Mass spectrometer

Details Mass spectrometer Mass spectrometer

2003-08-08

2004-09-21

Nguyen, Kiet T. (Department: 2881)

Radiant energy

Ionic separation or analysis

C250S282000, C250S287000, C250S292000

Reexamination Certificate

active

06794642

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a mass spectrometer and a method of mass spectrometry. The preferred embodiment relates to 3D quadrupole ion traps (“QIT”) and Time of Flight (“TOF”) mass analysers.
2. Discussion of the Prior Art
Known 3D (Paul) quadrupole ion trap mass spectrometers comprise a doughnut shaped central ring electrode and two end-cap electrodes. Such known 3D (Paul) quadrupole ion trap mass spectrometers typically have a relatively low resolution and a relatively low mass measurement accuracy when scanning the complete mass range compared with other types of mass spectrometers such as magnetic sector and Time of Flight mass spectrometers. 3D quadrupole ion traps do however exhibit a relatively high sensitivity in both MS and MS/MS modes of operation. One particular problem with 3D quadrupole ion traps is that they suffer from having a relatively limited mass range and exhibit a low mass to charge ratio cut-off limit below which ions cannot be stored within the quadrupole ion trap. In a MS/MS mode of operation only about a 3:1 ratio of parent mass to fragment mass can be stored and recorded.
Orthogonal acceleration Time of Flight mass spectrometers have relatively higher resolving powers and higher mass measurement accuracy for both MS and MS/MS modes. Typically, orthogonal acceleration Time of Flight mass spectrometers are coupled to ion sources which provide a continuous beam of ions. Segments of this continuous ion beam are then orthogonally extracted for subsequent mass analysis. However, about 75% of the ions are not extracted for mass analysis and are thus lost.
It is therefore desired to address the mass range limitation inherent with conventional quadrupole ion traps and to increase the duty cycle of an orthogonal acceleration Time of Flight mass analyser when performing MS and MS/MS experiments.
SUMMARY OF THE INVENTION
According to the present invention there is provided a mass spectrometer comprising:
a first ion trap and a second ion trap wherein the first ion trap is arranged to have, in use, a first low mass cut-off and the second ion trap is arranged to have, in use, a second low mass cut-off, the second low mass cut-off being lower than the first low mass cut-off so that at least some ions having mass to charge ratios lower than the first low mass cut-off which are not trapped in the first ion trap are trapped in the second ion trap.
Advantageously, the combination of two or more ion traps in series having different low mass cut-offs increases the overall ion trapping volume or capacity and hence the dynamic range of the ion trapping system.
A mass spectrometer according to the preferred embodiment is capable of performing both MS and MS/MS modes of operation and comprises an ion source, a series of coupled quadrupole ion traps and an orthogonal acceleration Time of Flight mass analyser. The combination of multiple quadrupole ion traps and the orthogonal acceleration Time of Flight mass analyser provides a mass spectrometer with an increased mass range (especially in MS/MS), increased sensitivity, increased mass measurement accuracy and increased mass resolution compared with other known arrangements.
According to a less preferred embodiment fragment ions may be generated externally to the first ion trap by surface induced disassociation (SID), collision induced disassociation (CID) or post source decay (PSD) and then transferred to the first ion trap.
According to the preferred embodiment collisional cooling with a bath gas may be employed in one or more of the ion traps and/or in the transfer region(s) between the ion traps. Collisional cooling advantageously reduces both the kinetic energy of the ions and the spread of kinetic energies of the ions. Collisional cooling also has the effect of improving the trapping efficiency within the ion trap whilst preparing the ions for subsequent mass analysis in a Time of Flight mass analyser, preferably an orthogonal acceleration Time of Flight mass analyser, which may optionally include a reflectron.
The first ion trap preferably comprises a quadrupole ion trap. According to the one embodiment the first ion trap comprises a 3D (Paul) quadrupole ion trap comprising a ring electrode and two end-cap electrodes, the ring electrode and the end-cap electrodes having a hyperbolic surface.
According to another embodiment the first ion trap comprises one or more cylindrical ring electrodes and two substantially planar end-cap electrodes.
According to another embodiment the first ion trap comprises one, two, three or more than three ring electrodes and two substantially planar end-cap electrodes.
One of the end-cap electrodes may comprise a sample or target plate. The sample or target plate may comprise a substrate with a plurality of sample regions arranged preferably in a microtitre format wherein, for example, the pitch spacing between samples is approximately or exactly 18 mm, 9 mm, 4.5 mm, 2.25 mm or 1.125 mm. Up to or at least 48, 96, 384, 1536 or 6144 samples may be arranged to be received on the sample or target plate. A laser beam or an electron beam is preferably targeted in use at the sample or target plate.
One of the end-cap electrodes of the first ion trap may comprise a mesh or grid.
The first ion trap may comprise a 2D (linear) quadrupole ion trap comprising a plurality of rod electrodes and two end electrodes.
According to other less preferred embodiments the first ion trap may comprise a segmented ring set comprising a plurality of electrodes having apertures through which ions are transmitted or a Penning ion trap.
A first AC or RF voltage having a first amplitude is preferably applied to the first ion trap. The first amplitude is preferably selected from the group consisting of: (i) 0-250 V
pp
; (ii) 250-500 V
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; (iii) 500-750 V
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; (iv) 750-1000 V
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; (v) 1000-1250 V
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; (vi) 1250-1500 V
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; (vii) 1500-1750 V
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; (viii) 1750-2000 V
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; (ix) 2000-2250 V
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; (x) 2250-2500 V
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; (xi) 2500-2750 V
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; (xii) 2750-3000 V
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; (xiii) 3000-3250 V
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; (xiv) 3250-3500 V
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; (xv) 3500-3750 V
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; (xvi) 3750-4000 V
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; (xvii) 4000-4250 V
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; (xviii) 4250-4500 V
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; (xix) 4500-4750 V
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; (xx) 4750-5000 V
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; (xxi) 5000-5250 V
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; (xxii) 5250-5500 V
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; (xxiii) 5500-5750 V
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; (xxiv) 5750-6000 V
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; (xxv) 6000-6250 V
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; (xxvi) 6250-6500 V
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; (xxvii) 6500-6750 V
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: (xxviii) 6750-7000 V
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; (xxix) 7000-7250 V
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; (xxx) 7250-7500 V
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; (xxxi) 7500-7750 V
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; (xxxii) 7750-8000 V
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; (xxxiii) 8000-8250 V
pp
; (xxxiv) 8250-8500 V
pp
; (xxxv) 8500-8750 V
pp
; (xxxvi) 8750-9000 V
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; (xxxvii) 9250-9500 V
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; (xxxviii) 9500-9750 V
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; (xxxix) 9750-10000 V
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; and (xl) >10000 V
pp
.
The first AC or RF voltage preferably has a frequency within a range selected from the group consisting of: (i) <100 kHz; (ii) 100-200 kHz; (iii) 200-400 kHz; (iv) 400-600 kHz; (v) 600-800 kHz; (vi) 800-1000 kHz; (vii) 1.0-1.2 MHz; (viii) 1.2-1.4 MHz; (ix) 1.4-1.6 MHz; (x) 1.6-1.8 MHz; (xi) 1.8-2.0 MHz; and (xii) >2.0 MHz.
The second ion trap preferably comprises a quadrupole ion trap.
The second ion trap may comprise a 3D (Paul) quadrupole ion trap comprising a ring electrode and two end-cap electrodes, the ring electrode and the end-cap electrodes having a hyperbolic surface. Alternatively, the second ion trap may comprise a cylindrical ring electrode and two substantially planar end-cap electrodes.
The second ion trap may comprise one, two, three or more than three ring electrodes and two substantially planar end-cap electrodes. One or more of the end-cap electrodes of the second ion trap may comprise a mesh or grid.
According to another embodiment the second ion trap may comprise a 2D (linear) quadrupole ion trap comprising a plurality of rod electrodes and two end electrodes.
According to less preferred embodiments the second ion trap may comprise a segmented ring set comprising a plurality of electrodes having apertures through which ions are transmitted or a Penning ion trap.
A second AC or RF voltage having a second amplitude is pre

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