WO1999039368A2

WO1999039368A2 - Time-of-flight mass spectrometer

Info

Publication number: WO1999039368A2
Application number: PCT/GB1999/000084
Authority: WO
Inventors: Eizo Kawato
Original assignee: Shimadzu Research Laboratory (Europe) Ltd.
Priority date: 1998-01-30
Filing date: 1999-01-12
Publication date: 1999-08-05
Also published as: DE69906699D1; JP4132667B2; EP1051730B1; WO1999039368A3; JP2002502095A; AU2065199A; GB9802111D0; DE69906699T2; US6380666B1; EP1051730A2

Abstract

A time-of-flight spectrometer comprises a quadrupole ion trap (10) as an ion source, a drift tube (11) defining a field-free drift space, an ion reflector (12) and an ion detector (13). The quadrupole ion trap (10) has two end-cap electrodes (22, 23) and a ring electrode (21). End-cap electrode (22) has a central hole (24) through which ions to be extracted can pass. High voltage power supplies (34, 35) and associated switching devices (32, 33) are provided to supply extraction voltages to the end-cap electrodes (22, 23). The extraction voltage supplied to end-cap electrode (22) has the opposite polarity to the extraction voltage supplied to the other end-cap electrode (23) being respectively negative and positive voltages for positive ion extraction and respectively positive and negative voltages for negative ion extraction. The magnitude of the extraction voltage supplied to electrode (23) is in the range from 0.5 to 0.8 that of the extraction voltage supplied to electrode (22).

Description

TIME-OF-FLIGHT MASS SPECTROMETER

FIELD OF THE INVENTION

The present invention relates to a time-of-flight mass

spectrometer. More specifically, the invention relates to a

time-of-flight mass spectrometer comprising an ion source in

the form of a quadrupole ion trap, an ion detector and a

field- free drift space between the ion source and the ion

detector. Usually, though not necessarily, there will also be

provided an ion reflector between the ion source and the ion

detector .

BACKGROUND OF THE INVENTION

A quadrupole ion trap comprises a pair of end-cap electrodes

and a ring electrode. One of the end-cap electrodes has a

central hole through which ions can be extracted for

transmission along a field-free drift space. The invention is

particularly concerned with the optimal extraction of ions

from the quadrupole ion trap .

A quadrupole ion trap device is widely used in mass analysis

of ions and/or molecular structure analysis of a chemical

composite by trapping ions using a high voltage

radio-frequency (RF) , selecting specific ions in dependence on

their mass-to-charge ratio, cooling ions by collisions with buffer gas, and many other associated techniques. This area of

application of quadrupole ion trap devices has been discussed

in various articles, for example, in "Practical Aspects of Ion

Trap Mass Spectrometry volume 1 (1995, CRC Press)".

Recently attempts have been made to use a quadrupole ion trap

as an ion source for a time-of-flight mass spectrometer due to

the superior ability of the quadrupole ion trap to cool the

ion energy to a level which is sufficiently low to be suitable

for high resolution analysis of the time-of-flight . While a

time-of-flight mass spectrometer compensates for a spread of

flight times for a certain range of initial ion energies

emitted from the ion source, a reduced spread of flight times

using the smaller range of initial energies in the quadrupole

ion trap gives higher resolution. The disclosure in U.S.

patent 5,569,917 suggests that it is important to optimize the

operational parameters of the quadrupole ion trap to obtain a

high-resolution mass spectrum and a high sensitivity for trace

analysis. This patent discloses a quadrupole ion trap (shown

in Figure 1) utilizing a bipolar extraction field whereby

extraction voltages of the same magnitude (between 200V and

550V) , or almost the same magnitude, but of opposite polarity

are applied to the end-cap electrodes. In a particular example, voltages of +500V and -420V were used, the positive

voltage having a slightly larger value so as to produce a

parallel ion beam after the ions have been emitted into the

field-free drift space of the time-of-flight mass

spectrometer. Post acceleration is also used whereby ions

initially accelerated to an energy of about 500eV in the

quadrupole ion trap continue to be accelerated by an electric

field outside the quadrupole ion trap to obtain an energy

required for time-of-flight mass analysis, usually in the

range from 5keV to 30keV. Ion beam focusing is also affected

by this post-acceleration and this effect is allowed for by

adjustment of the magnitudes of the voltages applied to the

two end-cap electrodes.

It is an object of the present invention to provide a time-of-

flight mass spectrometer incorporating a quadrupole ion trap

having an improved performance .

SUMMARY OF THE INVENTION

According to one aspect of the invention there is provided a

time-of-flight mass spectrometer comprising a quadrupole ion

trap as an ion source, an ion detector and a field-free drift

space between the quadrupole ion trap and the ion detector,

the quadrupole ion trap having a ring electrode and two end- cap electrodes, at least one of the end-cap electrodes having

at least one hole at its centre through which ions can be

extracted in use, and voltage supply means for supplying to

said at least one end-cap electrode a first extraction voltage

relative to the ring electrode and for supplying to another

said end-cap electrode a second extraction voltage relative to

the ring electrode having the opposite polarity to said first

extraction voltage, said first and second extraction voltages

being respectively negative and positive voltages for positive

ion extraction and being respectively positive and negative

voltages for negative ion extraction, the second extraction

voltage having a magnitude in the range from 0.5 to 0.8 of

that of said first extraction voltage.

According to another aspect of the invention there is provided

a method for forming an ion beam using a quadrupole ion trap

having a ring electrode and two end-cap electrodes, at least

one of the end-cap electrodes having at least one hole at its

centre through which ions can be extracted, in use, the method

comprising supplying to said at least one end-cap electrode a

first extraction voltage relative to the ring electrode and

supplying to another said end-cap electrode a second

extraction voltage relative to the ring electrode having the opposite polarity to said first extraction voltage, said first

and second extraction voltages being respectively negative and

positive voltages for positive ion extraction and being

respectively positive and negative voltages for negative ion

extraction, the second extraction voltage having a magnitude

in the range from 0.5 to 0.8 of that of said first extraction

voltage .

According to a yet further aspect of the invention there is

provided a quadrupole ion trap having a ring electrode and two

end-cap electrodes, at least one of the end cap electrodes

having at least one hole at its centre through which ions can

be extracted, in use, and voltage supply means for supplying

to said at least one end-cap electrode a first extraction

voltage relative to the ring electrode and for supplying to

another said end-cap electrode a second extraction voltage

relative to the ring electrode having the opposite polarity to

said first extraction voltage, said first and second

extraction voltages being respectively negative and positive

voltages for positive ion extraction and being respectively

positive and negative voltages for negative ion extraction,

the second extraction voltage having a magnitude in the range

from 0.5 to 0.8 of that of said first extraction voltage. Recent investigations by the inventor into the operation of a

time-of-flight mass spectrometer incorporating a quadrupole

ion trap as the ion source and into the systematic design of

an ion reflector in order to achieve high resolution gave

unexpected results.

Initially, a relatively high extraction field was used inside

the quadrupole ion trap with a view to obtaining the highest

possible electric field for ion extraction whereby to reduce

turn-around time. This was done because turn-around time

tends to dominate time spread in the spectrometer which should

be reduced to achieve higher resolution. The turn-around time

is the time taken by an ion having a small initial velocity

directed away from the extraction end-cap electrode to return

to the initial position with the same velocity but in the

opposite direction. A high extraction field was used inside

the quadrupole ion trap to enable ions to acquire enough

energy for time-of-flight analysis without the need for any

post acceleration following their extraction.

Quite unexpectedly, and contrary to the teaching of US Patent

No. 5,569,917, it was found as a result of these

investigations that the optimum electric field configuration inside the quadrupole ion trap was established when the

magnitude of the second extraction voltage (a positive voltage for positive ions) was only 0.6 that of the first extraction

voltage (a negative voltage for positive ions) and it was also

found that relative magnitudes having a ratio in the range

from 0.5 to 0.8 also gave desirable results. It was found

that when the magnitude of the second extraction voltage was

0.6 that of the first extraction voltage ions inside the ion

trap experienced an acceleration voltage as high as 90% that

of the first extraction voltage.

Slightly curved equi-potential lines concentric to the surface

of the extraction end-cap electrode smoothly accelerate ions

into the hole in the end-cap electrode with slightly

convergent trajectories. However, termination of the electric

field at and around the hole causes a slight divergence which

compensates for the convergent trajectories giving a parallel

ion beam outside the quadrupole ion trap. The curvature of

the equi-potential lines causes a slight shift in the energies

of ions initially occupying a plane perpendicular to the

extraction axis. However, an ion reflector has the capability

to cancel out the effect of this energy shift from the total

flight time measured at the surface of the ion detector. In an example, a first extraction voltage of -lOkV was applied

to the extraction end-cap electrode and a second extraction voltage of +6kV was applied to the other end-cap electrode,

where both extraction voltages are expressed relative to the

voltage on the ring electrode. Ions originating from the

centre of the quadrupole ion trap were found to have an energy

of 9keV after being emitted into a field-free drift space. In

the field- free drift space the ions had almost parallel

trajectories without the need for post-acceleration or an

electrostatic lens to focus the beam, and so the ions were

reflected in the ion reflector towards the ion detector

without any significant loss of intensity thereby achieving

high sensitivity. The inventor considered another possibility

of accelerating the ions using a much higher extraction field

followed by post-deceleration so as to reduce the ion energy

in front of the field-free drift space. The beam divergence

caused by post -deceleration can be compensated by further

reducing the ratio of the extraction voltages. However, this

seems less effective because of the requirement to apply much

higher voltages to the end-cap electrodes than those in

previous examples .

There is another type of voltage configuration in which the field-free drift space and the extraction end-cap electrode

are maintained at ground voltage and the ring electrode and

the other end-cap electrode have applied positive voltages,

namely +10kV and +16kV, respectively. This configuration

doesn't change the relative voltage differences between the

electrodes, but only shifts all the voltages by lOkV. This

has the advantage that the field-free drift space can be at

ground voltage thereby eliminating the need to apply a

floating voltage to the flight tube. Otherwise the voltage

+16kV must be switched at ion extraction, which increases the

practical difficulty of handling higher voltage.

It was found that an approximate time focussing occurs in the

field- free drift space, about 37.4mm from the centre of the

quadrupole ion trap. However, this is not regarded as

important or necessary. The ion reflector can be so designed

as to take into account the time spent in the quadrupole ion

trap so as to produce a much smaller time spread at the ion

detector surface than at the aforementioned approximate time

focussing plane.

In the investigations carried out by the inventor, the

extraction end-cap electrode had a surface provided with a cone-shaped hump around the central hole. The end-cap

electrodes were nominally positioned such that the asymptotes

of the ring and end-cap electrodes were coincident at the

centre of the quadrupole ion trap. Another well known form of

the quadrupole ion trap has a stretched geometry in which both

end-cap electrodes are each moved apart by 0.76mm from their

nominal positions. In this case the optimum electric field

configuration was achieved by applying a first extraction

voltage of -lOkV to the extraction end-cap electrode and a

second extraction voltage of +7kV to the other end-cap

electrode, a ratio of 0.7. It was found that the optimum

ratio of the second voltage to the first voltage increases as

the geometry of the quadrupole ion trap becomes more

stretched. The diameter of the hole in the end-cap electrodes

also affects the optimum ratio of the voltages, but this has

a lesser effect than does the extent of stretching.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention are now described, by way of

example only, with reference to the accompanying drawings of which:

Figure 1 shows a cross-sectional view through a known

quadrupole ion trap and associated drift tube, Figure 2 is a schematic representation of a time-of-flight

mass spectrometer according to the invention, and

Figure 3 is an enlarged cross-sectional view through the

central parts of a quadrupole ion trap used in the time-of-

flight mass spectrometer of Figure 2.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

Referring to Figure 2, the time-of-flight mass spectrometer

comprises a quadrupole ion trap 10, a drift tube 11 defining a field-free drift space, an ion reflector 12 and an ion

detector 13. The quadrupole ion trap 10 itself comprises a

ring electrode 21 and two end-cap electrodes 22 and 23.

End-cap electrode 22 has a hole 24 through which ions are

extracted to form an ion beam 28. End-cap electrode 23 also

has a hole 25 through which ions produced by an external ion

injector 14 can pass for injection into the trap volume 26 of

the quadrupole ion trap 10. In an alternative arrangement,

the ions to be analysed are formed inside the quadrupole ion

trap 10. In this case, the external ion injector 14 is

replaced by an electron injector and ions are produced inside

the trap volume 26 of the quadrupole ion trap 10 by electron impact ionization of sample atoms and/or molecules. Three switching devices 31, 32 and 33 normally connect the

ring electrode 21 to an RF generator 15 and end-cap electrodes

22 and 23 to ground through a transformer 17 which produces a dipole electric field inside the quadrupole ion trap 10. The

form of the dipole electric field is determined by the output

of a waveform generator 16 also connected to the transformer

17. This arrangement facilitates a range of different methods

for handling ions, such as selecting or eliminating specific

ions and/or causing fragmentation to perform MS/MS analysis.

The transformer could be replaced by low impedance amplifiers

with opposite polarities.

The switches 31, 32 and 33 have another connection which is

used in an extraction mode when ions are to be extracted from

the trap volume 26 of the quadrupole ion trap 10 and ejected

into the field-free drift space. In the extraction mode,

switch 31 connects the ring electrode 21 to ground whereby to

terminate the RF voltage during the extraction period. Switch

32 connects end-cap electrode 22 to a negative high-voltage

power supply 34 and switch 33 connects end-cap electrode 23 to

a positive high-voltage power supply 35. The negative

high-voltage power supply 34 is also connected to drift tube

11. The described polarities apply when the ions to be analysed are positive. The polarities would be reversed in the case of negative ions.

Referring now to Figure 3 parts of the ring electrode 41, the

end-cap electrodes 42 and 43 having holes 44 and 45,

respectively, part of the drift tube 46 and a part of the

external ion injector 47 are shown on an enlarged scale. This

Figure shows equi-potential lines 49, in steps of IkV produced

when a voltage of -lOkV is applied to the extraction end-cap

electrode 42 and to the drift tube 46 and when a voltage of

+6kV is applied to the other end-cap electrode 43, these

voltages being expressed with respect to the grounded ring

electrode 41. Accordingly, in this embodiment, the ratio of

the applied voltages has the aforementioned optimum value of

0.6. The ions around the centre of the quadrupole ion trap

where the electric potential is about -IkV relative to ground

form an ion beam 48 which initially converges in the direction

of the end-cap electrode 42 and is subsequently caused to

diverge around the hole 44 to form a parallel ion beam in the

field- free drift space.

The ions to be mass analysed in time-of-flight mass

spectrometer of this embodiment are prepared by an external ion injector such as by matrix-assisted laser desorption/

ionization (MALDI) and are selected depending on their

mass-to-charge ratio and concentrated into a small region at

the centre of the quadrupole ion trap 10 using standard

techniques usually adopted in this field. At this moment ions

are trapped by RF electric field produced by the RF generator

15. Before ion extraction, the trapping field is switched off

by the switching device 31 and the extraction voltages are

applied to the end-cap electrodes 22 and 23 using switching

devices 32,33. Provided the switching of the switching device

31 is fast enough the trapping field can be switched off and

the extraction voltages applied at exactly the same time.

However, because of the high voltages involved, the voltages

appearing at the end-cap electrodes may have delays and/or may

exhibit a finite rise time to reach the required values.

Variations in delay times and rise times of the extraction

voltages were investigated and it was found that mass

resolution does not show a significant change, whereas the

time-of-flight suffers a time shift equal to half of the rise time of the switching devices measured from appearance of the

voltages . It will be understood that the positive voltage and

the negative voltage need not necessarily be switched at the

same time nor do they need to have a linear slope to reach their final voltage values nor need they exhibit the same

voltage variation as they approach those values. There may

also be a delay between activation of the two switching

devices 32,33. Ideally, switching of the voltages should have

been completed, and the voltages should have settled to their

final values, within about 200 nanoseconds, and preferably

within about 100 nanoseconds. However, it is necessary that

the switching delay and the pulse shape resulting from the

variation in voltage as a function of time be highly

reproducible so that the same compensating shift in flight

time can be applied each time ions are extracted from the ion

trap.

Claims

1. A time-of-flight mass spectrometer comprising a

quadrupole ion trap as an ion source, an ion detector and a

field-free drift space between the quadrupole ion trap and the

ion source, the quadrupole ion trap having a ring electrode

and two end-cap electrodes, at least one of the end-cap

electrodes having at least one hole at its centre through

which ions can be extracted in use, and voltage supply means

for supplying to said at least one end-cap electrode a first

extraction voltage relative to said ring electrode and for

supplying to another said end-cap electrode a second

extraction voltage relative to said ring electrode having the

opposite polarity to said first extraction voltage, said first

and second extraction voltages being respectively negative and

positive voltages for positive ion extraction and being

respectively positive and negative voltages for negative ion

extraction, the second extraction voltage having a magnitude

in the range from 0.5 to 0.8 of that of said first extraction

voltage .

2. A time-of-flight mass spectrometer as claimed in claim 1,

wherein the ions to be extracted are positive ions, said first extraction voltage is a negative voltage and said second

extraction voltage is a positive voltage.

3. A time-of-flight mass spectrometer as claimed in claim 1,

wherein the ions to be extracted are negative ions, said first

extraction voltage is a positive voltage and said second

extraction voltage is a negative voltage.

4. A time-of-flight mass spectrometer as claimed in any one

of claims 1 to 3 , wherein said second extraction voltage has

a magnitude which is 0.6 that of said first extraction

voltage .

5. A time-of-flight mass spectrometer as claimed in any one

claims 1 to 4 , wherein said first extraction voltage is also

applied to the field-free drift space.

6. A time-of-flight mass spectrometer according to claims 1

to 5, wherein said end-cap electrodes and said ring electrode

enclose a trap volume, the voltage supply means is arranged to

supply to the end cap electrodes further voltages to confine

and/or control ions within said trap volume, and includes

switching means for switching between said further voltages and said first and second extraction voltages.

7. A time-of-flight mass spectrometer as claimed in claim 6

wherein said switching means effects switching from said

further voltages to said first and second extraction voltages

within a time interval of less than 200 nanoseconds.

8. A time-of-flight mass spectrometer as claimed in any one

of claims 1 to 7 wherein the field-free drift space includes

an ion reflector.

9. A method for forming an ion beam using a quadrupole ion

trap having a ring electrode and two end-cap electrodes, at

least one of the end-cap electrodes having at least one hole

at its centre through which ions can be extracted in use, the

method comprising supplying to said at least one end-cap

electrode a first extraction voltage relative to said ring

electrode and supplying to another said end-cap electrode a

second extraction voltage relative to said ring electrode,

having the opposite polarity to said first extraction voltage,

said first and second extraction voltages being respectively

negative and positive voltages for positive ion extraction and

being respectively positive and negative voltages for negative ion extraction; the second extraction voltage having a

magnitude in the range from 0.5 to 0.8 of that of said first

extraction voltage.

10. A method as claimed in claim 9, wherein the ions to be

extracted are positive ions, said first extraction voltage is

a negative voltage and said second extraction voltage is a

positive voltage.

11. A method as claimed in claim 9, wherein the ions to be

extracted are negative ions, said first extraction voltage is

a positive voltage and said second extraction voltage is a

negative voltage.

12. A method as claimed in any one of claims 9 to 11, wherein

said second extraction voltage has a magnitude which is 0.6

that of said first extraction voltage.

13. A method as claimed in any one of claims 9 to 12

including applying said first extraction voltage to a field-

free drift region of a time-of-flight mass spectrometer

incorporating the quadrupole ion trap.

14. A method according to any one of claims 9 to 13 including

applying to the end cap electrodes further voltages suitable

for confining and/or controlling ions within a trap volume

enclosed by the end-cap electrodes and said ring electrode and

including switching between said further voltages and said

first and second extraction voltages.

15. A method as claimed in claim 14 including switching from

said further voltages to said first and second extraction

voltages within a time interval of less than 200 nanoseconds.

16. A quadrupole ion trap having a ring electrode and two

end-cap electrodes, at least one of said end cap electrodes

having at least one hole at its centre through which ions can

be extracted in use, and voltage supply means for supplying to

said at least one end-cap electrode a first extraction voltage

relative to said ring electrode and for supplying to another

said end-cap electrode a second extraction voltage relative to

said ring electrode having the opposite polarity to said first

extraction voltage, said first and second extraction voltages

being respectively negative and positive voltages for positive

ion extraction and being respectively positive and negative

voltages for negative ion extraction, the second extraction voltage having a magnitude in the range from 0.5 to 0.8 of

that of said first extraction voltage.

17. A quadrupole ion trap as claimed in in claim 16, wherein

said second extraction voltage is 0.6 that of said first

extraction voltage.

18. A time-of-flight mass spectrometer substantially as

herein described with reference to Figures 2 and 3 of the

accompanying drawings .

19. A method for forming an ion beam using a quadrupole ion

trap substantially as herein described with reference to

Figures 2 and 3 of the accompanying drawings.

20. A quadrupole ion trap substantially as herein described

with reference to Figures 2 and 3 of the accompanying

drawings .