US2772364A - Mass spectrometry - Google Patents

Description

1956 H. w. WASHBURN 2,772,364

MASS SPECTROMETRY Filed May 6, 1955 TO El/ACUAT/NG SYSTEM i P BUNCH/N6 f GRIDS 24 22, 80 29 AMPLIFIER Q IZ J W L AMPLIFIER /L/L/I 1 50 GATE PULJE ggxggg g VOL T465 GENERATOR INVENTOR.

. HAROLD W WASHBURN ATTORNEYS United States Patent MASS SPECTROMETRY Harold W. Washburn, Pasadena, Calif., assignor, by mesne assignments, to Consolidated Electrodynamics Corporation, Pasadena, Calif., a corporation of California Application May 6, 1955, Serial No. 506,644

8 Claims. (Cl. 250-413) This invention relates to mass spectrometry and more particularly to an improve-d instrument of the type which has become known as a time of flight mass spectrometer. This is a continuation of my co-pending application Serial No. 426,998 filed May 3, 1954, now abandoned.

In mass spectrometry a sample of material to be analyzed is ionized by any of several known means, and the ions thus formed are separated as a function of mass to charge ratio. Separation of the ions is generally accomplished under the influence of electric or magnetic fields, or both, effecting spatial or temporal separation. Ions of a given mass to charge ratio may then be directed upon an ion collector or target and discharged, whereby a measure of the resulting ion current may be obtained. This ion current in turn is a function of the partial pressure in the original sample of the components from which ions of the given mass to charge ratio are derived.

In a time of flight mass spectrometer, separation of ions is normally accomplished by taking advantage of the characteristic accelerations imposed on ions of differing mass to charge ratio by an accelerating electrical field. Generally the ions, after formation or upon formation, are subjected to the influence of a propelling electrical field and are allowed to travel along a so-called drift tube wherein the ion masses separate as a consequence of their characteristic and differing response to the imposed accelerating field. In one instrument of this type a collector electrode is provided in the drift tube remote from the ion source, and a series of grid-type electrodes are disposed in front of the collector electrode and are pulsed to act as a gate, limiting access to the collector electrode to ions which reach the gate at a particular instant or instants. Such selective gating provides a degree of resolution between the spatially separated ion bunches.

In a certain embodiment the acceleration of the ions is accomplished by so-called bunching grids or electrodes disposed adjacent the ion source. The bunching grids have impressed on them a rising voltage so that the energy imparted to an ion by the bunching grid source increases with time. The various parameters which affect the ion movement including the injection voltage, the

rise rate of the bunching voltage, the tube length, and the ion mass to charge ratio can be so correlated that all of the ions of a given mass are bunched, whereby they all occupy the same coordinate in space at a pro-assigned instant, and they can be made to bunch at the gating grids. The gating grids can be driven with a voltage pulse such that only ions which arrive at a particular instant gain access to the collector electrode. The reason for bunching the ions is that if the ions arrive at the gate in a very short bunch, the gate need be open for only a very short time and this feature improves the resolution over what would be attainable if no bunching were imposed.

The voltage on the bunching grids cannot continue to rise indefinitely and must therefore interrupted periodically. If the voltage applied to the bunching grids is a continuous saw-tooth wave and if the magnitude of this voltage wave is held within reasonable limits, there will usually be several bunches of ions of each of the several ion masses in the drift tube between the bunching and gating grids at any one instant, because an ion of a particular mass may, and usually will, require a time equal to several cycles of the bunching voltages to traverse the drift space. If the gate grid is set to pass ions which require a specified number of cycles, say n, then unwanted ions which require nm cycles, in being any integer, can also pass through the gate to the collector and be measured together with the wanted ions. Such unwanted ions are called harmonics. The appearance of harmonics can be mitigated by increasing the amplitude and decreasing the frequency of the bunching voltage so as to minimize the number of groups of ions Within the drift space at any one time. Harmonics can be eliminated entirely by admitting ions in a single pulse and waiting until all the ions admitted escape from the drift tube before admitting a second pulse. This procedure, however, cuts down the duty cycle to a serious extent since it is necessary to wait for the drift tube to clear itself between one pulse and the next. Not only must the tube be cleared of the heaviest ion of interest but also of the heaviest ions present in the tube.

The present invention is directed to improving the duty cycle of a time of flight mass spectrometer of the type above described, i. e. wherein ions are admitted to the drift tube in pulses by artificially removing from the drift tube ions other than those of primary interest so as to minimize the passage of such harmonic ions through the gate. The invention to this end contemplates a mass spectrometer comprising an evacuable envelope, means for developing ions within a restricted region of the, envelope, an ion collecting electrode disposed in the can velope remote from the region of ion formation, means for propelling ions from the region of ion formation toward the collector electrode in pulses whereby the ions separate into mass discriminated bunches, gating means disposed adjacent the collector electrode to alternately block and permit ion access to the collector, and means for producing in the region between the propelling means and the gating means a field operable to deflect ions from their line of travel between the source and collector electrode.

One means of developing this deflecting force comprises a pair of electrostatic deflector plates spaced on opposite sides of the drift tube and extending parallel to the line of ion travel therein. The plates are pulsed in matched relation to the operation of the gate grids and function to sweep ions from the drift space approximately at the instant that the ions of interest have traversed the gate.

Another means for developing the desired deflecting force whereby the duty cycle of a pulsed time of flight mass spectrometer can be improved consists of establishing a weak transverse magnetic field across the drift space of the instrument. This field is established perpendicular to the direction of the motion of the ions so that the various ion trajectories are bent by the magnetic field by amounts which depend on the mass and on the initial energies of the ions. The field can be arranged so that particular ions of interest will reach the gate while ions having substantially different masses from those of inthrough the gate.

The invention will be more clearly understood by reference to the following description taken in conjunction with the accompanying drawing in which:

Fig. l is a diagram in longitudinal section of a time of flight mass spectrometer, including electrostatic means for developing the desired component of field energy; and

Fig. 2 is a simplified diagram of a time of flight mass spectrometer in accordance with the invention, including magnet means for developing the desired transverse field component.

The mass spectrometer shown in Fig. 1 includes an envelope which is evacuated through an outlet 11 adapted to be connected to an evacuating system (not shown). An ion source is disposed in one end of the envelope and includes a repeller electrode 12, an electron gun 13 for developing an ionizing electron beam (illustrated by a dotted line) projected from the gun .13 toward a target 14. Target 14 is conventionally biased as with a battery 15. An inlet line 16 provides means for introducing a sample to be analyzed to the ion source. A pair of electrodes 17, 13 referred to as bunching grids, are disposed adjacent the ion source and together with the repeller electrode 12 determine the introduction of ions from the source into the longitudinally central portion of the envelope, which portion is identified as drift space 19. A battery 20 is connected to energize the electron gun 13.

Bunching grids 17 and 18 are energized by a bunching voltage generator 21 which is adapted to develop 21 voltage wave approximately as illustrated at 22. The bunching grids and repeller electrode are connected to the voltage generator across .slidewire resistor 23 as illustrated and the repeller electrode 12 is biased with respect to the bunching grid 17 by a bias battery 24.

A gate consisting of grids 25, 26, 27 is disposed at the opposite end of the drift space 19 in front of a collector electrode 28. Grids 26 and 27 function as a shield to screen the collector from stray R. F. fields and the like. A gating pulse generator 29 is connected between the grid 25 and the grounded grids 26, 27, and is adapted to be matched to the operation of the bunching voltage generator as through a variable delay line 30 to energize the gate at predetermined intervals to pass ions of interest from the drift tube to the collector 28. Other than during these predetermined intervals, the gate system blocks such ion passage to prevent ion access to the collector electrode. The collector electrode is connected to an amplifier 31 in conventional manner, such amplifier usually being connected to some form of sensing or recording circuit which forms no part of the present invention and is not illustrated.

A pair of electrostatic plates 32, 33 are disposed parallel to each other in the drift space 19 and are symmetrically arranged about the longitudinal axis of the drift space. The plates 32 and 33 are interconnected, in the embodiment shown, to the gating system. A bias battery 34 is connected in circuit with the plate 32 so that the plates 32 and 33 will be at the same potential, except when the gating pulse is applied. In this manner the plates do not interfere with the normal travel of ions through the drift space.

The operation of the apparatus illustrated in Fig. 1 is as follows: The sample to be analyzed is admitted into the ionization region through the sample inlet and is ionized under the influence of the electron beam. A pulsed propelling potential is applied across the repeller and bunching electrodes to deliver ions into the drift space in time-spaced pulses or bursts. The pulsed po tential is supplied to the bunching grids or electrodes from the bunching voltage generator which is set to deliver a pulse wave of the desired repetition rate and pulse amplitude.

As previously mentioned, the problem of interfering ions can be eliminated if the pulse frequency at the bunching grids is such that all ions delivered to the drift space by one accelerating pulse are allowed to. collect or discharge before the next pulse. Such practice, however, results in an extremely low duty cycle of the order of approximately 2% or less.

in the present instrument the bunching grids are pulsed at a more rapid rate so that several ion groups would be in the drift space at any given instant save for the action of the sweep field developed by plates 32, 33.

The gating grids are operated to reject ions, i. e. to block. their access to the collector electrode, except at periodic momentary intervals determined by the application of a pulse to the gating grids derived from the gating pulse generator. Normally grid 25 is maintained at a positive potential higher than that of the repeller so as to block ion travel, this potential being periodically changed in the negative direction by the application of pulses from the gating pulse generator. The negative bias applied to plate 32 is selected to balance this positive potential on grid 25 so that when the gate is closed the electrostatic plate will be at the same potential and the drift space will be field-free.

The gating pulse generator is matched in its operation with the bunching voltage generator, which matching may be accomplished by separate frequency adjustments of the two generators or by interconnection of the two generators through a variable delay line as illustrated, and which practice is familiar in the art. The gating pulse generator is matched so that the application of pulses to the gating grids is timed to coincide with the arrival at the gating grids of successive bunches of ions of a given mass. These ions are thus passed through the gating grids and are collected at the collector electrode.

Simultaneously with the application of a pulse to the gating grids causing these grids to allow the passage of ions to the collector electrode, the electrostatic electrodes 32 and 33 are similarly pulsed so as to sweep the ions from the drift space 19 with the consequent result that,

the bunching grids can be operated to deliver another burst of ions into the drift tube at this instant. This means that a second bunch of ions can be introduced into the drift space immediately upon traversal of the gating grids by the ion bunch of interest, whereas to insure proper resolution without the electrostatic electrodes it is necessary to operate the bunching grids so as to deliver the second pulse of ions only after all of the ions in the first pulse pass through or discharge at the gating grids. In this fashion the electrostatic electrodes increase the duty cycle of the device many-fold for a given degree of resolution.

A second means of sweeping unwanted ions from a drift space of a time of flight mass spectrometer is illustrated in Fig. 2 which is a schematic diagram of an instrument similar to the one shown in Fig. 1. The instrument shown schematically in Fig. 2 includes an ion source comprising a repeller electrode 40, an electron gun 41 and an electron target 42, the gun and the target operating to develop an electron beam traversing the region intervening between the two. A pair of bunching grids 43, 44 are connected to be energized in a pulsed manner by a bunching voltage generator 45. The ion source and the bunching grids are adjacent one end of a drift space 46 and as illustrated are oriented to propel ions from the ion source obliquely toward the collector electrode. The same oblique relation can be achieved by mounting the collector off the center axis of the envelope.

A pair of gating grids 47, 48 are disposed adjacent the opposite end of the drift space 46 and are connected to be energized by a gating pulse generator 49. A variable delay line 50 is connected between the bunching voltage generator and the gating pulse generator to match their operation. A collector electrode 51 is disposed to receive ions passing from the drift space through the gating grids, and is connected to an amplifier 52 which is in turn conventionally connected to asensing or recording circuit (not shown). A grounded shield grid 53 is disposed between the gate and the collector electrode.

A magnet pole 54, and a second pole piece (not shown) are disposed on opposite sides of the drift space 46 to develop across the drift space a relatively weak magnetic field transverse to the direction of ion travel in the drift space. In the operation of the apparatus of Fig. 2, ions are manipulated under the influence of the accelerating potentials applied on the repeller electrode and bunching grids and the operation of the gating grids in essentially the same manner as described with the relation of the instrument of Fig. 1. In this instance, however, a weak transverse magnetic field is continuously impressed across the drift space to bend the trajectory of the ions traveling between the bunching grids and gating grids, the radius of curvature of the ions under the influence of the magnetic field being a function of the strength of the magnetic field, the specific mass of the ions and the initial energies of the ions. As illustrated in the figure, the accelerating pulses applied to the bunchcing grids are of saw-tooth configuration so that the ions formed by the electron beam during the period of one of these pulses will vary in initial energy with the age of the pulse.

It can be shown mathematically that with the magnetic field superposed on the drift space as illustrated, the geometrical spread of the resonant ions due to the action of the magnetic field and to the fact that they are given a substantial energy spread by the bunching grids as explained above, is just twice the physical separation between a group of resonant ions and the nearest harmonic group; the term resonant ions is used in this instance to apply to those ions which traverse the space between the bunching grids and the gating grids in an integral multiple of the pulse repetition rate applied to the gating grids. As a consequence of the geometrical spread, mentioned above, the magnetic field operates successfully to suppress harmonics when the incoming ion beam is interrupted at such a rate as to give the instrument a duty cycle of as high as up to about 33 /3 At this interruption rate the magnetic field will not function to suppress the two nearest harmonics on each side of the resonant mass, but will suppress more distant harmonics. This means that it is possible to achieve a substantially harmonic-free duty cycle of as high as 33 /a% with a large number of groups of ions in the drift space, thus eliminating the need of high voltage on the bunching and gating grids.

The problem faced in the operation of a time of flight mass spectrometer is the problem of harmonics attendant upon the presence in the drift space of more than one ion bunch at a given instant. The term hunch is used in this sense to indicate each separate burst of ions emanating from the bunching grids under the influence of the pulse potential applied thereto. Heretofore it has been necessary in this type of time of flight mass spectrometer to operate at such a low duty cycle that only one bunch of ions was in the drift space at any given instant. By the means of the present invention whereby a transverse component of influence is provided in the drift space either periodically, in the case of electrostatic sweep field, or continuously, in the case of the transverse magnetic field, it is possible to operate at a much higher duty cycle. With the electrostatic sweep field still, only one bunch of ions is in the drift space at a given time but the delay required between one bunch and the rest is considerably shortened by virtue of the removal of all of the unwanted ions of the bunch at the instant that the ions of interest pass through the gating system. With the transverse magnetic field several bunches of ions can be in the drift space at the same time so that in either of these embodiments the duty cycle of the instrument is materially improved.

I claim:

1. In a mass spectrometer, the combination comprising an evacuable envelope, means for developing ions within a restricted region of the envelope, an ion collecting elec trode disposed in the envelope remote from the region of ion formation, propelling means for propelling ions from the region of ion formation toward the collector electrode whereby the ions separate into mass discriminated bunches, gating means disposed adjacent the collector electrode to alternately block and permit ion access to the collector, and means for producing in the region between the propelling and gating means a force operable to deflect ions from their line of travel between the source and the collector electrode.

2. In a mass spectrometer, the combination comprising an evacuable envelope, means for developing ions within a restricted region of the envelope, an ion collecting electrode disposed in the envelope remote from the region of ion formation, propelling means for propelling ions from the region of ion formation toward the collector electrode whereby the ions separate into mass discriminated bunches, gating means disposed adjacent the collector electrode to alternately block and permit ion access to the collector, and electrostatic means disposed between the propelling and gating means to periodically deflect ions from their line of travel between the source and the collector electrode.

3. Apparatus according to claim 2 wherein the electrostatic means comprises a pair of electrodes disposed on opposite sides of the longitudinal axis of the envelope and connected to develop a periodic transverse field during the intervals that the gating means passes ions to the collector.

4. In a mass spectrometer, the combination comprising an evacuable envelope, means for developing ions within a restricted region of the envelope, an ion collecting electrode disposed in the envelope remote from the region of ion formation, propelling means for propelling ions from the region of ion formation toward the collector electrode whereby the ions separate into mass discriminated bunches, gating means disposed adjacent the collector electrode to alternately block and permit ion access to the collector, and magnet means for producing a magnetic field transverse to the direction of ion travel in the region between th propelling and gating means.

5. In a mass spectrometer, the combination comprising an evacuable envelope, means for developing ions within a restricted region of the envelope, an ion collecting electrode disposed in the envelope remote from the region of ion formation, propelling means for propelling ions from the region of ion formation obliquely toward the collector electrode whereby the ions separate into mass discriminated bunches, gating means disposed adjacent the collector electrode to alternately block and permit ion access to the collector, and magnet means for producing a transverse magnetic field in the region between the propelling and gating means.

6. In a mass spectrometer, the combination comprising an evacuable envelope, an ion source disposed adjacent one end of the envelope and including in serial arrangement a repeller electrode means for ionizing sample molecules and a plurality of bunching grids, a bunching voltage generator connected between the repeller electrode and bunching grids and adapted to deliver a periodic saw-tooth pulse, a collector electrode disposed adjacent the other end of the envelope, an electrode gate disposed in front of the collector electrode, a gating pulse generator connected to the electrode gate and adapted to deliver periodic negative pulses to the gate during which periods only the gate passes ions, means matching the operation of the gating pulse generator with the bunching voltage generator, and field-forming means for producing in the region between the bunching grids and the electrode gate a force operable to deflect ions from their line of travel between the ion source and the collector electrode.

7. Apparatus according to claim 6 wherein the fieldforming means comprises a pair of electrostatic electrodes disposed on opposite sides of the longitudinal axis of the envelope, means for producinga field between the electrodes during the periods that the electrode gate passes ions, and means for maintaining the electrostatic electrodes at the same potential during the intervals intervening between said periods.

8. Apparatus according to claim 6 wherein the repeller electrode and bunching grids are oriented to propel ions from the ion source obliquely in the direction of the collector electrode, and the field-forming means comprises a pair of magnet poles disposed on opposite sides of the region of ion travel between the bunching grids and electrode gate.

No references cited.

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