Abstract:
Novel components reduce background noise caused by secondary ions generated by metastable entity bombardment in a mass spectrometric system. Layered structures for exit electrodes and deflector plates confine secondary ions in a local low-energy well, preventing them from entering the detector.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The invention relates to mass spectrometry. In particular, this invention provides method and apparatus for reducing background noise caused by neutral metastable entities in a mass spectrometer. More particularly, instrument components are described for trapping secondary ions generated by bombardment of components by metastable entities. 
         [0003]    2. Background Information 
         [0004]    Mass spectrometry is an analytical technique that exploits the dependence of an ion trajectory through electric and magnetic fields on the ion mass/charge ratio. Typically the prevalence of component ions is measured as a function of mass/charge ratio and the data are assembled to generate a mass spectrum of a physical sample. The mass spectrum is useful, for example, for identifying compounds of unknown identity, determining the isotopic composition of elements in a known compound, resolving the structure of a compound and, with the use of calibrated standards, quantitating a compound in a sample. 
         [0005]    Analysis by mass spectrometry entails a sequence of three component processes, each of which can be performed by any one of several types of devices. First, an ion source converts the sample into constituent ions. Second, after leaving the ion source, the charged species in the fragmented sample undergo sorting according to mass/charge ratio in a mass analyzer. Finally, the sorted ions enter a detector chamber, in which a detector converts each separated ion fraction into a signal indicative of its relative abundance. The attributes of the particular ion source, mass analyzer, and detector assembled to constitute a mass spectrometer tailor the capabilities of the instrument to analysis of particular sample types or to acquisition of specialized data. 
         [0006]    For some applications, analysis by mass spectrometry can be enhanced by combination with other analytical techniques that separate the sample into constituents before ionization in the mass spectrograph. For example, in a common enhancement a gas chromatograph separates the sample into constituent components before it meets the spectrometer ion source, to improve distinction between compounds of relatively low molecular weight. This arrangement, termed gas chromatography-mass spectrometry (“GC/MS”), is widely used to identify unknown samples, especially in environmental analysis and drug, fire and explosives investigations. 
         [0007]    The separative powers of gas chromatography enable GC/MS to identify substances to a much greater certainty than is possible using a mass spectrometry assembly alone. However, its necessary use of an inert carrier gas also introduces analytical difficulties in the form of background noise. 
         [0008]    Some atoms of an inert carrier gas such as helium are excited to higher-energy metastable states in the mass spectrometer due, for example, to electron impact in the ion source or by collision with helium ions accelerated by the focusing elements. The common helium metastable states, e.g., 2 3 S 1 , have energy levels of approximately 20 eV and can persist for several seconds. 
         [0009]    The metastable atoms are uncharged and thus not focused by any of the ion optics. They tend to follow a line-of-sight path and bombard instrument components in their paths. The collisions generate secondary ions by a process known as Penning ionization, whereby ionization occurs due to a transfer of potential energy between atoms in an excited metastable state and a source of secondary ions. The secondary ion sources are believed primarily to be contaminants (for example, hydrocarbons)—arising from the pump oil, sample residue, and the reduced pressure atmosphere—on component surfaces. 
         [0010]    Secondary ions created early in the matter stream, such as in the ion source or in the upstream portion of the analyzer, have the opportunity to be sorted by the analyzer and counted by the detector as representative of their chemical composition and structure. However, if the secondary ions are instead created near the exit from the analyzer, such as by striking the ion-focusing lens gating the detector chamber, or in the detector chamber itself, the secondary ions are not resolvable by the analyzer. If these late-created secondary ions enter the detector, they do so randomly, generating background noise. Metastable helium atoms are a major source of noise in GC/MS systems that use helium carrier gas. 
         [0011]    Secondary ions can also be generated by excited neutral particles of other elements introduced, for example, by an inductively coupled plasma (“ICP”) ion source or by liquid chromatography-mass spectrometry (“LC/MS”) and other approaches that ionize the sample at atmospheric or reduced pressure. 
       SUMMARY OF THE INVENTION 
       [0012]    The invention provides novel components for reducing background noise caused by metastable neutral atoms and molecules in a mass spectrometric system and related novel methods of analysis by mass spectrometry. 
         [0013]    In one aspect the invention provides a novel multi-layer lens for admitting ions from the mass analyzer to the detector system. The lens, which has a central aperture for transmitting the subject ions, includes external and middle electrodes biased to create within the lens a local potential-energy well for secondary ions. Secondary ions created by particle bombardment of the middle electrode are trapped in the potential-energy well and remain confined on the surface of the middle electrode. Accordingly, such secondary ions are unable to contribute to background noise in the detector. 
         [0014]    In particular, the lens comprises a layered structure of front, middle and back electrodes, electrically isolated from one another. The front electrode includes a grid which distributes the potential of the front electrode over the front of the lens to provide electrostatic shielding of the middle electrode while permitting neutral and charged particles to pass. Subject ions are focused to the central aperture while neutral particles pass through the front electrode and strike the surface of the middle electrode behind the grid. 
         [0015]    The middle electrode is biased with respect to the front and back electrodes so that a secondary ion at the middle electrode is at a lower potential energy than it would be at either of the front and back electrodes. Namely, when negatively charged secondary ions are to be captured, the middle electrode is at a higher potential than is each of the front and back electrodes; conversely, for positively charged secondary ions the middle electrode is at a lower potential than is each of the front and back electrodes. 
         [0016]    In a preferred embodiment, the external electrodes shielding the subject ions from the potential-energy well are grounded. This configuration contains the electric field created by the middle electrode and limits the influence of the middle electrode on the trajectories of the subject ions through the central aperture, such that, to the ions, the structure appears similar to a single grounded electrode. 
         [0017]    A similarly layered deflector plate confines secondary ions generated by the impact of neutral metastable particles passing from the mass analyzer into the detector chamber. The grid-covered, low-potential-energy middle electrode surface of the layered deflector plate faces the admitting aperture so that neutral particles entering the chamber pass through the grid and strike the surface. Secondary ions thus generated are confined to the deflector plate middle electrode surface. 
         [0018]    These layered biased structures reduce the system background noise caused by neutral metastable entities. The improved signal-to-noise ratio translates into a lower detectability limit for the mass spectrometric systems of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    The invention description below refers to the accompanying drawings, of which: 
           [0020]      FIG. 1  schematically depicts a mass spectrometry system compatible with an embodiment of the invention; 
           [0021]      FIG. 2  is an exploded view of an ion-focusing lens constructed in accordance with an embodiment of the invention; 
           [0022]      FIGS. 3A-3B  show prospective views of an embodiment of the ion-focusing lens of the invention,  FIG. 3A  showing the complete assembly and  FIG. 3B  showing the lens with the grid removed for ease of viewing; 
           [0023]      FIG. 4  shows a mass spectrometry system having a deflector plate constructed in accordance with an embodiment of the invention; and 
           [0024]      FIG. 5  depicts a cross-section of a deflector plate embodiment of the invention. 
       
    
    
       [0025]    Features in the drawings are not, in general, drawn to scale. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0026]    With reference to  FIG. 1 , a mass spectrometry system  10  of the prior art includes three principal components: an ion source  16 , a mass analyzer  18  and a detector system  20 . Techniques for accomplishing sample ionization, ion sorting and detection, and considerations informing assembly of these techniques to perform analysis by mass spectrometry are known to those skilled in the art of mass spectrometry. 
         [0027]    The ion source  16  effects ionization of the sample by any one of several techniques, including electron ionization, chemical ionization, electrospray ionization, matrix-assisted laser desorption/ionization, and inductive coupling of a plasma. 
         [0028]    The ionization technique may incidentally introduce neutral particles unrelated to the physical sample into the ion stream entering the mass analyzer. For example, argon or helium atoms are typically present downstream of an ICP ion source, whereas ions transferred from an ion source operating at atmospheric pressure are at risk for contamination by nitrogen molecules. Pre-ionization separation techniques are another source of extraneous neutral particles such as the excited helium atoms normally seen with GC/MS, which typically uses a helium carrier gas. LC/MS may also introduce nitrogen molecules from an active agent of the ion source—such as a nebulizing gas—or from the atmosphere in which it operates. 
         [0029]    After treatment by the ion source  16 , the adventitious neutral particles are electrostatically propelled with the constituent ions of the sample through an inlet  22  in a gate  24  into the mass analyzer  18 . The gate  24  may be a focusing lens, a collimator or any other well-known apparatus, compatible with the function of the other components of the spectrometry system, for admitting ions into the analyzer. 
         [0030]    The mass analyzer  18 —for example, a sector field, time-of-flight, or quadrupole analyzer—sorts the ions according to their mass/charge ratio. The sorted ions pass through an aperture in an exit lens  30 , for example, a grounded plate with a standard 8 mm central aperture, to be counted by the detector system  20 . 
         [0031]    Neutral particles in the analyzer  18  are not sorted by the applied electric and magnetic fields and principally move through the analyzer  18  along straight paths between collisions. Sufficiently energetic neutral particles striking surface contaminants on instrument components generate secondary ions. Secondary ions generated from bombardment of the lens  30  near its aperture exit the analyzer through the aperture. Also, excited neutral particles leaving through the aperture may generate secondary ions by striking elements of the detector system  20 . Secondary ions originating from these locations enter the detector unsorted and are counted randomly by the detector system  20 , contributing to background noise. 
         [0032]      FIG. 2  shows in exploded view the layers of an illustrative embodiment of a noise-reducing composite exit lens  34  of the invention suitable for use in place of the prior art lens  30  in the mass spectrometry system  10 . The lens  34  comprises a middle electrode  36  sandwiched between two external electrodes  40  and  60  with intervening insulating layers  50  and  55 . The front electrode  40  consists of a solid conductive ring  42  around a central hole  44  with an attached conductive grid  46  covering the hole  44 . 
         [0033]    The front insulating layer  50  has a window  52  corresponding in size and shape to the hole  44 . The conductive middle electrode  36 , back insulating layer  55  and back electrode  60  respectively have aperture holes  62  of common shape and size, which are smaller than the window  52 . 
         [0034]      FIG. 3A  shows the assembled composite lens of  FIG. 2 .  FIG. 3B  shows the lens  34  without the grid  46  to facilitate explanation. Referring now to FIGS.  2  and  3 A-B, the grid-covered hole  44  and window  52  leave exposed on the middle electrode  36  a front surface  64  that is oriented toward the mass analyzer  18 . The holes  62  in the middle electrode, back insulating layer  55 , and back electrode  60  form a common aperture  66  through the lens  34  along an axis perpendicular to the exposed surface  64  of the middle electrode. In the embodiment, the common aperture  66  is centered with respect to the window  52 . Optionally, the grid  46  has an opening (not shown) such that the aperture  66  extends through the front electrode  40 . 
         [0035]    In operation, the middle electrode  36  is maintained at a potential differing from the potential of the front electrode  40  and from the potential of the back electrode  60  so that an ion on the middle electrode  36  experiences a local minimum in potential energy. A middle electrode  36  at a more positive potential than the front  40  and back  60  electrodes will create a potential energy well for a negative ion. A middle electrode  36  at a less positive potential than the front  40  and back  60  electrodes creates a potential energy well for a positive ion. In one embodiment, the potential of the middle electrode  36  differs from those of the external electrodes  40  and  60  by 10 to 75 volts, or more. 
         [0036]    In a preferred embodiment the two external electrodes  40  and  60  are grounded and the middle electrode  36  is at a potential differing from ground by 20 to 75 volts, or more. In a lens configured to confine negative secondary ions, the middle electrode potential is positive with respect to ground. To confine positive secondary ions, the middle electrode potential is negative with respect to ground. The grounded external electrodes  40  and  60  contain the electric field formed by the potential on the middle electrode  36  and limit the influence of the middle electrode on the trajectories of the subject ions through the aperture  66 . A voltage supply (not shown) may be used to maintain the middle electrode  36  at the desired relative potential. 
         [0037]    Ions approaching the lens  34  from the mass analyzer  18  pass through the grid  46  and are focused through the aperture  66 . The lens  34  does not electrically focus any neutral particles. Neutral particles striking the lens  34  with sufficient energy generate secondary ions. Secondary ions generated near the aperture  66 , by neutral particles that penetrate the grid  46  and then collide with the exposed surface  64  of the middle electrode, are prevented from leaving the surface  64  due to the local potential-energy minimum in the layered electrode  34 . The localized secondary ions do not reach the detector  20  and the noise they would have generated is preempted. This is in contrast to the prior art lens  30  of  FIG. 1 , the front surface of which releases secondary ions, thus allowing them to enter the detector system  20  and contribute to background noise. 
         [0038]    In another aspect, an embodiment of which is illustrated in  FIG. 4 , the invention provides a deflector plate  68  for confining secondary ions in a detector chamber  69  having an off-axis detector  70 . 
         [0039]    With reference to  FIG. 5 , the deflector plate  68  of the embodiment preferably comprises the following layers: a front electrode  72 , a front insulating layer  80 , a middle electrode  86 , a back insulating layer  90  and a back electrode  92 . 
         [0040]    The front electrode  72  is a solid conductive ring  74  around an interior hole  76  with an attached conductive grid  78  covering the interior hole  76 . The front insulating layer  80  is a solid frame  82  around a window  84  coextensive with the interior hole  76 . The middle electrode  86  has a surface  88 , facing the exit lens  30 , exposed through the interior hole  76  and window  84 . 
         [0041]    The middle electrode  86  is maintained at a potential about 20 to 75, or more, volts higher or lower, depending on whether negative or positive secondary ions are targeted, than the potentials of each of the front electrode  72  and back electrode  92  by a voltage supply  94 . In a preferred embodiment, the front electrode  72  and back electrode  92  are grounded. 
         [0042]    Ions leaving the mass analyzer  18  pass through the exit lens  30  into the chamber  69  and are pulled into the off-axis detector  70 , which is negatively biased by several thousand volts. Neutral particles entering the chamber  69  continue their trajectory until striking the exposed surface  88  of the middle electrode  86  facing the lens  30 . Resulting secondary ions are held on the surface  88  and prevented from making their way into the detector  70 . This is in contrast to mass spectrometry systems of the prior art, in which neutral particles collide with the chamber walls or other surfaces in the chamber  69 , thereby generating secondary ions which are pulled into the detector and contribute to background noise. 
         [0043]    The deflector plate of the invention  85  could in principle function without the back insulating layer  90  and the back electrode  92 . The grounded back electrode  92  ensures that the electric field created by the middle electrode  86  is contained so as to minimize its influence the trajectories of ions entering the detector chamber  69 . 
         [0044]    The layered structures of the embodiments are readily constructed from stainless steel plate, poly(tetrafluoroethylene) sheet, and tungsten mesh. For example external and middle electrodes may be made of 0.5 mm-thick stainless steel with mesh on the front electrode and separated by 0.25 mm-thick plastic insulating layers. The mesh may be tungsten wire mesh of 50×50 wires/inch and 0.003 inch wire diameter, which does not unduly interfere with transmission of the subject ions. The layers may be held together by conventional means such as clamps or screws. 
         [0045]    In other embodiments, the front electrode may be constituted entirely of mesh, without any solid border. As used herein, mesh denotes not only an interwoven or intertwined structure, but may equivalently be a grid or perforated material capable of distributing the potential of the middle electrode while allowing neutral and charged particles to pass. The relative sizes and positions of the holes and windows are not necessarily as described in the embodiments. Rather, the holes and windows may be in any relationship that establishes the middle electrode surface behind the mesh and, in the case of an exit lens, an aperture to pass subject ions out of the analyzer. Furthermore, the insulating layers adjacent the middle electrode may be absent altogether. For example, the electrodes may be captured at the edges and their mutual insulation maintained in the low-pressure atmosphere of the apparatus by gaps. 
         [0046]    The specified voltage ranges were determined using a GC/MS system with a quadrupole analyzer and dynode detector. It is expected that similar voltage ranges would be effective for mass spectrometry systems having different principal components. 
         [0047]    Although specific features of the invention are included in some embodiments and drawings and not in others, it should be noted that each feature may be combined with any or all of the other features in accordance with the invention. 
         [0048]    It will therefore be seen that the foregoing represents a highly advantageous approach to mass spectrometry, especially for technique varieties dependent upon introducing an inert gas into the instrument. The terms and expressions employed herein are used as terms of description and not of limitation, and there is no intention, in the use of such terms and expressions, of excluding any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the invention claimed.