CA1249381A - Low noise tandem quadrupole mass spectrometers and method - Google Patents
Low noise tandem quadrupole mass spectrometers and methodInfo
- Publication number
- CA1249381A CA1249381A CA000531311A CA531311A CA1249381A CA 1249381 A CA1249381 A CA 1249381A CA 000531311 A CA000531311 A CA 000531311A CA 531311 A CA531311 A CA 531311A CA 1249381 A CA1249381 A CA 1249381A
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- Prior art keywords
- ions
- mass spectrometer
- collision cell
- quadrupole
- mass
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
- H01J49/00—Particle spectrometers or separator tubes
- H01J49/26—Mass spectrometers or separator tubes
- H01J49/34—Dynamic spectrometers
- H01J49/42—Stability-of-path spectrometers, e.g. monopole, quadrupole, multipole, farvitrons
- H01J49/4205—Device types
- H01J49/421—Mass filters, i.e. deviating unwanted ions without trapping
- H01J49/4215—Quadrupole mass filters
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
- H01J49/00—Particle spectrometers or separator tubes
- H01J49/004—Combinations of spectrometers, tandem spectrometers, e.g. MS/MS, MSn
- H01J49/0045—Combinations of spectrometers, tandem spectrometers, e.g. MS/MS, MSn characterised by the fragmentation or other specific reaction
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- Analytical Chemistry (AREA)
- Chemical Kinetics & Catalysis (AREA)
- Electron Tubes For Measurement (AREA)
- Other Investigation Or Analysis Of Materials By Electrical Means (AREA)
Abstract
Abstract of the Invention A tandem mass spectrometer which includes an ion source for projecting ions along a predetermined path, a detector offset from said path and a quadropole ion filter or analyzer disposed adjacent said detector to provide its output to the detector. Quadropole means for directing ions away from said path into said ion filter and analyzer.
Description
A-4195~ JT
38~
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.
~5~
~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.
~, 8~l 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 g~2~38~
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.
2a ~z~g~
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.
3~
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 38~L
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 3?~l 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 38~
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 3338~l 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 3~
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-3~3~
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 Z ~3i38~L
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.
38~
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.
~5~
~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.
~, 8~l 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 g~2~38~
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.
2a ~z~g~
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.
3~
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 38~L
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 3?~l 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 38~
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 3338~l 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 3~
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-3~3~
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 Z ~3i38~L
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.
Claims (17)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A mass spectrometer including:
an ion source for providing ions of a sample, means for directing ions from said ion source along a pre-determined path;
a multiple 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 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.
an ion source for providing ions of a sample, means for directing ions from said ion source along a pre-determined path;
a multiple 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 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.
2. A mass spectrometer as in claim 1 in which said multipole collision cell is a quadrupole.
3. A mass spectrometer as in claim 1 including a quadru-pole filter or analyzer having its entrance aperture positioned to receive ions from said ion source and its output positioned to deliver filtered or analyzed ions to said collision cell.
4. A mass spectrometer as in claim 3 in which said multi-pole collision cell is a quadrupole.
5. A mass spectrometer as in claim 1 or 2 in which said elongated rods are bent or curved rods.
6. A mass spectrometer as in claim 1 in which said multi-pole collision cell comprises straight rods at an angle with res-pect to said path.
7. A mass spectrometer as in claim 3 in which said multi-pole collision cell has bent or curved rods.
8. A mass spectrometer as in claim 7 in which said multi-pole collision cell is a quadrupole collision cell.
9. A mass spectrometer as in claim 1 including means for introducing a collision gas into said multipole collision cell.
10. A mass spectrometer as in claim 3 including means for introducing a collision gas into said multipole collision cell.
11. A mass spectrometer including:
an ion source for providing sample ions;
means for directing ions from said ion source along a pre-determined path;
a quadrupole filter or analyzer for receiving said ions and providing output ions within a selected mass-to-charge range;
a multipole collision cell for receiving said ions of selec-ted 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.
an ion source for providing sample ions;
means for directing ions from said ion source along a pre-determined path;
a quadrupole filter or analyzer for receiving said ions and providing output ions within a selected mass-to-charge range;
a multipole collision cell for receiving said ions of selec-ted 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.
12. A mass spectrometer as in claim 11 in which said multipole collision cell is a quadrupole collision cell.
13. A mass spectrometer as in claim 11 in which said collision cell comprises a plurality of bent rods.
14. A mass spectrometer as in claim 13 including means for introducing a collision gas into said collision cell.
15. A mass spectrometer as in claim 14 including means for applying both RF and DC voltages to the bent rods of the collision cell.
16. A mass spectrometer as in claim 1 in which said ion source is a double focusing sector mass analyzer.
17. A mass spectrometer as in claim 1 in which said ion source includes a high energy collision cell.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US83760086A | 1986-03-07 | 1986-03-07 | |
| US837,600 | 1986-03-07 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1249381A true CA1249381A (en) | 1989-01-24 |
Family
ID=25274923
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000531311A Expired CA1249381A (en) | 1986-03-07 | 1987-03-06 | Low noise tandem quadrupole mass spectrometers and method |
Country Status (3)
| Country | Link |
|---|---|
| EP (1) | EP0237259A3 (en) |
| JP (1) | JPS62264546A (en) |
| CA (1) | CA1249381A (en) |
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-
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- 1987-03-04 EP EP87301870A patent/EP0237259A3/en not_active Withdrawn
- 1987-03-06 JP JP62051825A patent/JPS62264546A/en active Pending
- 1987-03-06 CA CA000531311A patent/CA1249381A/en not_active Expired
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Also Published As
| Publication number | Publication date |
|---|---|
| EP0237259A2 (en) | 1987-09-16 |
| EP0237259A3 (en) | 1989-04-05 |
| JPS62264546A (en) | 1987-11-17 |
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