US3342993A - Time-of-flight mass spectrometer having an accelerating tube with a continuous resistive coating - Google Patents

Description

Sept- 1967 c. J. QHALLORAN ETAL 3,

TIME-OF-FLIGHT MASS SPECTROMETEH HAVING AN ACCELERATING TUBE WITH A CONTINUOUS RESISTIVE COATING Filed Sept. 21, 1964 POWER SUPPLY 0 SUPPLY DETECTING PULSE APPARATUS SOURCE as X Fig. 2

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INVENTORS LOWELL D. FERGUSON ATTO-RNEY United States Patent 3,342,993 TIME-OF-FLIGHT MASS SPECTROM'ETER HAVING AN ACCELERATING TUBE WITH A CONTINU- OUS RESISTIVE COATING Gerald J. OHalloran, Detroit, and Lowell D. Ferguson, Royal Oak, Mich., and Hayden M. Smith Kendall Park, N.J., assignors to The Bendix Corporation, Southfield, Mich., a corporation of Delaware Filed Sept. 21, 1964, Ser. No. 397,814'

' 3 Claims. (Cl. 250-413) This inve ntion relates to mass spectrometers and more particularly to a time-of-flight mass spectrometer utilizing a rotationally symmetric electrostatic focusing field to improve the efficiency of ion transport within the instrument.

In its simplest embodiment, the known type of time-offlight mass spectrometer comprises an ion source and a collector disposed at opposite ends of an evacuated fieldfree drift tube. Upon the introduction of gas molecules into the ionization region of the spectrometer source, ions are formed, usually by electron bombardment, which are periodically pulsed out of the source by, source grids toward the collector by either one or .several electric fields established between appropriately spaced grids. Since the velocity of the ions in the field-free drift tube region is a function of the ratio of their charge q to their mass m, ion separation occurs corresponding to q/m, the amount of separation depending strongly on the length of the tube. Therefore, when the ions reach the collector they have separated into q/m: bunches with the lightest group reaching the collector first. As such, proper electrical circuitry connected to the collector will show a complete mass spectrum (in time) of the gas molecules ionized in the source;

Although the principles of operation of the time-offlight mass spectrometer presented supra are suflicient to enable one skilled'in the art to comprehend the invention herein disclosed, a more detailed description ofpresent time-of-fiight mass spectrometers is available in The-Review of Scientific Instruments, volume 26, No. 12, pages 1150-1157, December 1955.; j

, It is well known that in time-of-flight mass spectrometers of the above mentioned known type, a substantial portion of the ions emanating from the source grids preceding the field-free drift. tube region are subsequently lost to the driftztu-be walls prior to striking the collector.

This occurs because ions exiting from "thejsource grids do so in a broad band of exit angles and only those ions having favorable exit vectors are able' to reach the collector without impinging upon and becoming absorbed by the drift tube walls. As a result of this loss, spectrometer sensitivity and resolution are substantially depreciated for a specified geometrical'configuration. Sensitivity is impaired because the yield of ions formed per.

unit ionization source current is eifectively reduced. Reso-. lution is impaired because the field-free drift tube length is effectively shortened to insure that a sufiicient number.

of ions reach the collector. Thus, present time-of-flight mass spectrometers are limited to something less than optimum performance due to the inefficient transport of ions from the ionization source region to the'collector.

. It has been found however, that ions may be transported from the ionization source region to the collector of a time-of-flight mass spectrometer with substantially greater efiiciency than heretofore attained. This means that by utilization of the invention substantially all of the ions pulsed from the source region are constrained to pass to a collector without loss. Of course, since the number of ions reaching the collector compare favorably with the number of ions formed per unit ionization source current, spectrometer sensitivity is greatly enhanced.

3,342,993 Patented Sept. 19, 1967 ice tion of a novel ion lens to gather ions diverging from.

the source region and to focus .them upon a collector remotely disposed at the terminal portion of a field-free drift region. The lens, as hereinafter explained in greater detail, comprises an electric two-dimensional field region having a symmetry about an axis of rotation. It is formed by passing a current along the entire surface of a cylindrical envelope whose resistivity varies in a manner to produce a potential distribution upon its axis which is parabolic. Such a lens, as utilized to form the new timeof-fli'ght mass spectrometer, is a converging lens for positive ions, and is characterized by having a radial force component which is independent of axial or longitudinal position, and which is proportional to the radius or off axis position throughout the lens region.

More specifically, the lens portion of the new time-offiight mass spectrometer is characterized by having an axial parabolic potential distribution of the general quadratic form V( z)=f( 2 where V is potential, r is the radial position in the cylindrical region, and z represents the axial coordinate of the region. The components of the electric field of this potential region are E =ar and E =bz where E represents the radial electric field, E represents the axial electric field and a and b contain the potential boundary conditions and geometrical constants. It is seen from the equations that the component electric fields are independent. Further, the radial field, E,, is a central field which constrains the ions toward the central axis of the field while the axial field E accelerates the ions in the longitudinal axial direction. Thus ions in their transit through the lens region describe substantially simple harmonic motion about the central axis of the region as they are accelerated toward the collector. The importance of the radial field is readily appreciated as it is this component which prevents the ions from striking the Walls of the lens region and becoming lost thereby. In addition, since the radial field is independent of longitudinal position and proportional to radius throughout the lens region, all ions of the same charge to mass ratio having the same axial velocity will arrive simultaneously at the collector. The reason all ions reach the collector at the-same time is that all the longitudinal field vectors in any given transverse plane are equal therefore-particles near the walls will receive the same acceleration as particles near the center of tube. This means that the time of flight of an ion through the lens region is independent of its trajectory through the region. More concretely it means that all ions of the same q/m ratio which enter the lens region with equal axial velocities, but perhaps having different radial velocities, will reach the collector at the same time although their trajectories through the field will be different.

It is, therefore, an object of this invention to provide a new time-'of-fiight mass spectrometer in which the ion transport characteristic of known time-of-fiight mass spectrometers is improved.

This and other objects and advantages will become more apparent from the following detailed description and from the appended claims and drawings:

In the drawings: 7

FIGURE 1 is a sectional view comprising a time-offlight mass spectrometer in accordance with the invention;

FIGURE 2 is an enlarged view of the electrostatic focusing field element of FIGURE 1;

FIGURE 3 illustrates the continuity of the axial parabolicpotential profile of the focusing field element shown in FIGURES 1 and 2; and

FIGURE 4 illustrates the continuity of the axial potential gradient of the focusing field element of FIGURES l and 2.

In the time-of-fiight mass spectrometer of FIGURE 1, in envelope 9 is shown an ion source region 10 positioned between a grounded backing plate 12 and an ion accelerating grid 14, which is spaced a distance S plus or minus a distance AS/2 from the aperture 16 of the grid. Region 10 designates the region of ion formation typically accomplished by passing an electron beam 11 from a cathode 13 through a gas to be analyzed introduced into the region 10. A power supply 15 is shown supplying a heater current which may be 2.5 to 3 amps to the cathode 13. Cathode 13 has a negative potential bias supplied by source 15 to provide the ionizing electrons with energy sufficient to ionize the gas in region 10. This potential may include 1 to -l volts, depending on the gas to be ionized.

Disposed between grid 14 and ion collector 18 is a tubular electrode lens element 20 of length L which is cylindrically symmetric about the central axis 40 and has.

an ion entrance aperture 22 and a field terminating grid 24. Element 20 is characterized by having a continuous resistive surface 52 and is shown having two lead wires 26 and 28, one at each end of the element through which a constant potential V which may be 12Q0 volts, and V which may be -3600 volts applied from the power supply 30. Connected to the lenes element 20 is housing 34 which is of an electrically conductive material, for example stainless steel, shown having a length D. Housing 34 shares a common lead 28 with element 20 at one end of the housing and at the opposite end lead wire 32 is shown, both leads 28, 32 having the same potential V of -3600 volts from power supply 30. A potential distribution having predetermined characteristics is thus produced along the resistive coating 52 of electrode 20 forming an electrostatic focusing field therein of specific lens focal properties. However, a field free region is formed within housing 34 since there is no potential drop across the length D' of the housing.

Upon application of a voltage pulse U which may be 1200 volts, to lead 36 of grid 14 from the pulse source 38, the heterogeneous group of ion masses formed in region are accelerated a distance S to aperture 16 of grid 14, each ion acquiring substantially the same kinetic energy but differing in exit velocity through aperture 16 in proportion to the square root of its charge to mass ratio, the heavier ions having lower velocities and therefore becoming separated from the lighter ions. As a result, discrete ion bunches of different charge to mass ratios pass through aperture 16 and either drift or accelerate, depending on the specific U and V potentials through aperture 22 into the electrostatic focusing field of lens element 20.

In lens element 20 the ions are subjected to a focusing field wherein a radial electric field component directs them toward the central axis 40 at the same time an axial electric field component accelerates them longitudinally along the central axis 40 toward the field terminating grid 24. Thus, ions entering the focusing field region of element 20 are restrained from straying to the walls and becoming absorbed therein such that substantially all of the incoming ions are focused into a relatively narrow beam of spatially separated ion masses which exit through grid 24 and drift a length D, where mass separation is enhanced, to the collectorlS as'shown by the two typical trajectories 39 and 41. If only ions of the same charge are present,

the lightest group reaches the collector 18 first followed by groups of successively heavier mass.

The performance of the new mass spectrometer may be illustrated with reference to its resolving power. A convenient measure of resolution is the largest mass m, for which adjacent masses are essentially completely separated. If all the ions formed in region 10 were formed at an infinitesimal point spaced from the accelerating grid 14 with the same initial velocity vector, the flight time from the source to the collector 18 would be the same for all ions having the same charge to mass ratio, and the spectrometer resolution would be limited only by the detecting apparatus 42. However, in practice the over-all resolving power of the spectrometer depends on its ability to reduce ion flight time spread through the instrument caused by the ever present initial space and initial kinetic energy distributions. The ability of the spectrometer to resolve masses despite the initial space distribution is termed space resolution, whereas its ability to cope with the initial velocity spread is called energy resolution. Thus, space resolution refers to ions formed at different distances about the S distance and is shown in FIGURE 1 occurring in a deviation of AS/ 2. Energy resolution refers to ions formed within region 10 having velocity vectors both parallel and non-parallel to the spectrometer axis 40 immediately before the time they are pulsed out of region 10. The overall resolution of the spectrometer is determined by the respective space and energy resolutions which are in turn complex functions of the pulsed potential U,,; the ratio of the potentials V /V and VO/UO; nd the distance parameteres S d, L and D. For example, where the aforementioned potentials V V and U of 3600, 1200 and -1200 volts are applied respectively to leads 28, 26 and 36 of the figure, the corresponding S,,, d, L and D distances in centimeters are 2.224, 0.675, 20 and 81.7. Accordingly, if the ions are formed within region 10 in a AS/ 2 distance of 0.05 millimeters, the spectrometer space and energy resolution capabilities are respectively 650 and 443 atomic mass units for ions formed at room temperature.

In FIGURE 2, an enlarged view of the lens element 20 of FIGURE 1 is shown. In both figures the orientation of the electric field is illustrated 'by' the coordinate axes r', z,

where r is the radial direction of the field and z is the mesh grid 24 disposed at the opposite end of the element in. the ion exit plane, both elements 54 and 24 being in electrical contact with the resistive material 52. In the figure material 52 is shown upon the external wall of the shell 50. It is not, however, requiredthat the resistive material 52 be placed upont-he external wall since, as will be apparent to those skilled in the art, the same eflective electric field will be formed .within' the lens element 20 if material 52 is placed upon the internal walls of shell 50. v v

The specific focusing field for-med within lens element 20 is one characterized by being rotationally symmetric about the axis 40 and having anaxial potential distribution along its length which is parabolic as shown in FIGURE 3. Rotationally' symmetric means that any point a distance r fromthe longitudinal axis 40 and in a plane transverse to the axis 40 has exactly the same field vector as every other point in that'plane a'distance r from the axis 40. To obtain this parabolic field, the resistance per unit length p of material 52 must continuously increase along the length of the lens element 20 from aperture 22 to grid 24 so that the potential gradient dV/dz (the potential drop per unit length), increases along the resistive material 52 in proportion to the axial displacement from aperture 22' as hereinafter explained in connection with FIGURE 4. One method which may be used to obtain a continuously increasing resistance is shown in FIG- URE 2 where material 52 is gradually decreasing in thickness in an axial direction from aperture 22 to grid 24 and satisfies the equation where K is a constant. Upon application of the hereinbefore mentioned potentials V and V;,, which may respectively by 1200 and 3600 volts, to leads 26 and 28 of element 20 of FIGURE 2, an axial current flows through the resistive material 52 establishing a continuous variation of potential along the cylindrical surface forming an electric field within the lens element 20 which has the axial potential distribution of FIGURE 3. This electric field is further illustrated in FIGURE 2 by the equipotential surfaces 56 which are hyperboloids of revolution increasing in radius of curvature from aperture 22 to grid 24 and which characterize the lens as a converging hyperbolic lens. The conical conductive surface 58 of electrode 54 preserves the field geometry at the ion entrance aperture 22 of FIGURE 2 because it forms an angle a of 54 44 with axis 40 which is also the asymptotic equipotential boundary of the hyperboloidal equipotential surfaces 56. Grid 24 at the opposite end of element 20 serves the purpose of providing an abrupt discontinuity of the field region.

In FIGURE 3, as hereinbefore discussed, the axial potential distribution of the electric field formed within element 20 of FIGURE 2 is shown. The important feature shown in FIGURE 3 is that the parabolic potential distribution is continuous along the entire axis of the field. As shown in the figure, the highest point of the parabola corresponds to the entrance aperture plane 22 of FIGURE 2 and the lowest point corresponds to the exit grid plane 24.

The continuity of the electric field within element 20 of FIGURE 2 is further illustrated in FIGURE 4. Here the potential gradient dV/ dz is shown increasing in an axial direction proportional to axial displacement and, as in FIGURE 3, represents the potential gradient along the axis of rotation 40 of the lens element 20 of FIGURE 2.

Although this invention has been disclosed and illustrated with reference to particular applications, the principles involved are susceptible of numerous other applications which will be apparent to persons skilled in the art. The invention is, therefore, to be limited only as indicated by the scope of the appended claims.

Having thus described our invention, we claim:

1. A mass spectrometer comprising an ion source,

means for accelerating the ions from said source in a specified direction, means for providing a rotationally symmetric field region having an entrance and exit means disposed to receive and discharge the accelerated ions,

said field region having an axial potential distribution in the form of a parabola with the highest value and lowest value of the parabola corresponding respectively with said field entrance and exit means. whereby the entrance field is small and exit field is large, so that accelerated ions are caused to converge,

said field region having a radial field which varies in proportion to the distance from the axis of the region but which is constant along the axis,

and means to collect the ions.

2. A mass spectrometer as set forth in claim 1 with said means for providing an electric field region comprising a continuous resistive electrode bounding said field region,

means for applying a potential to opposite ends of said electrode causing a current flow therethrough.

3. A mass spectrometer comprising an ion source,

a collector means,

means for accelerating the ions from said source to the collector means in a region of acceleration,

said field region having an axis disposed between said source and said collector means,

means for providing a rotationally symmetric electric field about said axis in said region of acceleration,

said field region having an axially descending parabolic potential distribution with the highest value and the lowest value of the parabolic curve corresponding respectively with the source end of said region of acceleration on said axis and the collector end of said region of acceleration on said axis, whereby the accelerated ions are caused to converge as they approach the collector end of said region of acceleration,

said rotationally symmetric electric field having a radial field which varies in proportion to the distance from the axis of the acceleration region but which is uniform in the axial direction,

said electric field providing a focusing action on ions entering said region of acceleration which ions after a single journey through said accelerating region are subsequently collected by said collector means,

means to provide said region of acceleration in an evacuated environment.

References Cited UNITED STATES PATENTS 2,570,158 10/1951 Schissel 25041.9 2,782,316 2/ 1957 Robinson 250-419 2,971,118 2/1961 'Burdick 31516 3,258,591 6/1966 Blauth et al. 25041.9

RALPH G. NILSON, Primary Examiner.

A. L. BIRCH, Assistant Examiner.

Claims (1)

1. A MASS SPECTROMETER COMPRISING AN ION SOURCE, MEANS FOR ACCELERATING THE IONS FROM SAID SOURCE IN A SPECIFIED DIRECTION, MEANS FOR PROVIDING A ROTATIONALLY SYMMETRIC FIELD REGION HAVING AN ENTRANCE AND EXIT MEANS DISPOSED TO RECEIVE AND DISCHARGE THE ACCELERATED IONS, SAID FIELD REGION HAVING AN AXIAL POTENTIAL DISTRIBUTION IN THE FORM OF A PARABOLA WITH THE HIGHEST VALUE AND LOWEST VALUE OF THE PARABOLA CORRESPONDING RESPECTIVELY WITH SAID FIELD ENTRANCE AND EXIT MEANS, WHEREBY THE ENTRANCE FIELD IS SMALL AND EXIT FIELD IS LARGE, SO THAT ACCELERATED IONS ARE CAUSED TO CONVERGE, SAID FIELD REGION HAVING A RADIAL FIELD WHICH VARIES IN PROPORTION TO THE DISTANCE FROM THE AXIS OF THE REGION BUT WHICH IS CONSTANT ALONG THE AXIS, AND MEANS TO COLLECT THE IONS.

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