WO1997048120A1

WO1997048120A1 - Time-of-flight mass spectrometer

Info

Publication number: WO1997048120A1
Application number: PCT/GB1997/001552
Authority: WO
Inventors: Alexander Alekseevich Makarov; Stephen Charles Davis
Original assignee: Hd Technologies Limited
Priority date: 1996-06-10
Filing date: 1997-06-10
Publication date: 1997-12-18
Also published as: GB9612091D0; AU3041197A

Abstract

A mass spectrometer of the time-of-flight kind includes an ion source which produces ions for analysis which on emergence from the source have a velocity in a first direction. The ions pass between a pair of electrodes to one of which a voltage is provided which imposes a velocity in a second direction onto the ions to carry them into a measurement chamber containing a detector. Ions of interest of a particular m/z ratio are selected, fragmented in a fragmentation device and detected by the detector which produces a mass spectra in accordance with the detected smaller mass ions. The ions of interest are selected by passing the ions through a pair of electrodes, one of which has a voltage applied to it which creates an electric field which reduces only the ions of interest to substantially zero velocity in the second direction in the vicinity of the fragmentation device. The velocity in the first direction then causes only the ions of interest into the fragmentation device for fragmentation.

Description

ΗME-OF-FLIGHT MASS SPECTROMETER

This invention relates to improvements in or relating to mass

spectrometers of the time-of-flight kind.

For some measurements it can be advantageous to use a tandem

time-of-flight mass spectrometer. A tandem mass spectrometer usually

comprises two linked mass spectrometers, the first mass spectrometer being

used to separate ions of interest from inorganic, bio-organic or organic

compounds and then the ion of interest is fed into a fragmentation device

where this ion is fragmented into smaller ions which produce a mass

spectrum which is characteristic of the structure of the ion of interest and

is detected using a second mass spectrometer.

It will be appreciated that whilst use of a tandem mass spectrometer

in the manner mentioned above increases the amount of information which

can be determined about an ion of interest, the cost and complexity of

apparatus necessary to perform such measurements can be excessive,

especially bearing in mind that two separate or linked mass spectrometers

are required.

It is an object of the present invention to provide a mass

spectrometer of the time-of-flight kind which can provide an equivalent

amount of information about an ion of interest to that which can be

provided by a conventional tandem mass spectrometer. It is further an object of the present invention to provide a mass

spectrometer which is relatively simple in construction and therefore

relatively low in cost compared to other tandem time-of-flight mass

spectrometers.

It is still further an object of the present invention to provide a mass

spectrometer which can operate both as a single spectrometer and also as

a tandem spectrometer.

According to the present invention therefore there is provided a mass

spectrometer of the time-of-flight kind comprising an ion source to produce

ions for analysis, said ions having a velocity in a first direction, means to

impose a velocity in a second direction on said ions to cause the ions to

enter a measurement chamber in which is disposed a detector, means to

reduce the velocity of ions of interest of a selected m/z ratio in said second

direction to substantially zero whereby the velocity in a first direction

causes only said ions of interest to enter a fragmentation means which acts

to fragment the ions into smaller mass ions, means to impose a further

velocity in a second direction on said smaller mass ions to cause said

smaller ions to move towards the detector for detection, said detector being

operable to produce a mass spectrum in accordance with said detected

smaller mass ions.

With this arrangement it is possible to provide a time-of-flight mass

spectrometer which can, if desired, provide the same amount of measured information as a conventional tandem spectrometer but in which it is only

necessary to use a single measurement chamber, thereby reducing cost and

complexity. Furthermore, measurements can be made in the same manner

as a single time-of-flight spectrometer or a tandem spectrometer.

The invention will now be described further by way of example only

and with reference to the accompanying drawings of which:-

Fig. 1 shows a diagrammatic representation of

one form of time-of-flight mass spectrometer in

accordance with the present invention;

Fig. 2 shows the spectrometer of Fig. 1 with the

path of travel of fragmented ions shown; and

Fig. 3 shows a diagrammatic sectional side view

of one form of fragmentation device for use in the

mass spectrometer of Fig. 1 ; and

Fig. 4 shows a schematic perspective view of an

alternative form of time-of-flight spectrometer in

accordance with the present invention.

Referring now to Fig. 1 , there is shown a mass spectrometer of time-

of-flight form in accordance with the present invention.

The spectrometer comprises an ion source 10 which produces a beam

of ions, an ion optical arrangement 20 which acts to focus and accelerate

the ion beam produced by the ion source, an extraction device 30 which acts to orthogonally extract from the beam of ions produced by the ion

source 10, a small part thereof which is to be fed into the measurement

chamber 50 of the mass spectrometer for analysis, an ion mirror 40, a

fragmentation device 60 which acts to fragment selected ions of interest

into a number of smaller mass ions and a detector device 70 to detect

incident ions and from the output of which a mass spectrum can be

produced which is characteristic of the incident ions. All of the

abovementioned components are contained within a vacuum envelope.

The ion source 10 is of conventional form and may form ions utilising

any suitable means of ionization as desired or as appropriate. For example,

the ion source may use electron impact, chemical ionization, thermo or

electro spray ionization, fast atom bombardment or any other suitable

means of ionization. The ion source can generate either a continuous beam

or a pulsed beam of ions, as desired or as appropriate.

The ion optical arrangement 20 contains any suitable arrangement

which can act to focus the beam produced by the ion source to a narrow

low-divergence beam.

The extraction device 30 comprises electrodes 31 and 32 which

extend generally in the direction of travel of the beam of ions produced by

the ion source. In the embodiment shown in Fig. 1 , the electrodes 31 and

32 are generally similar in shape, but of course it will be appreciated that

the electrodes can be of any suitable shape. At least electrode 32 which is positioned adjacent to the measurement chamber 50 is either in the form

of a perforated grid or has a slot or aperture (not shown) therein through

which ions can pass for a reason to be described hereinafter.

The measurement chamber 50 is generally of conventional form and

is provided with a perforated grid electrode 34, or an electrode having slits

therein through which ions can pass, in a wall substantially adjacent and

parallel to the perforated grid electrode 32, or slit electrode, of the

extraction device 30 whereby ions can pass from the extraction device 30

into the measurement chamber 50, or vice versa, via the perforations or slit

in the electrodes 32 and 34. A detector 70 of conventional form is located

within the measurement chamber 50. The position of the detector within

the measurement chamber is of importance for a reason which will become

apparent hereinafter. At an opposite end of the measurement chamber 50

to the perforated grid or slit electrode 34, there is provided an ion mirror 40

of conventional form.

The fragmentation device 60 acts to fragment selected ions of

interest into ions of smaller masses. The fragmentation device 60 is located

adjacent an end 35 of the extraction device 30 opposite to the ion source

10. The fragmentation device 60 can operate to fragment the ions in any

of a number of conventional ways, for example by collision with gas

molecules at high pressure (collision induced dissociation - CID), by

smashing the ions against a fixed surface (surface induced dissociation - SID) or by fragmentation of the ions within a light beam (photo induced

dissociation - PID).

One form of fragmentation device 60 is shown more clearly in Fig. 3.

This fragmentation device comprises a fragmentation region defined

between the conducting restrictors 61 which are shaped so as to define an

inlet aperture at one end. An opposite end of the fragmentation region is

defined by an end cap 62 which may have a transparent or at least

translucent window 63 there for a purpose which will become apparent

hereinafter. A gas leak valve 64 may also be provided linked to the

fragmentation device, along with an ionization gauge 65. A radio frequency

ion guide 66 extends generally along a horizontal axis of the device for a

reason which will also become hereinafter apparent. Any form of ion

optics, static or dynamic may be used within the fragmentation device 60.

In use, an ion beam is produced by an ion source 10 which exits the

source in a direction generally parallel to the direction indicated Y in Figure

1 . The ion beam passes through the ion optical arrangement 20 in which

the beam is focused into an ion beam of low divergence. The narrow low-

divergent beam of ions are directed so as to pass between electrodes 31

and 32 which form a part of the extraction device 30. As the ion beam

enters the region between the electrodes 31 and 32, a pulse of voltage is

applied to the electrode 31 using a pulsed voltage supply 33. The electrode

32 is maintained at a fixed potential whilst electrode 31 is pulsed to a voltage V greater than the fixed potential at which electrode 32 is

maintained. Alternatively, electrode 32 can be pulsed to a voltage lower

than a fixed voltage at which electrode 31 is maintained. The difference in

voltage V between electrode 31 and electrode 32 causes an electric field

orthogonal to the direction Y and generally in the direction X indicated on

Fig. 1. If electrode 31 is pulsed to voltage V, it will be understood that a

section of the beam of ions travelling along the extraction device 30

between electrodes 31 and 32 will experience an electric field £ which will

cause them to accelerate transversely of electrode 31 towards electrode 32.

Electrode 32 is arranged to be formed either from a perforated grid or to

have a slit therein in order that ions can pass through this electrode under

the influence of the field £ and into the measurement chamber 50 of the

spectrometer through a similar electrode 34 provided in the wall of the

chamber adjacent electrode 32. If desired, the beam can be further

accelerated between electrodes 32 and 34. If desired further electrodes

may be provided for this purpose. If the voltage is applied rapidly, it can be

seen that a rapid and efficient acceleration of a segment of ion beam into

the measurement chamber 50 of the spectrometer can be achieved.

When the section of the ion beam enters the measurement chamber

50 of the mass spectrometer after its orthogonal extraction in the device

30, the ions within the section of the beam will not only have a velocity in

the direction X, due to acceleration between electrodes 31 , 32 and 34, but will also substantially retain their component of velocity in the direction Y

and therefore the path followed by the ion beam section will be that shown

in the diagram of Fig. 1 marked P_v Hereafter any component of velocity in

the direction Y will be referred to as Y-velocity and correspondingly any

component of velocity in the direction X will be referred to as X-velocity.

Since ions within the ion beam produced by the ion source 10 which are

closer to electrode 31 prior to application of the voltage V to this electrode

31 , will inevitably acquire a greater amount of energy as they move a

greater distance through the electric field £, such ions will inevitably have

a greater X-velocity when they enter the chamber 50 of the mass

spectrometer than ions which were initially in a part of the ion beam closer

to electrode 32. Due to their greater X-velocity, there will be a plane,

shown as a dotted line Q in Fig. 1 , within the measurement chamber 50 in

which these ions will have caught up with those ions which started from a

position closer to electrode 32 and this plane Q. The detector may be

situated at this plane Q. As the ions move beyond plane Q within the

measurement chamber 50 along their path of travel, the ions with a higher

X-velocity will move ahead of the ions with a lower X-velocity. In order to

compensate for this, at one end of the measurement chamber 50 opposite

to the point of entry of the ions into the measurement chamber, an ion

mirror 40 is provided. The ions, as they move through the measurement

chamber 50, will interact with the ion mirror 40. The ion mirror acts to reflect ions which interact with it so as to cause them to travel in the

opposite X-direction. However, the interaction with the ion mirror 40 varies

according to the X-velocity of the ions as they enter the ion mirror. The

greater the X-velocity of ions, the greater the depth with which these ions

penetrate the ion mirror 40 and accordingly it will be realised that it can be

arranged by suitable choice of ion mirror 40 for the higher X-velocity ions

to penetrate sufficiently far into the ion mirror 40 compared to lower X-

velocity ions whereby after reflection the lower X-velocity ions emerge from

the ion mirror 40 travelling in the opposite X-direction ahead of the higher

X-velocity ions. An ion mirror 40 of this type is usually referred to as a

"reflectron".

After reflection at the ion mirror 40 it can be arranged that the

section of ion beam is directed so as to impinge upon the detector 70

whereby the masses of the ions can be detected in conventional single time-

of-f light mass spectrometer manner. In this case it may be arranged such

that, after reflection, the higher X-velocity ions catch up with the lower X-

velocity ions, at the detector 70.

However, the present invention is primarily concerned with operation

of the mass spectrometer in a tandem manner.

By arranging for the voltage applied to the electrode 31 to be at a

selected predetermined value, it can be arranged that after reflection within

the ion mirror 40 the beam section is directed so as to pass the detector 70 along a second path P₂ shown in Fig. 1 and to exit the measurement

chamber 50 via electrode 34 and electrode 32. Ions will then be

decelerated by the voltage difference between electrodes 31 and 32 and 32

and 34 in the opposite manner to which they are accelerated on entry to the

chamber 50. At some point in time, ions of interest of a given mass/charge

ratio will be decelerated to substantially zero X-velocity and after this point

in time will be turned around and accelerated in the opposite direction. It

can be seen in Fig. 1 that the arrangement of the present invention is

substantially symmetrical with respect to forces in the X-direction and, if the

voltage V on electrode 31 is switched off at the time the ion of interest

(with the given mass/charge ratio) has substantially zero X-velocity, the ion

of interest will continue to move along the direction Y due to the Y-velocity

imposed from extraction from the ion source. Thus the ion of interest will

enter the fragmentation device through the aperture 61 . The ions of

interest which enter the fragmentation device 60 will appear to all intents

and purposes as if they had simply travelled directly from the ion source 10

between electrodes 31 and 32 to the fragmentation device 60. At this point

in time at which the ion of interest is at substantially zero X-velocity, ions

of other mass/charge values which are not of interest will not have

substantially zero X-velocity and will therefore have an X-Velocity which will

carry them towards electrode 31 or, if they have already been turned round

by electrode 31 , towards electrode 32. Ions of mass/charge value close to the ion of interest will also retain an X-velocity when the voltage to

electrode 31 is switched off and these will therefore run at an angle to the

direction Y and therefore if the aperture to the fragmentation device 60 is

arranged to be of a permitted size, ions of mass/charge value, other than

that of the ion of interest can be prevented from entering the fragmentation

device 60. Therefore, in this way, an ion of interest with a particular

mass/charge ratio can be selected for fragmentation. Within the

fragmentation device the ion of interest is fragmented into ions of smaller

mass utilising one of the fragmentation methods mentioned above and, to

achieve highest transmission of ions and maximum mass resolution, angular

acceptance (α) of the fragmentation device 60 should be

where E is the mean initial acceleration voltage in the ion source 10 and ion

optical arrangement, δv_x is the spread of velocities in direction X resulting

from aberrations in the mass spectrometer, v_β is the mean velocity in the

direction Y, R is the mass resolution as a single mass spectrometer

(measured on the same level of ion distribution as δv,). V is a voltage

applied across the gap d between electrodes 31 and 32, L_M is effective path

length of ions between pulses (i.e. the mass-independent multiplication of

mean velocity in the direction X and time-of-flight between moments of push-out and turning in the extraction device 30), V_a is the mean voltage

difference passed by ions during the transverse acceleration. Once the

fragmented ions have been formed, they are ejected in a manner to be

described from the fragmentation device into the extraction device 30 and

a pulsed voltage applied to electrode 31 will once again cause the

fragmented ions to move generally transversely in the direction X back into

the measurement chamber 50 via electrode 32 and electrode 34. The ions

of interest as mentioned above enter the fragmentation device 60 via

aperture in the conductivity restrictor 61 . Within the fragmentation device

60, the ions of interest are fragmented into smaller ions using either multiple

low energy collisions with gas molecules (CID), or can be directed towards

the end cap 62 against which they smash and are fragmented (SID).

Alternatively an intense light beam can be directed at the ions of interest

through window 63 in order that the ions can be fragmented (PID). When

collision with gas molecules is being used for fragmentations, gas is

introduced into the fragmentation region through a leak valve 64 and may

be a pulsed or continuous supply thereof. The presence of gas in the

fragmentation region is detected using ionization gauge 65. After

fragmentation, the Y-velocity of the ions, both ion of interest and ions

formed by fragmentation thereof is reversed by applying a voltage, static or

dynamic to the end cap 62. This causes the ions to accelerate away from

the end cap 62 and towards the aperture in the fragmentation device 60. The beam of fragmented ions will exit the fragmentation device 60 and may

pass through a further ion optical arrangement 36 and will enter a field free

region between electrodes 31 and 32. If electrode 31 is pulsed to voltage

V once again, then the beam of fragmented ions will be accelerated

substantially orthogonally of the Y direction through electrode 34 and into

the measurement chamber 50.

When collision with a gas is used to fragment an ion of interest, the

collision energy will be partitioned amongst the fragment ions formed. The

fragment ions have a similar velocity to the ion of interest from which they

are formed which means that the collision energy in the case of singly

charged ions will be partitioned approximately as a fraction md/Mp of the

energy of the ion of interest wherein md is the mass value for the fragment

ion and Mp is the mass value of the ion of interest. This therefore means

that the fragment ions will have different angles after entry back into the

measurement chamber 50 and therefore there will be a different path of

travel for each different fragment ion of different m/z ratio. In order to

ensure that all of the fragment ions ultimately impinge upon the detector 70,

it is necessary to minimise the difference in Y distance travelled. This can

be achieved in a number of different ways. Firstly, if desired, a voltage can

be applied to the fragmentation device 60, such that the fragment ions can

receive an additional acceleration on exit. The lighter fragments are

accelerated to a higher Y-velocity than the heavier fragments which assists in minimisation of the difference in path of travel in the Y-direction of the

fragment ions in the measurement chamber 50.

The energy and coordinates f ions of interest and of the fragment ions

may also be controlled using collisional damping, collisional focusing or

collisional reflection.

Alternatively, if gas collisions using other energies are necessary then

there are several possibilities which may be employed. Firstly, the energy

could be changed by scaling voltages on all electrodes of the whole mass

spectrometer so that the shape of trajectories does not change while

efficiency of CID is strongly varied. Secondly, it could be changed by

applying additional accelerating or decelerating voltage along the gap

between the extraction device 30 and the fragmenting means 60 prior to

the entrance of the precursor ions into this gap. While all beam section S

of the ions is already travelling inside the fragmenting means 60, this

voltage could also be changed in order to optimise forthcoming extraction

of fragments. Any type of ion optical arrangement 36 either transmitting

or reflecting, static or dynamic could be disposed between the extraction

device 30 and fragmentation device 60. For example, a controlled retarding

voltage could be used as a means for distinguishing fragment ions from

other charge state ions. Thirdly, energy of collisions could be varied inside

the fragmenting means 60 by an additional radio-frequency or impulsive

excitation of the ions using voltages applied across or along the focusing means 66. In particular, cycles of excitation/de-excitation could be the

most promising as after each cycle the precursor ions (as well as heavier

fragments) return to the axis making their focusing easier. By suitable

arrangement of the voltages on the various electrodes it can be arranged

such that the beam of fragmented ions travel through measurement

chamber 50, are reflected by the ion mirror 40 and then impinge upon the

detector 70 whereby measurement of a mass spectrum can be made from

these fragmented ions. This situation is illustrated in Fig. 2. It can be seen

therefore that using the apparatus shown in Fig. 1 and Fig. 2 of the

drawings that a tandem time-of-flight mass spectrometer can be provided

without the necessity to utilise two separate mass spectrometer chambers

linked together. Furthermore, the arrangement of the present invention is

clearly versatile insofar as the mass spectrometer can operate either as a

single mass spectrometer or as a tandem mass spectrometer simply by

arranging for the extracted ion beam to either be fragmented prior to

detection or by arranging for the extracted beam simply to be detected

without fragmentation. It will be appreciated that this gives rise to

considerable savings as to cost and complexity of the apparatus.

An alternative embodiment of apparatus in accordance with the

present invention is shown in Fig. 4. In this embodiment, additional pairs

of deflector plates 51 and 52 are provided in the measurement chamber.

The pairs of deflector plates 51 , 52 act to deflect an ion beam passing through the measurement chamber 50. The first pair of deflector plates 51

deflects the incoming ion beam so as to avoid the detector 70 after

reflection. The second pair of deflector plates 52 deflects the ion beam by

an equal amount in the opposite direction to plates 51 . The nett effect of

plates 51 and 52 is the parallel displacement of the ion beam in a direction

illustrated as Z in Fig. 4 with minimum time-of-flight aberrations. The beam

enters the fragmentation device 60 which is vertically displaced relative to

the detector 70 and ion source 10. Of course, if the deflection plates 51

do not have voltage applied then the incoming ion beam will simply, after

reflection in the ion mirror 40, impinge upon the detector without

fragmentation having occurred.

It is of course to be understood that the invention is not intended to

be restricted to the details of the above embodiment which are described

by way of example only.

Thus, for example, it is possible to improve the duty cycle of the

spectrometer of the invention by arranging for the voltage pulse which

causes the ion beam to enter the measurement chamber 50 from the ion

source 10 to be coincident in time with the pulse to cause the fragment ions

to enter the measurement chamber 50 from the fragmentation device 60.

In the described embodiment (Figs. 1 and 2) there is an absence of any

obstacles in the direction Y between the ion optical arrangement 20 and

the fragmentation device 60 along extraction device 30. The extraction device may of course be constructed in several sections. In these

circumstances, a further electrode 37 is provided between the ion source

10 and fragmentation device 60 which can be pulsed appropriately with a

voltage to ensure that ions do not pass from ion source 10 to fragmentation

device 60 directly along extraction device 30. Also, for example,

successive tandem experiments (usually referred to as (MS)ⁿ) could be

carried out in this instrument by having a second fragmentation module or

other means of fragmentation between the ion source 10 and the extraction

means 30, or located above or below the ion source. Alternatively, ion

optical arrangement 20 could be used to reflect the beam.

Furthermore, if desired, more than one detector 70 may be provided

in the measurement chamber 50.

Furthermore, it is possible if desired for the initial acceleration into the

chamber 50 and the deceleration on exit from the chamber 50 and

fragmentation to be carried out by suitable means disposed at any

convenient position in the spectrometer, and hence it may be possible to

omit the ion mirror from the measurement chamber 50.

Claims

1 . A mass spectrometer of the time-of-flight kind comprising an ion

source to produce ions for analysis, said ions having a velocity in a first

direction, means to impose a velocity in a second direction on said ions to

cause the ions to enter a measurement chamber in which is disposed a

detector, means to reduce the velocity of ions of interest of a selected m/z

ratio in said second direction to substantially zero whereby the velocity in

a first direction causes only said ions of interest to enter a fragmentation

means which acts to fragment the ions into smaller mass ions, means to

impose a further velocity in a second direction on said smaller mass ions to

cause said smaller ions to move towards the detector for detection, said

detector being operable to produce a mass spectrum in accordance with

said detected smaller mass ions.

2. A mass spectrometer according to claim 1 wherein said means to

reduce the velocity of ions of interest comprises a pair of parallel spaced

apart electrodes, one of fixed voltage and the other of variable voltage

(relative thereto) whereby an electric field can be created therebetween

which acts to decelerate all ions passing into the field, the variable voltage

being selected such that only ions of interest are decelerated to substantially

zero velocity and can enter the fragmentation means.

3. A mass spectrometer according to claim 1 wherein said means to

impose a velocity in said second direction comprises a pair of parallel spaced apart electrodes, one of fixed voltage and the other of variable

voltage relative thereto whereby an electric field can be created

therebetween which imposes a velocity in the second direction in the ions.

4. A mass spectrometer according to claim 2 or claim 3 wherein one of

said electrodes is perforated so as to allow ions to pass therethrough into

the measurement chamber.

5. A mass spectrometer according to any one of claims 1 to 4 wherein

the ion source and the fragmentation means are axially aligned in said first

direction and are outside said chamber.

6. A mass spectrometer according to any one of claims 1 to 5 wherein

said means to impose a velocity in said second direction to cause the ions

to enter the measurement chamber and the means to reduce the velocity of

ions of interest to substantially zero velocity are formed by a single pair of

parallel spaced apart electrodes, one of fixed voltage and the other of

variable voltage, each electrode being split into at least two separate

electrode parts.

7. A mass spectrometer according to any one of claims 1 to 6 wherein

an ion reflector is provided in the measurement chamber which acts to

selectively direct ions in the chamber towards the detector or to the

fragmentation means for fragmentation prior to detection.

8. A mass spectrometer according to any one of claims 1 to 7 wherein

the fragmentation means causes fragmentation of the ions by one of. collision induced dissociation, surface induced dissocation or photo induced

dissociation.

9. A mass spectrometer according to claim 8 wherein said fragmentation

means includes means to reverse the direction of travel of the ions of

interest and smaller mass ions after fragmentation to allow ejection from the

fragmentation means.

10. A mass spectrometer according to claim 8 or claim 9 wherein said

fragmentation means also includes ion focussing means to focus ions on

ejection from the fragmentation means.

1 1 . A mass spectrometer according to any one of claims 1 to 10 wherein

a pair of deflection means are provided in said measurement chamber, one

of which acts upon entry of ions into said chamber to direct said ions

around said detector towards said fragmentation means and a second of

said pair of deflection means acts to direct said fragmented ions towards

the detector for detection.

12. A mass spectrometer according to claim 1 1 wherein each of said pair

of deflection means comprises a pair of parallel spaced apart plates

connected to a voltage supply.

13. A mass spectrometer according to any one of claims 2 to 12 wherein,

in use, said means to impose a velocity in a second direction to cause said

ions to enter said measurement chamber is operable to impose said velocity

simultaneously with ejection of fragmented ions from the fragmentation means.