Abstract:
A mass spectrogram employs a set of controllable electrodes to produce a time varying axially inhomogenous electric field and enhance separation of charged particles by exposing the charged particles to different electric field strengths based on their spatial positions. The fields may be tailored to provide a traveling wave that expands portions of a spectrographic plot of the particles and/or to provide focusing or other effects.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
     CROSS-REFERENCE TO RELATED APPLICATIONS 
     BACKGROUND OF THE INVENTION 
     The present invention relates to mass spectrometry and, in particular, to a spectrometer providing variable and improved sensitivity. 
     In a typical mass spectrometer, particles, such as different molecular species, are ionized and accelerated in an electric field. The acceleration of particles having the same charge will be principally dependent on the mass of the particles and thus particles may be separated by mass according to their final velocity in the electric field. Differences in velocity may be detected by a time-of-flight detector positioned after a drift region or by passing the particles through a magnetic or electric field to separate them into curving trajectories determined by mass and velocity to be received by a spatial detector. 
     For a larger mass species, the relative difference in velocities between the particles becomes much smaller. For example, in biological molecules with a mass around 1000 amu with a 0.01 amu difference, the time of flight (TOF) separation, normalized to one of the species can be on the order of: 
     
       
         
           
             
               
                 Δ 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 TOF 
               
               
                 TOF 
                 0 
               
             
             = 
             
               5.0 
               × 
               
                 10 
                 
                   - 
                   6 
                 
               
             
           
         
       
     
     For a 1 m drift following a 25 kV acceleration potential, the time of flight of the reference species (TOF 0 ) can be on the order of 14 μs. Distinguishing these two species thus requires a time resolution of 72 ps in the time-of-flight detector, a resolution equal to the time for light to travel less than an inch. A similar problem, albeit in the spatial dimension, occurs with a bending magnet/spatial detector system. 
     SUMMARY OF THE INVENTION 
     The present inventors have recognized that increased velocity separation between species can be obtained through the use of a spatial- and time-variant electric field for accelerating the species. This more sophisticated accelerating field allows different species to experience different accelerating potentials increasing their separation without the need for greater accelerating voltages, increased drift regions, or increased detector size. 
     In prior art systems, ions of the same charge in the same field gain the same amount of energy and the TOF variation is just mass dependent, 
                 Δ   ⁢           ⁢   TOF       TOF   0       =         m         m   0         -   1.           
In constrast, in the present invention with a more sophisticated accelerating field in which the ions experience different potentials, different ions gain different amounts of kinetic energy (KE) and the subsequent drift TOF can be expressed
 
                 Δ   ⁢           ⁢   TOF       TOF   0       =           m         m   0         ⁢         KE   0         KE         -   1.           
If the fields are such that heavier ions gain less kinetic energy, this kinetic energy ratio serves as an “enhancement factor” to the spread in the TOF.
 
     The ability to produce an spatial- and time-varying electric field can be used to flexibly and selectively magnify the axes of the mass spectrogram, allowing the user to “zoom” in on particular peaks while accommodating a wide range of masses. The spatial- and time-varying electric field also permits sophisticated focusing techniques to be used to reduce peak width. 
     Specifically then, the present invention provides a mass spectrometer having a source presenting multiple species of charged particles along an axis. The particles enter the axially inhomogeneous field chamber having a series of independently controllable electrodes that expose the particles to a spatially- and time-variant electric field as the charged particles move along the axis. A detector system positioned to receive the charged particles from the spatially- and time-variant field chamber detects differences in the speed of the particles passing through the field. An electronic computer executes a stored program: (i) to apply different electric fields to spatially-separated species within the spatially- and time-variant field chamber over a continuous range of electric fields to increase the velocity separation of the spatially-separated species, and (ii) to read the detector system and output mass spectrogram data reflecting the different electric fields. 
     It is thus one object of the invention to provide a versatile mass spectrogram that may better differentiate between charged particles. 
     The electronic computer may control the spatially- and time-variant field chamber to produce a traveling wave moving along the axis. 
     It is thus an object of the invention to use the spatial separation of the particles during acceleration along the axis to differentiate the electric field experienced by the particles. 
     The traveling wave may move along the axis at a varying rate of speed. 
     It is thus an object of the invention to allow the force differences produced by the spatially-variant field to track the particles as they move through the chamber. 
     The electronic computer may determine the energy gained by particles by integrating the value of the spatially-variant and time-variant electric field over the trajectory of the particles along the axis. 
     It is thus an object of the invention to permit a calibrated spectrogram to be produced with an arbitrary accelerating waveform. 
     The location of each species in the spatially-variant field chamber may be determined iteratively at a series of locations based upon an average electric field at a previous location. 
     It is thus an object of the invention to provide a method of managing the complex interaction between the force experienced by a particle in the traveling wave and its acceleration with respect to the traveling wave. 
     The spectrometer may further include a static field chamber positioned along the axis exposing the particles to a static electric field as they move through the static field chamber or the spatially-variant field chamber itself may apply a static electric field in addition to the spatially-variant time-variant electric field. 
     It is thus an object of the invention to provide an additional degree of freedom in producing an arbitrary spatially-variant, time-variant accelerating field. 
     The mass spectrogram data may be output as a graph of species amount versus mass/charge ratio providing two scale portions on the mass/charge ratio axis having different resolutions and the electronic computer may accept user inputs of a mass range to determine the location of the different scale portions. 
     It is thus an object of the invention to provide for a flexible spectrographic display that may simultaneously provide a high degree of magnification for some mass ranges while still providing a large range of masses necessary to include display of a calibrant or the like. 
     The axially inhomogeneous field chamber may extend along a line or may extend along a circle. 
     It is thus an object of the invention to permit an arbitrarily long acceleration region. 
     The axially inhomogeneous field chamber may include a set of stacked, electrically insulated electrodes each separately controlled by a solid-state amplifier controlled by the electronic computer to vary the speed and shape of the electric field within the axially inhomogeneous field chamber. The solid-state amplifiers may provide continuous control of amplitude of electrical voltage applied to the electrodes. 
     It is thus an object of the invention to provide an acceleration chamber that may produce an arbitrary waveform shape and amplitude in both position and time. 
     The electronic computer may further execute the stored program to apply different electric fields to spatially-separated species within the axially inhomogeneous field chamber to decrease separation of spatially separated species. 
     It is thus an object of the invention to use the arbitrary waveform chamber to provide for focusing of spectrographic peaks. 
     These particular objects and advantages may apply to only some embodiments falling within the claims, and thus do not define the scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic representation of a prior art mass spectrometer having a spatially- and time-variant accelerator chamber and showing two alternative detector configurations; 
         FIG. 2  is a figure similar to that of  FIG. 1  showing a mass spectrometer according to one embodiment of the present invention providing a spatially- and time-variant field chamber controlled by an electronic computer; 
         FIG. 3  is a schematic representation of the forces experienced by different charged species at a first and second time within the axially inhomogeneous field chamber of  FIG. 2 ; 
         FIG. 4  is a flow chart showing steps executed by the electronic computer of  FIG. 2  in implementing the present invention; 
         FIG. 5  is a first and second representation of a mass spectrogram showing a zooming feature enabled by the present invention; 
         FIGS. 6   a - 6   c  are a set of graphs with aligned distance axes showing the iterative determination of an enhancement factor using complex field shapes; 
         FIG. 7  is a detailed flowchart evaluation of the field profile per  FIGS. 6   a - 6   c  and  FIG. 4 ; 
         FIGS. 8   a - e  are a graphical representation of a refocusing function implement using the arbitrary waveform chamber; 
         FIG. 9  is a plan view in partial cutaway of the present invention in an embodiment providing a circular axially inhomogeneous field chamber; and 
         FIG. 10  is a schematic representation of the axially inhomogeneous field chamber applied to electrophoresis machine. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to  FIG. 1 , a conventional mass spectrometer  10  includes an analyte source  12  presenting a stream or pulse of charged particles  14  directed along an axis  16  into an accelerating chamber  18 . 
     The accelerating chamber  18  typically presents a uniform and time invariant electrostatic field  20  (measured along the axis  16 ) that accelerates the particles  14  into a drift region  22  or a bending field  24 . The former drift region  22  allows the particles  14  to separate from their velocity differences before being received by a time-of-flight detector  26  differentiating among particles by their time of arrival. 
     The latter bending field  24  disperses the particles  14  into a set of curved trajectories determined by the velocity differences of the particles  14  times their mass (i.e., the radius of curvature goes as mass times velocity), thus separating the particles  14  spatially along a spatial detector  28 , the latter of which may distinguish among particles  14  by their spatial arrival points. Preferably the field  24  is created by a magnet providing separating radii proportional to the mass times the velocity of the particles  14 . 
     Detectors  26  or  28  may connect with a computer  30  analyzing the data from the detectors  26  or  28  to produce a spectrogram  32  typically being a plot of particle number versus species, the latter differentiated by mass (or technically mass/charge also designated m/z). 
     Referring still to  FIG. 1 , within the accelerating chamber  18 , the electric field is generally static (time invariant) and uniform between plates  34  of the chamber  18  along axis  16 . Consequently each of the like charged particles  14   a ,  14   b , and  14   c  (having successively decreasing masses in this example) experience identical electric forces  36 . Because of the mass differences of charged particles  14   a ,  14   b , and  14   c , however, the charged particles  14   a ,  14   b , and  14   c  experience different accelerations to different velocities  38 . These different velocities  38  ultimately produce the velocity differences in the drift region  22  or in the bending field  24  used separate the particles  14 . 
     Referring now to  FIG. 2 , a spectrometer  40  of the present invention also provides for an analyte source  12  presenting charged particles  14  along axis  16 . In this case, the charged particles  14  are received by an arbitrary field chamber  42  which produces a controllable, time-variant, spatially-variant field along the axis  16 . 
     The anisotropic field chamber  42  may, for example, be composed of a set of parallel rings  44  spaced along and coaxial with axis  16 . Each of the rings  44  is electrically isolated from the others and connected to an output of a separate amplifier  46  allowing independent control of the voltage of the rings  44  throughout a range of voltages. Each amplifier  46  receives a waveform from a waveform generator  50  which may simultaneously generate a different independent waveform for each ring  44 . The waveform generator  50  may independently control the voltages on each of the rings  44  to create, in one embodiment, a traveling wave  60  that moves at a controlled acceleration  52  along the axis  16  through the arbitrary chamber  42 . The ability to provide a different control waveform of arbitrary shape to each ring  44  allows the generation of a wide variety of arbitrary time-variant electric fields for a variety of purposes as will be described. 
     An optional static field chamber  18  providing an initial uniform acceleration of the particles  14  may be positioned before the chamber  42  and aligned with axis  16 . Alternatively, the voltages on the rings  44  may be controlled to provide a similar static field. 
     Referring still to  FIG. 2 , an electronic computer  30  executing a stored program  55  may communicate with the waveform generator  50  to determine the shape and speed of the traveling wave  60  which may be synchronized with signals received from detector  26  or  28  and modified according to user input. The user input may be received by the computer  30  through a keyboard or cursor control device  58  according to methods well known in the art. The signals from the detectors  26  or  28  may be processed by the computer  30  to produce a spectrogram  56  representing the actual time or position separation magnification experienced by the particles as will be described. 
     As shown in  FIG. 2 , the field  24 ′ may be, in this case, optionally provided by an electric dipole which provides separating radii proportional to kinetic energy of the particles providing improved peak separation in the context of the present invention where differences in particle momentums are not as pronounced. 
     Referring now to  FIG. 3 , in a simple embodiment, the electronic computer  30  may be programmed to drive the waveform generator  50  to provide a ramp-shaped traveling wave  60  having constant width along axis  16  and accelerating away from the analyte source  12  to track and embrace particles  14   a - 14   c  representing different species of particles with the identical charge. In this example, the center of the traveling wave  60  is aligned with particle  14   b . In an initial region  62  of the anisotropic field chamber  42 , the particles  14   a - 14   c  will have separated slightly based on their different masses under the influence of the electric field provided by the traveling wave  60  or earlier static wave chamber  18 . As the particles  14   a - 14   c  separate, the slower, heavier particles  14   a  move backward with respect to the center of the traveling wave  60  to experience a lower electric force  36  as a result of the ramp shape of the traveling wave  60 . In contrast the faster particles  14   c  move forward with respect to the traveling wave  60  to experience a higher electric force  36  based on the upward ramping of the traveling wave  60 . In this respect, the forces  36  experienced by the different particles  14   a - 14   c  differ, with the leading and faster particles  14   c  receiving additional accelerative force  36  to accelerate faster than the trailing and slower particles  14   a , both increasing the difference in velocities  38  experienced by the particles  14   a - 14   c  and imparting different amounts of energy to the particles based on the different fields. 
     Referring still to  FIG. 3 , at a later time when the particles  14   a - 14   c  are in a later region  64 , additional separation of the particles  14   a - 14   c  caused by their differences in velocity further decreases of the electric force  36  on particle  14   a  and further increases the electric force  36  on particles  14   c . Thus, the traveling wave  60  produces two effects which increase the velocity separation of the particles  14   a - 14   c : (i) the difference in electric fields experienced by the spatially separated particles at any time, and (ii) the change in the electric fields experienced by the spatially separated particles over time. 
     If the traveling wave  60  is properly shaped to provide a substantially linear function with distance and is accelerated to match the center of mass of the particles  14   a - 14   c  and expanded in axial width as the particles  14   a - 14   c  disperse, a simple expansion in the horizontal axis (m/z) of the spectrogram  56  by a constant amount is produced providing essentially a zoom feature based on actual physical changes allowing particular portions of the spectrogram  56  to be arbitrarily enlarged. 
     Referring now to the  FIGS. 4 and 5 , the electronic computer  30  may implement this zoom feature executing the stored program  55  to receive a first input designating a lower m/z boundary for the expanded portion of the spectrogram  56  and a second input designating an upper m/z boundary for the expanded portion of the spectrogram  56  as indicated by process blocks  70  and  72  respectively. As shown in  FIG. 5 , a normal spectrogram  56  using an isotropic static acceleration field may be displayed and the low m/z value input and high m/z value input entered by positioning a first cursor  76  at the lower m/z value and second cursor  80  at the upper m/z value, for example, about a peak  78  designating a range for expansion. 
     At process block  82 , based on these inputs  76  and  80 , the computer  30  may generate a traveling wave  60 , for example, as shown in  FIG. 3 , to expand the species between the cursors  76  and  80  by aligning the traveling wave  60  with those species as they move through the arbitrary chamber  42 . The effect of the arbitrary traveling wave  60  is to expand or magnify the region between the cursors  76  and  80  to an expanded portion highlighted by region  85 . The amount of expansion may be controlled by the user within the ranges of the spectrometer  40  by controlling the amplitude and length of the traveling wave. 
     The actual amount of the expansion is computed at process blocks  84  accommodating possible variations in the physically obtainable traveling wave  60 . A new spectrogram  56 ′ is then produced, as indicated by process block  88 , applying the enhancement factor produced by the traveling wave  60  to expand the m/z axis of the spectrogram  56  appropriately. 
     Generally, for a simple traveling wave  60  as in  FIG. 2 , the expanded region  85  will extend rightward to the end of the spectrogram  56  to prevent overlap of different species caused by the discontinuous accelerating fields. Nevertheless, provided that the range of the detector  26  or  28  is not exceeded, the region to the right of the cursor  80 , while expanded by the traveling wave  60 , may be re-scaled at process block  84  to visually eliminate the expansion and thus to produce the limited expansion of region  85  rather than a full expansion of all spectrographic data to the right of cursor  76 ′. Note in either case, a low m/z calibrant peak  90  may remain unexpanded to provide for a robust reference value and context for the spectrogram reading. 
     Referring now to  FIG. 6   a - 6   c , generally the traveling wave  60 ′ will be more complicated than the single-polarity ramp depicted in  FIG. 3 , accommodating practical restraints on waveform generation. Nevertheless, a complicated traveling wave  60 ′ may still provide for the expansion features of the present invention by modeling particle movement through the chamber  42  to deduce its total accelerating field. This different total accelerating field for different species provides an enhancement factor between separate species. 
     Referring now to  FIGS. 6 and 7 , the process of computing the total accelerating field may begin as indicated by process block  92  with the determination of the force experienced by each species at each location, starting with the entrance of the axially inhomogeneous field chamber  42 . At an initial time to, the particles  14   a - 14   c  will have well-defined initial positions  101  with respect to the traveling wave  60 ′ so that a first data point on field profile  100  (shown in  FIG. 6   b ) associated with each particle  14   a - 14   c  may be determined per process block  92 . The force at this initial position  101  may be used to calculate an incremental movement  104  (shown in  FIG. 6   c ) of each particle  14  to a later time to provide a new location of the particle  14  designated (d i , t i ) as indicated by process block  94  and local field experienced (si) as indicated by process block  94 . This location may be compared against the waveform trajectory  102  (shown in  FIG. 6   c ) to compute a new instantaneous force acting on the particle in an iterative loop back to process block  92 . Again, this new instantaneous force may be used to deduce the next position of the particle with respect to the waveform trajectory  102 . This iterative process accommodates the fact that the position of the particles  14   a - 14   c  at each point d i  will depend on their history of positions with respect to the traveling wave  60  at all previous points. 
     The known endpoint of the trajectory  103  of a calibrant at the detector may be used to correct errors accumulating in the iteration by tipping the trajectory  103  to fit between the known initial position  101  and the final detector position. 
     This iterative process may be repeated for each time t i  to generate a particle trajectory  103  passing through the waveform trajectory  102  and generating a stream of field data providing field profiles  100  for each of particles  14   a - 14   c . The area under these field profiles  100  may be used to determine the average force acting on the particle and thus to provide calibration of the data from detector  26  or  28 . Generally, since the energy gained by the particle is proportional to the integral of the field profile, the calibration factor or enhancement factor C will be proportional to the square root of the integral of the field profile  100  per process block  106 . 
     This same methodology may be used to produce a desired shape of traveling wave  60  and to define its trajectory  102 , for example by inverse planning techniques known in the art. 
     Referring to  FIG. 6   a , it will be understood that that the traveling wave  60 ′ may have two portions with different polarities  61  and  63 , where polarity  61  accelerates the particles  14  and polarity  63  decelerates the particles  14 . At certain times t n  particles  14  may be allowed to pass up from the positive polarity  61  where they are accelerated to the negative polarity  63  where they are decelerated with respect to the lab reference frame. This deceleration may be used to compress portions of the spectrogram  56 , for example to the right of region  85  as shown in  FIG. 5 . Also the particles experiencing accelerating fields may not keep pace with the accelerating wave form, decelerating with respect to the wave reference frame, which also affects the compression. 
     Referring now to  FIG. 8   a - d , a complex traveling wave  60 ′ may be used to effect a re-focusing of particles  14  and  14 ′ of the same species having slightly different initial velocities. As shown in  FIG. 8   a , these initial velocities may, for example, differ because of the ejection speed from the analyte source  12  at time to. At a later time t n  (shown in  FIG. 8   b ) this initial velocity difference will cause a separation of the particles  14  and  14 ′, a separation accentuated by the traveling wave  60 ′ and resulting in a spread of the peak associated with particles  14  and  14 ′. 
     As shown in  FIG. 8   c , in the present invention, at time t m  the traveling wave  60 ′ may be positioned so that it slopes down in the direction of travel providing relatively greater force on particles  14 ′ having lesser initial velocity and lesser force on particles  14  having greater initial velocity. This force difference may be adjusted so that particles  14  and  14 ′ align at subsequent time t p  (shown in  FIG. 8   d ) aligned with the detector  26  or  28  thus refocusing the peak by eliminating this initial velocity spread. This re-focusing by improving signal strength and thus signal-to-noise ratio, may improve resolution of the spectrogram  56 . Alternatively, as shown in  FIG. 8   e , at time t m , the traveling wave  60  may be positioned so that a crest  110  of the traveling wave  60  is between particles  14  having the greater initial velocity and particles  14 ′ having lesser initial velocity to provide the former particles  14  with less accelerating force relative to particles  14 ′. This approach provides a refocusing of particles near  14  with the slow ion cut-off of particles near  14 ′ 
     In the present invention, the technique of reflectometry bunching can be achieved within the device by providing a repelling field in front of the ions we seek to bunch. This field may be timed to affect only a range of ion species. In reflectometry, faster ions of the same mass take longer to reflect back from a repelling field than slower ions, and so travel a longer path which gives the slower ions, more quickly reflected, a head start in the reflected path. The faster ions overtake the slower ions at some point in the reflected path. Reflectometry focuses the ions in time, reducing the individual species spread for TOF measurements. 
     Referring now to  FIG. 9 , the ability to produce a traveling wave allows the generation of a cyclic axially inhomogeneous field chamber in which a traveling wave  60  circulates indefinitely. The acceleration of particles  14  along a circular axis  16 ′ is enforced by a radially increasing gradient traveling wave provided as indicated by partial rings  44 ′ together with full rings  44  to accelerate and curve particles  14  about axis  16 . The radial containment may also be via magnetic field, with the angular acceleration via traveling wave. As the radius of curvature in a magnetic field follows the particle momentum mv, using the technique in which heavier masses gain less velocity, the fields may be set such that several mass species may be contained in the same orbital radius. After an appropriate period of acceleration, the particles may be released tangentially to a detector  26  or  28 . This system may be used for sorting and separation of particles with similar masses. 
     Referring now to  FIG. 10 , the present invention may provide application to other types of particle separation in which a block  114  of electrophoretic gel, filter medium, or a gas column may be placed in the arbitrary chamber  42  to be exposed to traveling waves  60  for separation of particles. 
     It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein, but include modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments as come within the scope of the following claims.