Abstract:
A reflectron lens for a time-of-flight mass spectrometer is disclosed. The reflectron lens includes a glass tube having a conductive surface along the length of the tube. The conductive surface has an electrical resistance gradient along its length. The electrical resistance gradient provides an electric field interior to the tube that varies in strength along the length of the tube when an electric potential is applied to opposing ends of the tube. A mass spectrometer incorporating the reflectron lens, a method of making the reflectron lens, and an apparatus for removing lead from the surface of a lead silicate glass tube are also disclosed.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims the benefit if U.S. Provisional Application No. 60/781,088, filed Mar. 10, 2006. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     This invention relates generally to mass spectrometers, and in particular to a reflectron type time-of-flight mass spectrometer and to a glass reflectron lens for such a spectrometer wherein the lens has a gradient electrical resistance on its surface.  
         [0003]     Mass spectrometers are analytical instruments capable of identifying unknown materials in complex mixtures with precision in the parts per billion range. Once used exclusively in research laboratories, mass spectrometers are now in use in a broad range of applications. They are used in screening for pesticides in canned foods, controlling semiconductor manufacturing processes, diagnosing disease, exploring for natural resources, discovering new pharmaceuticals, predicting volcano eruptions, and security systems. Indeed, these instruments have traveled beyond our world aboard the Galileo and Cassini spacecrafts to provide atmospheric analysis of neighboring worlds within our solar system.  
         [0004]     Time of Flight Mass Spectrometry (TOF-MS) is rapidly becoming the most popular method of mass separation in analytical chemistry. The development of low cost digitizers and extremely fast ion detectors has fueled this popularity. TOF-MS is easily deployed and can produce very high mass resolution. This technique of mass separation can be adapted for many forms of sample introduction and ionization. Unlike quadrupoles and ion traps, time of flight mass analyzers perform well with very high mass molecules of the type frequently found in protean applications. Wiley and McLaren in 1955, followed by Cotter in 1992, and Wollnik in 1993 have described time of flight mass analyzers.  
         [0005]     Time-of-flight mass spectrometers are produced in two main types: linear instruments and reflectron instruments.  
         [0000]     Linear Time of Flight Mass Spectrometers  
         [0006]      FIG. 1  illustrates a linear time of flight mass spectrometer embodied as a matrix assisted laser desorption ionization (MALDI) instrument  100 . In a linear time of flight mass spectrometer, an unknown sample is first converted to ions. The sample is deposited on a plate  102 . A light beam from a laser  104  is directed at the sample on plate  102  which causes the sample to ionize. The resultant ions are injected into a flight tube  106  wherein they travel towards the ion detector  108 . The detector may be embodied as a microchannel-plate type detector as described in U.S. Pat. No. 6,828,729, the entire disclosure of which is incorporated by reference.  
         [0007]     The motion of the ions within the flight tube can be described by the following equation. 
 
 t   2   =m/z ( d   2 /2 V   se ) 
 
 In Equation 1, m/z is the mass to charge ratio of the ion, d is the distance to the detector, and V se  is the acceleration potential. 
 
         [0008]     The lighter ions (i.e. ions having relatively lower masses) travel toward the detector  108  faster than the higher mass ions. If the flight tube is long enough, the ions will arrive at the detector according to their mass, i.e., lowest to highest.  
         [0009]     When the ions arrive at the detector  108 , they initiate a cascade of secondary electrons within the detector, which results in the generation of a series of very fast voltage pulses. The voltage pulses precisely signal the arrival of the ions. A high-speed oscilloscope or transient recorder is used to record the arrival times.  FIG. 2  illustrates the arrival time spectrum of a sample of Brandykinin analyzed on a linear time of flight mass spectrometer. Knowing the exact arrival times, Equation 1 can be used to solve for the mass-to-charge ratios of the ions.  
         [0000]     Reflectron-Type Time of Flight Mass Spectrometer  
         [0010]     The second type of time-of-flight mass spectrometer is the reflectron instrument.  FIG. 3  illustrates a known arrangement of a reflectron TOF mass spectrometer  300 . The reflectron design takes advantage of the fact that the further the ions are allowed to travel, the greater the distance between ions of slightly differing masses. Greater distances between ions with different masses will increase the arrival time differences between the ions and thereby increase the resolution at which ions having similar mass-to-charge ratios (m/z) can be differentiated. In addition, the reflectron design corrects for the energy dispersion of the ions leaving the source.  
         [0011]     In the reflectron analyzer  300 , the ions are injected into the flight tube  302  in the same manner as in a linear instrument described above. The ions travel down the flight tube and enter the reflectron lens  304 .  FIG. 4  shows the construction of a known reflectron lens. It consists of a plurality of stacked metal rings that are spaced and insulated from each other. An electrostatic field is created within the reflectron lens  304  by applying different high voltage potentials to each of the metal rings. The electrostatic field has a polarity that causes the ions to decelerate and eventually reverse their direction. The ions exit the lens  304  and are directed to the ion detector  306 . The action of the reflectron lens on the ions effectively doubles their length of travel in the flight tube. The additional travel time improves mass resolution without adding additional length to the flight tube.  
         [0012]     Most time of flight instruments manufactured today incorporate reflectron lenses. As shown in  FIG. 4 a  reflectron lens consists of a stack of precision ground metal rings  402  alternating with insulating spacers  404 . The rings and spacers are held together with threaded rods  406 . The reflectron lens assembly may have hundreds of components which must be carefully assembled and aligned (typically by hand) in a clean, dust free environment. Additionally, a voltage divider must be included in each row or layer in order to produce the electrostatic field gradient necessary to reverse the direction of the ions.  
         [0013]     An improved variant of the classical reflectron lens design utilizes a single resistive glass tube to generate the gradient electric field. A resistive glass tube reflectron lens is shown in  FIG. 5 . The monolithic structure of the resistive glass tube replaces the multi-component assembly of the metal ring type of reflectron lens.  
         [0014]     Reflectron lenses fabricated from resistive glass tubes have thus far been produced with a uniform resistance along the inside of the tube. That architecture is useful for many reflectron geometries. However, a significant performance advantage and greater design flexibility could be realized if the electrical resistance varied discretely or continuously along the length of the lens.  
         [0015]     Resistive glass reflectron lenses are fabricated from lead silicate glass that has been subjected to a hydrogen reduction process to produce a thin resistive layer on the inside surface of the tube. A resistive glass reflectron lens is described in U.S. Pat. No. 7,154,086, the entire disclosure of which is incorporated herein by reference.  
         [0016]     The hydrogen reduction process consists of loading the glass tube into a closed furnace through which pure hydrogen or a controlled mixture of hydrogen and oxygen is purged. The temperature is gradually increased, typically at a rate of 1-3 degrees per minute. Beginning at approximately 250° C., a chemical reaction occurs in the glass in which the lead oxide in the glass converts to a semi-conductive state. This reaction occurs in the first few hundred angstroms of the cross section of the glass. As the glass continues to be heated in the presence of the hydrogen, more of the lead oxide is chemically reduced, thereby providing lower electrical resistance. Temperature, time, gas pressure, and gas flow can be controlled to provide a desired amount of electrical resistance on the surface of the glass.  
         [0017]     The electrical resistance is also dependent on the composition of the glass. For example, a glass containing more lead oxide with a modifier such as bismuth can be used to produce lower resistances. The hydrogen reduction process makes all surfaces of the glass tube conductive. Unwanted conductive surfaces can be stripped by chemical or mechanical means.  
         [0018]     A known hydrogen reduction process has the following parameters:  
         [0019]     3 hour ramp up from RT to 200° C.;  
         [0020]     1 hour ramp up from 200 to 300° C.;  
         [0021]     12.5 hour ramp up from 300 to 445° C.;  
         [0022]     hold at 445° C. for 3 hours in hydrogen at a pressure of 34 psi and a hydrogen flow of 40 l/m.  
         [0023]     The reduction temperature is limited on the low end by the minimum temperature needed to sustain the reaction and is limited on the high side by the sag point of the glass.  
         [0024]     In some applications, it is desirable to produce a segmented resistive tube in which certain sections of the tube have significantly different values of electrical resistance. In other applications it is desirable to have a continuous resistance gradient in which the resistance along the wall varies continuously along the length of the tube. The variation may be linear or nonlinear. For example, an orthogonal geometry time of flight mass spectrometer utilizes a reflectron tube having a nonlinear resistance characteristic. U.S. Pat. No. 7,081,618, the entire disclosure of which is incorporated herein by reference, and U.S. Pat. No. 7,154,086 describe methods to produce a uniform electrical resistance in a lead silicate glass tube by subjecting the tube to a reducing environment within a hydrogen furnace.  
       SUMMARY OF THE INVENTION  
       [0025]     In accordance with a first aspect of the present invention there is provided a reflectron lens for a time-of-flight mass spectrometer that includes a glass tube having a conductive surface along the length of the tube. The conductive surface has an electrical resistance gradient along its length. The electrical resistance gradient provides an electric field interior to the tube that varies in strength along the length of the tube when an electric potential is applied to opposing ends of the tube.  
         [0026]     In accordance with a second aspect of the present invention there is provided a time-of-flight mass spectrometer that has a reflectron lens. The reflectron lens is formed of a glass tube having a conductive surface along the length of the tube. The conductive surface has an electrical resistance gradient along its length. The time-of-flight mass spectrometer according to this invention also includes a voltage supply electrically connected to opposite ends of the tube. With the foregoing arrangement, the reflectron lens provides an electric field interior thereto that varies in strength along the length of the lens.  
         [0027]     In accordance with a further aspect of the present invention there is provided a method of preparing a glass tube for use as a reflectron lens. The process includes the steps of providing a glass tube wherein the glass contains a lead compound and immersing a portion of the glass tube in an acidic solution. The portion of the glass tube is held in the acidic solution for a time sufficient to remove lead from the surface of the immersed portion of said glass tube. The surface of the glass tube is then chemically reduced to form an electrically resistive layer on the surface of the glass tube. In this manner, the electrical resistance of the portion held in the acidic solution will be lower than the electrical resistance of another portion of the glass tube.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0028]     The foregoing description of the background of the invention, as well as the following description of preferred embodiments of the invention, will be better understood by reference to the drawings wherein:  
         [0029]      FIG. 1  is a schematic diagram of a linear time-of-flight mass spectrometer;  
         [0030]      FIG. 2  is a graph of the response of a linear time-of-flight mass spectrometer for a sample of Brandykinin;  
         [0031]      FIG. 3  is a schematic diagram of a reflectron type mass spectrometer;  
         [0032]      FIG. 4  is a photograph of a metal-ring reflectron lens;  
         [0033]      FIG. 5  is a photograph of a glass reflectron lens;  
         [0034]      FIG. 6  is a schematic diagram of an apparatus for carrying out selective leaching of lead from a lead silicate glass tube in accordance with the present invention;  
         [0035]      FIG. 7  is a schematic diagram showing the step of selective immersion of a glass tube in a lead leaching solution;  
         [0036]      FIG. 8  is a graph of the electrical resistance per unit area of a glass tube prepared in accordance with the present invention wherein the electrical resistance varies in steps;  
         [0037]      FIG. 9  is a graph of the electrical resistance per unit area of a glass tube prepared in accordance with the present invention wherein the electrical resistance varies linearly;  
         [0038]      FIG. 10  is a graph of the electrical resistance per unit area of a glass tube prepared in accordance with the present invention wherein the electrical resistance varies nonlinearly; and  
         [0039]      FIG. 11  is a schematic diagram of a reflectron type mass spectrometer utilizing a glass reflectron lens made in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0040]     The lead oxide within lead silicate bulk glass is reacted with hydrogen at temperatures ranging from 350-550° C. The chemical reaction occurs on the surface of the work piece, not in the interior of the bulk material. The chemical reaction proceeds as follows: 
 
Pb 2 O 5 +H 2 →Pb 2 O x +H 2 O 
 
 The reaction yields a semi-conductive form of lead oxide that is uniformly distributed over all surfaces of the work piece. The electrical resistance along the surface is a function of the process parameters of temperature, time, the pressure and composition of the hydrogen/oxygen cover gas, and the composition of the lead silicate glass at the reduction surface. 
 
         [0041]     In order to obtain an electrical resistance that varies along the length of the work piece, it is necessary to either vary the process parameters locally along the piece or change the base material in some way. The hydrogen reduction process is performed in large furnaces with significant gas flows in order to prevent local process variations. The bulk glass material is fabricated in very large batch melts that weigh up to thousands of pounds. Manufacturers strive for uniformity within the melt, thereby making it nearly impossible to produce chemical differences over small distances in the work piece.  
         [0042]     One method to change the composition of the glass structure over small distances would be to preferentially remove lead from the surface of the substrate. Preferentially removing lead from the immediate surface in the desired locations would cause those areas to have higher resistances because less lead would be reduced to the semi-conducting form of the lead oxide during the hydrogen reduction process.  
         [0043]     Lead can be preferentially removed from the surface of a lead silicate glass in a leaching process by exposing the workpiece to certain acidic solutions. Either hydrochloric or nitric acid, in a concentration of 10%, has been used to preferentially remove lead in desired areas of a resistive glass work piece. Removing the lead from the surface will increase the electrical resistance in the area(s) exposed to the leaching process. Conversely, preventing exposure to the leachant in specific areas will ensure lower resistances in those areas. Experiments have been performed which indicate that the resistance obtained after the reduction process is inversely proportional to the exposure time to the leachant for a given concentration and temperature of the solution.  
         [0044]     In order to control the exposure of the work piece to the leaching solution, it is preferred to have an apparatus for performing the process in a controllable manner.  FIG. 6  illustrates such a device. The preferred machine  600  includes an acid tank  602  which holds the acidic solution used for leaching. The work piece  604  is held by a chuck  606  that is attached to a slide mechanism  608 . The slide  608  is moved by rotation of a lead screw (not shown). An L-shaped support fixture  610  is attached to the slide  608 . The chuck  606  is supported from the fixture  610  with a gimbal  612 . The slide mechanism  608  is supported by a mounting bracket  614  that is mounted on a table  616  or other support stand. A shield  618  is provided to protect the slide mechanism  608 .  
         [0045]     The slide mechanism  608  is connected to a DC stepper motor  620  that is mounted to the table  616 . The stepper motor  620  has a drive shaft to which a drive pulley  624  is attached. A second drive pulley  628  mounted on a drive shaft  630  that is connected to the lead screw in the slide mechanism  608 . A timing belt  626  connects the first drive pulley  624  to the second drive pulley  628  such that rotation of the stepper motor shaft results in rotation of the drive shaft  630 . The drive shaft  630  is coupled to the lead screw with a flexible connector  634  to complete the linkage between the stepper motor  620  and the lead screw. With the arrangement shown in  FIG. 6 , the glass tube work piece  604  can be precisely moved into and out of the acid tank  602  to permit selective leaching of the lead from the glass.  
         [0046]     As shown in  FIG. 7 , a step-wise resistance gradient is produced by stopping the immersion of the glass tube at a predetermined location along the length of the tube. The tube is held for a time sufficient to achieve a predefined amount of leaching. In this embodiment, the resistance of the portion of the tube immersed in the solution is significantly higher than the portion not exposed to the solution.  FIG. 8  illustrates the stepped resistance profile along the length of the glass tube. Please note the y-scale is resistance per unit area, not total end-to-point resistance along the wall of the tube.  
         [0047]     In another embodiment of the process, the apparatus shown in  FIG. 6  can be used to slowly lower the work piece into the acid solution at a controlled rate of speed. Lowering the piece at a fixed rate creates a linear resistance profile as illustrated in  FIG. 9 .  
         [0048]     In other embodiments of the process, the apparatus shown in  FIG. 6  is modulated to perform a process in which a combination of the steps described above is performed. For example, a first portion of the glass tube is exposed for a first time period, while a different section is moved continuously as it is exposed, and a third section is not exposed at all.  FIG. 10  illustrates the nonlinear resistance profile produced by such a process. In this manner, an unlimited number of resistance profiles can be produced.  
         [0049]     Glass tubes having low resistances in the middle portion of the tube length can be produced either by masking the work piece in the area of desired low resistance or flipping the work piece in the holder. Masking materials can be applied by brushing, spraying, taping, or they can be spun on.  
         [0050]     A glass tube made in accordance with the present invention can be used a reflectron lens in a reflectron type mass spectrometer. Such an arrangement is shown in  FIG. 11 . An ion mass spectrometer  1100  according to the present invention has a flight tube  1102  through which the ions travel. The mass spectrometer  1100  includes a reflectron lens  1104  that is formed from a glass tube having a gradient electrical resistance along the length of its surface. The reflectron lens  1104  is disposed at an end of the flight tube  1102  distal from an ion detector  1108 . Preferably, the ion detector  1108  is a microchannel plate-based ion detector. A linear detector may be provided at the reflectron lens end of the flight tube  1102 . An ionization section  1110  is provided at the same end of the flight tube  1102  as the ion detector  1108 , although spaced therefrom. The ionization section  1110  provides ionized sample material into the flight tube  1102 .  
         [0051]     It will be recognized by those skilled in the art that changes or modifications may be made to the above-described embodiments without departing from the broad inventive concepts of the invention. It is understood, therefore, that the invention is not limited to the particular embodiments which are described, but is intended to cover all modifications and changes within the scope and spirit of the invention as described above and set forth in the appended claims.