Mass spectrometer

Details Mass spectrometer Mass spectrometer

2002-11-22

2004-08-03

Lee, John R. (Department: 2881)

Radiant energy

Ionic separation or analysis

C250S282000, C250S287000, C250S292000

Reexamination Certificate

active

06770872

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a mass spectrometer.
2. Discussion of the Prior Art
The duty cycle of an orthogonal acceleration Time of Flight (“oaTOF”) mass analyser is typically in the region of 20-30% for ions of the maximum mass to charge ratio and less for ions with lower mass to charge ratios.
FIG. 1
illustrates part of the geometry of a conventional orthogonal acceleration Time of Flight mass analyser. In an orthogonal acceleration Time of Flight mass analyser ions are orthogonally accelerated into a drift region (not shown) by a pusher electrode
1
having a length L1. The distance between the pusher electrode
1
and the ion detector
2
may be defined as being L2. The time taken for ions to pass through the drift region, be reflected by a reflectron (not shown) and reach the ion detector
2
is the same as the time it would have taken for the ions to have travelled the axial distance L1+L2 from the centre of the pusher electrode
1
to the centre of the ion detector
2
had the ions not been accelerated into the drift region. The length of the ion detector
2
is normally at least L1 so as to eliminate losses.
If the Time of Flight mass analyser is designed to orthogonally accelerate ions having a maximum mass to charge ratio M
max
then the cycle time &Dgr;T between consecutive energisations of the pusher electrode
1
(and hence pulses of ions into the drift region) is the time required for ions of mass to charge ratio equal to M
max
to travel the axial distance L1+L2 from the pusher electrode
1
to the ion detector
2
.
The duty cycle D
cy
for ions with a mass to charge ratio M is given by:
D
cy
=
L1
L1
+
L2
·
M
M
max
For example, if L1 is 35 mm and the distance L2 is 90 mm then the duty cycle for ions of maximum mass to charge value is given by L1/(L1+L2) which equals 28.0%.
Increasing L1 and/or decreasing L2 will in theory increase the duty cycle. However, increasing L1 would require a larger and hence more expensive ion detector
2
and this would also place a greater demand on mechanical alignment including grid flatness. Such an option is not therefore practical.
On the other hand, reducing L2 would also be impractical. Reducing L2 per se would shorten the flight time in the drift region and result in a loss of resolution. Alternatively, L2 could be reduced and the flight time kept constant by reducing the energy of the ions prior to them reaching the pusher electrode
1
. However, this would result in ions which were less confined and there would be a resulting loss in transmission.
A person skilled in the art will therefore appreciate that for mechanical and physical reasons constraints are placed on the values that L1 and L2 can take, and this results in a typical maximum duty cycle in the range 20-30%.
It is known to trap and store ions upstream of the pusher electrode
1
in an ion trap which is non-mass selective i.e. the ion trap does not discriminate on the basis of mass to charge ratio but either traps all ions or releases all ions (by contrast a mass selective ion trap can release just some ions having specific mass to charge ratios whilst retaining others). All the ions trapped within the ion trap are therefore released in a packet or pulse of ions. Ions with different mass to charge values travel with different velocities to the pusher electrode
1
so that only certain ions are present adjacent the pusher electrode
1
when the pusher electrode
1
is energised so as to orthogonally accelerate ions into the drift region. Some ions will still be upstream of the pusher electrode
1
when the pusher electrode
1
is energised and other others will have already passed the pusher electrode
1
when the pusher electrode
1
is energised. Accordingly, only some of the ions released from the upstream ion trap will actually be orthogonally accelerated into the drift region of the Time of Flight mass analyser.
By arranging for the pusher electrode
1
to orthogonally accelerate ions a predetermined time after ions have been released from the ion trap it is possible to increase the duty cycle for some ions having a certain mass to charge ratio to approximately 100%. However, the duty cycle for ions having other mass to charge ratios may be much less than 100% and for a wide range of mass to charge ratios the duty cycle will be 0%.
The dashed line in
FIG. 2
illustrates the duty cycle for an orthogonal acceleration Time of Flight mass analyser operated in a conventional manner without an upstream ion trap. The maximum mass to charge ratio is assume to be 1000, L1 was set to 35 mm and the distance L2 was set to 90 mm. The maximum duty cycle is 28% for ions of mass to charge ratio 1000 and for lower mass to charge ratio ions the duty cycle is much less.
The solid line in
FIG. 2
illustrates how the duty cycle for some ions may be enhanced to approximately 100% when a non-mass selective upstream ion trap is used. In this case it is assumed that the distance from the ion trap to the pusher electrode
1
is 165 mm and that the pusher electrode
1
is arranged to be energized at a time after ions are released from the upstream ion trap such that ions having a mass to charge ratio of 300 are orthogonally accelerated with a resultant duty cycle of 100%. However, as is readily apparent from
FIG. 2
, the duty cycle for ions having smaller or larger mass to charge ratios decreases rapidly so that for ions having a mass to charge ratio≦200 and for ions having a mass to charge ratio≧450 the duty cycle is 0%. The known method of increasing the duty cycle for just some ions may be of interest if only a certain part of the mass spectrum is of interest such as for precursor ion discovery by the method of daughter ion scanning. However, it is of marginal or no benefit if a full mass spectrum is required.
It is therefore desired to provide a mass spectrometer which overcomes at least some of the disadvantages of the known arrangements.
SUMMARY OF THE INVENTION
According to an aspect of the present invention there is provided a mass spectrometer comprising: a mass selective ion trap; an orthogonal acceleration Time of Flight mass analyser arranged downstream of the ion trap, the orthogonal acceleration Time of Flight mass analyser comprising an electrode for orthogonally accelerating ions; and a control means for controlling the mass selective ion trap and the orthogonal acceleration Time of Flight mass analyser, wherein in a mode of operation the control means controls the ion trap and the orthogonal acceleration Time of Flight mass analyser so that: (i) at a first time t
1
ions having mass to charge ratios within a first range are arranged to be substantially passed from the ion trap to the orthogonal acceleration Time of Flight mass analyser whilst ions having mass to charge ratios outside of the first range are not substantially passed to the orthogonal acceleration Time of Flight mass analyser; (ii) at a later time t
1
+&Dgr;t
1
the electrode is arranged to orthogonally accelerate ions having mass to charge ratios within the first range; (iii) at a second later time t
2
ions having mass to charge ratios within a second range are arranged to be substantially passed from the ion trap to the orthogonal acceleration Time of Flight mass analyser whilst ions having mass to charge ratios outside of the second range are not substantially passed to the orthogonal acceleration Time of Flight mass analyser; and (iv) at a later time t
2
+&Dgr;t
2
the electrode is arranged to orthogonally accelerate ions having mass to charge ratios within the second range, wherein &Dgr;t
1
≠&Dgr;t
2
. Accordingly, ions are released from the ion trap and are orthogonally accelerated after a first delay and then further ions are released from the ion trap and are orthogonally accelerated after a second different delay time.
At the first time t
1
ions having mass to charge ratios outside of the first range are preferably substantially retained within the ion trap. Likewise, at the second time

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