Abstract:
A linear electric field ion mass spectrometer having an evacuated enclosure with means for generating a linear electric field located in the evacuated enclosure and means for injecting a sample material into the linear electric field. A source of pulsed ionizing radiation injects ionizing radiation into the linear electric field to ionize atoms or molecules of the sample material, and timing means determine the time elapsed between ionization of atoms or molecules and arrival of an ion out of the ionized atoms or molecules at a predetermined position.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention generally relates to mass spectrometers, and, more specifically, relates to a time-of-flight ion mass spectrometer using a linear electric field. This invention was made with Government support under Contract No. W-7405-ENG-36 awarded by the U.S. Department of Energy. The Government has certain rights in the invention. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     Mass spectrometers are used extensively in the scientific community to measure and analyze the chemical compositions of substances. In general, a mass spectrometer is made up of a source of ions that are used to ionize neutral atoms or molecules from a solid, liquid or gaseous substance, a mass analyzer that separates the ions in space or time according to their mass or their mass- per-charge ratio, and a detector. Several variations of mass spectrometers are available, such as magnetic sector mass spectrometers, quadrupole mass spectrometers, and time-of-flight mass spectrometers.  
         [0003]     All citations to publications contained within this application effectively include those publications herein for all purposes.  
         [0004]     The magnetic sector mass spectrometer uses a magnetic field or combined magnetic and electrostatic fields to measure the ion mass-per-charge ratio. In one type of magnetic sector geometry, {see A. O. Nier,  A Mass Spectrometer for Isotope and Gas Analysis, Review of Scientific Instruments , Vol.18, No. 6, June 1947, p. 398; L. Holmlid,  Mass Dispersion and Mass Resolution in Crossed Homogeneous Electric and Magnetic Fields: The Wien Velocity Filter as a Mass Spectrometer, International Journal of Mass Spectrometry and Ion Physics , Vol. 17 (1975) p. 403} only one mass-per-charge species is detected at any one time, so the magnetic field strength and, if present, the electric field strength must be varied in order to obtain a mass spectrum comprising multiple mass-per-charge species. Major limitations on this type of mass spectrometer are the high mass of the magnet and the time that is required to scan the entire mass range one mass at a time.  
         [0005]     Another type of magnetic sector mass spectrometer creates a monoenergetic beam of ions, which are spatially dispersed according to mass-per-charge ratio, and which are focused onto an imaging plate. While this type of spectrometer can detect multiple mass-per-charge species can be detected simultaneously, the poor spatial resolution it provides limits its use to a narrow mass range.  
         [0006]     Quadrupole mass spectrometers utilize a mass filter having dynamic electric fields between four electrodes. These fields are tailored to allow only one mass-per-charge ion to pass through the filter at a time. Major limitations of quadrupole mass spectrometers are the high mass of mass of the required magnet and the time required to scan the entire mass range one mass at a time.  
         [0007]     Time-of-flight mass spectrometers (TOFMS) can detect ions over a wide mass range simultaneously {see W. C. Wiley and I. H. McLaren,  Time-of-Flight Mass Spectrometer with Improved Resolution, Rev. Sci. Instrum. , Vol. 26, No. 12, December 1955, p. 1150. Mass spectra are derived by measuring the times for individual ions to traverse a known distance through an electrostatic field free region. In general, the mass of an ion is derived in TOFMS by measurement or knowledge of the energy, E, of an ion, measurement of the time, t 1 , that an ion passes a fixed point in space, P 1 , and measurement of the later time, t 2 , that the ion passes a second point, P 2 , in space located a distance, d, from P 1 . Using a ion beam of known energy-per-charge E/q, the time-of-flight (TOF) of the ion is t TOF =t 2 −t 1 , and by the ion speed is v=d/t TOF . Since E=0.5 mv 2 , the ion mass-per-charge m/q is represented by the following equation:  
               m   q     =         2   ⁢     Et   TOF   2         qd   2       .           10           
 
         [0008]     The mass-per-charge resolution, commonly referred to as the mass resolving power of a mass spectrometer, is defined as:  
                   Δ   ⁢           ⁢     m   /   q         m   /   q       =         Δ   ⁢           ⁢   E     E     +     2   ⁢       Δ   ⁢           ⁢     t   TOF         t   TOF         +     2   ⁢       Δ   ⁢           ⁢   d     d           ,         11           
 
 where ΔE, Δt TOF , and Δd are the uncertainties in the knowledge or measurement of the ion&#39;s energy, E, time-of-flight, t TOF , and distance of travel, d, respectively, in conventional time-of-flight spectrometers. 
 
         [0009]     In a gated TOFMS in which a narrow bunch of ions is periodically injected into the drift region, uncertainty in t TOF  may result, for example, from ambiguity in the exact time that an ion entered the drift region due to the finite time, Δt 1 , that the gate is “open,” i.e. Δt 1 ≈Δt TOF . The ratio of Δt TOF /t TOF  can be minimized by decreasing Δt TOF , for example, by decreasing the time the gate is “open.” This ratio can also be minimized by increasing t TOF , for example, by increasing the distance, d, that an ion travels in the drift region. Often, a reflectron device is used to increase the distance of travel without increasing the physical size of the drift region.  
         [0010]     Uncertainty in the distance of travel, d, can arise if the ion beam has a slight angular divergence so that ions travel slightly different paths, and, therefore, slightly different distances to the detector. The ratio of Δd/d can be minimized by employing a long drift region, a small detector, and a highly collimated ion beam.  
         [0011]     The uncertainty in the ion energy, E, may result from the initial spread of energies ΔE of ions emitted from the ion source. Therefore, ions are typically accelerated to an energy E that is much greater than ΔE.  
         [0012]     A further limitation of conventional mass spectrometry lies in the fact that the source of ions is a separate component from the time-of-flight section of a spectrometer, and it requires significant resources. First, most ion sources are inherently inefficient, so that few atoms or molecules of a gaseous sample are ionized, thereby requiring a large volume of sample and, in order to maintain a proper vacuum, a large vacuum pumping capacity. Second, the ion source typically generates a continuous ion beam that is gated periodically, creating an inefficient condition in which sample material and electrical energy are wasted during the time the gate is “closed.” Third, ions have to be transported from the ion source to the time-of-flight section, requiring, among other things, electrostatic acceleration, steering and focusing. Fourth, typical ion sources introduce a significant spread in energy of the ions so that the ions must be substantially accelerated to minimize the effect of this energy spread on the mass resolving power. Finally, having an ion source separate from the drift region creates an apparatus having large mass and volume.  
         [0013]     Still another problem with conventional time-of-flight mass spectrometers is that ions must be localized in space at time t 1  in order to minimize Δd and, therefore, minimize the mass resolving power. Typically, time t 1  corresponds to the time that the ion is located at the entrance to the drift region.  
         [0014]     In summary, the limitations on conventional TOFMS include a mass resolving power dependent on the energy spread of the ions emitted from the ion source; the uncertainty in the distance of travel of the ion in its flight path; the problems associated with an ion source that is separate from the drift region; and the need to localize ions in space at time t 1 . The present invention provides an apparatus that overcomes these limitations and provides more accurate data.  
       SUMMARY OF THE INVENTION  
       [0015]     In order to achieve the objects and purposes of the present invention, and in accordance with its objectives, time-of-flight ion mass spectrometer comprises an evacuated enclosure with means for generating a linear electric field located in the evacuated enclosure and means for injecting a sample material into the linear electric field. A source of pulsed ionizing radiation injects ionizing radiation into the linear electric field to ionize atoms or molecules of the sample material; and timing means determine the time elapsed between ionization of the atoms or molecules and arrival of an ion out of the ionized atoms or molecules at a predetermined position. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]     The accompanying drawing, which is incorporated in and forms a part of the specification, illustrates an embodiment of the present invention and, together with the description, serves to explain the principles of the invention. In the drawing:  
         [0017]      FIG. 1  is a schematic illustration of an embodiment of the present invention showing the elements of the invention and its operation. 
     
    
     DETAILED DESCRIPTION  
       [0018]     The present invention ionizes a sample atom or molecule within a drift region having a linear electric field. The electric field accelerates the ions toward a detector, such that the time-of-flight of an ion, from the time of its ionization to the time of its detection, is independent of the distance the ion travels in the drift region. The invention provides high mass resolving power, smaller resource requirements in such areas as mass, power, volume, and pumping capacity, and elimination of the prior art requirement that the location of an ion at time t 1  must be known in order to measure its time-of-flight in the drift region. The invention can be understood more easily through reference to the drawing.  
         [0019]     Referring to  FIG. 1 , there can be seen the time-of-flight mass spectrometer  10  of the present invention resides inside evacuated chamber  11 . The gaseous sample to be investigated is introduced into drift region  12  by sample inlet  13 , where the sample is a gas . Alternatively, a solid sample could be introduced, for example, at the surface of an electrode near end plate  17 . Concentric electrically conductive rings  14  surround drift region  12 , and are connected to resistors  15  that are connected between voltage V 1  and voltage V 2 , as shown, with V 1  negative with respect to V 2 . Also as shown, V 1  is connected to stop detector  16 , and V 2  is connected to end plate  17  at the opposite end of drift region  12 . This arrangement provides the linear electric field in drift region  12  that is required by the present invention. The resistor values are selected to generate the linear electric field along the central axis of the drift region. Generally, the resistor values increase quadratically from stop detector  16  (V 1 ) to end plate  17  (V 2 ) for a cylindrical drift region  12 .  
         [0020]     The linear electric field created by V 1  and V 2  across resistors  15  and concentric rings  14  is coaxial about central axis  18  (the z axis), and has a magnitude, ε(z), that is proportional to the distance, z, normal to stop detector  16 , as shown in U.S. Pat. No. 5,168,158, issued December, 1992, to McComas et al. Although concentric ring  14  and resistors  15  effectively provide the linear electric field for the present invention, other methods can be used. For example, a dielectric cylinder could surround drift region  12 , and have a resistive coating applied whose resistance varies with the distance from stop detector  16 . Another electric field arrangement could involve a conically shaped grid at stop detector  16  (V 1 ) and a hyperbolic shaped grid located at end plate  17  (V 2 ) as described by D. C. Hamilton et al., in New high resolution electrostatic ion mass analyzer using time-of-flight, Rev. Sci. Instrum. Vol. 61 (1990) 3104-3106. It is also possible that combinations of these methods could be used. Any method of effectively producing a linear electric field within drift region  12  could be used with the present invention.  
         [0021]     Stop detector  16  can be any effective single particle detector that can measure the time that an ion strikes the detector with time accuracy much less than the ion&#39;s TOF in the drift region. One appropriate stop detector  16  is an electron multiplier detector such as a microchannel plate detector or channel electron multiplier detector that would detect ionized sample atoms or molecules that have been accelerated through drift region  12 , and output a signal indicating the detection.  
         [0022]     Pulsed ionizing radiation source  19  emits pulses of ionizing radiation through concentric rings  14  and into drift region  12  where it ionizes atoms or molecules of the gas sample of interest. Pulsed ionizing radiation source  19  can emit any effective ionizing radiation, such as photons, electrons, or ions and could be a laser, a source of electrons, or a source of ions.  
         [0023]     Pulsed ionizing radiation source  19  ionizes sample atoms or molecules at time, t 1 , and the ionized atom or molecule is accelerated by the linear electric field toward stop detector  16 , where the ionized atom or molecule is detected at time, t 2 . The difference in times, t 2 −t 1 , corresponds to the time-of-flight of the ionized atom or molecule over the distance that it travels from the time it was ionized to the time it is detected at stop detector  16 .  
         [0024]     The general equation governing the motion of an ion in a linear electric field is:  
                 -   qkz     =     m   ⁢         ⅆ   2     ⁢   z       ⅆ     t   2             ,         12           
 
 where q is the ion charge and k is a constant that depends only upon the electromechanical configuration of the drift region. Equation 12 has the solution of:
 
 z=A  sin(ωt+φ)  13
 
 where A and φ are determined by the initial conditions and ω 2 =kq/m. A requirement of these relationships is that an ionized sample atom or molecule is initially at rest or partially at rest in the z direction. It is well known to those having skill in this art, that the mean kinetic energy of a gaseous atom or molecule is 1.5 kT, where k is the Boltzman constant, and T is the temperature of the gas. At room temperature (approximately 300 K), the mean energy is approximately 0.04 eV. This initial energy uncertainty ΔE can influence the mass resolving power according to Equation 11. To minimize ΔE/E the magnitude of the potentials generating the linear electric field must be sufficiently high to achieve the desired mass resolving power. 
 
         [0025]     Under the initial conditions that stop detector  16  is located at z=0, and that the ion is created at rest at a distance of z=d from stop detector  16 , the time-of-flight of the ion according to Equation 13 is:  
               t   TOF     =       π     2   ⁢   ω       =       π   2     ⁢         (     m   qk     )       1   2       .               14           
 
         [0026]     In contrast to a conventional linear electric field ion mass spectrometer in which an ion experiences a retarding electric field and follow a half-oscillation path of the harmonic oscillator analog, Equation 14 corresponds to acceleration over a quarter-oscillation path of the harmonic oscillator analog. Rearranging Equation 14 yields:  
                 m   q     =       4   ⁢     kt   TOF   2         π   2         ,         15           
 
 which, as seen, is independent of the distance of travel, d, of the ion in the accelerating linear electric field. Thus, it is clear that the advantage of an acceleration linear electric field, such as is generated in the present invention, in which sample atoms or molecules are ionized while they are considered to be at rest (or nearly so relative to the energy to which they are accelerated by the linear electric field in drift region  12 ) is that the ions can be created at any location in drift region  12  and they will have a time-of-flight that depends only on the mass-per-charge of the ion and on the electromechanical design of the apparatus. This also allows for a high mass resolving power according to Equation 11, since, for an ideal system, (a) the m/q is independent of the location that the ion is formed in the drift region, so that Δd/d=0, and (b) the sample atom or molecule is ionized at rest or nearly at rest and is accelerated to a high enough energy so that ΔE/E is smaller than or comparable to other factors that limit the mass resolving power described in Equation 11. Additionally, this eliminates the requirement of prior art TOFMS, including prior conventional linear electric field devices, that the ionizing radiation particles be localized at a known location at time t 1 . 
 
         [0027]     It should be noted that the prior art of retarding linear electric field devices teaches TOF mass spectrometry using half-sine-wave ion orbits in which an ion enters a drift region with high energy, but which is slowed down by the electric field so that it reverses direction at the point at which the ion has zero velocity in the z-direction. The ion then returns to and is detected at the same plane from which the ion was originally introduced into the drift region. In the present invention, an ion starts at rest from any position in drift region  12 , and is accelerated by the linear electric field in one direction toward stop detector  16 . This corresponds to a quarter-sine-wave particle orbit in the solution to the differential equation of motion, Equation 12.  
         [0028]     Those with skill in this art recognize that the invention requires a power supply to provide the necessary potential differences required for V 1  and V 2  and to produce the necessary linear electric field, and for powering pulsed ionizing radiation source  19 . Additionally, timing electronic circuits are required to measure the time between generation of the pulse from pulsed ionizing radiation source  19 , and the detection of an ion at stop detector  16 .  
         [0029]     As has been explained, the present invention ionizes the sample atoms or molecules inside drift region  12 , not in some external ion source. This allows the invention to be inherently compact, allowing the invention to provide TOFMS apparatus that has a small volume and mass, that requires smaller sample volume, and that requires reduced power resources. The ionization of sample atoms or molecules inside drift region  12  also allows the present invention to accelerate the ions from a condition of near rest independent of the ion&#39;s position within drift region  12 . This allows use of a spatially broad pulsed ionizing radiation source  19  that is efficient and requires little or no steering, collimation or focusing.  
         [0030]     In the present invention, the sample ion is formed when the sample atom or molecule is approximately at rest, and the time-of-flight of the sample ion in drift region  12  is independent of the location at which the sample ion was formed. Therefore, the mass resolving power of the sample ion is likely dependent primarily on the accuracy of the time-of-flight measurement, which includes, for example, the length of time that the ionizing radiation from pulsed ionizing radiation source  19  is admitted into drift region  12 , the timing accuracy of the stop detector  16 , and the timing accuracy of the time-of-flight measurement electronics.  
         [0031]     The present invention requires only a small volume of sample material because the pressure of the sample in the drift region is necessarily low to prevent high voltage arcing within the device and because most ionized sample atoms or molecules are detected. This is in contrast to prior art mass spectrometers, where few ions created in the ion source are injected into the drift region because of the low efficiency of extracting ions from the ion source and because of removal of ions from the ion beam by, among other things, collimating slits, and while the gate is “closed.” Additionally, due to the smaller volume of the present invention and the lower required volume of sample, the pumping requirements for evacuation of evacuated chamber  11  is reduced, allowing use of a smaller vacuum pump.  
         [0032]     Finally, the present invention requires lower voltage differences across drift region  12 . Since a sample atom or molecule is ionized while it is at thermal energies of approximately 0.04 eV at 300 K, the calculated mass-per-charge of the ion is dependent on knowledge accuracy of the ion&#39;s energy relative to its accelerated energy as it traverses drift region  12 . Because the spread in the initial energies of the sample ions is small, the acceleration voltage (V 1 −V 2 ) does not have to be high. To put this into perspective, in some conventional mass spectrometers, ions are extracted from the ion source by electrostatic means, and a potential gradient can exit within the ion source so that ions are created at different potentials that result in an energy spread that can range from about 1 eV to tens of eV, which requires acceleration of the sample ions to a high energy in order to remove the uncertainty of the energies of the sample ions. In one embodiment of the present invention, a single applied voltage (except for the signal electronics) could be applied both as the bias for stop detector  16  and for voltage V 1  at stop detector  16 . This voltage could be −3 kV at V 1 , and 0 V at V 2 .  
         [0033]     The foregoing description of the invention has been presented for purposes of illustration and description and is not intended to be exhaustive or to limit the invention to the precise form disclosed, and obviously many modifications and variations are possible in light of the above teaching. The embodiments were chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto.