Abstract:
The present invention relates to an apparatus and method for focusing, separating, and detecting gas-phase ions using the principles of electrohydrodynamic quadrupole fields at high pressures, at or near atmospheric pressure. Ions are entrained in a concentric flow of gas and travel through a high-transmission element into a RF/DC quadrupole, exiting out of the RF/DC quadrupole, and then impacting on an ion detector, such as a faraday plate; or through an aperture or capillary tube with subsequent identification by a mass spectrometer. Ions with stable trajectories pass through the RF/DC quadrupole while ions with unstable trajectories drift off-axis collide with the rods and are lost. Alternatively, detection of ions with unstable trajectories can be accomplished by allowing the ions to pass through the rods and be detected by an off-axis detector. Embodiments of this invention are devices and methods for focusing, separating, and detecting gas-phase ions at or near atmospheric pressure, when coupled to mass spectrometers.

Description:
GOVERNMENT SUPPORT 
     The invention described herein was made in part with United States Government support under Grant Number: 1 R43 RR15984-01 from the Department of Health and Human Services. The U.S. Government may have certain rights to this invention. 
    
    
     CROSS-REFERENCE TO RELATED APPLICATION 
     This application is entitled to the benefit of application Ser. No. 10/155,151, filed 2001 May 26, now U.S. Pat. No. 6,784,424, issued 2004 Aug. 31. In addition, this invention uses the high-transmission elements of our applications, Ser. No. 09/877,167, filed 2001 Jun. 8, now U.S. Pat. No. 6,744,041, issued 2004 Jun. 1; and Ser. No. 10/449,147, filed 2003 May 31, now U.S. Pat. No. 6,818,889, issued 2004 Nov. 16; Ser No. 10/862,304, filed 2003 Jun. 7, now U.S. patent publication No. 2005/0056776, issued 2005 Mar. 17; and Ser. No. 10/989,821, filed 2004 Nov. 15. 
     BACKGROUND 
     1. Field of Invention 
     This invention relates to an atmospheric RF/DC device, specifically to such RF/DC devices which are used for analyzing gas-phase ions at or near atmospheric pressure. 
     2. Description of Prior Art 
     Quadrupole Mass Spectrometry (QMS) 
     The analytical utility of a RF/DC (radio frequency/direct current) mass filter or analyzers, such as a quadrupole mass filter, as a device for continuous selection and separation of ions under conventional vacuum conditions is well established. It also has a highly developed theoretical basis (for example see, Paul et al. (1953), Dawson (1976), Miller et al. (1986), Steel et al. (1999), Titov (1998), Gerlich (1992). The desirable performance attributes of the quadrupole mass filter is the fact that motion in the x, y, and z directions are decoupled, (i.e. motion in each direction is independent of motion of the other directions in the Cartesian coordinate system, see Dawson (Chapter 2, 1976)). In general, a time varying potential is applied to opposite sets of parallel rods as illustrated in  FIG. 1 . 
     The “hyperbolic” geometry in the x-y plane coupled with the appropriate time-varying applied potential (an RF field) creates a pseudo-potential well that will trap ions within a “stable” mass range along the centerline of the x-y plane (the z-axis), while ejecting ions of “unstable” mass in the x and y directions. In a quadrupole operated a low pressures (under vacuum, &lt;10 −3  torr), motion along the z-axis is generally determined by the initial energy of the ions as they enter the quadrupole field, and can be generally considered equivalent to motion in a field free environment. One notable exception to this field-free model would be the effects the fringing fields at the entrance and exit of the quadruple. At the entrance and exit from quadrupoles the x, y and z motions are coupled. This results in the transfer of small amounts of translational energy between the different dimensions. The effects of which can generally be reduced dramatically through electrode design (e.g. the use of RF-only pre- and post-filters). 
     Ion motion within a quadrupole is well characterized, and is described by the various solutions of the Mathieu equation (see Dawson (Chapter 3, 1976), Miller et al. (1986), Steel et al. (1998)). Simply stated, for a given ion with a particular mass-to-charge ratio (m/z), there exist sets of RF (alternating at the radio frequency) and DC (direct current) voltages, which when applied to a quadrupole yield stable trajectories. These sets of RF and DC voltages can be plotted to represent regions of stability both in the x and y directions (as shown in  FIG. 2A ). Since motion in the x and y directions are de-coupled, it is convenient to plot both directions in a single plot, focusing on the region(s) where stable trajectories are possible simultaneously in both the x and y directions. This region of stability is designated the “bandpass region”. 
     According to the analytical theory based on the Mathieu equation, any set of voltages which do not lie within one of these regions of stability (in both x and y directions) will result in an unstable trajectory of ions, with exponentially increasing acceleration from the centerline of the quadrupole in the unstable direction (x or y). These stability boundaries tend to be very sharp, and can therefore be used to reject certain masses while accepting other masses. Since each mass has a unique set of stable voltages, judicious selection of voltages can allow selection of a narrow bandpass of masses (or one particular mass) to be transmitted through the quadrupole at the expense of all others as illustrated in  FIG. 2B . Quadrupole mass spectrometers are typically scanned through the mass range by increasing both RF and DC voltages while maintaining a constant ratio (see “Scan Line” in  FIG. 2B ). The slope of the scan line determines the resolution of the mass spectrometer. 
     There is evidence that these stability boundaries observed with convention quadrupole operation are independent of the operating pressure, and therefore achieving a specific mass resolution should be possible even for a quadrupoles operated at higher pressures, such as atmospheric pressure. The majority of research with higher pressures has occurred in the pressure range of 1×10 −5  to 1×10 −1  torr with the three-dimensional quadrupole ion trap (for example, Johnson et al. (1992), U.S. Pat. No. 4,540,884 to Strafford et al. (1985)) and recently with two-dimensional (2-D) quadrupole linear traps (for example, U.S. Pat. No. 5,420,425 to Bier et al. (1995) and U.S. Pat. No. 6,797,950 to Schwartz et al. (2004); and commercialized by Applied Biosystems/MDS Sciex of Foster City, Calif., USA (see http://www.appliedbiosystems.com) and Thermo Electron Corp. of San Jose, Calif., USA (see http:/www.thermo.com)). It has been clearly observed with three-dimensional quadrupole ion traps that stability boundaries may actually be sharpened at these higher pressures yielding improved resolution. But there are limits with the operating pressures. As the pressure is increased in quadrupole devices the incidence of a gas discharge increases as illustrated in the studies of ion pipes by Bruce Thomson and coworkers (Thompson et al. (1995)). 
       FIG. 3  illustrates that there are two pressure regimes where time-varying fields can be established at sufficient field strength to affect the radial displacement of unstable ions; the first is at low pressures (&lt;10 −2  torr, where existing 2- and 3-D quadrupole mass analyzers and traps are operated) and the second is at or near atmospheric pressure (760 torr, the present invention). The region marked forbidden at intermediate pressures is limited by gas discharge at the higher voltages required for quadrupole mass filtering. In addition, scattering effects from discrete collisions between ions and the surrounding gases deleteriously affect the motion of the ions in the intermediate pressure region as well. 
     Ion Mobility Spectrometry (IMS) 
     In recent years ion mobility spectrometry (IMS) has become an important analytical tool for measurement of ionized species created in a wide variety of atmospheric pressure ion sources; including but not limited to, discharge,  63 Ni, and photo-ionization (Eiceman et al. (1994), Hill et al. (1990)). Recently, a number of researchers have also incorporated LC/MS sources, such as, electrospray (ES) and atmospheric pressure chemical ionization (APCI) into IMS (Wyttenbach et al. (1996), Wittmer et al. (1994), Covey et al. (1993), Guevremont et al. (1997)). 
     One recent non-conventional implementation of IMS (known as FAIMS, high-field asymmetric waveform ion mobility spectrometry) utilizes an asymmetric waveform to isolate ions between parallel plates or concentric tubes (Buryakov et al. (1993), U.S. Pat. No. 5,420,424 to Carnahan et al. (1995), Purves et al. (1999), W.O. patents 00/08456 (2000) and 00/08457 (2000) both to Guevremont et al., and commercialized by lonalytics, Corp. (Ottawa, Calif., http://www.ionalytics.com) as an LC/MS interface). This technique demonstrates the principal that we propose with the present invention, in that it utilizes a flow of gas along the z-axis coupled with alternating field conditions to create a bandpass spectrometer. Of particular note is the ability to produce field strengths of well over 10,000 volts per cm without discharge occurring. When coupled to ES and mass spectrometry FAIMS has served as an effective means of fractionation of various molecular weight regimes (Ells et al. (1999)). 
     Recent work by Miller and coworkers (U.S. Pat. Nos. 6,495,823 (2002), 6,512,224 (2003), 6,690,004 (2004), 6,806,463 (2004), 6,815,668 (2004) 6,815,669 (2004), 6,972,407 (2005); and U.S. patent application publications 2003/0132380 (2003) and 2004/0094704 (2004)) have introduced a miniaturized differential mobility device, microDMx™ (see SIONEX, Corp., Bedford, Mass., USA, http://www.sionex.com) and are now selling the device complete with electronics and as a component for incorporation into analytical devices, for example, gas chromatography-differential mobility detectors: CP-4900 by Varian, Inc. (Palo Alto, Calif., USA, http://www.varianinc.com) and EGIS Defender™ by Thermo, Inc. (Waltham, Mass., USA, http://www.thermo.com). 
     In a separate implementation of ion mobility, an ion mobility storage trap, both 2- and 3-dimensional traps, with asymmetric alternating current (AC) and variable direct current (DC) potentials has been proposed—for example, in the U.S. Pat. No. 6,124,592 to Sprangler (2000). Although these ion trapping devices may be able to trap ions, but once the ions are trapped ejecting the ions from the trap is very difficult due to lack of inertia of the ions at higher pressures, especially at, near, and above atmospheric pressure. These devices must rely on ions drifting very slowly out of the trap. 
     Our patent U.S. Pat. No. 6,784,424 B1 (2004) disclosed many of the same components of the present invention; however, the present invention distinguishes itself from our own prior art by disclosing improved ion sample introduction, alternative operating modes, and improved ion detection alternatives that yield better specificity and selectivity. 
     Nevertheless all the RF/DC mass filters or analyzers, linear and three-dimensional quadrupoles, IMS, FAIMS, and DMS heretofore known suffer from a number of disadvantages: 
     (a) Conventional quadrupole mass filters require the need for components, such as vacuum chambers, high-vacuum electrical feed-throughs, etc., that can withstand large pressure differences (−1,000 torr). This necessitates the need for stainless steel, aluminum, or other materials; chambers with vacuum tight welds; or metal or rubber seals that can withstand the large pressure difference. 
     (b) Conventional quadrupole mass filters require the need for expensive high vacuum pumps, such as turbomolecular or diffusion pumps; and low vacuum pumps, such as mechanical vane pumps; both costing several thousands of dollars. The cost of these pumps can makeup approximately 20% of the total cost of an instrument. 
     (c) Atmospheric interfaces for quadrupole mass filters require expensive high vacuum pumps for operation, resulting in costly and complex interface designs. 
     (d) Quadrupole mass filters weight several hundred pounds and require a substantial amount of electrical power for operation, heating and cooling, etc.; all restricting their portability. 
     (e) These all add to the manufacturing cost of quadrupole mass spectrometers and filters thereby resulting in a large percentage (−50%) of the cost of mass analyzers being due to the cost of the vacuum system components, including the vacuum pumps (both high and low vacuum), chamber, vacuum feed-throughs; atmospheric pressure interfaces; etc. 
     (f) FAIMS and other IMS analyzers lack the precision and band pass capabilities of quadrupolar designs or other multi-pole designs, by utilizing only 2 parallel plates instead of multiple poles. For example, in FAIMS and other asymmetrical RF devices, by utilizing asymmetric RF voltages between parallel plates these devices are forming only one-half of the fields seen in quadrupolar designs, therefore stopping short of the precision and band-pass capabilities of quadrupolar devices. 
     (g) 2- and 3-dimensional ion trapping devices while having the ability to trap ions with symmetric (and asymmetric) RF and DC potentials, lack sufficient axial forces to move ions from inside the device to the outside where they may be detected or samples through apertures or capillaries. 
     (h) All of these designs suffer from a very inefficient sampling of atmospheric gas-phase ions into the area between the parallel plates. 
     OBJECTS AND ADVANTAGES 
     Accordingly, besides the objects and advantages of the atmospheric quadrupole device described in our above patent, several objects and advantages of the present invention are: 
     (a) to provide a RF/DC mass and mobility analyzer with an axial flow of gas that can be produced from a variety of materials without requiring the need for materials and/or construction that can withstand large pressure difference; 
     (b) to provide a RF/DC mass and mobility analyzer with an axial flow of gas which does not require the use of high vacuum pumps; 
     (c) to provide a RF/DC mass and mobility analyzer with an axial flow of gas which does not require high vacuum pumps for atmospheric pressure ion-source interfacing; 
     (d) to provide a RF/DC mass and mobility analyzer with an axial flow of gas which is both lightweight and portable; 
     (e) to provide a RF/DC mass and mobility analyzer with an axial flow of gas which can be inexpensive to manufacture and easily mass produced; 
     (f) to provide a RF/DC mass and mobility analyzer with an axial flow of gas which can provide a precise band-pass capability; 
     (g) to provide a RD/DC mass and mobility analyzer with an axial flow of gas which can efficiently sample gas-phase ions at atmospheric pressure. 
     Further objects and advantages are to provide an atmospheric RF/DC mass analyzer with an axial flow of gas which can be composed of plastic and other easily molded materials; the electrodes (traditionally call rods) can be solid, tubes, make of perforated metal sheets or axially oriented wires; ion source can be an atmospheric pressure ionization source, such as but not limited to, atmospheric pressure chemical ionization, electrospray, photo-ionization; corona discharge, inductively coupled plasma source, etc.; and ion detector can be but not limited to an active pixel sensor array. Still further objects and advantages will become apparent for a consideration of the ensuing descriptions and drawings. 
     SUMMARY 
     In accordance with the present invention an atmospheric RF/DC mass and mobility analyzer comprises an atmospheric ion source, an ion-focusing region, an RF/DC quadrupole, an atmospheric gas-phase ion detector, and a source of gas which flows down the axis of the device. 
     REFERENCES 
     
         
         Buryakov, I. A., Krylov, E. V., Nazarov, E. G., Rasulev, U. Kh., “A new method of separation of multi-atomic ions by mobility at atmospheric pressure using a high-frequency amplitude-asymmetric strong electric filed,” Int. J. Mass Spectom. Ion Processes. 128, pages 143-148 (1993). 
         Covey, T., Douglas, D. J., “Collision cross sections for protein ions,” J. Am. Soc. Mass Spectrom. 4, pages 616-623 (1993). 
         Dawson, P. H., “Quadrupole Mass Spectrometry and Its Applications,” Elsevier: New York (1976). 
         Dawson, P. H., “Chapter 2: Principals of operation,” IN: Quadrupole Mass Spectrometry and Its Applications, Dawson, P. H. (ed.), pages 9-64, Elsevier: New York (1976). 
         Dawson, P. H., “Chapter 3: Analytical Theory,” IN: Quadrupole Mass Spectrometry and Its Applications, Dawson, P. H. (ed.), pages 65-78, Elsevier: New York (1976). 
         Eiceman, G. A., Karpas, Z., “Ion Mobility Spectrometry,” CRC Press: Boca Raton (1994). 
         Ells, B., Bameft, D. A., Froese, K., Purves, R. W., Hrudey, S., Guevremont, R., “Detection of chlorinated and brominated by products of drinking water disinfection using electrospray ionization-high-field asymmetric waveform ion mobility spectrometry-mass spectrometry,” R., Anal. Chem. 71, pages 4747-4752 (1999). 
         Guevremont, R., Siu, K. W. M., Ding, L., “Ion mobility/TOF mass spectrometric investigation of ions formed by electrospray of proteins,” Proceedings of the 45 th  ASMS Conference on Mass Spectrometry and Allied Topics, page 374, Palm Springs, Calif., Jun. 1-5, 1997. 
         Guevremont, R, Purves, R., Barrett, D., “Method for Separation and Enrichment of Isotopes in gaseous Phase,” WO Patent 00/08456 (Feb. 17, 2000). Guevremont, R, Purves, R., “Apparatus and Method for Atmospheric Pressure 3-Dimensional Ion Trapping,” WO Patent 00/08457 (Feb. 17, 2000). Purves, R., Guevremont, R, “Electrospray ionization high-field asymmetric waveform ion mobility spectrometry-mass spectrometry,” Anal. Chem. 71, pages 2346-2357 (1999). 
         Hill, H. H., Siems, W. F., St. Louis, R. H., McMinn, D. G, “Ion mobility spectrometry,” Anal. Chem. 62, pages 1201A-1209A (1990). 
         Gerlich, D., “Inhomogeneous RF fields: A vsersatile tool for the study of processes with slow ions,” IN: State-Selected and State-To-State Ion-Molecule Reaction Dynamics. Part 1. Experiments, Ng, C-Y, Baer, M. (eds.), pages 1-176, John Wiley &amp; Sons: New York (1992). 
         Johnson, J. V., Pedder, R. E., Yost, R. A. “The stretched quadrupole ion trap: implications for the Mathieu a u  and q u  parameters and experimental mapping of the stability diagram,” Rapid Commun. Mass Spectrom. 6, pages 760-764 (1992). 
         Miller P. E., Denton, M. B., “The quadrupole mass filter: Basic operating concepts,” J. Chem. Ed. 63, pages 617-622 (1986). 
         Paul, W., Steinwedel, H., “Mass spectrometer without magnetic field,” Z. Naturforsch, 8a, pages 448-450 (1953). 
         Stafford, G. C., Kelly, P. E., Stephens, D. R., “Method of Mass Analyzing a Sample by Use of a Quadrupole Ion Trap”, U.S. Pat. No. 4,540,884 (Sep. 10, 1985). 
         Steel, C., Henchman, M., “Understanding the quadrupole mass filter through computer simulation,” J. Chem. Ed. 75, pages 1049-1054 (1998). 
         Thomson, B. A., Douglas, D. J., Corr, J. J., Hager, J. W., Jolliffe, C. L., “Improved collisionally activated dissociation efficiency and mass resolution on a triple quadrupole mass spectrometer,” J. Am. Soc. Mass Spectrom. 6, pages 1696-1704 (1995). 
         Titov, V. V., “Detailed study of the quadrupole mass analyzer operating within the first, second, and third, (intermediate) stability regions. I. Analytical approach,” J. Am. Soc. Mass Spectrom 9, pages 50-69 (1998). 
         Wittmer, D., Chen. Y. H., Luckenbill, B. K, Hill, H. H., “Electrospray ionization ion mobility spectrometry,” Anal. Chem. 66, pages 2348-2355 (1994). 
         Wyttenbach, T., von Helden, G., Bowers, M. T., “Gas-phase conformation of biological molecules: Bradykinin,” J. Am. Chem. Soc. 118, pages 8335-8364 (1996). 
       
    
    
    
     
       FIGURES 
       In the drawings, closely related figures have the same number but different alphabetic suffixes. 
         FIG. 1  (Prior Art). Rod assembly and polarity configuration for a conventional (vacuum) quadrupole. The applied potentials, variable in time t and at frequency Ω, showing both the DC component V dc ; and the alternating component V rf . V ion energy  is a fixed DC potential on the rods (commonly referred to as pole bias) that determine the energy of the ion in the z-direction. 
         FIG. 2A  (Prior Art). Bandpass Region: x, y-stability regions for a given mass in a quadrupole mass filter, with axis label with rf and dc functions rather than traditional a and q values. The overlap indicates the bandpass region. 
         FIG. 2B  (Prior Art). Scanning the Mass Range: the bandpass region of the stability diagram for three masses (M 1 , M 2 , and M 3 ) indicating how one mass is resolved from another through rejection of adjacent masses due to instabilities. 
         FIG. 3  Applied voltage of the RF (V rf )(peak-to-peak) versus observed discharge limit as a function of pressure. Both conventional (vacuum) and atmospheric pressure-operating regimes are shown. 
         FIG. 4  represents the geometry of the potential surface in the x-y plane within a quadrupole device for operation in the DC mode. Motion of ions in DC fields tend to follow the electric field lines (moving perpendicular to the equipotential lines) at atmospheric pressure. Ions introduced near the rods (top-of-the-saddle) are directed downhill into the pseudo-potential well. While ions in the pseudo-potential well are directed downhill into the rods. 
         FIG. 5  represents the geometry of the potential surface in the x-y plane within a quadrupole device operating in the RF mode. Motion of ions in RF fields oscillate about a fixed point with little if any motion towards or away from the rods. Little inertial focusing occurs at atmospheric pressure because most of the inertial energy is dissipated through random collisions within a few collisions. 
         FIGS. 6A and 6B  are cross-sectional representations of the essential features of the atmospheric RF/DC mass and mobility analyzer, depicting a quadrupole device, with an ion source, an ion focusing region at the entrance of the quadrupole RF/DC filter (for introducing ions at the top-of-the-saddle), sample and carrier gas inlets, a gas exhaust, and a detector region at the exit of the quadrupole device with a tubular faraday detector on-axis with the quadrupoles to sample ions from the field-free axis.  FIG. 6B  is enlarged view of the Focusing and Quadrupole Regions. 
         FIG. 7  is a cross-sectional representation of the essential features of a similar atmospheric RF/DC mass and mobility analyzer, depicting the quadrupole device, with the ion source, the ion focusing region at the entrance of the quadrupole RF/DC filter (for introducing ions at the top-of-the-saddle), the sample and carrier gas inlets, the gas exhaust, and the detector region at the exit of the quadrupole device with a tubular conductance pathway leading into a low pressure chamber occupied by a mass spectrometer. 
         FIG. 8  is a cross-sectional schematic representation of a similar atmospheric RF/DC mass and mobility analyzer operating as a high-pass filter with top-of-the-saddle sample introduction and collection of high-passed ions (Species B) on-axis with the quadrupole; for the removal of excess low mass reagent ions (Species A). 
         FIG. 9A  is a cross-sectional schematic representation of a similar atmospheric RF/DC mass and mobility analyzer operating as a low-pass filter with top-of-the-saddle sample introduction and DC collection of low-passed ions (Species B) on-axis of with the quadrupole; for the removal of particles (Species A). 
         FIG. 9B  is a cross-sectional schematic representation of a similar atmospheric RF/DC mass and mobility analyzer comprised of RF-only pre-quadrupoles and RF/DC quadrupoles, operating as a band-pass filter. Note the trajectories of low mass or high mobility ions (Species A) resulting in these ions colliding with the rods in the RF-only region. Ions of appropriate mobility (Species B) are directed towards the axis of the quadrupole in the RF/DC region, primarily under the influence of net DC fields and passed through the analyzer. While high molecular weight ions or particles (low mobility components, Species C) pass through the analyzer without being directed into the axis of flow. 
         FIG. 10  is a cross-sectional schematic representation of a similar atmospheric RF/DC mass and mobility analyzer operating as a band-pass filter with a physical stop on-axis to stop or prevent ions of a specific mobility (in this case Species A) from passing through the analyzer. The mobility of Species C is less than Species B while the mobility of Species B less than Species A. 
         FIGS. 11A to 11B  are a cross-sectional schematic representations of a similar atmospheric RF/DC mass and mobility analyzer with off-axis detection of unstable ions (Species B) sampled through the rods as an alternative to axial sampling.  FIG. 11B  is a cross-sectional slice along the plane at the opening in the rod looking down the assembly from the detector region, shows the motion of positive ions (Species B) from the top-of-the-saddle, through an opening in the rod (at negative potential) continuing through the central axis of the rod, and finally impacting onto the detector. 
         FIGS. 12A and 12B  are schematic representations of a similar atmospheric RF/DC mass and mobility analyzer, comprised of two sets of quadrupole assemblies in series operating as an ion trap.  FIG. 12A  showing the focusing and accumulation of Species B in the RF-only quadrupole assembly, while  FIG. 12B  shows the release and detection of the trapped ions. 
         FIG. 13  is a representation of the essential features of a RF/DC mass and mobility analyzer operated below atmospheric pressure depicting a quadrupole device, with an atmospheric or near atmospheric pressure ion source, a top-of-the-saddle ion focusing region at the entrance of a low pressure quadrupole RF/DC filter; sample and carrier gas inlets; a low pressure detector region at the exit of the quadrupole RF/DC filter comprised of a hemispherical high-transmission element for collecting and focusing ions into or onto an ion detection apparatus; and a vacuum exhaust for maintaining the RF/DC filter and detector region below atmospheric pressure but still operating in the viscous flow regime; and below the discharge boundaries as prescribed by the boundaries of the Paschen curve (see  FIG. 3 ). 
     
    
    
     REFERENCE NUMBERS IN DRAWINGS 
     
         
           10  Ion Source Region 
           12  Gas inlet 
           13  gas inlet 
           14  cylindrical electrically conductive analyzer housing 
           20  Focusing Region 
           22  electrical lead 
           30  Quadrupole Region 
           32  electric lead 
           40  Detector Region 
           42  electrical lead 
           44  electrical lead 
           46  gas-exhaust port 
           47  vacuum pump 
           50  conductive electrospray ionization chamber 
           52  ionization region 
           54  electrospray needle 
           56  insulator 
           60  laminated high-transmission element 
           64  insulator 
           65  entrance lens 
           66  entrance apertures 
           67  slotted or tubular openings 
           68  axial gas inlet tubes 
           72  atmospheric or near atmospheric RF/DC quadrupole filter or assembly 
           73  axial stop 
           74  individual primary electrodes 
           76  insulator 
           77  insulator 
           90  housing 
           94  exit lens 
           95  detector insulator 
           96  ion detector 
           97  off-axis detector 
           98  ion exit opening 
           99  conductance tube 
           101  opening 
           180  region 
       
    
     DETAILED DESCRIPTION 
     Preferred Embodiment— FIGS. 6A ,  6 B,  7 ,  8 ,  9 A, and  9 B (Basic Focusing Device, On-Axis Detection) 
     A preferred embodiment of the atmospheric RF/DC device of the present invention is illustrated in  FIGS. 6A and 6B . Basic parts include an Ion Source Region  10 , Focusing Region  20 , RF/DC Quadrupole Region  30 , and Detector Region  40 . The Ion Source Region  10  is mounted at one end of the cylindrical electrically conductive analyzer housing  14  and is symmetrically disposed about the central axis Z. The ion source may comprise, for example, a conductive electrospray ionization chamber  50  comprised of an ionization region  52 , an electrospray needle  54 , an insulator  56 , and a gas inlet  12 . A carrier gas is supplied upstream of the Ion Source Region  10  through the gas inlet  12  from the regulated and metered gas supply source. The gas is generally composed of, but not limited to nitrogen. 
     This device is intended for use in collection and focusing of ions from a wide variety of ion sources at atmospheric or near atmospheric pressure; including, but not limited to electrospray, atmospheric pressure chemical ionization, photo-ionization, electron ionization, laser desorption (including matrix assisted), inductively coupled plasma, and discharge ionization. Both gas-phase ions and charged particles emanating from the Ion Source Region  10  are collected and focused with this device. Samples can be derived directly from gases or from surfaces at or near atmospheric pressure. Samples may also emanate from flow streams of liquid, gas, or aerosols and have any number of conditioning or selectivity steps before entering the present device. 
     A laminated high-transmission element or lens  60  is positioned symmetrically about the Z-axis adjacent to an atmospheric or near atmospheric RF/DC quadrupole filter or assembly  72  and downstream of the Ion Source Region  10 , in the Focusing Region  20 . The laminated high-transmission element  60  is comprised of an entrance lens  65  and two slotted or tubular openings  67  directing ions into the top-of-the-saddle (near the rods). Element  60  is electrically isolated from the housing  14  and RF/DC quadrupole assembly  72  by insulator  64 . The two tubular openings  67  of the laminated lens defines entrance apertures  66 . Electric lead  22  schematically depict the connections required to operate the high-transmission element  60  and entrance lens  65 . Additional gases can be added to the analyzer through axial gas inlet tubes  68 , the gas being delivered through inlet  13  from the Regulated and Metered Gas Supply. 
     Downstream of the Focusing Region  20  is the Quadrupole Region  30 , which contains the atmospheric RF/DC quadrupole filter assembly  72 . Individual primary electrodes  74  in assembly  72  are held in place and electrically isolated from the cylindrical electrically conductive housing  14  by a series of insulators  76   a ,  76   b ,  76   c . The primary electrodes  74  are in the form of cylindrical conducting rods or poles extending parallel to one another and disposed symmetrically about the central axis. The X rods lie with their centers in the X-Y plane, and the Y rods lie with their centers on the Y-Z plane. Electric lead  32  schematically depict the connections required to operate the quadrupole filter. The four rods  74  in standard positive and negative polarity sets are held in an equally spaced position and equal radial distance from the centerline by attachment to insulators  76   a ,  76   b ,  76   c.    
     An exit lens  94  is located downstream of the Quadrupole Region  30 , in the Ion Detector Region  40 , while a housing  90  encloses the Ion Detector Region  40 . Electric lead  42  schematically depict the connections required to operate the exit lens  94 . A series of insulator  77   a ,  77   b  isolates lens  94  from the housing  90 . An ion detector  96  with an ion exit opening  98 , such as a faraday plate, cup, or tube, or a tessellated array detector is symbolically provided with electrical leads  44 , and may be conveniently mounted on the exit lens  94  with detector insulator  95  isolating the exit lens  94  from the ion detector  96 . In addition, a gas-exhaust port  46  is located at the end of the housing  90 ; downstream of the detector  96 . 
     In  FIG. 7  the RF/DC atmospheric focusing device shows a conductance tube  99  for an atmospheric interface to a mass spectrometer mounted in the Detector Region  40  symmetrically disposed about the central Z-axis. Tube  99  has a diameter appropriate to restrict the flow of gas from the Ion Detector Region  40 , at or near atmospheric pressure, into region  180 . In the case of utilizing a mass spectrometer in region  180  for analysis and detection, typical aperture diameters of 100 to 500 micrometers of the ion exit opening  98  are generally required to maintain the mass spectrometer at low pressures. Alternatively, the conductance tube can be replaced by an array of tubes or apertures as described in our U.S. Pat. No. 6,878,930 (2005). 
     Additional Embodiment—( FIG. 10 ) On-Axis Detection with an Axial Stop. 
     An additional embodiment is shown in  FIG. 10 . An axial stop  73  is placed within the RF/DC quadrupole assembly  72  for removal of ions that have been focused into the center of the assembly. 
     Alternative Preferred Embodiment—( FIGS. 11A and 11B ) Off-Axis Detection. 
     An alternative configuration is to place a detector electrode  97  off-axis from the flow of gas behind or within a particular rod  74 . Ions that are unstable under the influence of the DC fields are directed at the appropriate polarity rod so that the ions will travel through an aperture or opening  101  in the rod and be detected by the off-axis detector  97 . Multiple discrete detectors  97  (along with accompanying apertures or openings  101 ) can be place at specific locations along the rod to simultaneously detect specific analytes under fixed voltage conditions, or a single detector can detect multiple analytes by scanning RF and DC voltages. The off-axis mode of sample collection can alternatively serve as a means to select ions through a conductance tube or opening into vacuum with the conductance opening location at an appropriate position off-axis for subsequent mass spectrometric analysis. 
     Alternative Preferred Embodiment—( FIGS. 12A and 12B ) Trapping Mode 
     An alternative configuration is to place the exit lens  94  in a position to retard the motion of ions downstream at the exit of the RF/DC Quadrupole Region  30 . This mode of operation will serve to trap ions in the pseudo-potential well, particularly if the downstream quadrupole assembly  72  is operated in RF-only mode. 
     Alternative Preferred Embodiments—( FIG. 13 ) Lower Pressure Mode 
     An alternative configuration is to place a vacuum pump  47  on the exhaust of the Detector Region  40  to enable reduction of pressure in the RF/DC Quadrupole Region  30  relative to the Ion Source  10  and Focusing  20  Regions. The lower pressure allows a higher degree of inertial focusing and better selectivity in the RF/DC Quadrupole Region  30 . Care has to be taken not to reduce pressure to the point where discharge occurs (See  FIG. 3 ). This mode may require only inexpensive pumps. 
     Alternative Embodiments—(Shapes, Multi-poles, Monopoles, and Manufacturing) 
     There are various possibilities with regard to the shape and number of poles  74  of the RF/DC atmospheric filter  72 , including hexapoles and octapoles. In addition, each electrical element or electrode  74  can be fabricated from solid metal stock, extruded and coated, formed from sheer stock (solid or perforated), or define by axially aligned wires to minimize turbulence. Alternatively, assembly  72  may be manufactured by using the techniques of microelectronics fabrication: photolithography for creating patterns, etching for removing material, and deposition for coating the surfaces with specific materials; or combinations of macro and microelectronic techniques. 
     Operation of the Basic Device (As Shown in  FIGS. 4  thru  10 ) 
     The manner of the using the RF/DC atmospheric mass and mobility analyzer with an axial flow of gas to collect, focus, and separate ions based on their mobility is as follows. Ions supplied or generated in the Ion Source Region  10  from the electrospray source are attracted to the laminated high-transmission element  60  by an electrical potential difference between the Ion Source Region  10  and the potential on element  60 . The ions will tend to follow the electrical field lines through the Ion Source Region  10 , pass through the entrance lens  62 , traverse the element  60 , enter the entrance apertures  66 , and be direct through laminated openings  67 . Such means are described and illustrated in our U.S. Pat. Nos. 6,818,889 (2004), 6,878,930 (2005), and 6,643,347 (2005); and U.S. patent applications Ser. Nos. 10/862,304 (2004), 10/989,821 (2004), and 11/173,377 (2005). In addition a sweep gas is also added into the Ion Source Region  10 . The combination of the potential difference and the flow of the sweep gases cause the ions, as they exit the laminated lens, to be focused at or near a small cross-sectional area at the entrance to the Quadrupole Region  30 , near an individual rod  74  (at the-top-of-the saddle). 
     As the ions or charged particles are swept into the Quadrupole Region  30  the RF or RF and DC potentials effectively select the ions of specific mobilities into the pseudo-potential well preventing their dispersion in the radial (X-Y) plane. While their movement along the longitudinal z-axis is driven by the gas flow supplied from the Ion Source Region  10  and the axial gas inlet tubes  68 . RF and DC potentials can be selected to select specific ions or a range of ions that are stable within the quadrupole assembly  72 . At the appropriate RF and DC ratios ions that are not stable will drift off the central axis and eventually collide with the rods (Species A in  FIGS. 8 and 9B ) or pass through the filter without falling into the pseudo-potential well (as shown by Species A in  FIG. 9A  and Species C in  FIG. 9B ). The ions that remain in the center or fall into the center (Species B in  FIGS. 8 ,  9 A, and  9 B) are swept out of the quadrupole cylinder exiting into the ion exit opening  98 . 
       FIG. 4  illustrates the motion of ions under the influence of DC fields in the Quadruple Region  30 . The X-Y potential surface shows the saddle-shaped geometry from the opposite polarity sets of quadrupole rods. In order for ions of specific mobility to be effectively collected in the potential well of the quadrupolar field, the ions must be introduced at the top-of-the-saddle where electric fields are highest and the mobility is greatest. This is the key operating condition of the present device. Also, the cross-section of ions being introduced into the quadrupolar fields should be small relative to the dimensions of the rods. The ions will follow the electric fields from the top-of-the-saddle to the axis of the quadrupole assembly  72 . 
       FIG. 5  illustrates the motion of ions under the influence of RF fields in the Quadruple Region  30 . Because the inertia of ions from electric fields is minimized at atmospheric pressure due to randomization from collisions, the focusing of ions in RF is minimized as well; ions can oscillate towards then away from the electrodes. We can utilize RF fields to remove ions from the stream if the amplitude of the RF induced motion extends to the rods where ions can collide with rods and be neutralize. Higher mobility or lower mass ions (Specie A in  FIGS. 8 and 9B ) with large amplitudes can be lose at the rod boundary when ions are introduced near the rod as shown with the present invention. Higher mass (or lower mobility) ions (Specie A in  FIG. 9A , Specie C in  FIGS. 9B ,  10 , and  11 A) will pass downstream and through the quadrupole assembly, only focused by DC fields. We envision that the present invention will operate with both RF and DC fields to affect the desired operating performance. In its simplest mode of operation, the present device doses not require RF fields, only DC. 
       FIG. 8  illustrates one mode of operation where high mobility ions (Species A) are lost due to RF displacement into the rods at the entrance to the quadrupole assembly while lower mobility (higher mass, Species B) species are focused in DC fields to the axis of the quadrupole assembly and transported to the axial detector  96  or conductance tube  99  ( FIG. 7 ). Note the sampling into a conductance tube from the axis of the quadrupole assembly minimizes rim loses associated with sampling from higher electric fields. This device operates as a “high-pass filter” and has an important operational utility of removing low mass reagent ions before conductance openings (ion exit opening  98 ) resulting in minimizing the effects of space charge at openings at higher currents. 
       FIG. 9A  illustrates the “low-pass filter” mode of operation where high mass species (low mobility species, Species A) are removed by virtue of the lack of radial displacement in the gas flow while lower mass species (higher mobility species, Specie B) fall into the pseudo-potential well and are samples on-axis. This operational mode has the utility to remove particles, charged or uncharged, while effectively transmitting sample. 
       FIG. 9B  illustrates the “band-pass filter” mode of operation utilizing a quadrupole assembly comprised of RF-only pre-quads and RF/DC quadrupoles. Low mass species (higher mobility species, Species A) are lost through RF displacement to the pre-quads, higher mass species (lower mobility species, Specie C) are lost due to lack of radial displacement from gas flow, and only intermediate species (Specie B) are transmitted on-axis and detected. 
       FIG. 10  illustrates the “band-pass filter” mode of operation where low mass species (higher mobility species, Specie A) are removed by virtue of an axial stop, higher mass species (lower mobility species, Specie C) are lost due to lack of radial displacement from gas flow, and intermediate species (Specie B) are transmitted on-axis and detected. 
     In the operation of this device as an atmospheric inlet to the mass spectrometer ( FIG. 7 ), the detector  96  is replace with the conductance tube  99  through which focused ions will travel on their path into a vacuum system. Both focusing fields and viscous forces will cause ions upstream of the ion exit opening  98  to travel into the vacuum system of the mass spectrometer in region  180 . It is intended that this atmospheric RF/DC focusing device be coupled to the vacuum inlet of any conventional mass spectrometer or the atmospheric pressure inlet to any ion mobility spectrometer. 
     Operation of Off-Axis Device (as Shown in  FIGS. 11A and 11B ) 
     This device operates in a similar manner to the axial devices with the notable exception that ions are allowed to fall off-axis under the influence of sufficient DC fields to drive the target analyte to an off-axis detector or conductance tube at or near the opposite polarity of the analyte ion. At fixed RF and DC potentials, specific ions will deposit at specific positions along the length of the rods; higher mobility species falling off the saddle first and lower mobility species later. Detectors  97   a ,  97   b  can be placed at an appropriate position along the axis to collect specific analytes. The rod voltages can also be scanned to direct a range of analytes to the detector  97 . Conversely, the rod voltages can be fixed to collect a specific target ionic species or a range of species. 
     Operation of a Trapping Device (As Shown in  FIGS. 12A and 12B ) 
     This embodiment operates in a sequential rather than a continuous manner. Sample is introduced into the quadrupole assembly from any of a wide variety of pulsed (i.e., MALDI) or continuous (i.e., electrospray) sources. The ions collected are directed onto the axis of the quadrupole assembly and gas flow directs them downstream toward exit lens  94 . In this embodiment, a retarding potential can be applied to retard transmission of some or all of the ionic species directed down the quadrupole assembly. When the quadrupole pseudo-potential well becomes full, the ions can then be released following out through the ion exit opening  98  or conductance tube into vacuum  99  for detection, mass analysis, or even conventional ion mobility analysis. 
     Operation of Low Pressure Mode (As Shown in  FIG. 13 ) 
     Reducing the pressure of the Quadrupole Region  30  to pressures somewhat below atmospheric allows some increase in the inertial components of motion relative to atmospheric pressure. Operating at lower pressures allows more effective RF focusing and potentially higher selectivity with the limitation of operating potentials below the breakdown potentials prescribed in  FIG. 3 . Operating from 10 to 300 torr and lower RF and DC potentials provide an operating mode where RF fields can contribute more to the collection, focusing, and detection of gas-phase ions. 
     Advantages 
     From the description above, a number of advantages of our atmospheric RF/DC mass and mobility analyzer become evident: 
     (a) Without the need for a vacuum interface between the ion source and the RF/DC mass and mobility analyzer there is no need for high vacuum pumps, vacuum interlocks and feed-throughs, small apertures for interfacing—all of which are expensive and can complicate the interface design. 
     (b) Without the need for a vacuum chamber, high vacuum pumps, vacuum feed-throughs, etc., all of which add to the cost of the analyzer, the RF/DC mass and mobility analyzer can be mass produced inexpensively. 
     (c) Being at atmospheric pressure there is no need for vacuum interlocks, thus avoiding the need to vent the system for maintenance or repair. 
     (d) Not requiring a vacuum chamber and large power requirements of the high vacuum pumps, the mass analyzer can be made of light weight material and not be tethered to one location. 
     CONCLUSION, RAMIFICATION, AND SCOPE 
     Accordingly, the reader will see that the atmospheric RF/DC mass and mobility filter of this invention can be used to separate gas-phase ions from an electrospray ion source or other atmospheric pressure ion sources based on mobility characteristics, and can be used as an atmospheric inlet to a mass analyzer, a ion mobility analyzer, or a combination thereof; and also can be used to pass a wide or a narrow mass range of ions. In addition, segmented quadrupole assemblies or assemblies arranged in parallel can be operated with independent values of frequency and RF and DC potentials; thus optimizing the passage of ions while eliminating charged and uncharged particles which may contaminate ion detectors or clog small apertures. 
     Furthermore, the atmospheric RF/DC mass and mobility analyzer has the additional advantages in that:
         it permits the production of RF/DC mass and mobility analyzers to be inexpensive;   it provides an atmospheric RF/DC mass and mobility analyzer which can be made from molded materials;   it provides an atmospheric RF/DC mass and mobility analyzer which is both lightweight and portable;   it allows access to and maintenance of RF/DC mass and mobility analyzers to be simple and accomplished without specialized tools;   it allows atmospheric or near-atmospheric ionization sources to be easily interfaced to RF/DC mass and mobility analyzers without the need for complex and costly vacuum system interface; and   it allows for all or nearly all ions formed at atmospheric pressure to be introduced into the RF/DC mass and mobility analyzer.       

     Although the description above contains many specifications, these should not be construed as limiting the scope of the invention but as merely providing illustrations of some of the presently preferred embodiments of this invention. For example, the RF/DC mass and mobility analyzer can be composed of multiple RF/DC, RF/DC-RF, or RF-RF/DC filters in parallel or in series; the rods of the RF/DC mass and mobility analyzer can have other shapes such as, tapered, hourglass, barrel, etc.; the rods can have various cross-sectional shapes, such as circular, oval, hyperbolic, circular trapezoid, etc.; the rods can be composed of solid cylinders, tubes, tubes made of fine mesh, composites, etc.; the ion source region can be composed of other means of atmospheric or near atmospheric ionization, such as photoionization; corona discharge, electron-capture, inductively couple plasma; single or multiple ion sources can be configured with individual or arrays of RF/DC mass and mobility analyzers; the ion detector can be have other means of detecting gas-phase ions, such as active pixel sensors, etc. 
     Thus the scope of the invention should be determined by the appended claims and their legal equivalents, rather than by the examples given.