Mass spectrometer

Details Mass spectrometer Mass spectrometer

2002-06-25

2004-07-13

Wells, Nikita (Department: 2881)

Radiant energy

Ionic separation or analysis

C250S287000, C250S282000

Reexamination Certificate

active

06762404

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to mass spectrometers.
In many tandem mass spectrometers ions are fragmented in a collision or fragmentation cell. A known fragmentation cell comprises a multipole (e.g. a quadrupole or hexapole) rod set wherein adjacent rods are connected to opposite phases of an RF voltage supply. The quadrupole or hexapole collision cell is housed in a cylindrical housing which is open at an upstream end and at a downstream end to allow ions to enter and exit the collision cell. The housing includes a gas inlet port through which a collision or buffer gas, typically nitrogen or argon, is introduced into the collision cell. The collision cell is maintained at a pressure of 10
−3
-10
−2
mbar.
Ions entering the collision cell are arranged to be sufficiently energetic so that when they collide with the collision or buffer gas at least some of the ions will fragment into daughter or fragment ions by means of Collisional Induced Dissociation/Decomposition (“CID”). Ions in the collision cell will also become thermalised after they have undergone a few collisions i.e. their kinetic energy will be considerably reduced, and this leads to greater radial confinement of the ions in the presence of the RF electric field. In order to ensure that ions are sufficiently energetic so as to fragment when entering the collision cell, the collision cell is typically maintained at a DC potential which is offset from that of the ion source by approximately −30V DC or more (for positive ions). Once ions have fragmented and have been thermalised within the collision cell, their low kinetic energy is such that they will tend to remain within the collision cell. In practice, ions are observed to exit the collision cell after a relatively long period of time, and this is believed to be due to the effects of diffusion and the repulsive effect of further ions being admitted into the collision cell.
Accordingly, one of the problems associated with the known collision cell is that ions tend to have a relatively long residence time within the collision cell. This is problematic for certain types of mass spectrometry methods since it is necessary to wait until ions have exited the collision cell before further ions are admitted into it. For example, in MS/MS (i.e. fragmentation) modes of operation if a quadrupole mass filter Q
1
(MS
1
) upstream of a collision cell Q
2
is scanned rapidly compared to the typical empty time (~30 ms) of ions to exit the collision cell Q
2
, then the peaks in the resulting parent ion scanning mass spectrum will suffer from peak tailing towards higher mass and thus the resulting mass spectrum will suffer from relatively poor resolution. An example of this is shown in FIG.
16
(
a
).
Similarly, in Multiple Reaction Monitoring (MRM) experiments the upstream quadrupole mass filter Q
1
(MS
1
) is switched rapidly to cyclically transmit a number of parent ions (e.g. P
1
, P
2
. . . Pn) in a multiplexed manner, and the long empty times of ions to exit the collision cell Q
2
may result in cross-talk between the various channels.
Long empty times of ions to exit the collision cell Q
2
is also problematic when the mass spectrometer is being used in on-line chromatography applications since each peak only elutes over a short period of time and the mass spectrometer will have to acquire data very rapidly if a full parent (precursor) ion spectrum is desired.
It is therefore desired to provide an improved collision or fragmentation cell for use in a mass spectrometer which does not suffer from some or all of the problems discussed above.
SUMMARY OF THE INVENTION
According to a first aspect of the present invention, there is provided a mass spectrometer comprising: a fragmentation cell in which ions are fragmented in use, the fragmentation cell comprising a plurality of electrodes having apertures through which ions are transmitted in use, wherein at least some of the electrodes are connected to both a DC and an AC or RF voltage supply and wherein an axial DC voltage gradient or difference is maintained in use along at least a portion of the length of the fragmentation cell.
The preferred collision or fragmentation cell differs from a conventional multipole collision cell in that instead of comprising four or six elongated rod electrodes, the fragmentation cell comprises a number (e.g. typically >100) of ring, annular or plate like electrodes having apertures, preferably circular, through which ions are transmitted. Furthermore, an axial DC voltage gradient is preferably maintained across at least a portion of the length of the fragmentation cell, preferably the whole length of the fragmentation cell.
The fragmentation cell according to the preferred embodiment is capable of being emptied of and filled with ions much faster than a conventional collision cell. Mass spectra obtained using the preferred fragmentation cell exhibit improved resolution and greater sensitivity.
The fragmentation cell may comprise 10-20, 20-30, 30-40, 40-50, 50-60, 60-70, 70-80, 80-90, 90-100, 100-110, 110-120, 120-130, 130-140, 140-150, or >150 electrodes. The fragmentation cell may have a length <5 cm, 5-10 cm, 10-15 cm, 15-20 cm, 20-25 cm, 25-30 cm, or >30 cm. Preferably, at least 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, or 95% of the electrodes are connected to both a DC and an AC or RF voltage supply. According to a one embodiment, an axial DC voltage difference of approximately 3V may be maintained along the whole length of the fragmentation cell (i.e. for positive ions, electrodes at the downstream end of the fragmentation cell are maintained at a DC voltage approximately 3V below electrodes at the upstream end of the fragmentation cell). In other embodiments the axial DC voltage difference maintained along at least a portion, preferably the whole length, of the fragmentation cell is 0.1-0.5 V, 0.5-1.0 V, 1.0-1.5 V, 1.5-2.0 V, 2.0-2.5 V, 2.5-3.0 V, 3.0-3.5 V, 3.5-4.0 V, 4.0-4.5 V, 4.5-5.0 V, 5.0-5.5 V, 5.5-6.0 V, 6.0-6.5 V, 6.5-7.0 V, 7.0-7.5 V, 7.5-8.0 V, 8.0-8.5 V, 8.5-9.0 V, 9.0-9.5 V, 9.5-10.0 V or >10V.
In terms of V/cm, the axial DC voltage gradient maintained along at least a portion of the fragmentation cell, and preferably along the whole length of the collision cell, may be 0.01-0.05 V/cm, 0.05-0.10 V/cm, 0.10-0.15 V/cm, 0.15-0.20 V/cm, 0.20-0.25 V/cm, 0.25-0.30 V/cm, 0.30-0.35 V/cm, 0.35-0.40 V/cm, 0.40-0.45 V/cm, 0.45-0.50 V/cm, 0.50-0.60 V/cm, 0.60-0.70 V/cm, 0.70-0.80 V/cm, 0.80-0.90 V/cm, 0.90-1.0 V/cm, 1.0-1.5 V/cm, 1.5-2.0 V/cm, 2.0-2.5 V/cm, 2.5-3.0 V/cm or >3.0 V/cm.
The voltage gradient may be a linear voltage gradient, or the voltage gradient may have a stepped or curved stepped profile similar to that shown in FIG.
4
. The term “voltage gradient” should be construed broadly to cover embodiments wherein the DC voltage offset of electrodes along the length of the fragmentation cell relative to the DC potential of the ion source varies at different points along the length of the fragmentation cell. This term should not, however, be construed to include arrangements wherein all the electrodes forming the fragmentation cell are maintained at substantially the same DC potential.
According to the preferred embodiment, the electrodes forming the fragmentation cell are supplied with an AC or RF voltage which can be considered to be superimposed upon the DC potential supplied to the electrodes. Preferably, adjacent electrodes are connected to opposite phases of an AC or RF supply but according to other less preferred embodiments adjacent electrodes may be connected to different phases of the AC or RF supply i.e. voltage supplies having more than two phases are contemplated. Furthermore, although according to the preferred embodiment the AC or RF voltage supplied to the electrodes has a sinusoidal waveform (with a frequency 0.1-3.0 MHz, preferably 1.75 MHz), non-sinusoidal waveforms including square waves may be supplied to the electrodes.
According to a particularly preferred embodiment, the fragmentation cell

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