operated in electronimpact (EI3 mode or chemical ionization (CI) mode. Other
types of ion sources used in mass spectrometry and suitable
in the present invention are those which generate secondary
ions from a sample liquid matrix or solid sample by bom-
15 bardment with a beam of fast atoms or ions. These ionsources are used for analysis of high mass organic compounds.
There are other ionization techniques for use in elemental
or inorganic mass spectrometry. These -types of ion source
provide more neutral particles and fast ions giving rise to
20 higher noise levels. In any event, the ion source generates
ions which are accelerated and directed in a predetermined
path by the lens 16 into the quadrupole mass filter or
analyzer 17. Neutral particles and fast ions also travel
to the quadrupole mass filter.
25 The quadrupole analyzer or filter 17 operates with a peri-
odical voltage comprising an RF voltage and a d.c. voltage.
The analyzer or filter 17 passes only ions of a selected
charge to mass ratio. That is, it filters the ions and only
selects those having charge to mass ratio within a prede-
30 termined range. The range is determined by the RF and d.c.voltages applied to the quadrupole rods 18. The ions which
are not trapped or passed by the quadrupole filter or ana-
lyzer strike the walls of the enclosure or the quadrupole
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rods 18 and are neutralized. The selected or filtered ions
pass through the analyzer 17.
A lens 19 focuses the ions of selected mass to charge ratio
which are passed by the analyzer 17 into the quadrupole
region 21 which includes rods 22 operated RF only. By
operating the quadrupole RF only, it passes substantially
all the ions, that is, it acts as a very broad band high
pass filter.
The RF quadrupole 21 is in a separate volume defined by the
10 walls 25 which also form part of the associated lens 19 and
24. A collision gas is introduced into the volume via a
suitable inlet 23. The ions passed by quadrupole 21 collide
with the gas to form daughters or fragments of the selected
ions. The fragments or daughters are passed through a lens
24 into a second quadrupole mass filter or analyzer 26 where
particles of selected mass are selected and passed through
the aperture 27 through the openings formed in the X-ray
shield 28 to either dynodes 31 or 32 depending whether
negative or positive ions are to be analyzed. The secon-
20 dary ions or electrons leaving the dynodes are then collect-
ed by an electron multiplier 33 which provides an output
signal. A preferred detector is described in U. S. Patent
4,423,324. Operation of a tandem mass spectrometer including
a collision induced dissociation region is described in
25 U. ~. Patent 4,234,791 and Patent 4,329,582.
As described above, a tandem ~uadrupole mass spectrometer of
the type just described suffers from noise because of ex-
cited and fast neutral particles and fast ions which are
generated in the ion source. It is believed that these
30 neutral particles and fast ions travel in a straight line
through the various quadrupoles and lenses and strike sur-
faces in the vicinity of the dynodes 31, 32. When they
strike these surfaces, they cause emission of positive and
negative ions (possibly electrons) which are attracted by
35 the dynodes 31 or 32 and sensed by multiplier 32 and de-
tected as a signal. This noise seems to be strictly a line-
- s
of-sight phenomenon since it does not occur in magnetic
sector instruments which have a magnetic field and an elec-
trostatic sector and therefore have a curved ion path be-
tween the ion source and the detector. In these instruments
5 the detector region is remote from the line of sight of the
ion source.
In view of the above, it is proposed that in a triple tandem
quadrupole system of the type described in Figure 1 noise
would be reduced if the detector assembly were placed remote
10 from the ion source ion path whereby neutral particles could
not travel in a straight line to and strike surfaces adjac-
ent the detector.
Figure 2 shows a triple tandem mass spectrometer in accord-
ance with the present invention. Like reference numera~s
15 have been applied to parts which correspond to those in
Figure 1.
In accordance with the present invention, the RF only quad-
rupole is bent as illustrated by quadrupole 36 so that the
detector is no longer in line with the ion source 11. Neu-
20 tral particles traveling in a straight line from the ionsource 11 then strike either the walls of the enclosure or
the quadrupole rods 37. Thus, any secondary particles are
collected and dissipated, never finding their way to the
vicinity of the detector. In summary, the excited neutrals
25 impinge upon intervening surfaces and never reach the reyion
of the detector where they can add to the signal. The
neutrals are effectively filtered by the RF only quadrupole
and never travel to the mass filter. Even if a secondary
from an excited neutral would have a mass appropriate to
30 allow it to traverse through the last quadrupole 26 to the
detector, its initial position would most likely be such
that its transmission through the quadrupole and to the
detector would be highly unlikely.
The mass spectrometer of Figure 2 operates in the same man-
35 ner as the prior art as the bent quadrupole acts like a
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straight one for the ions of interest.
Bent quadrupoles have been known for bending of ion beams.
The book "Quadrupole Mass Spectrometry and its Applications"
edited by Peter H. Dawson provides criteria for operation
5 of a bent quadrupole ion beam guide. Generally, the radius
of curvature of the axis of the quadrupole structure must
be much larger than the characteristic dimension of the
quadrupole electrode structure.
The theory of operation of quadrupoles and their ion trans-
10 mission characteristics is well known and is described in
U. S. Patent 2,939,952. Also the early chapters of the
previously mentioned book "Quadrupole Mass Spectrometry and
its Applications" discuss this subject in detail. In sum-
mary, there are two modes of operating a quadrupole: RF
15 only and RF/DC. When only RF voltage is applied between
rod pairs then, theoretically, the device will only trans-
mit ions above some threshold or cutoff mass. When a com-
bination of RF and DC voltages is applied between pole
pairs there is both an upper cutoff mass as well as a lower
20 cutoff mass. As the ratio of DC to RF voltage increases,
the transmission band of ion masses narrows. The quadruple
mass filter operation occurs when the applied ratio of DC
to RF is such that the pass band of the device is so narrow
as to allow only a single ion mass to transmit. The spec-
25 ific range of ion masses passed by the quadrupole istheoretically solely a function of the device character-
istic dimension, rO, the magnitudes of the applied RF and
DC voltages, and the frequency of the applied RF. However,
since real devices are of finite length, ion transmission
30 also depends upon the velocity with which ions travel down
the length of the quadrupole. If ions enter the device
with axial velocities such that they do not experience a
sufficient number of field cycles in transit through the
quadrupole, some ions with masses outside the ion mass pass
35 band will be transmitted anyway. In general the effect of
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increased axial ion velocity is a progressive widening and
smearing of the ion mass pass band limits. At very high
axial velocities ion transmission can become virtually in-
dependent of ion mass~
For a bent quadrupole as in Figure 2, the effect of axial
velocity on ion transmission is substantially different
from the straight quadrupole. At low axial velocities the
strong focusing nature of the quadrupole field is sufficie~t
to divert all ions within the pass band along the curved
axis of the quadrupole. However, at higher axial veloc-
ities ions with masses near the limits of the pass band do
not experience sufficient strong focusing to follow the
curved ion path and are not transmitted. In general, for
a curved quadrupole the effect of increased axial ion
velocity is a progressive narrowing and smearing of the
ion mass pass band limits. At very high axial ion veloc-
ities no ions can be transmitted. Naturally the larger the
radius of curvature of the device the slower the onset of
this velocity discrimination effect. A gently curved struc-
ture like the RF only quadrupole in Figure 2, operating with
a conventional frequency, RF voltage, and range of ion
axial velocities behaves substantially like a straight one.
However, its curvature is sufficient such that very fast
ions, which are sometimes part of the noise problem, are
not transmitted.
The invention is also applicable to a mass spectrometer in-
cluding a single mass filter or analyzer such as the mass
spectrometer shown in Figure 3 wherein like reference
numerals have been applied to like parts. In this mass
spectrometer the first tandem analyzer or filter stage of
Figure 2 has been eliminated.
The bent quadrupole 36 could be operated such that it is
both a neutral noise filter and an ion prefilter for the
mass analyzer. Appropriate RF/DC voltages are applied to
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the bent quad such that only a broad ~and of ions around the
ion mass being analyzed in the mass filter are allowed to
reach the mass filter 26.
This is a particularly appropriate mode of operation when
5 the ion source emits an ion beam that consists overwhelm-
ingly of one or a few ion masses. Independent of neutral
noise there is an interfering noise current at the detector
that is associated with the magnitude of the total ion
current entering the mass analyzer. Elimination of the
10 dominant ion masses from the ion beam in the bent quadru-
pole prior to its entrance to the quadrupole mass analyzer
should commensurately diminish this ion current related
noise and thus enhance the detection and measurement of
important but weak component ion masses. ---
15 The bent quadrupole can be used in other tandem mass spec-
trometers such as those used for study of photo dissocia-
tion and metastable ion structures.
One such tandem mass spectrometer where this invention finds
application is a hybrid sector quadrupole instrument. A
20 simplified hybrid sector quadrupole instrument of the BEQQ
geometry of the prior art is shown in Figure 4. Essentially
this instnlment performs the same function as the tandem
quadrupole instrument in Figure 1, only a high resolution
double focusing sector mass analyzer has been substituted
25 in place of the first quadrupole mass filter, 17. Like
reference numerals have been assigned to parts that corres-
pond to those in Figure 1. The high resolution analyzer
79 consists of entrance slit 71, ~- slit 72, magnetic sector
73, ~- slit 78, electrostatic sector 74, and exit slit 75.
30 The ions created in the ion source 11 are mass analyzed at
high resolution and the parent ion beam exits at the exit
slit 75. At this point there are two possible experiments
that can be performed: Low energy CID and high energy CID.
The low energy CID experiment is the same experiment that
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is performed with the instrument in Figure 1. The parent
ion beam is transmitted through and deceleratea in the
deceleration lens system 77 into a conventional RF quad-
rupole collision cell 21 where it undergoes low energy CID
at kinetic energies ranging from 2 to 200 eV. The
daughter ions are in turn mass analyzed in the RF/DC quad-
rupole mass analyzer and detected at the detector. This
experiment is very quiet as the noise causing particles
generated in the ion source, as discussed before, do not
transmit through the sector analyzer.
The other experiment, high energy CID, is not as
quiet. In this experiment the parent ion beam undergoes
CID in a needle collision cell, 76, at a kinetic energy of
thousands of volts prior to entering the deceleration lens.
This collision cell consists of a capillary tube 80 that
introduces a jet of collision gas to the parent ion beam.
The daughter ions produced in this region are decelerated
in the deceleration lens and are transmitted through the
RF only quadrupole into the mass analyzing quadrupole for
mass analysis and then detected. The high energy collision
process generates a couple of particle entities that can
produce substantial noise at the detector. Neutral
daughter fragments are created with high kinetic or
internal energies. These neutrals can transmit directly
to the region of the detector in a line-of-sight fashion
and produce noise causing secondaries. Also the daughter
ions have well defined but widely spread kinetic energies.
The kinetic energy of the parent ion is distributed to
its ionic and neutral daughter fragments in proportion to
their masses. Low mass daughter ions may have hundreds or
thousands of electron volts less kinetic energy than
higher mass daughters of the same parent. So when a low
mass daughter ion is decelerated to a velocity appropriate
for mass analysis in the quadrupole mass filter other
higher mass daughter ions present will still have very
high kinetic energies and may transmit through the quad-
rupoles at such a high velocity that they are not effec-
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tively mass analyzed. These fast daughters can produce avery substantial interfering noise current at the detector.
While this noise can be taXen care of by the addition of a
kinetic energy analyzing means prior to the quadrupole mass
analyzer, this causes an increase in complexity and often
a decrease in sensitivity of the instrument. Substitution
of a bent quadrupole for the straight quadrupole in the
low energy collision cell would be a simple means by which
to eliminate both the neutral and fast daughter ions
generated during high energy CID. Figure 5 shows a BEQQ
geometry hybrid instrument with a bent quadrupole deposed
as the low energy collision cell. Like reference numerals
have been applied to parts w~ich correspond to those in
Figures 2 and 4. In the high energy CID mode an RF voltage
or combination of RF and DC voltages is applied to the--~ent
quadrupole as to efficiently transmit the particular mass
daughter ion being analyzed in the quadrupole mass anal-
yzer while accentuating the velocity filtering characteris-
tic of the bent quadrupole so as to prevent any fast higher
mass daughter ions from transmitting. The excited and fast
neutrals emitted from the high energy collision cell will
also be removed from the beam as they will not be diverted
toward the mass analyzer and detector by the curved
quadrupole. In the low energy CID mode the bent quadrupole
is operated RF only and as discussed above behaves like a
straight RF only collision cell.
In the RF/DC or in an RF only mode, the bent quadrupole
reduces the number of neutral particles or fast ions which
reach the detector region of the mass spectrometer and
thereby reduces the neutral noise caused by secondary ions
generated in the vicinity of the detector.
Although a bent quadrupole RF only stage is preferred, a
straight quadrupole stage 41 disposed at an angle will per-
form to minimize the travel of neutral particles and fast
ions from the ion sources to the detector. Figure 6 shows
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a mass spectrometer as in Figure 2 with like reference
numerals. However, the RF quadrupole stage includes
straight rods 41 at an an~le with respect to the rods of the
quadrupole 17.
The mass spectrometer may use one or more bent or angled
sections to provide enhanced noise immunity. Figure 7 shows
a system using three RF sections 46, 47 and 48, two of which
are curved and one of which is straight. The center
straight section 47 may have a gas inlet 49 and perform in
the CID mode. The RF only quadrupole may be composed of
combined straight and curved sections. Figure 8 shows a
mass spectrometer including an RF section having outer
straight sections 52 and 53 separated from a CID section 54
which includes a curved rod portion 56 and a straight po~-
tion 57.
It is apparent that there are many combinations of quadru-
pole filters or analyzers and RF only sections, CID or
otherwise, possible. In common with all is the fact that
the detector is offset from the ion source whereby there is
no direct transmission or travel of neutral particles or
fast ions.
