Mass spectrometer

Details Mass spectrometer Mass spectrometer

2002-06-25

2004-11-02

Lee, John R. (Department: 2881)

Radiant energy

Ionic separation or analysis

Ion beam pulsing means with detector synchronizing means

C250S281000, C250S282000, C250S289000

Reexamination Certificate

active

06812453

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to mass spectrometers.
Orthogonal acceleration time of flight (“oaTOF”) mass spectrometers sample ions travelling in a first (axial) direction by periodically applying a sudden accelerating electric field in a second direction which is orthogonal to the first direction. Because the ions have a non-zero component of velocity in the first direction, the result of the pulsed electric field is that ions are accelerated into the field free or drift region of the time of flight mass analyser at an angle &thgr; with respect to the second direction. If the ions have an initial energy eVa in the first direction, and they are accelerated to an energy eVo in the orthogonal direction, then tan(&thgr;)=(Va/Vo)
0.5
. For a continuous stream of ions travelling in the axial direction, all with the same energy eVa, the ion sampling duty cycle of the orthogonal acceleration time of flight mass analyser is typically of the order of 20-30% for ions having the maximum mass to charge ratio. The duty cycle is less for ions with lower mass to charge ratios. For example, if it is assumed that the length of the pusher region of the time of flight mass analyser is L
1
, the length of the detector is at least L
1
(to eliminate unnecessary losses at the detector) and the distance between the pusher and the detector is L
2
, then if ions with the maximum mass to charge ratio have an mass to charge ratio mo, then the duty cycle Dcy for ions with a mass to charge ratio m is given by: Dcy=L
1
/(L
1
+L
2
).(m/mo)
0.5
. Accordingly, if L
1
=35 mm and L
2
=120 mm, then L
1
/(L
1
+L
2
)=0.2258. Hence the maximum duty cycle is 22.6% for ions with the maximum mass to charge ratio mo, and is correspondingly less for ions with lower mass to charge ratios.
SUMMARY OF THE INVENTION
According to a first aspect of the present invention, there is provided a mass spectrometer comprising:
an ion guide wherein in use a DC potential travels along a portion of the ion guide.
As will be explained in more detail below, the ion guide with a travelling DC wave is particularly advantageous in that all the ions preferably exit the ion guide with essentially the same velocity. The ion guide can therefore be advantageously coupled to an orthogonal acceleration time of flight mass analyser which can be operated in conjunction with the ion guide so as to have an ion sampling duty cycle of nearly 100% across the whole mass range i.e. the ion sampling duty cycle is improved by a factor of approximately ×5 and furthermore is substantially independent of the mass to charge ratio of the ions. This represents a significant advance in the art.
Most if not all of the electrodes forming the ion guide are connected to an AC or RF voltage supply. The resulting AC or RF electric field acts to radially confine ions within the ion guide by creating a pseudo-potential well. According to less preferred embodiments, the AC or RF voltage supply may not necessarily output a sinusoidal waveform, and according to some embodiments a non-sinusoidal RF waveform such as a square wave may be provided. 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 the preferred embodiment, a repeating pattern of DC electrical potentials is superimposed along the length of the ion guide such as to form a periodic waveform. The waveform is caused to travel along the ion guide in the direction in which it is required to move the ions at constant velocity. In the presence of a gas the ion motion will be dampened by the viscous drag of the gas. The ions will therefore drift forwards with the same velocity as that of the travelling waveform and hence ions will exit from the ion guide with substantially the same velocity, irrespective of their mass.
The ion guide preferably comprises a plurality of segments. The ion guide is preferably segmented in the axial direction such that independent transient DC potentials can be applied, preferably independently, to each segment. The DC travelling wave potential is preferably superimposed on top of the AC or RF radially confining voltage and any constant or underlying DC offset voltage which may be applied to the segment. The DC potentials at which the various segments are maintained are preferably changed temporally so as to generate a travelling DC potential wave in the axial direction.
At any instant in time a moving DC voltage gradient is generated between segments so as to push or pull the ions in a certain direction. As the DC voltage gradient moves along the ion guide, so do the ions.
The DC voltage applied to each of the segments may be independently programmed to create a required waveform. The individual DC voltages on each of the segments are preferably programmed to change in synchronism such that the waveform is maintained but shifted in the direction in which it is required to move the ions.
The DC voltage applied to each segment may be programmed to change continuously or in a series of steps. The sequence of DC voltages applied to each segment may repeat at regular intervals, or at intervals that may progressively increase or decrease. The time over which the complete sequence of voltages is applied to a particular segment is the cycle time T. The inverse of the cycle time is the wave frequency f. The distance along the RF ion guide over which the waveform repeats itself is the wavelength &lgr;. The wavelength divided by the cycle time is the velocity v of the wave. Hence, the wave velocity, v=&lgr;/T=&lgr;f. Under correct operation the velocity of the ions will be equal to that of the travelling wave. For a given wavelength, the wave velocity may be controlled by selection of the cycle time. The preferred velocity of the travelling wave may be dependent on a number of parameters. Such parameters may include the range of ion masses to be analysed, the pressure and composition of the bath gas and the maximum collision energy where fragmentation is to be avoided. The amplitude of the travelling DC waveform may progressively increase or decrease towards the exit of the ion guide. Alternatively, the DC waveform may have a constant amplitude. In one embodiment the amplitude of the DC waveform grows to its full amplitude over the first few segments of the ion guide. This allows ions to be introduced and caught up by the travelling wave with minimal disruption to their sequence.
One application of the preferred ion guide is to convert a continuous ion beam into a synchronised pulsed beam of ions. The ability to be able to convert a continuous beam of ions into a pulsed beam of ions is particularly advantageous when using an orthogonal acceleration time of flight mass analyser since it allows the pulsing of an orthogonal acceleration time of flight mass spectrometer to be synchronised with the arrival of ions at the orthogonal acceleration region. The delay time between the time the ions exit the travelling wave ion guide and the pulsing of the orthogonal acceleration stage of the time of flight mass spectrometer depends on the distance to be travelled and the ion velocity. If all the ions have the same velocity, irrespective of their mass, then the ion sampling duty cycle will be optimised for all ions simultaneously, irrespective of their mass.
Another application of the preferred ion guide is to convert an asynchronous pulsed ion beam into a synchronous pulsed ion beam. The travelling wave ion guide may be used to collect and organise an essentially random series of ion pulses into a new series with which an orthogonal acceleration time of flight mass analyser may be synchronised. Again, if all the ions have the same velocity, irrespective of their mass, then the ion sampling duty cycle may be optimised for all ions simultaneously, irrespective of their mass.
Preferably, ions are not substantially fragmented within the ion guide so that all the ions received by the ion guide are essentially onward

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